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Review

Review on the Research and Development of Ti-Based Bulk Metallic Glasses

1
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China
2
School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
*
Author to whom correspondence should be addressed.
Metals 2016, 6(11), 264; https://doi.org/10.3390/met6110264
Submission received: 7 July 2016 / Revised: 11 October 2016 / Accepted: 18 October 2016 / Published: 4 November 2016

Abstract

:
Ti-based bulk metallic glasses (BMGs) are very attractive for applications because of their excellent properties such as high specific strength and high corrosion resistance. In this paper, we briefly review the current status of the research and development of Ti-based bulk metallic glasses. Emphasis is laid on glass-forming ability, mechanical properties, corrosion resistance, and biocompatibility.

1. Introduction

Metallic glasses are alloys which possess disordered atomic-scale structure and contain short- to medium-range ordered clusters. In 1960, the first metallic glass was fabricated by rapid quenching a metallic liquid of Au75Si25 with a high cooling rate of 106 K/s [1]. In the 1970s and 1980s, metallic glasses could be directly made in bulk form by solidifying the melt at relatively low cooling rates (typically 103 K/s or less), where “bulk” is defined as that the minimum dimension of the alloy sample exceeds 1 mm. In 1974, the first reported bulk metallic glass (BMG) was developed in the Pd–Cu–Si alloy system by Chen et al. and only φ1–2 mm glassy samples could be prepared [2]. Then, extensive work was performed in exploring novel BMGs with good glass-forming ability (GFA). For instance, Inoue’s group developed many classic glass-forming alloy systems, e.g., Pd–Cu–Ni–P [3,4], Zr–Cu–Ni–Al [5], La–Al–Cu–Ni [6], and Mg–Cu–Y [7]. Recently, the world’s largest BMG with a diameter of 80 mm and a length of 85 mm was successfully prepared based on the Pd42.5Cu30Ni7.5P20 alloy [8]. Johnson’s group developed a good glass former Zr41.2Ti13.8Cu12.5Ni10Be22.5 (vit1) with a low critical cooling rate of ~1 K/s, which is the first commercial BMG and has been widely studied [9]. BMGs have been discovered in many alloy systems, such as Pd-based [10], Zr-based [11,12,13], Cu-based [14,15,16], Mg-based [17,18], Fe-based [19,20], Ni-based [21,22], Co-based [23,24], real earth-based [25,26], Pt-based [27], and Au-based [28] systems.
Among the metallic elements in the periodic table, titanium possesses ultrahigh specific strength together with high corrosion resistance and good biocompatibility. Accordingly, titanium alloys are widely used for structural, functional, and biomedical applications. Compared with conventional crystalline titanium alloys, Ti-based BMGs show higher specific strength and other unique properties because of the amorphous structure and are more attractive for practical applications as structural and functional materials [29,30]. In recent decades, Ti-based BMGs have received significant attention, and a large number of Ti-based BMGs have been developed. In this review, we summarize the details of the developments on the glass-forming ability, mechanical properties, corrosion resistance, and biocompatibility of Ti-based BMGs.

2. Glass-Forming Ability of Ti-Based BMGs

The formation of the first Ti-based amorphous alloys was reported in 1977 in Ti–Be–Zr ternary systems by Tanner [31]. Since then, several Ti-based glass-forming systems, such as Ti–Si [32], Ti–Ni [33], Ti–Be [34], Ti–Nb–Si [35], Ti–Al–Ni [36], and Ti–Zr–Cu [37], have been developed subsequently. However, the glass-forming ability (GFA) of these alloys is too low to form bulk metallic glass. In 1994, Peker et al. [38] reported Ti–Zr–Be–Ni alloys with wide supercooled liquid regions up to 45 K. As the large supercooled liquid region always entails a high thermal stability of the supercooled liquid against crystallization, these Ti–Zr–Be–Ni alloys were inferred to be potential bulk glass formers. In 1998, the first Ti-based BMG was successfully prepared by a copper mold casting in a Ti–Cu–Ni–Sn alloy system [39]. Encouraged by this success, with the effort of scholars all over the world, a number of Ti-based BMGs, such as Ti–Cu–Ni [40,41], Ti–Zr–Be [42,43], Ti–Zr–Cu–Ni–Sn [44,45], Ti–Zr–Cu–Pd–(Sn, Si, Nb) [46,47,48,49,50], Ti–Zr–Cu–Ni–Be [51,52,53], Ti–Zr–Hf–Cu–Ni–Si–Sn [54], and Ti–Cu–Zr–Fe–Sn–Si–(Ag, Sc) [55,56,57], have been developed over the last few decades. Figure 1 shows the maximum diameters for glass formation obtained in different alloy systems and the year in which they were discovered [8,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73]. Compared with other alloy systems (e.g., Zr-, Pd-, Mg-, Fe-, Co-, Ni-, and Cu-based So far, the largest known critical diameter for Ti-based BMGs has been over 50 mm in the Ti–Zr–Cu–Ni–Be [72] and Ti–Zr–Cu–Fe–Be [73] quinary alloy systems. Only Pd- and Zr-based BMGs possess higher GFA. Figure 2 shows the X-ray diffraction (XRD) patterns and appearances of the developed (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 and Ti32.8Zr30.2Cu9Fe5.3Be22.7 fully glassy rods with critical diameters up to 50 mm [72,73].
The GFA of BMGs can be directly evaluated by the critical cooling rate for glass formation, which is difficult to measure accurately. The critical diameter for glass formation (tmax) is a more practical measure of GFA, which has been widely used to compare the GFA of different BMGs but strongly depends on the fabrication condition. Hence, many quantitative criteria based on the thermal properties that can be easily measured by differential scanning calorimetry (DSC) have also been proposed to evaluate the GFA of BMGs [74,75,76,77,78]. The following three parameters are the most widely used: the width of the supercooled liquid region ∆Tx (∆Tx = TxTg) [74], the reduced glass temperature Trg (Trg = Tg/Tl, sometimes use Tm instead of Tl) [75], and the γ parameter (γ = Tx/(Tg + Tl)) [76]. Here, Tg, Tx, Tm, and Tl are the glass transition temperature, the onset crystallization temperature, the melting temperature, and the liquidus temperature, respectively. Table 1 presents the details of the composition, the synthesis method, and the thermal properties of representative Ti-based BMGs that have been developed recently [39,40,41,45,47,49,52,55,56,57,73,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110]. Copper mold casting is found to be the most frequently used method to prepare Ti-based BMGs. Compared with ∆Tx, the parameters Trg and γ correlate better with tmax of Ti-based BMGs. All the Ti-based BMGs can be classified into three groups: Be- and Pd-free, Pd-containing, and Be-containing alloys. Based on the results summarized in Table 1, it has been found that although great progress has been achieved in the GFA of Ti-based BMGs, some technological drawbacks still exist. First, the titanium content of most developed Ti-based BMGs with high glass-forming ability is relatively low (<50 atom %). The addition of heavy elements (e.g., Pd, Cu, and Ni) improves the GFA but also increases the density. Second, all the developed centimeter-sized Ti-based BMGs contain toxic elements (e.g., Be) or noble elements (e.g., Pd). The largest critical diameter of Be- and Pd-free Ti-BMGs is only 7 mm [56]. Developing low cost and nontoxic Ti-based BMGs with high GFA is still very challenging.

2.1. Alloy Development Studies

As most bulk metallic glasses are multicomponent alloys, the composition design of bulk metallic glasses has been acknowledged to be a difficult task. However, all the developed BMGs can be traced on the binary glassy alloys. According to the binary phase diagrams, Ti can form deep eutectics with Cu, Be, Pd, Si, Co, Ni, etc. By rapid cooling (e.g., sputtering or single roller spun-melt technique), amorphous thin films or ribbons were fabricated in these binary systems [32,33,34,111,112,113]. In order to improve the GFA of these binary alloys and obtain Ti-based BMGs, different alloying elements have been added to the binary alloys to explore multicomponent Ti-based bulk metallic glass-forming alloy systems. Except for the trial-and-error method, which is widely used but tedious and time-consuming, some empirical criteria have also been used for choosing the alloying elements. According to Inoue’s classic three rules [114], the common characteristics of most bulk glass-forming alloy systems are (1) multicomponent systems consisting of more than three constituent elements; (2) significant differences in atomic sizes with a size ratio above 12% among the main constituent elements; and (3) negative heats of mixing among the main constituent elements. In this sense, the alloying elements should possess large atomic size mismatches and large values of negative heats of mixing against Ti. According to Table 1, we summarize the candidate constituent elements for developed Ti-based BMGs. Their atomic radii (R) and heats of mixing (∆Hmix) against Ti are listed in Table 2 [115]. Generally, the classic empirical rules work well. There are also some special cases such as Sc, Y (∆HSc-Ti = 8 kJ/mol, ∆HY-Ti = 15 kJ/mol), and Ag (RAg is similar as RTi). However, the content of these elements in Ti-based BMGs is relatively low. It has also been found that the substitution of similar atoms can markedly improve the glass-forming ability of BMGs [116]. Ti exhibits similar properties such as Zr, Hf, etc., while Cu can be replaced by similar atoms—Ni, Pd, or Ag. Therefore, based on the Ti–Cu binary system, multicomponent Ti(Zr, Hf)–Cu(Ni, Pd, Ag) alloys with higher GFA have been developed [82,86,98,99], indicating that this substitution method is also effective for exploring novel Ti-based BMGs.
As mentioned earlier, Ti-based BMGs can be classified into three groups: Be- and Pd-free, Pd-containing, and Be-containing alloys. In the rest of this section, typical alloy systems for each group are introduced in detail.

2.1.1. Be- and Pd-Free Ti-Based BMGs

This type of Ti-based BMG is mainly developed based on Ti–Cu binary systems [111]. The Ti–Cu–Ni alloy system, one of the typical representatives, is among the few Ti-based ternary systems with bulk glass-forming ability. In 2008, Wang et al. [41] investigated the GFA of the Ti–Cu–Ni ternary system systematically. Figure 3 shows the reported BMG-forming composition map of the Ti–Cu–Ni system. It can be seen that the BMG-forming composition region for tmax = 1 mm is located in the triangular region enclosed by the three intermetallic compounds TiNi, TiCu, and Ti2Cu (50–57 atom % Ti, 34–44 atom % Cu, and 6–10 atom % Ni). The Ti50Cu43Ni7 and Ti53Cu39Ni8 alloys possess the best GFA, which enables the preparation of fully glassy rods with diameters up to 1.5 mm. It has also been found that the critical diameter increases to 2 mm by more fine-tuning of Ti50Cu43Ni7 to Ti50Cu42Ni8 [40]. Subsequently, it has been shown that the replacement of Ti by Zr effectively improves the GFA of Ti–Cu–Ni glassy alloys. By the “3D pinpoint approach,” the BMG-forming maps for tmax = 1 mm was found in the composition range of 51–53 atom % (Ti + Zr), 38–41 atom % Cu, and 8–10 atom % Ni (as shown in Figure 4) [41]. This incentive played a trigger effect on the subsequent development of new Ti-based BMGs with critical diameters ranging from 1 to 6 mm, such as Ti–Zr–Cu [82,83], Ti–Zr–Cu–Ni [82], Ti–Zr–Cu–Ni–Sn [45,95], Ti–Zr–Cu–Ni–Sn–Si [104], Ti–Zr–Hf–Cu–Sn–Si [99], and Ti–Zr–Hf–Cu–Ni–Si–Sn [54]. Recently, a novel strategy for designing BMG with high mixing entropy and large GFA has been proposed. Using this method, a series of Ti–Cu–Zr–Fe(Co)–Sn–Si–Ag(Sc) [56,57] BMGs has been discovered by multi-substitution of similar elements. After multi-substitution, the mixing entropy of the alloys is significantly enhanced, and the crystallization process exhibits a higher level of complexity, implying a better GFA. It is worth mentioning that the critical diameter for glass formation was reported to be 7 mm for Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 alloy [56], which is the largest among the Be- and Pd-free Ti-based BMGs.

