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Article

The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass

by
Aytekin Hitit
* and
Hakan Şahin
Department of Materials Science and Engineering, Afyon Kocatepe University, Afyonkarahisar 03200, Turkey
*
Author to whom correspondence should be addressed.
Metals 2017, 7(1), 7; https://doi.org/10.3390/met7010007
Submission received: 23 November 2016 / Revised: 25 December 2016 / Accepted: 26 December 2016 / Published: 29 December 2016
(This article belongs to the Special Issue Bulk Metallic Glasses)
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Abstract

:
In this study, change in glass forming ability (GFA) and thermal stability of Co–Fe-based bulk metallic glasses were investigated as a function of iron content. Cylindrical samples of alloys with diameters of up to 4 mm were synthesized by a suction casting method in an arc furnace. Structures and thermal properties of the as-cast samples were determined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. It was found that the critical casting thickness of the alloys reduced as iron content was increased and cobalt content was decreased. It was determined that GFA parameters, reduced glass transition temperature (Tg/Tl) and δ (= Tx/(Tl − Tg)), show a very good correlation with critical casting thickness values. It was also observed that changing iron content did not effect thermal properties of the alloys.

1. Introduction

Co- and Fe-based bulk metallic glasses have received considerable attention because of their good soft magnetic properties [1,2,3,4,5,6,7,8,9,10], ultrahigh fracture strength values [1,3,6,7,9,10,11,12,13,14,15,16] and high corrosion resistance [17,18]. Besides, they have a potential to be used in biomedical applications [17,19,20]. In addition to these attractive properties, they also have very high thermal stability [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,21,22,23]. Additionally, it is known that Co- and Fe-based metallic glasses that contain high amounts of boron can be used as precursors to develop nanocomposites that have ultrahigh hardness values [8,24,25]. Because such ultrahigh hardness values of these composites result from the precipitation of borides which form upon devitrification, it is reasonable to expect that, if the boron content of Co- or Fe-based bulk metallic glass is increased, the hardness of the composite becomes higher due to increased volume fraction of borides.
A comparison of boron contents of Co- and Fe-based bulk metallic glasses shows that, in general, boron contents of Co-based bulk metallic glasses [1,14,15] are higher than those of Fe-based bulk metallic glasses [2,4,6,7,13]. In fact, the boron content of Co-based bulk metallic glasses can be as high as 37.5 atom % [14]. However, the highest boron content of a Fe-based bulk metallic glass is 25 atom % [6]. When compared in terms of cost, it is obvious that Fe-based bulk metallic glasses are more attractive than Co-based bulk metallic glasses. Therefore, replacing cobalt with iron in a Co-based bulk metallic glass containing high amount of boron without degrading its GFA will definitely make the resulting metallic glasses more cost-effective precursors to develop composites having ultrahigh hardness values. In addition, the cost of composites can be lowered further by using low-cost industrial raw materials such as ferro-boron, ferro-niobium, and ferro-tantalum.
In this study, we report the effect of iron content on GFA and the thermal properties of a Co-based bulk metallic glass, Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5, which has a critical casting thickness of 4 mm [26]. For this reason, cobalt is partially replaced with iron and Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 alloys (where x = 10, 20, and 30) were synthesized. The critical casting thicknesses, the thermal properties, and the microhardnesses of the alloys were determined as a function of iron content.

2. Materials and Methods

Co–Fe-based alloy ingots with composition of Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 (where x = 10, 20, 30) were prepared by arc melting the mixtures of pure Co (99.8 wt %), Fe (99.9 wt %), Ni (99.9 wt %), Ta (99.9 wt %), Nb (99.8 wt %), and Si (99.9 wt %) metals and pure crystalline B (98 wt %) in a Ti-gettered ultrahigh purity argon atmosphere. Master alloys were melted three times in order to ensure homogeneity. Cylindrical samples of the alloys with diameters up to 4 mm and a length of 40 mm were synthesized by suction casting method in a vacuum arc furnace. Structures of the samples were examined by X-ray diffraction (XRD, Shimadzu XRD-6000, Kyoto, Japan) with Cu Kα radiation. The glass transition temperatures (Tg), the crystallization temperatures (Tx), the solidus temperatures (Tm), and the liquidus temperatures (Tl) of the alloys were determined by differential scanning calorimetry (DSC, Netzsch STA 449 F3, Selb, Germany) at a heating rate of 0.33 K/s. Microhardness measurements were carried out with a Vickers microhardness tester (Shimadzu HMV 2 L, Kyoto, Japan) under a load of 2.94 N. For each alloy, microhardnesses of as-cast samples were measured. Twenty microhardness measurements were carried out for each sample, and the arithmetic mean of the measurements were considered as the microhardness of the alloy.