2.1.2. Pd-Containing Ti-Based BMGs

This type of Ti-based BMG was firstly developed in the Ti–Zr–Cu–Pd system in 2007 [86]. Since Pd possesses properties similar to Cu, Pd was introduced to partially substitute for Cu to improve the GFA. It was noticed that a series of 6 mm-diameter Ti–Zr–Cu–Pd alloys were produced, and the largest critical diameter of Ti–Zr–Cu–Pd quaternary alloy was up to 7 mm. In order to further improve the GFA, Zhu et al. investigated the addition of Sn on the GFA of Ti–Zr–Cu–Pd BMG [47]. With the addition of 2–4 atom % Sn, the critical diameter was markedly increased to 10 mm, which is larger than that of other Be-free Ti-based BMGs. Figure 5 shows the appearance of the as-cast 10 mm-diameter Ti40Zr10Cu34Pd14Sn2 and Ti40Zr10Cu32Pd14Sn4 rods. Except Sn, Si [49], Nb [50] and Co [117] have also been adopted as alloying elements for Ti–Zr–Cu–Pd quaternary alloys. The maximum critical diameters of Ti–Zr–Cu–Pd–Si, Ti–Zr–Cu–Pd–Nb, and Ti–Zr–Cu–Pd–Co were reported to be 5 mm, >2 mm, and 10 mm, respectively. Without biological toxic elements such as Be, Ni, and Al, these Ti-based BMGs are expected to be used as biomedical materials. The limitation is that, because of the noble element Pd, the cost of this type of Ti-based BMG is relatively high.

2.1.3. Be-Containing Ti-BMGs

This type of Ti-based BMG is mainly developed based on the Ti–Be binary system [34]. Be is a magic metallic element with unique properties. The density of Be is only 1.85 g/cm3, which is much lower than that of Ti (4.50 g/cm3). Therefore, Be–bearing Ti-based BMGs exhibit relatively low density and high specific strength compared with Be-free Ti-based BMGs. Moreover, as the atomic radius of Be is much smaller than that of most metallic atoms (as shown in Table 1), it has been found that the addition of Be significantly improves the GFA of Zr- [118,119], Cu- [120], and Ti-based BMGs [79,80,81]. Except for the Ti–Cu–Ni system mentioned in Section 2.1.1, the Ti–Zr–Be system is another important Ti-based ternary system with bulk glass-forming ability. In 2008, Duan et al. firstly reported a series of lightweight Ti–Zr–Be BMGs with critical diameters larger than 6 mm [79]. Without containing heavy late transition metals (e.g., Cu, Ni, and Fe), the density of these alloys was lower than 5 g/cm3, resulting in a high specific strength larger than 400 J/g, which is more than twice that of the commercial Ti–6Al–4V (the most widely used crystalline titanium alloy). In 2010, Zhang et al. systematically studied the GFA of the Ti–Zr–Be system [80,81]. As shown in Figure 6, two amorphous regions were found in the Ti–Zr–Be system. The first amorphous region includes two intermetallic compounds (BeTi and Be2Zr) and one solid solution (α-Ti). The second amorphous region contains only one intermetallic compound (Be2Zr) and two solid solutions (α-Ti and β-Zr) and exhibits better GFA (tmax = 5 mm) compared with the first one (tmax = 3 mm). This may be because intermetallic compounds are easier to be suppressed as they are always ordered phases. Based on experimental results, the best glass former was located at Ti41Zr25Be34, which is close to the predicted composition by binary-eutectic rule (Ti41Zr32Be27). Then, a series of Ti–Zr–Be–(Fe, Al, Ag, Cu, Ni, V, Cr) BMGs [79,80,81,87,88,89,90,91,92,93,94] with improved GFA was reported, and some of the developed quaternary alloys were found to possess a larger critical size up to a centimeter scale. A pseudo Ti–Zr–Cu–Ni–Be system, which possesses a much higher GFA, was discovered based on the Ti–Zr–Be system. In 2005, Park et al. evaluated the GFA of Ti–Zr–Cu–Ni–Be alloys by injection copper mold casting and systematically outlined the alloy composition ranges with a tmax larger than 1, 6, and 10 mm, as shown in Figure 7 [121]. In 2010, a breakthrough was made in the GFA of Ti–Be-based BMGs by Tang et al [72]. They found that the microstructure of a Ti50Zr23Ni3Cu6Be18 as-cast pancake mainly consisted of a primary β-Ti phase and an amorphous phase. Based on the composition of the amorphous phase, which promises higher GFA than that of Ti50Zr23Ni3Cu6Be18, they prepared a high number of pancakes with compositions nearby and measured the compositions of the corresponding amorphous phases. Using this technique, a series of Ti–Zr–Ni–Be–Cu BMGs with high GFA have been developed. It was reported that (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 is formable as a BMG alloy with a larger diameter of 50 mm via water quenching, which is a new record in the GFA of Ti-based BMGs [72]. By replacing Ni with similar atom Fe, Ti–Zr–Cu–Fe–Be BMGs with critical diameters up to 50 mm have also been developed [73]. By direct solidification, a 150 g Ti36.2Zr30.3Cu8.3Fe4Be21.2 amorphous ingot was successfully prepared, indicating a superior GFA. Ti–Be-based BMGs possess low density and high strength together with high GFA, which favors their application as high performance structural materials. However, their application is also constrained to an extent because of the toxicity of Be.

2.2. Role of Alloying Elements on GFA

Alloying additions have been widely applied to develop new metallic crystalline materials and optimize the properties of these alloys. Recently, this technique was proved to be a simple but effective way to improve the GFA of BMGs [122,123,124]. In this section, we will focus on the effect of alloying on the GFA of Ti-based BMGs. Table 3 summarizes the effects of different alloying elements on the thermal stability (reflected by ∆Tx) and GFA of Ti-based BMGs [39,49,52,54,57,79,87,90,91,92,99,105,106,107,108,110,117,125,126,127,128,129,130].

2.2.1. The Additions of Metalloid Elements

Metalloid elements (such as O, N, B, and Si) have a strong affinity with Ti and can be absorbed during the preparation process of Ti-based BMGs. Therefore, it is necessary to investigate the effect of metalloid elements on the GFA of Ti-based BMGs. It was reported that a high oxygen concentration has a detrimental effect on glass formation for Zr-based BMGs [131,132]. As Ti possesses similar properties as Zr and easily reacts with oxygen, it can be inferred that oxygen introduced from the raw materials or the low vacuum may have a negative effect on GFA of Ti-based BMGs. Experimental results indicate that the GFA of Ti42.5Cu40Zr10Ni5Sn2.5 is sensitive to the nitrogen doping level [125]; with the addition of less than 0.1 atom % nitrogen, the GFA is improved because of the suppression of the formation of a competing eutectic structure. However, if the nitrogen concentration exceeds 0.1 atom %, the formation of quasicrystals can be promoted, and the GFA is deteriorated. A minor Si addition also shows a great effect on the thermal stability and GFA of Ti-based BMGs. For example, a 2 atom % Si addition to Ti–Zr–Cu–Pd BMG can extend the ∆Tx from 50 K to 65 K, indicating an improvement in the thermal stability [49]. Substituting 1 atom % Ti with Si in a Ti42.5Zr2.5Hf5Cu42.5Ni7.5 alloy increases the critical diameter from 2 mm to 5 mm [99]. Because of the relatively small atomic size, the minor amount of Si may facilitate the formation of a denser atomic structure and stabilize the supercooled liquid. However, as Si exhibits large negative heats of mixing against most constituent elements of Ti-based BMGs (e.g., ∆HSi–Ti = −66 kJ/mol), an excessive addition of Si is detrimental to the GFA due to the formation of intermetallic compounds.

2.2.2. The Additions of Metallic Elements

The intermediate metallic atoms such as Fe, Ni, Cu, Co, and Nb have been widely used as alloying elements in various alloy systems. It was reported that only when the alloying quantities exceed 5 atom % have they been shown to be beneficial in bulk glass formation [123]. For Ti-based BMGs, this argument is supported by some examples listed in Table 3. For example, in order to improve the GFA of a Ti41Zr25Be34 alloy, the alloying quantities of Fe, Al, Cu, Ni, or Cr have to be over 5 atom % [79,87,88,89,90,91,92]. However, there are contrary examples. According to Guo et al.’s research [52], the optimum Ni content in the Ti40Zr25Cu15−xNixBe20 system is no more than 3 atom %. Moreover, the GFA of Ti–Zr–Cu–Pd BMG can be markedly improved by the addition of only 1 atom % Co, and the critical diameter is increased from 3 mm to 5 mm [117].
The addition of large atoms (e.g., Zr, Sn, and Sc) has been proved to be beneficial in increasing the atomic size difference and improving the GFA of Ti-based BMGs. For instance, Sn is an effective alloying element to developing Pd- and Be-free Ti-based BMGs with high GFA. By replacing 5 atom % of Cu with Sn in a Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 alloy, the supercooled liquid region can be extended by 29 K, and the critical diameter is also drastically enhanced from 2 mm to 6 mm [54]. Moreover, Sc has been found to be able to not only alleviate the harmful effect of oxygen impurity but also influence the formation of topological and chemical short-range orders. With the addition of 2 atom % Sc, the critical diameter of the Ti47Cu40Zr7.5Fe2.5Sn2Si1 alloy is increased from 3 mm to 6 mm [57].
Among all the metallic elements, Be possesses the smallest atomic size. As shown in Table 1, the Be-containing Ti-based BMGs possess higher GFA compared with Be-free Ti-based BMGs, indicating that the addition of Be is effective in enhancing the GFA of Ti-based BMGs. Moreover, the addition of Be also increases the specific strength of Ti-based BMGs. As Be is a toxic element, it is hoped that the GFA of Ti-based BMGs can be significantly enhanced with minimal Be content. However, the Be content in Ti-based BMGs is always over 5 atom %. Little work has been done on the effect of the minor addition of Be (<2 atom %) on the GFA of Ti-based BMG forming systems.

2.2.3. The Additions of Rare-Earth Elements

Rare-earth elements possess unique physical and chemical properties, so they have been widely used as minor additions for metallic materials. Yttrium is the most widely used real earth element to tailor the properties of BMGs [133,134]. It was found that the addition of 0.5 atom % yttrium can effectively increase the GFA of Ti40Zr25Be20Cu12Ni3 alloys and enables the formation of 5 mm-diameter glassy rods using low purity raw materials [106,127]. It is well known that oxygen is suspected to form titanium oxide at higher temperatures, which acts as crystallization nuclei. A small and proper amount of yttrium addition can scavenge oxygen from the supercooled liquid to suppress the precipitation of the Laves phase and lower the liquidus temperature. The large atomic size of yttrium also plays an important role in increasing the atomic packing efficiency and improving the GFA.

2.3. Prediction of GFA of Ti-Based BMGs

The search for novel BMGs is still mainly based on trial-and-error although a few empirical guides (e.g., Inoue’s three rules) have been proposed. It is important to develop a simple and unified criterion that can characterize the GFA of BMGs. The GFA of BMGs is directly reflected by the critical cooling rate Rc and the critical diameter tmax. However, Rc measurement is complex, while tmax is affected by the preparation conditions. Therefore, researchers have established GFA parameters or criteria based on the characteristic temperatures (e.g., Tg, Tx, Tm, and Tl), which can be determined by DSC, such as ∆Tx, Trg, and γ, mentioned at the beginning of this section. These parameters are good in practicability but cannot intrinsically reflect the origin of GFA.
The GFA is an inherent property and should relate to the intrinsic factors of BMGs. Thus, it is important to determine the key factors influencing the GFA. Much research shows that atomic size differences between the constituent elements and the heat of mixing play important roles on the glass formation of BMGs [135,136,137]. Recently, it has been found that the electronegativity difference also influences the GFA of Al- [138,139], Fe-[140], and Ti-based BMGs [90,91,92,93]. In order to evaluate the GFA of Ti-based BMGs, Zhao et al. [93] calculated the parameters including the atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆Hmix for typical Ti-based BMGs. As shown in Figure 8, the high GFA of Ti-based BMGs (tmax ≥ 10 mm) requires an optimal combination of atomic size difference δ, electronegativity difference ∆x, and the enthalpy of mixing ∆Hmix: 0.1176 ≤ δ ≤ 0.1333, 0.1194 ≤ ∆x ≤ 0.1837, and −23.81 kJ/mol ≤ ∆Hmix ≤ −33.15 kJ/mol. These rules provide new insight and valuable guidance for the future development of Ti-based BMGs with higher GFA.

3. The Mechanical Properties of Ti-Based BMGs

Compared with crystalline Ti alloys, Ti-based BMGs exhibit outstanding mechanical properties such as large elastic limit, higher specific strength, and higher hardness [141,142,143]. Consequently, Ti-based BMGs can be exploited for structural applications. However, as other BMGs, Ti-based BMGs also have drawbacks that make their use for engineering applications appear challenging.

3.1. Elastic Properties

Figure 9 summarizes the relationship between the Young’s modulus and tensile fracture strength of typical BMGs together with classic engineering materials [144]. Table 4 lists the Young’s modulus and Poisson’s ratio of typical Ti-based BMGs [40,41,45,47,49,52,53,54,55,56,57,72,79,80,81,82,84,86,87,88,89,90,91,92,93,94,95,99,100,101,102,103,104,105,106,110,126,145,146,147]. Compared with conventional Ti crystalline alloys, Ti-based BMGs possess a lower Young’s modulus but higher fracture strength and larger elastic strain (close to 2.0%). The elastic strain energy of Ti-BMGs is larger than 20.0 MJ/m2 (e.g., Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMG), which is more than eight times that of the best spring steel [148]. There is also a correlation between the elastic properties (e.g., Young’s modulus E and Poisson’s ratio ν) and room temperature plasticity of BMGs. It was reported that BMGs with a higher Poisson’s ratio (e.g., >0.32) may possess larger plasticity [149,150]. In this sense, Ti-based BMGs can be classified as “ductile” because of the relatively high Poisson’s ratio.

3.2. Strength and Ductility

As a typical class of light metal-based BMGs, the development of Ti-based BMGs has been closely linked with high specific strength. Table 4 lists the density and compressive mechanical properties of typical Ti-based BMGs together with other BMGs and crystalline lightweight alloys [40,41,45,47,49,52,53,54,55,56,57,72,79,80,81,82,84,86,87,88,89,90,91,92,93,94,95,99,100,101,102,103,104,105,106,110,126,145,146,147]. As shown in Table 4, Ti-based BMGs exhibit high yield strengths of 1700–2300 MPa, which is the same level as that of Zr-based BMGs and much higher than that of other light metal-based BMGs (e.g., Mg- and Al-based BMGs). For comparison, the yield strength of the Ti–6Al–4V alloy is 825–895 MPa, which is only about half that of Ti-based BMGs. Although the density of Ti is higher compared with Mg and Al, the specific strength σc of Ti-based BMGs is significantly higher, especially Be-containing Ti-based BMGs that possess densities of less than 5 g/cm3 and high specific strength over 4 × 105 N·m/kg. Figure 10 shows the Ashby diagram of strength versus density for engineering materials [29]. It is clear that Ti-based BMGs possess higher specific strength than that of most other engineering materials. In order to further enhance the specific strength of Ti-based BMGs, the content of heavy constituent elements should be decreased without sacrificing the GFA. Unlike Fe- and Mg-based BMGs, which are completely brittle and fracture via fragmentation even under compression [151,152,153], most Ti-based BMGs exhibit a certain plastic strain and fracture mainly by shearing. This is mainly because of the relatively high Poisson’s ratio of Ti and main constituent elements (e.g., Cu, Zr, and Pd). However, compared with Pd- and Zr-based BMGs, the room temperature plasticity of Ti-based is still relatively low.
Some scholars have also investigated the tensile properties of Ti-based BMGs. Figure 11 shows stress–strain curves of Ti41.5Cu42.5Ni7.5Zr2.5Hf5Si1 BMG obtained by tension and compression tests [99]. It was found that the tensile strength is slightly lower than the compressive strength. Moreover, although this Ti-based BMG exhibits certain ductility under compression, no plastic deformation is observed during the tension test. At room temperature, the plastic deformation of BMGs is dominated by localized shear bands. Under compression, the propagation of the primary shear band is restricted by the friction and confinement at the sample-loading platen surface. Then, multiple shear bands form and interact, leading to the substantial development of plastic deformation. However, this effect is not available under tension. Under the unconfined tensile loading, once a shear band penetrates the sample and bears the entire load, catastrophic fracture will immediately occur. Therefore, it is more difficult to obtain tensile ductility for BMGs including Ti-based BMGs. Few Ti-based BMG matrix composites have exhibited tensile plasticity [154,155,156]. Developing Ti-based BMGs with tensile ductility is still a challenge.
It is worthy noticing that sample size plays an important role on the plasticity of Ti-based BMGs. Huang et al. reported a “smaller is softer” phenomenon in Ti40Zr25Ni3Cu12Be20 BMG under compression [157]. According to their experiments, Ti-based BMG samples with a smaller diameter exhibit a larger plasticity, while the strength does not markedly change. Similar results have also been reported in other BMGs [158,159,160,161,162,163]. The size effect has been explained from the viewpoints of free volume content [157,162,163], flaw sensitivity [159], plastic zone [160], and elastic energy dissipation [161]. Except for the sample diameter, the aspect ratio is another important size factor that affects the ductility of BMGs [164]. By decreasing the aspect ratio of BMG samples, the geometrical constrain effect becomes more obvious and the compressive plasticity can be dramatically improved.
It was also found that the compressive mechanical properties of Ti-based strongly depends on the service temperature. A transition from ductile to brittle behavior in Ti40Zr25Ni3Cu12Be20 BMG at cryogenic temperatures has been reported by Huang et al. [165]. Because of the cryogenic surroundings, the diffusion of the atoms slows down, and the nucleation and growth of nanocrystals is suppressed, resulting in an improvement of both strength and plasticity.

3.3. Fracture Toughness

The lack of room temperature plasticity has been considered as the Achilles’ heel of BMGs. In this section, typical techniques for improving the room temperature plasticity of Ti-based BMGs have been introduced.

3.3.1. Poisson’s Ratio Control Strategy

Because of the correlation between the Poisson’s ratio and plasticity, BMGs with good plasticity can be developed via composition optimization based on Poisson’s ratio control strategy [166,167]. A simple but effective way is replacing one of the constituent elements with alloying elements that possess a higher Poisson’s ratio. Gong et al. investigated the alloying effect on the compressive plasticity of Ti–Zr–Be BMGs [87,88,89,90,91,92,93,94]. Among the constituent elements in the Ti–Zr–Be system, Be possesses a very low Poisson’s ratio of 0.032, while Zr possesses a high Poisson’s ratio of 0.34. Therefore, it is not surprising that replacing Be by alloying elements (e.g., Fe, Cu, Ni, and Al) in the Ti–Zr–Be BMG improves the plasticity, while the substitution of alloying elements to Zr degrades room temperature plasticity. Park et al. [53] also reported that partial substitution of Zr by Ti in the Ti–Cu–Ni–Be system increases the plastic strain from 0.7% to 8.3%. The addition of Zr is beneficial for increasing the Poisson’s ratio and the shear transformation zone volume, resulting in larger plasticity.