3. Results

XRD patterns of samples are given in Figure 1. The base alloy, Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5, has a critical casting thickness of 4 mm. For the casting thickness of 5 mm, (Co,Fe)21Ta2B6 and (Co,Fe)2B phases form. The Co31Fe30Ni2Ta2.75Nb2.75B26.5Si5 alloy has a critical casting thickness of 3 mm. For the casting thickness of 4 mm, the precipitation of (Co,Fe)2B phase was observed for this alloy. The critical casting thicknesses of alloys Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5 and Co11Fe50Ni2Ta2.75Nb2.75B26.5Si5 were found to be 2 mm and 0.5 mm, respectively. For both of these alloys, the precipitation of the (Co,Fe)2B, (Co,Fe)16Ta6Si7, and Fe3Si phases was observed in the samples having diameters larger than the critical casting thicknesses.
Thermal properties of the alloys were determined by DSC (Figure 2). During heating, all the DSC traces showed an endothermic event, which is the indication of the glass transition and followed by exothermic reactions, which are the signs of crystallization of the glassy structure. Tg and Tx of the base alloy, Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5, are 891 and 947 K, respectively [21]. The Tg of Co31Fe30Ni2Ta2.75Nb2.75B26.5Si5 alloy is 890 K. Additionally, the Tx of Co31Fe30Ni2Ta2.75Nb2.75B26.5Si5 alloy is 957 K, which is about 10 K higher than the Tx of Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5 alloy. The Tg of Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5 and Co11Fe50Ni2Ta2.75Nb2.75B26.5Si5 alloys are found to be 895 and 894 K, respectively. Moreover, the Tx of Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5 and Co11Fe50Ni2 Ta2.75Nb2.75B26.5Si5 alloys are determined as 960 and 964 K, respectively. In addition, the Tl of the base alloy is 1443 K [26], and the Tl of 30, 40, and 50 atom % iron-containing alloys are 1458, 1488, and 1532 K, respectively. Thermal properties of the alloys are given in Table 1. In addition, the microhardnesses of the alloys were determined to be around 1200 Hv.

4. Discussion

The critical casting thickness decreases as iron content is increased. It is quite obvious that the reduction in critical casting thickness results from the fact that the Tl of the alloys increases with iron content. For a constant Tg, if liquidus temperature increases, the minimum cooling rate necessary to obtain a completely amorphous structure also increases. As a result, the casting thickness must be decreased to achieve this cooling rate. Well-known GFA parameters, reduced glass transition temperature (Tg/Tl) [27] and δ (= Tx/(Tl − Tg)), show a very good correlation with the critical casting thickness values (Figure 3). However, the correlation between the GFA parameter Tx/(Tg + Tl) and the critical casting thickness values is not as satisfactory as those of the others.
(Co,Fe)2B, (Co,Fe)16Ta6Si7, and Fe3Si phases form during cooling. It is shown that (Co,Fe)2B phase forms at temperatures higher than temperatures at which the (Co,Fe)16Ta6Si7 phase forms [25]. In other words, (Co,Fe)2B is the first phase that precipitates during cooling. Since the melting temperature of the Fe2B phase is higher than the Co2B phase, increasing iron content increases the melting temperature of the (Co,Fe)2B phase. As a result, the liquidus temperatures of the alloys increase.
Glass transition temperatures of the alloys do not change with iron content. However, crystallization temperatures increase slightly. Similarly, microhardnesses of the alloys remain almost constant. Cohesive energies of Fe–Fe and Co–Co bonds are 413 and 424 kJ/mol, respectively [28]. It is quite reasonable to assume that cohesive energy of Fe–Co bond is close to these values. When the iron content of the alloys is increased, the number of Co–Co bonds decreases, but the number of Fe–Co bonds increases. Since cohesive energies of these bond are almost the same, the total cohesive energy of the amorphous structure remains constant. For this reason, the thermal properties and the microhardnesses of the alloys do not change.
The Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5 alloy has a reasonably high iron content and critical casting thickness. Therefore, it is believed that it can used as a precursor to develop composites with ultrahigh hardness values at a low cost.