3.3.2. Nanocrystallization

Producing BMG composites reinforced by ductile crystalline phases is believed to be an effective way to overcome the poor room temperature ductility of BMGs [168,169]. Table 5 summarizes the composition, synthesis method, and mechanical properties of typical Ti-based BMG composites [153,170,171,172,173,174,175,176,177,178,179,180]. It is found that most developed Ti-based BMG composites are of an in situ β-phase dendrite-reinforced type, as β-Ti possesses good ductility and a low modulus compared with hexagonal α-Ti. In general, Ti-based BMG composites exhibit better plasticity compared with Ti-based BMGs. For instance, Hofmann et al. developed Ti–Zr–V–Cu–Be BMG-matrix composites reinforced by a dendritic phase with room temperature tensile ductility over 10% [154,155]. However, the preparation process of BMG-matrix composites is always very complex [181]. The processing parameters should be precisely controlled to obtain a perfect microstructure. Therefore, the preparation of BMG composites puts a greater demand on the equipment. For example, a copper mold casting is the most widely used method of preparing BMGs. However, the microstructure of BMG composites prepared via copper mold casting is not always uniform because of the cooling rate difference. Zhang et al. [178] introduced a new technology named Bridgman solidification, which ensures the uniform microstructure of the prepared BMG composites by precisely controlling the processing parameters, but the corresponding equipment is more complex than traditional arc-melter. Moreover, the enhancement of ductility is always obtained at the price of lower strength. By introducing nanocrystals in situ formed in the glassy matrix, the propagation of primary shear band is disturbed when it reaches the nanocrystals, which act as high energy barriers; then, the primary shear band may be forced to be deflected and branched to initiate new shear bands. As a result, the ductility of BMGs can be improved without sacrificing strength. For example, Park et al. [121] reported a Ti40Zr29Be14Cu9Ni8 BMG with a large plastic strain of ~7%. In order to find out the reason of the superior plasticity, they investigated the microstructure and crystallization behavior of this alloy with a high resolution transmission electron microscope (HRTEM) and via differential scanning calorimetry (DSC). It was found that, during the deformation, the nuclei transform to precipitations with a size of several nanometers and disperse into the amorphous matrix, resulting in an improvement in ductility. There are several different ways to introduce nanocrystallization in BMG samples. The first method is composition design. For instance, the addition of alloying elements may promote nanocrystallization of BMGs during the deformation process. Typical examples are Nb for Ti–Zr–Cu–Pd alloys [50] and Si for Ti–Zr–Cu–Ni–Sn alloys [104]. Another widely used method is annealing treatment. Jun et al. [182] reported that the compressive strain of Ti43.3Zr21.7Ni7.5Be27.5 BMG can be significantly enhanced to 42% after sub-Tg annealing. Nanocrystalline phases formed during the annealing were believed to be responsible for the rise of the compressive plastic strain. Moreover, as structural relaxation also occurs during the annealing process, the free volume content of the amorphous matrix decreases, which maintains the high strength of the annealed BMG samples.

3.3.3. Pre-Plastic Deformation

For conventional crystalline alloys, after plastic deformation, the microstructure can be changed (e.g., dislocation density), and the flow behaviors can be quite different. For BMGs, pre-plastic deformation is supposed to introduce microstructural inhomogeneities and induce controlled stress distributions or activate multiple shear bands, which are beneficial to enhancing the room temperature plasticity [183,184]. Huang et al. utilized prior compressive plastic deformation to tune the room temperature plasticity of Ti40Zr25Ni3Cu12Be20 BMG [185]. It was found that, with the increase of prior plastic strain, the plastic strain of the deformed BMG samples first increases and then decreases with the optimized prior plastic strain of 10%. The improvement of compressive plasticity was explained in view of the free volume content. Park et al. [186] investigated the effect of strain-induced internal state modulation created by cold rolling on the compressive plasticity of Ti40Zr25Ni8Cu9Be18 BMG. The compressive plastic strain can be dramatically improved from 1.5% to 14.5% with a 50% thickness reduction (as shown in Figure 12). Because of rolling, a network-like structure of hard and soft regions was found to be introduced uniformly in the BMG samples, which is beneficial to the uniform distribution of multiple shear bands.

3.3.4. Surface Treatment

Surface treatment has been widely applied to improve the room temperature ductility of BMGs by retarding the initiation and propagation of the shear band [187]. It has been reported that the room temperature plasticity of Zr-based BMGs can be improved by various surface modification technologies including shot peening [188] and surface coating [189,190]. As Ti-based BMGs possess properties similar to Zr-based BMGs, these strategies can also be adopted to improve the room temperature plasticity of Ti-based BMGs. Fan et al. [191] proposed a novel technology called surface mechanical attrition treatment (SMAT) to improve the plasticity of BMGs. By surface crystallization, isolated crystallite islands are formed in the top surface layer, which act as the obstacles to restrict the localization of shear bands and avoid the development of cracks. With the optimization of SMAT processing parameters, the plastic strain of Ti40Zr25Ni3Cu12Be20 BMG can be enhanced to 3.78%, which is nearly four times that of the untreated sample.

3.4. Fracture Toughness

For engineering materials, damage tolerance is a very important mechanical design parameter. As BMGs usually possess high strength but a lack of plasticity, fracture toughness Kc, which assesses a material’s resistance to crack propagation and can be measured by the energy needed to cause fracture, is a more important indicator of mechanical performance compared with yield strength [192]. The fracture toughness of BMGs strongly depends on the alloy composition. For instance, some brittle BMGs (e.g., Mg- and Fe-based BMGs) exhibit an ideally brittle behavior (Kc < 10 MPa·m1/2) [193,194], while Pd-based BMGs are remarkably tough (Kc ~200 MPa·m1/2) [195]. Regarding the fracture toughness of Ti-based BMGs, it has been reported that the Kc of Ti50Ni24Cu20B1Si2Sn3 alloy is ~50 MPa·m1/2 [153]. Gu et al. [196] investigated the effects of changes in sample dimensions and the stress state on the fracture toughness of Ti40Zr25Cu12Ni3Be20 BMG. It was found that the measured fracture toughness ranges from 98.6 to 126.3 MPa·m1/2. The fatigue pre-cracked Ti40Zr25Cu12Ni3Be20 sample exhibited a slightly higher toughness than that of the notched sample. The notched plate sample and the notched rod sample were also found to have different fracture toughness values. In summary, the fracture toughness of Ti-based BMGs is higher than that of brittle BMGs but lower than that of Pd-BMGs, which is comparable to those for age-hardened Al-based alloys (24–36 MPa·m1/2), commercial Ti crystalline alloys (Kc = 54–98 MPa·m1/2), and 4340 high strength steels (Kc ~50 MPa·m1/2). However, it should be noticed that, even for the same alloy, a wide scatter in Kc has been reported. The cooling rate during the preparation process, the stress state, the impurity inclusions, the sample size, and the sharpness of notch/crack fabrication are all possible reasons. In order to reduce the extrinsic effect, Chen et al. [197] proposed a novel sample preparation method for fracture toughness test via the thermoplastic forming of BMGs and Si photolithography. Using this strategy, they measured the notch toughness of 86 ± 3 MPa·m1/2 for a Ti41Zr25Be28Fe6 BMG. The small scatter is believed to reflect an intrinsic origin rather than extrinsic sample preparation effects.

3.5. Fatigue Properties

The fatigue behavior is one of the dynamic mechanical properties and also a very important characteristic for the applications of BMGs as structural materials. Yamaura et al. [198] and Fujita et al. [199] have investigated the fatigue behavior of Ti40Zr10Cu34Pd14Sn2 and Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 alloys, respectively. Table 6 lists the fatigue properties of Ti-based BMGs together with other typical BMGs and Ti–6Al–4V alloy [198,199,200,201,202]. Compared with the Ti–6Al–4V alloy, Ti40Zr10Cu34Pd14Sn2 BMG, which is a typical representative of biomedical Ti-based BMG, exhibits higher fatigue strength although the fatigue ratio is lower. Moreover, it is worthy noticing that the Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMG shows a fatigue ratio of 0.79, which is significantly higher than that of other BMGs such as Zr-, Cu-, Co-, and Fe-based BMGs listed in Table 6. The fatigue strength of Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMG is up to 1610 MPa, which is much larger than that of Zr-based BMGs and close to the value of Ni–based BMGs. The superior fatigue properties of Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 BMG are attributed to the dispersion of nano-scaled crystalline particles in the glassy matrix. The initiation and the growth of shear bands are constrained by the nanocrystals. The shear bands branch, kink, and rotate, resulting in the reduction in the shear stress value near their tips. There have been few reports on the effect of frequency on the fatigue properties of Ti-based BMGs, and this will need to be further studied.

3.6. Thermoplastic Formability

BMGs usually present superplasticity in their supercooled liquid region with behavior similar to a Newtonian viscosity of conventional glass materials. Because of this unique property, thermoplastic forming (TPF) and patterning have been widely used to precisely fabricate BMG parts on length scales ranging from the nanometer scale to several centimeters [203,204,205]. There are two key factors influencing the thermoplastic formability of BMGs: the temperature-dependent viscosity and the temperature-dependent crystallization time. It was already known that the temperature dependence of the viscosity and the temperature dependence of crystallization time among BMGs vary significantly. Thus, the thermoplastic formability of BMGs strongly depends on the alloy composition. Schroers et al. [206] introduced an experimental method to characterize the thermoplastic formability of different BMGs using the maximum diameter to which the BMG can be deformed for a standardized set of processing parameters. It was found that the S parameter, which is defined based on the characteristic temperatures (S = ∆Tx/TlTg), is the best indicator for thermoplastic formability of BMGs so far. Table 7 summarizes the S values of developed Ti-based BMGs together with some typical Zr-, Pd-, Pt-, Au-, Fe-, and Mg-based BMGs [39,40,47,54,56,79,84,86,95,206,207,208,209,210,211,212]. It is found that Ti-based BMGs possess relatively small S values compared with other BMGs, implying a relatively low thermoplastic formability. Moreover, as titanium is a very active element, during the heating process in air, the oxidation is more serious compared with Pd-, Au-, and Pt-based BMGs, which is harmful to the thermoplastic forming [213,214,215]. In order to perform the thermoplastic forming of Ti-based BMGs, especially in air, it is necessary to first attempt composition optimization in order to develop novel Ti-based BMGs with better thermoplastic formability and oxidation resistance. Be alloying may be an effective way as Be-containing Ti-based BMGs possess relatively higher S value (as shown in Table 7). Other strategies, e.g., vibrational loading [216] or introducing a wetting layer [217], are also recommended.

4. The Corrosion Resistance of Ti-Based BMGs

Due to the formation of stable and protective surface oxide film, titanium alloys are inert and resistant to corrosion. Because of the potential application as structural and biomedical materials, the corrosion behavior of Ti-based BMGs in different solutions such as acid, alkaline, salt, and simulated body solutions have been investigated. Compared with conventional Ti alloys, Ti-based BMGs always exhibit better corrosion resistance in different kinds of solutions because of the unique amorphous structure. Figure 13 [110] shows the potentiodynamic polarization curves of Ti46Cu27.5Zr11.5Co7Sn3Si1Ag4 BMG and Ti–6Al–4V alloys in PBS, 0.9 wt. % NaCl, 1 mol/L HCl, and 1 mol/L NaOH solutions, respectively. It was found that, compared with the commercial Ti–6Al–4V alloy, Ti46Cu27.5Zr11.5Co7Sn3Si1Ag4 BMG possesses a higher corrosion potential and lower corrosion current density, implying a better corrosion resistance. In general, the corrosion-penetration rates (CPRs) of less than ~76 μm/year are considered acceptable for chemical and industrial applications. According to Morrison et al.’s research [218], the CPR of Ti43.3Zr21.7Ni7.5Be27.5 BMG in a PBS electrolyte at 37 °C is 2.9 ± 2.6 μm/year, which is well within the expected range for corrosion resistant materials and equivalent to, or better than, Zr-based BMGs and 316L stainless steel. Except for PBS, Ti-based BMGs also exhibit good bio-corrosion resistance in other simulated body fluids such as SBF, Ringer’s solution, and Hanks’ solution [219].
Chemical composition also plays an important role on the corrosion resistance of Ti-based BMGs. Qin et al. [220] investigated the effects of different alloying elements on the corrosion behavior of Ti47.5Cu42.5Ni7.5Zr2.5 BMG in a 0.14 kmol/m3 NaCl solution and a 0.2 kmol/m3 phosphate buffer solution with 0.14 kmol/m3 Cl ions. It was found that the addition of Nb or Ta significantly improves corrosion resistance. The passive current density of the Nb- or Ta-containing BMGs is between 10−2 and 10−3 A/m, which is one order of magnitude lower than that of the base alloy. The addition of Nb or Ta facilitates the enrichment of Ti, and certain amounts of Nb or Ta existing in the surface film, resulting in higher corrosion resistance. The corrosion resistance of the (Ti40Zr10Cu38Pd12)97Nb3 BMG has also been shown to be higher than that of the Ti40Zr10Cu38Pd12 alloy due to the presence of Nb. The pitting resistance is enhanced by the addition of Nb because of the improvement of the passive layer properties.
The effect of crystallization on the corrosion resistance of Ti-based BMGs has also been studied. Qin et al. [221] investigated the corrosion behavior of the Ti40Zr10Cu36Pd14 alloy in Hanks’ solution in three different conditions: in the as-cast fully amorphous condition, after annealing it for 10 min at 723 K to obtain BMG matrix composite, and after fully crystallizing the sample at 823 K for 10 min. As shown in Figure 14, it was found that the as-cast and partially crystalline samples have lower passive current densities located ~10−2 A/m2, markedly lower than that of the commercial Ti–6Al–4V alloy and pure Ti. However, the fully crystallized sample exhibits a much lower pitting potential, implying the least corrosion resistance. The enhancement of the pitting potential of the partially nanocrystalline alloy annealed at 723 K may be due to the formation of the Ti3Cu4 phase. This results in the enrichment of Pd in the matrix, which is helpful in forming a protective passive film. Moreover, because of the large number of interface defects that are expected at the nano scale, the breakdown of the passive film is uniform, which allows the partially crystallized alloy to maintain passivity. Correspondingly, the crystalline phases of the fully crystallized sample are Ti3Cu4, Ti2Pd, and Ti2Pd3 with a larger size. The lower corrosion resistance is mainly attributed to the serious micro-galvanic corrosion between the Cu-rich and Pd-rich phases. In conclusion, both the size and composition of the crystalline phase play important roles in controlling the corrosion behavior of Ti-based alloys.

5. Biocompatibility of Ti-Based BMGs

Titanium and its alloys are believed to be excellent implant materials in the fields of trauma and orthopedic surgery. Compared with conventional Ti alloys, Ti-based BMGs are more suitable for biomedical applications for the following reasons [222,223]: (1) high strength and hardness, which may lead to good loadbearing capability and high wear resistance; (2) low Young’s (elastic) modulus, which implies better load transfer to the surrounding bone and a potential for mitigating stress-shielding; (3) excellent corrosion resistance, resulting in reduced ion release in the human body environment; and (4) excellent thermoplastic formability that allows for the production of precise and versatile geometries on different length scales, which is of great interest for biomaterials processing.
Young’s modulus is one of the most important performance indicators of biomedical materials. According to Table 4, the Young’s modulus value of Ti-based BMGs is 80–110 GPa, which is smaller than commercial biomedical materials such as biomedical 316L stainless steel (230 GPa), Co–Cr alloys (230 GPa), and the Ti–6Al–4V alloy (110–114 GPa). However, the Young’s modulus of human bone tissue is only 20 GPa, which is still much smaller than that of Ti-based BMGs. Some developed β-type titanium alloys (e.g., Ti–Nb–Ta–Zr) possess relatively small Young’s modulus values of ~40 GPa [224], which are close to that of human bone tissue. In order to minimize the stress shielding effect, it is still necessary to decrease the Young’s modulus value of biomedical Ti-based BMGs. In order to improve the GFA, Ti-based BMGs always contain several alloying elements. As the Young’s modulus of BMGs shows a rough correlation with a weighted average of the Young’s modulus for the constituent elements [225], the elements with a low Young’s modulus value should be selected preferentially as constituent elements for biomedical Ti-based BMGs. On the other hand, the biological toxicity of different alloying elements has also been considered. Calin et al. investigated the biological safety of the constituent elements in Ti-based BMGs. Based on their research, it was concluded that Ti, B, Mg, Si, P, Ca, Sr, Zr, Nb, Mo, Pd, In, Sn, Ta, Pt, and Au are biocompatible elements, while harmful elements include Be, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ag [223]. In this sense, Ti-based BMGs for biomedical use should only contain biocompatible and low-modulus elements. According to this principle, a series of Ti–Zr–Si–(Nb, Ta) metallic glasses that contain no harmful elements has been successfully developed [226,227]. However, the GFA of this class of Ti-based alloys is too poor to form BMG samples. No Ti-based BMGs that do not contain either Ni, Cu, or Be has been reported. It is known that Cu ions are necessary nutrients for the human body, but excess Cu ions may lead to biological toxicity. An amount of 10–12 mg per day may possibly be the maximally safe concentration, which is suggested by the World Health Organization. According to Huang et al.’s research, after 1 day of immersion in a cell culture medium (DMEM), the concentration of Cu ions released from Zr50Cu43Al7 BMGs was below 50 ppb [228]. The addition of Cu in Ti-based BMGs results in no bio-toxicity. Similarly, a small amount of Fe, Ag, or both may also be added to Ti-based BMGs. Thus, Ti–Zr–Cu–Pd–(Sn, Nb) [47,49,86] and Ti–Zr–Cu–Fe–Sn–Si–(Ag) BMGs [56,57], potential biomaterials, were successfully discovered.
Biocompatibility is described as the ability of the material to exist in contact with tissues of the human body without causing a non-acceptable degree of harm. As a typical class of non-degradable BMGs, the biocompatibility of Ti-based BMGs can be mainly related to its cell-biological activity in the body environment. Wang et al. [229] studied the biocompatibility of Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG and pure Ti via in vitro cell response and in vivo animal implants. It was found that, although the cell viability is relatively lower due to the release of Cu ions, Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG shows good compatibility from in vivo evaluation for one month implantation compared with pure Ti. As shown in Figure 15, both the pure Ti and Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG are well integrated with the bone tissue, and the gap between the bone tissue is no more than 5 μm. However, because of the high content of Cu and the addition of allergenic element Ni, Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 is not an ideal biomaterial, and long-term implantation is still needed for further investigation. Liu et al. [55] investigated the cytocompatibility of Ti–Zr–Cu–Fe–Sn–Si BMGs and the Ti–6Al–4V alloy via adopted mouse MC3Ts-E1 pre-osteoblast. The results demonstrated that the cell viabilities in the Ti–Zr–Cu–Fe–Sn–Si BMG extracts are slightly higher than that in the Ti–6Al–4V alloy extract. Kokubun et al. [230] conducted a thorough in vivo evaluation of biocompatibilities of Ti40Zr10Cu34Pd14Sn2 BMG. They implanted bars of Ti40Zr10Cu34Pd14Sn2 BMG in the femoral bone of rats. A typical macroscopic view of the Ti40Zr10Cu34Pd14Sn2 bar at 12 weeks after implantation is shown in Figure 16. No inflammatory reaction, implant dislocation, or loosening was observed, implying that Ti40Zr10Cu34Pd14Sn2 BMG has an excellent biocompatibility and integration to bone tissue. As shown in Figure 17, the histological images revealed that both the BMG sample and the Ti sample were well covered by surrounding bone tissue. No abnormal finding in surrounding bone tissue was observed, and no component ion diffusion was detected up to 3 months post-implantation.
Based on the above experimental results, the possibility and efficacy of its use for bone implant is preliminarily confirmed. However, further long-term implant study is still recommended. Another issue is that the number of developed Ti-based BMGs for biomedical applications is very limited. It is still necessary to explore novel biomedical Ti-based BMGs with improved GFA and good biocompatibility.

6. Applications of Ti-Based BMGs

Ti-based BMGs have been applied to fabricate a sensing tube in a Coriolis flow meter, which is used to measure the Coriolis force of a liquid or gas that is flowing inside the pipe subjected to reinforced oscillation (as shown in Figure 18) [231,232]. A Ti-based BMG with the composition of Ti50Cu25Ni15Zr5Sn5 was selected to fabricate the metallic glass tubes using a copper mold suction casting technique. The sensitivity of the Coriolis flow meter using the Ti-based glassy alloy pipe was reported to be 28–53 times higher than that of a conventional Coriolis flow meter using a SUS316 pipe. The significant improvement in sensitivity allows the possibility of the use of a new type of Coriolis flow meter in various industries such as fossil-fuel, chemical, environmental, semiconductor, and medical science fields.
Ti-based BMGs are also potentially useful in many other serious applications. It was reported that Zr-based BMGs have been used to fabricate micro-geared motor parts [231,232,233,234], pressure sensors [231,235,236], watch cases [204,237], cell phone cases [204,237], and sports goods (e.g., golf plate) [238]. Replacing Zr-based BMGs by Ti-based BMGs is beneficial for enhancing the specific strength and reducing the cost. Because of the good corrosion resistance and biocompatibility, Ti-based BMGs are also suitable for medical components such as prosthetic implants and surgical instruments (e.g., the surgical razor and micro-surgery scissors). Aerospace engineering is another important applied field of Ti-based BMGs. Ti-based BMGs possess outstanding mechanical properties such as high specific strength, high hardness, and large elastic elongation. Moreover, although BMGs are metastable in thermodynamics, Ti-based BMGs exhibit high space environment applicability. According to Wang et al.’s experimental results [239], the microstructure, thermodynamics, and mechanical properties of Ti-based BMGs are all relatively stable after simulated thermal cycling treatment in vacuum in a temperature range of −196 °C to 150 °C. The effect of atomic oxygen (AO) on the BMGs has been also studied in a plasma type ground-based AO effect simulation facility. The results show that the structure on the surfaces of Ti-based BMGs samples do not change much, suggesting that Ti-based BMGs have high AO erosion resistance [240]. The combination of good mechanical properties and high space environment applicability implies that Ti-based BMGs possess a potential for applications in aerospace environments.