5. Conclusions

The following conclusions can be reached from this study:
  • The critical casting thicknesses of the alloys decrease with iron content due to the increase in liquidus temperatures.
  • The critical casting thicknesses of the alloys show a very good correlation with reduced glass transition temperature, Tg/Tl and Tx/(Tl − Tg).
  • Thermal properties and microhardnesses of the alloys do not change with iron content because of the fact that cohesive energies of the Co–Co- and Fe–Fe bonds are almost the same.

Acknowledgments

This study was supported by grant No. 10.MUH.10 from the Research Council of Afyon Kocatepe University.

Author Contributions

Aytekin Hitit and Hakan Şahin synthesized the alloys and cast the samples. Hakan Şahin performed XRD and DSC experiments and microhardness measurements. Aytekin Hitit and Hakan Şahin analyzed the results. Aytekin Hitit wrote and edited the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 07 00007 g001 550
Figure 1. XRD patterns of the Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 (x = 10, 20, and 30) alloys.
Figure 1. XRD patterns of the Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 (x = 10, 20, and 30) alloys.
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Metals 07 00007 g002 550
Figure 2. DSC curves of the Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 (x = 10, 20 and 30) alloys. (a) low temperature; (b) high temperature.
Figure 2. DSC curves of the Co41 − xFe20 + xNi2Ta2.75Nb2.75B26.5Si5 (x = 10, 20 and 30) alloys. (a) low temperature; (b) high temperature.
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Metals 07 00007 g003 550
Figure 3. Relationship between the critical casting thickness (Dmax) for the formation of a glassy phase and GFA parameters. (a) Reduced glass transition temperature (Tg/Tl); (b) Tx/(Tl − Tg); (c) Tx/(Tg + Tl).
Figure 3. Relationship between the critical casting thickness (Dmax) for the formation of a glassy phase and GFA parameters. (a) Reduced glass transition temperature (Tg/Tl); (b) Tx/(Tl − Tg); (c) Tx/(Tg + Tl).
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Table
Table 1. Thermal properties (Tg, Tx, Tl, Tm), parameters for GFA, critical casting thickness and microhardnesses of Co–Fe–Ni–Ta–Nb–B–Si alloys.
Table 1. Thermal properties (Tg, Tx, Tl, Tm), parameters for GFA, critical casting thickness and microhardnesses of Co–Fe–Ni–Ta–Nb–B–Si alloys.
AlloyTg (K)Tx (K)Tm (K)Tl (K)Tg/TlTx/(Tl − Tg)Tx/(Tg + Tl)Dmax (mm)Hv
Co41Fe20Ni2Ta2.75Nb2.75B26.5Si5 [26]891947132314430.6171.7160.40641197
Co31Fe30Ni2Ta2.75Nb2.75B26.5Si5890957129514580.6101.6850.40831242
Co21Fe40Ni2Ta2.75Nb2.75B26.5Si5895960131814880.6011.6190.40321240
Co11Fe50Ni2Ta2.75Nb2.75B26.5Si5894964134315320.5841.5110.3970.51238

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MDPI and ACS Style

Hitit, A.; Şahin, H. The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass. Metals 2017, 7, 7. https://doi.org/10.3390/met7010007

AMA Style

Hitit A, Şahin H. The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass. Metals. 2017; 7(1):7. https://doi.org/10.3390/met7010007

Chicago/Turabian Style

Hitit, Aytekin, and Hakan Şahin. 2017. "The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass" Metals 7, no. 1: 7. https://doi.org/10.3390/met7010007

APA Style

Hitit, A., & Şahin, H. (2017). The Effect of Iron Content on Glass Forming Ability and Thermal Stability of Co–Fe–Ni–Ta–Nb–B–Si Bulk Metallic Glass. Metals, 7(1), 7. https://doi.org/10.3390/met7010007

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