7. Conclusions and Future Research Directions

Ti-based BMGs have attracted much attention due to the combination of unique properties such as high specific strength and good anti-corrosion properties. In recent decades, many Ti-based BMGs have been successfully developed, and their GFA, mechanical properties, corrosion resistance, and biocompatibility have been investigated. In this paper, the development of Ti-based BMGs is reviewed. In terms of GFA, a maximum diameter for glass formation of over 50 mm is now achieved in (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 and Ti32.8Zr30.2Cu9Fe5.3Be22.7 alloys. In the aspect of mechanical properties, Ti–Be-based BMGs possess very high specific strength over 4 × 105 N·m/kg, which is more than twice that of the Ti–6Al–4V alloy. Because of the relatively high Poisson’s ratio of titanium, Ti-based BMGs are classified as “ductile”, and most developed Ti-based BMGs possess a certain plastic strain under compression. Ti-based BMGs also exhibit excellent corrosion resistance in acid, alkaline, and salt solutions compared with crystalline Ti alloys and typical engineering materials such as 316L stainless steel. Ti–Zr–Cu–Pd–(Sn, Nb) BMGs do not contain noxious elements and show good GFA together with a high potential for biomedical applications.
Although rapid progress has been made, several challenges for future applications of Ti-based BMGs still exist, including the need to (1) understand the glass-forming mechanisms and further improve the GFA of Ti-based BMG especially non-toxic, low-modulus, and biocompatible Ti-based BMGs; (2) develop a complete understanding of the processing-structure-mechanical property relations of Ti-based BMGs and propose new strategies for achieving the room temperature tensile ductility of Ti-based BMGs; (3) address the formability and oxidation resistance of Ti-based BMGs in their supercooled region and improve the thermoplastic formability of Ti-based BMGs; and (4) explore new application areas for Ti-based BMGs.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grants No. 51601063, 51575522, and 51271095) and the New Century Talent Supporting Project (Grant No. NCET-11-0185). The authors are grateful to the Analytical and Testing Center, Huazhong University of Science and Technology, for technical assistance. The valuable discussions and support from Yang Shao, Xin Wang, Shaofan Zhao, Shengbao Qiu, Zhen Peng, Guannan Yang, Peng Gui, Zhenyu Song, Jialun Gu, and Zhidog Han are appreciated.

Author Contributions

Pan Gong prepared the manuscript. Pan Gong, Lei Deng, Junsong Jin, and Sibo Wang collected the data. Xinyun Wang and Kefu Yao designed the scope of the paper. All authors discussed the conclusions and reviewed the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Maximum diameters of the bulk metallic glass (BMG) rods achieved in different alloy systems and the years in which they were discovered. Data are taken from [8,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73].
Figure 1. Maximum diameters of the bulk metallic glass (BMG) rods achieved in different alloy systems and the years in which they were discovered. Data are taken from [8,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73].
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Figure 2. (a) X-ray diffraction (XRD) patterns of water-quenched (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 rods with different diameters (inset is the picture of the 50 mm-diameter rod sample and 100 g pancake). Reproduced with permission from [72]. Copyright 2010, Elsevier. (b) The appearance of the 50 mm-diameter Ti32.8Zr30.2Cu9Fe5.3Be22.7 rod sample prepared by copper mold casting. Reproduced with permission from [73]. Copyright 2015, Elsevier.
Figure 2. (a) X-ray diffraction (XRD) patterns of water-quenched (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 rods with different diameters (inset is the picture of the 50 mm-diameter rod sample and 100 g pancake). Reproduced with permission from [72]. Copyright 2010, Elsevier. (b) The appearance of the 50 mm-diameter Ti32.8Zr30.2Cu9Fe5.3Be22.7 rod sample prepared by copper mold casting. Reproduced with permission from [73]. Copyright 2015, Elsevier.
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Figure 3. BMG formation composition zone with tmax = 1 mm in the ternary Ti–Cu–Ni system. Reproduced with permission from [41]. Copyright 2008, Springer.
Figure 3. BMG formation composition zone with tmax = 1 mm in the ternary Ti–Cu–Ni system. Reproduced with permission from [41]. Copyright 2008, Springer.
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Figure 4. Composition maps of BMG formation in the (Ti1−xZrx)–Cu–Ni system. The two panels, (a) and (b), show the BMG-forming composition ranges at two ratios of Ti1−xZrx, (a) x = 0.1 and (b) x = 0.15, for the as-cast rods of 2 and 3 mm in diameter, respectively. Reproduced with permission from [41]. Copyright 2008, Springer.
Figure 4. Composition maps of BMG formation in the (Ti1−xZrx)–Cu–Ni system. The two panels, (a) and (b), show the BMG-forming composition ranges at two ratios of Ti1−xZrx, (a) x = 0.1 and (b) x = 0.15, for the as-cast rods of 2 and 3 mm in diameter, respectively. Reproduced with permission from [41]. Copyright 2008, Springer.
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Figure 5. Outer surface of cast Ti40Zr10Cu34Pd34Sn2 and Ti40Zr10Cu32Pd14Sn4 BMG rods with a diameter of 10 mm. Reproduced with permission from [47]. Copyright 2008, Elsevier.
Figure 5. Outer surface of cast Ti40Zr10Cu34Pd34Sn2 and Ti40Zr10Cu32Pd14Sn4 BMG rods with a diameter of 10 mm. Reproduced with permission from [47]. Copyright 2008, Elsevier.
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Figure 6. Glass-formation zones of 3 mm and 5 mm rods in the Ti–Zr–Be ternary alloy system. Reproduced with permission from [43]. Copyright 2010, Springer.
Figure 6. Glass-formation zones of 3 mm and 5 mm rods in the Ti–Zr–Be ternary alloy system. Reproduced with permission from [43]. Copyright 2010, Springer.
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Figure 7. Schematic pseudo Ti–Zr–(Cu, Ni, Be) ternary diagram showing the outline of composition range for maximum diameter for glass formation of 1, 6, and 10 mm. Reproduced with permission from [121]. Copyright 2005, Elsevier.
Figure 7. Schematic pseudo Ti–Zr–(Cu, Ni, Be) ternary diagram showing the outline of composition range for maximum diameter for glass formation of 1, 6, and 10 mm. Reproduced with permission from [121]. Copyright 2005, Elsevier.
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Figure 8. The distribution of ∆x and ∆Hmix values of all the developed Ti-based BMGs with their atomic size difference δvalues. Reproduced with permission from [93]. Copyright 2015, Elsevier.
Figure 8. The distribution of ∆x and ∆Hmix values of all the developed Ti-based BMGs with their atomic size difference δvalues. Reproduced with permission from [93]. Copyright 2015, Elsevier.
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Figure 9. The relations between mechanical properties of typical BMGs: (a) tensile fracture strength σt,f with Young’s modulus E; (b) Vickers hardness Hv with Young’s modulus E. Reproduced with permission from [144]. Copyright 2002, The Japan Institute of Metals and Materials.
Figure 9. The relations between mechanical properties of typical BMGs: (a) tensile fracture strength σt,f with Young’s modulus E; (b) Vickers hardness Hv with Young’s modulus E. Reproduced with permission from [144]. Copyright 2002, The Japan Institute of Metals and Materials.
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Figure 10. The Ashby diagram of strength versus density for engineering materials. Reproduced with permission from [29]. Copyright 2014, John Wiley and Sons.
Figure 10. The Ashby diagram of strength versus density for engineering materials. Reproduced with permission from [29]. Copyright 2014, John Wiley and Sons.
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Figure 11. Stress–strain curves of BMG samples obtained by tension and compression tests. Reproduced with permission from [99]. Copyright 2004, The Japan Institute of Metals and Materials.
Figure 11. Stress–strain curves of BMG samples obtained by tension and compression tests. Reproduced with permission from [99]. Copyright 2004, The Japan Institute of Metals and Materials.
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Figure 12. Compressive stress–strain curves of as-cast and cold-rolled Ti40Zr25Ni8Cu9Be18 BMG samples. Reproduced with permission from [186]. Copyright 2012, Elsevier.
Figure 12. Compressive stress–strain curves of as-cast and cold-rolled Ti40Zr25Ni8Cu9Be18 BMG samples. Reproduced with permission from [186]. Copyright 2012, Elsevier.
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Figure 13. Potentiodynamic polarization curves of BMG and the Ti–6Al–4V alloy in (a) PBS, (b) 0.9 wt % NaCl, (c) 1 mol/L HCl, and (d) 1 mol/L NaOH solutions. Reproduced with permission from [110]. Copyright 2015, Elsevier.
Figure 13. Potentiodynamic polarization curves of BMG and the Ti–6Al–4V alloy in (a) PBS, (b) 0.9 wt % NaCl, (c) 1 mol/L HCl, and (d) 1 mol/L NaOH solutions. Reproduced with permission from [110]. Copyright 2015, Elsevier.
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Figure 14. Anodic and cathodic polarization curves of the Ti40Zr10Cu36Pd14 BMG and its crystalline alloys at 310 K in Hanks’ solution. Reproduced with permission from [221]. Copyright 2007, The Japan Institute of Metals and Materials.
Figure 14. Anodic and cathodic polarization curves of the Ti40Zr10Cu36Pd14 BMG and its crystalline alloys at 310 K in Hanks’ solution. Reproduced with permission from [221]. Copyright 2007, The Japan Institute of Metals and Materials.
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Figure 15. Implantation of Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG and pure Ti samples. (a) BMG sample; (b) representative X-ray images for the implants; (c,d) representative histological images stained by methylene blue after 1 month of implantation [229]. Reproduced with permission from [229]. Copyright 2013, Elsevier.
Figure 15. Implantation of Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 BMG and pure Ti samples. (a) BMG sample; (b) representative X-ray images for the implants; (c,d) representative histological images stained by methylene blue after 1 month of implantation [229]. Reproduced with permission from [229]. Copyright 2013, Elsevier.
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Figure 16. Macroscopic view of the Ti40Zr10Cu34Pd14Sn2 BMG sample at 12 weeks after implantation in femoral diaphysis. Reproduced with permission from [230]. Copyright 2015, IOS press.
Figure 16. Macroscopic view of the Ti40Zr10Cu34Pd14Sn2 BMG sample at 12 weeks after implantation in femoral diaphysis. Reproduced with permission from [230]. Copyright 2015, IOS press.
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Figure 17. Histological views of Ti-based BMG implant and Ti implant. Both samples are well covered by surrounding bone tissue. Objective magnification of upper images is ×4, and that for lower images is ×20. Reproduced with permission from [230]. Copyright 2015, IOS press.
Figure 17. Histological views of Ti-based BMG implant and Ti implant. Both samples are well covered by surrounding bone tissue. Objective magnification of upper images is ×4, and that for lower images is ×20. Reproduced with permission from [230]. Copyright 2015, IOS press.
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Figure 18. An outer appearance of the self-made Coriolis flow meter using the Ti–Cu–Ni–Zr–Sn BMG pipe. Reproduced with permission from [232]. Copyright 2011, Elsevier.
Figure 18. An outer appearance of the self-made Coriolis flow meter using the Ti–Cu–Ni–Zr–Sn BMG pipe. Reproduced with permission from [232]. Copyright 2011, Elsevier.
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Table 1. Thermal properties of typical Ti-based BMGs [39,40,41,45,47,49,52,55,56,57,73,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110].
Table 1. Thermal properties of typical Ti-based BMGs [39,40,41,45,47,49,52,55,56,57,73,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110].
Composition (atom %)Synthesis Methodtmax (mm)Tg (K)Tx (K)Tm (K)Tl (K)Tx (K)TrgγHeating Rate (K/min)Ref.
Ti50Cu43Ni7Cu mold casting1.5667704 37 40[41]
Ti53Cu39Ni8Cu mold casting1.565869811971219400.5390.37240[41]
Ti55Cu36Ni9Cu mold casting165269211981221400.5820.36940[41]
Ti50Cu42Ni8Cu mold casting265771311141168560.5630.39140[40]
Ti40Zr25Be35Cu mold casting6598675 1125760.5320.39220[79]
Ti45Zr20Be35Cu mold casting6597654 1123570.5310.38020[79]
Ti41Zr25Be34Cu mold casting5578631 53 20[80,81]
Ti50Zr5Cu45Cu mold casting1 [82,83]
Ti40Zr10Cu50Cu mold casting367069111211199210.560.37020[82,83]
Ti50Ni20Cu25Sn5Cu mold casting 7107701229 60 40[39]
Ti50Ni15Cu32Sn3Cu mold casting168675912051283730.530.38520[84]
Ti38Ni16.2Cu37.8Sn8Cu mold casting1717774 57 20[85]
Ti50Zr5Cu40Ni5Cu mold casting2634685 1155510.550.38340[82]
Ti45Zr5Cu45Ni5Cu mold casting3673715 1203430.560.38140[82]
Ti42.5Zr10Cu42.5Ni5Cu mold casting3651695 1213450.540.37240[82]
Ti41.3Cu43.7Hf13.9Si1.1Cu mold casting3680720 40 40[82]
Ti40Zr10Cu36Pd14Cu mold casting666971811141191490.560.38640[86]
Ti40Zr25Be30Cr5Cu mold casting8599692 1101930.540.40720[79]
Ti41Zr25Be28Fe6Cu mold casting10608725 11431170.530.41420[87]
Ti40Zr26Be28Fe6Cu mold casting10615715 11491000.540.40520[88]
(Ti41Zr25Be34)92Fe8Cu mold casting10619730 11601110.530.41020[89]
Ti41Zr25Be28Al6Cu mold casting7624691 67 20[90]
Ti41Zr25Be28Ag6Cu mold casting10597655 1118580.5340.38220[91]
Ti41Zr25Be28Cu6Cu mold casting15587684 1130970.5200.39820[92]
Ti41Zr25Be28Ni6Cu mold casting 20[93]
Ti50Zr16Be24Ni10Cu mold casting5605661 1103560.5490.38320[94]
Ti42.5Zr7.5Cu40Ni5Sn5Cu mold casting4683747 1217640.5610.39320[95]
Ti43.15Zr9.59Cu36.24Ni9.06Sn1.96Cu mold casting3649.5699.5986.81167.3500.5560.38520[45]
Ti50Cu20Ni24Si4B2Cu mold casting173580011831214650.6050.41040[96]
Ti48Ni32Cu8Si8Sn4Cu mold casting3783.8878.6 94.8 20[97]
Ti32.38Cu42.34Ni9.28Zr7.6Hf8.4Cu mold casting468272211111168390.5840.39020[98]
Ti42.5Zr2.5Hf5Cu42.5Ni7.5Cu mold casting267772611431203490.560.38640[99]
Ti40Zr10Cu34Pd14Sn2Cu mold casting1068973911261187500.5800.39440[47]
Ti40Zr10Cu38Pd10Si2Cu mold casting568575011171193650.5740.39940[49]
Ti32.8Zr30.2Ni5.3Cu9Be22.7Water quenching>50611655 961440.6360.41720[72]
Ti40Zr25Ni3Cu12Be20Cu mold casting14601643 ~985420.6100.40520[52]
Ti40Zr25Ni8Cu9Be18Cu mold casting86216689481009470.630.41040[84]
Ti32.8Zr30.2Cu9Fe5.3Be22.7Cu mold casting>505786589461064800.5430.40120[73]
Ti36.2Zr30.3Cu8.3Fe4Be21.2Cu mold casting>305766389281039620.5540.42120[100]
(Ti41Zr25Be28Fe6)91Cu9Cu mold casting>326166819501108650.5560.39520[101]
(Ti41Zr25Be29Al5)91Cu9Cu mold casting106377059381079700.5900.41020[102]
Ti50Ni15Cu25Sn3Be7Cu mold casting268874111221207450.570.38740[84]
Ti50Ni24Cu20B1Si2Sn3Cu mold casting172680012301310740.590.39340[103]
Ti45.8Zr6.2Cu39.9Ni5.1Sn2Si1Cu mold casting4670711 35 20[104]
Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1Cu mold casting568073011431199500.570.38940[99]
Ti47Zr7.5Cu40Fe2.5Sn2Si1Cu mold casting3646702 56 20[55]
Ti45Cu25Ni15Sn3Be7Zr5Cu mold casting5685741 1142560.600.40640[105]
(Ti40Zr25Be20Cu12Ni3)99.5Y0.5Cu mold casting5623644866953210.6530.40810[106]
Ti53Cu15Ni18.5Al7Sc3Si3B0.5Cu mold casting2709767 1240580.6190.39440[107]
Ti53Cu15Ni18.5Al7Hf3Si3B0.5Cu mold casting2695749 1230540.6090.38940[107]
Ti53Cu15Ni18.5Al7Zr3Si3B0.5Cu mold casting2.5703765 1237620.570.39440[108]
Ti53Cu27Ni12Zr3Al7Si3B1Cu mold casting26857541105 69 40[109]
Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5Cu mold casting6693.33752.521116.661176.0764.190.5900.40520[54]
Ti46Cu27.5Zr11.5Co7Sn3Si1Ag4Cu mold casting564069110681207510.5300.37420[110]
Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2Cu mold casting764169311161180520.570.38120[56]
Ti45Cu40Zr7.5Fe2.5Sn2Si1Sc2Cu mold casting6637692 55 20[57]
Ti44Cu40Zr7.5Fe2.5Sn2Si1Sc3Cu mold casting6641694 53 20[57]
Table 2. Atomic radii, Pauling electronegativity, and heats of mixing for some elements [115].
Table 2. Atomic radii, Pauling electronegativity, and heats of mixing for some elements [115].
ElementRadiun (nm)Hmix (Ti) (kJ/mol)RTi/RA|RARTi|/RTiPauling Electronegativity
Ti0.14621.00001.54
Be0.1128−301.2960.2281.57
B0.0820−581.5890.3712.04
Al0.1432−301.0210.0211.61
Si0.1153−661.2490.2001.90
Sc0.164180.8910.1221.36
V0.1316−21.1110.1001.63
Cr0.1249−71.1710.1461.66
Fe0.1241−171.1780.1511.83
Co0.1251−281.1690.1441.88
Ni0.1246−351.1730.1481.91
Cu0.1278−91.1440.1261.90
Y0.1802151.2330.2331.22
Zr0.160300.9130.0961.33
Ag0.1445−21.0120.0121.93
Sn0.1620−210.9250.0811.96
Table 3. Summary of the effects of alloying elements on the glass-forming ability (GFA) of various Ti-based BMGs [39,49,52,54,57,79,87,90,91,92,99,105,106,107,108,110,117,125,126,127,128,129,130].
Table 3. Summary of the effects of alloying elements on the glass-forming ability (GFA) of various Ti-based BMGs [39,49,52,54,57,79,87,90,91,92,99,105,106,107,108,110,117,125,126,127,128,129,130].
Alloying ElementOptimum Content x (atom %)Base Alloy (atom %)tmax (mm)Tx (K)Ref.
WithoutWithIncrementWithoutWithExtension
B0.2–0.8Ti53Cu22−xNi12Zr3Al7Si3Bx Improved [108]
N0.1(Ti42.5Cu40Zr10Ni5Sn2.5)100−xNx34146.744.1−2.6[125]
Si1Ti46Cu31.5Zr12.5−xCo7Sn3Six231 [126]
1Ti42.5−xZr2.5Hf5Cu42.5Ni7.5Six25349501[99]
2Ti40Zr10Cu40−xPd10Six451506515[49]
Al5Ti41Zr25Be34−xAlx572577013[90]
Sc2Ti47−xCu40Zr7.5Fe2.5Sn2Si1Scx36354551[57]
Cr5Ti40Zr25Be35−xCrx682539744[79]
Fe6Ti41Zr25Be34−xFex51055711760[87]
Co1(Ti0.4Zr0.1Cu0.36Pd0.14)100−xCox583495910[117]
Ni3Ti40Zr25Cu15−xNixBe2010–14>14Improved4842−6[52]
Cu6Ti41Zr25Be34−xCux51510579740[92]
Y0.5(Ti40Zr25Be20Cu12Ni3)100−xYx<55Improved4221−21[106,127]
Zr5Ti50−xCu25Ni15Sn3Be7Zrx561455611[105]
Nb3(Ti40Zr25Cu9Ni8Be18)100−xNbx 40466[128,129]
Ag4Ti46Cu31.5−xZr11.5Co7Sn3Si1Agx35251510[110]
6Ti41Zr25Be34−xAgx510557581[91]
Sn2(Ti0.45Cu0.378Zr0.10Ni0.072)100−xSnx12139423[130]
5Ti41.5Zr2.5Hf5Cu42.5−xNi7.5Si1Snx264356429[54]
Sb3Ti50Cu25Ni25−xSbx 40455[39]
Hf5Ti47.5−xZr2.5HfxCu42.5Ni7.512147492[99]
Table 4. Mechanical properties of typical Ti-based BMGs together with other BMGs and crystalline alloys [40,41,45,47,49,52,53,54,55,56,57,72,79,80,81,82,84,86,87,88,89,90,91,92,93,94,95,99,100,101,102,103,104,105,106,110,126,145,146,147].
Table 4. Mechanical properties of typical Ti-based BMGs together with other BMGs and crystalline alloys [40,41,45,47,49,52,53,54,55,56,57,72,79,80,81,82,84,86,87,88,89,90,91,92,93,94,95,99,100,101,102,103,104,105,106,110,126,145,146,147].
Composition (atom %)Density (g/mm3)σy (MPa)σmax (MPa)εp (%)σc (N·m/kg)Sample Size (mm)Stain Rate (s−1)E (GPa)νRef.
Ti50Cu43Ni7 20505–11 φ1 × 21 × 10−4 [41]
Ti53Cu39Ni8 21600–6 φ1 × 21 × 10−4 [41]
Ti55Cu36Ni9 [41]
Ti50Cu42Ni8 20080 φ2 × 43 × 10−4100 [40]
Ti45Zr20Be354.59 18602.24.05 × 105φ3 × 61 × 10−496.80.36[79]
Ti41Zr25Be344.88189122388.23.88 × 105φ2 × 41 × 10−4 [80,81,87]
Ti45Zr5Cu45Ni5 19261.8 φ2 × 44 × 10−4110 [82]
Ti41.3Cu43.7Hf13.9Si1.1 16850 φ3 × 65 × 10−495 [145]
Ti40Zr10Cu36Pd14~7.20 19500.52.7 × 105φ2.5 × 55 × 10−482 [86]
Ti40Zr25Be30Cr54.76172019003.53.61 × 105φ3 × 61 × 10−494.80.35[79]
Ti41Zr25Be28Fe64.88196422681124.02 × 105φ2 × 44 × 10−4 [87]
Ti40Zr26Be28Fe64.93198921627.44.03 × 105φ2 × 44 × 10−4 [88]
(Ti41Zr25Be28)94Fe64.94201821010.84.09 × 105φ2 × 44 × 10−4 [89]
Ti41Zr25Be28Al64.801996219016.34.16 × 105φ2 × 44 × 10−4 [90]
Ti41Zr25Be28Ag65.27196120132.33.73 × 105φ2 × 44 × 10−4 [91]
Ti41Zr25Be28Cu65.02186919262.73.72 × 105φ2 × 44 × 10−4 [92]
Ti41Zr25Be28Ni65.07188919334.83.73 × 105φ2 × 44 × 10−4 [93]
Ti50Zr16Be24Ni104.99190020005.73.81 × 105φ2 × 44 × 10−4 [94]
Ti65Cu9Ni8Be184.84218322500.74.51 × 105φ2 × 41 × 10−41100.351[53]
Ti42.5Zr7.5Cu40Ni5Sn5 197821620.9 φ2.5 × 54 × 10−4 [95]
Ti43.15Zr9.59Cu36.24Ni9.06Sn1.966.60236026402.243.58 × 105φ1 × 24 × 10−4103 [45]
Ti40Zr10Cu34Pd14Sn26.85200020503.52.9 × 105φ2 × 45 × 10−4 [47]
Ti40Zr10Cu38Pd10Si2 19350 φ2.5 × 55 × 10−480 [49]
Ti60Zr5Cu9Ni8Be184.98218322500.74.3 × 105φ2 × 41 × 10−41060.352[53]
Ti55Zr10Cu9Ni8Be185.03205021138.34.1 × 105φ2 × 41 × 10−4980.356[53]
Ti40Zr25Cu12Ni3Be205.381680178053.1 × 105φ3 × 6 92.6 [52]
Ti32.8Zr30.2Ni5.3Cu9Be22.75.541 183103.30 × 105φ2 × 41 × 10−497.80.354[72]
(Ti41Zr25Be29Al5)91Cu95.07209322090.64.13 × 105φ2 × 44 × 10−4 [102]
(Ti41Zr25Be28Fe6)91Cu95.36199920831.23.7 × 105φ2 × 44 × 10−4 [101]
Ti36.2Zr30.3Cu8.3Fe4Be21.2 168019408 φ2 × 41 × 10−4 [100]
Ti50Ni15Cu25Sn3Be7 2170~1.8 φ1 × 21 × 10−4 [84]
Ti45.8Zr6.2Cu39.9Ni5.1Sn2Si1 ~20005 φ2 × 42.1 × 10−4 [104]
Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 2080<2 φ3 × 63 × 10−4103 [99]
Ti47Zr7.5Cu40Fe2.5Sn2Si1 >20001.5 φ2 × 42.1 × 10−4100 [55]
Ti45Cu25Ni15Sn3Be7Zr5 2480~4 φ1 × 21 × 10−4 [105]
(Ti40Zr25Be20Cu12Ni3)99.5Y0.5 18470 φ5 × 101 × 10−4 [106]
Ti46Zr11.5Cu31.5Co7Sn3Si1 2477.92623.30.83 φ2 × 4 [126]
Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn57.0 22600.53.31 × 105φ3 × 64 × 10−4108 [54]
Ti46Cu27.5Zr11.5Co7Sn3Si1Ag46.442126 1.033.30 × 105φ2 × 41 × 10−497 [110]
Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag26.30201020802.53.20 × 105φ2 × 42.1 × 10−4100 [56]
Ti45Cu40Zr7.5Fe2.5Sn2Si1Sc26.27200321505.93.20 × 105φ2 × 42 × 10−497.10.362[57]
Ti44Cu40Zr7.5Fe2.5Sn2Si1Sc36.20196320422.03.20 × 105φ2 × 42 × 10−495.40.362[57]
Zr41.2Ti13.8Cu12.5Ni10Be22.56.07173717741.02.92 × 105φ2 × 4 960.366[53]
Zr46.75Ti8.25Cu7.5Ni10Be27.55.99182018902.03.04 × 105φ 2 × 4 99.60.364[53]
Zr52.5Ti5Cu17.9Ni14.6Al106.55185019002.02.82 × 105φ2 × 4 87.20.370[53]
Zr57Cu15.4Ni12.6Al10Nb56.68178518000.52.67 × 105φ2 × 4 87.30.365[53]
Pd77.5Cu6.0Si16.510.591476160011.41.39 × 1052 × 2 × 44 × 10−492.90.411[146]
Mg57Cu31Y6.6Nd5.43.81112611881.22.96 × 105φ1 × 21 × 10−454.40.312[17]
Al86Ni7Y4.5Co1La1.53.141050 ~43.34 × 105φ1 × 21 × 10−4 [147]
Ti–6Al–4V4.40729 a954 a 1.66 × 105 110–1140.349
AZ911.82160 a280 a 0.88 × 105 ~45~0.35
7075-T62.81503 a572 a 1.79 × 105 71~0.28
σy: Yield strength; σmax: ultimate strength; εp: plastic strain; σc: specific strength (defined as σy /ρ); E: Young’s modulus; ν: Poisson’s ratio. a Tensile data.
Table 5. A brief summary of Ti-based bulk metallic glass composites [153,170,171,172,173,174,175,176,177,178,179,180].
Table 5. A brief summary of Ti-based bulk metallic glass composites [153,170,171,172,173,174,175,176,177,178,179,180].
Alloys (atom %)Reinforced PhaseTypeSynthesis MethodDensity (g/mm3)Test Methodσy (MPa)σmax (MPa)εp (%)σc (N·m/kg)Ref.
Ti48Zr20V12Cu5Be15BCC Ti66V19Zr14Cu1 dendritesIn situArc melting5.15Tensile1362142910.22.64 × 105[153]
Ti50Cu23Ni20Sn7HCP-Ti dendrites + Ti3Sn + β-(Cu, Sn)In situArc melting & Cu mold casting Compression119021306 [170]
Ti52.9Zr34.5Ni1.6Cu4.2Be6.8BCC Ti61.5Zr36.4Cu2.1 dendritesIn situArc melting & Cu mold casting Compression1243191910.7 [171]
Ti50Zr20Nb12Cu5Be13Β-Ti(Zr, Nb) dendritesIn situArc melting & copper mold casting Compression18392425>21 [172]
Ti43.2Cu38Ni10Zr7.8Al0.5Si0.5B2-(Ti, Zr)(Cu, Ni) dendritesIn situArc melting & Cu mold casting Compression1255296016.5 [173]
Ti56Zr6Cu19.8Pd8.4Sn1.8Nb8Nb-rich β-Ti dendritesIn situArc melting & Cu mold casting Compression1690268020 [174]
Ti34.3Zr21.6Ni4.6Cu8.5Be17Nb15β-Ti dendritesIn situArc melting & Cu mold casting Compression1720179025 [175]
(Ti58Ni28Cu8Si4Sn2)94Mo6β-Ti dendritesIn situArc melting & Cu mold casting6.54Compression1134237212.181.73 × 105[176]
(Ti38.8Zr28.8Cu6.2Be16.2Nb10)92Ta8BCC β(Ti, Zr)(Nb, Ta) dendritesIn situArc melting & copper mold casting Compression815203227.2 [177]
Ti46Zr20V12Cu5Be17β-TiZr dendritesIn situArc melting & Bridgman solidification Compression1956270615.3 [178]
Ti47Zr19Be15V12Cu7β-Ti dendritesIn situArc melting & Bridgman solidification Compression1601302432.6 [179]
Ti42.5Cu40Zr7.5Ni5Sn5DiamondEx-situSpark plasma sintering Compression 18500 [180]
Table 6. Fatigue-endurance limits and fatigue ratios of various BMGs [198,199,200,201,202].
Table 6. Fatigue-endurance limits and fatigue ratios of various BMGs [198,199,200,201,202].
Composition (atom %)Ultimate Tensile Strength (MPa)Fatigue Endurance Limit (MPa)Fatigue RatioFrequency (Hz)RRef.
Ti40Zr10Cu34Pd14Sn220507620.3720000AX *-1[198]
Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1204016100.7910AX 0.1[199]
Zr50Al10Cu4018217520.4110AX 0.1[200]
Zr50Al10Cu30Ni1019008650.4610AX 0.1[200]
Zr41.2Ti13.8Cu12.5Ni10Be22.518507030.3810AX 0.1[200]
[(Co0.6Fe0.4)0.75B0.2Si0.05]96Nb4417023700.5710AX 0.1[200]
(Fe0.5Co0.5)72B20Si4Nb4420022800.5410AX 0.1[200]
Ni60Zr20Nb15Al5290016800.5810AX 0.1[200]
Cu60Zr30Ti1020009800.4910AX 0.1[199,201]
Ti–6Al–4V9605350.5620000AX-1[202]
* AX: Axial Loading.
Table 7. Thermoplastic formability of typical Ti-based BMGs together with various BMG forming alloys [39,40,47,54,56,79,84,86,95,206,207,208,209,210,211,212].
Table 7. Thermoplastic formability of typical Ti-based BMGs together with various BMG forming alloys [39,40,47,54,56,79,84,86,95,206,207,208,209,210,211,212].
Composition (atom %)S parameterRef.
Ti50Zr42Ni80.11[40]
Ti45Zr20Be350.11[79]
Ti40Zr10Cu36Pd140.10[86]
Ti40Zr10Cu34Pd14Sn20.10[47]
Ti42.5Zr7.5Cu40Ni5Sn50.12[95]
Ti50Ni15Cu25Sn5Zr50.10[39]
Ti40Zr25Ni8Cu9Be180.12[84]
Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn50.13[54]
Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag20.10[56]
Pd43Ni10Cu27P100.31[206,207]
Pt57.5Cu14.7Ni5.3P22.50.34[206,208]
Au49Ag5.5Pd2.3Cu26.9Si16.30.24[206,209]
Zr41.2Ti13.8Ni10Cu12.5B22.50.21[206,210]
Zr44Ti11Cu10Ni10Be250.25[206,211]
Mg65Cu25Y100.19[212]

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Gong, P.; Deng, L.; Jin, J.; Wang, S.; Wang, X.; Yao, K. Review on the Research and Development of Ti-Based Bulk Metallic Glasses. Metals 2016, 6, 264. https://doi.org/10.3390/met6110264

AMA Style

Gong P, Deng L, Jin J, Wang S, Wang X, Yao K. Review on the Research and Development of Ti-Based Bulk Metallic Glasses. Metals. 2016; 6(11):264. https://doi.org/10.3390/met6110264

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Gong, Pan, Lei Deng, Junsong Jin, Sibo Wang, Xinyun Wang, and Kefu Yao. 2016. "Review on the Research and Development of Ti-Based Bulk Metallic Glasses" Metals 6, no. 11: 264. https://doi.org/10.3390/met6110264

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