The Influence of Thermomechanical Treatments on the Structure, Microstructure, and Mechanical Properties of Ti-5Mn-Mo Alloys

Abstract

With the increase in the world’s population, the rising number of traffic accidents, and the increase in life expectancy, the need for implants, dental work, and orthopedics is growing ever larger. Researchers are working to improve the biomaterials used for these purposes, improve their functionality, and increase the human body’s life span. Thus, new titanium alloys are being developed, usually with β-stabilizer elements (which decrease the elastic modulus), with the Ti-Mn-Mo alloys being one example of these. This study of the Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys only showed signs of the β phase in the structure and microstructure, presenting a combination of low modulus of elasticity and high corrosion resistance compared to the values of commercial alloys. In this sense, this work presents an analysis of the influence of some thermomechanical treatments, such as homogenization, hot-rolling, solution, and annealing, on the structure, microstructure, and selected mechanical properties of the Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys.

1. Introduction

Metallic biomaterials are widely used in pins, plates, screws, needles, and implants (orthopedic, dental, and cardiovascular) [1]. They are divided into precious metals (such as gold and silver), stainless steel, Co-Cr alloys, and titanium alloys. Stainless steel and Co-Cr alloys began to be widely used during the 21st century due to them having mechanical properties, having good corrosion resistance, and being low cost. After World War II, titanium has aroused great interest due to its properties, such as high mechanical, corrosion, and wear resistance; good elastic modulus value; and high biocompatibility [2]. The first titanium alloys used were α-type alloys, which were developed for use in aircraft and missiles. Titanium is as tough as steel but 45% lighter and 60% heavier than aluminum but twice as strong [3]. α- + β-type alloys gained attention due to their excellent properties, with the Ti-6Al-4V alloy being widely used until today [4,5,6]. However, studies have found that vanadium is cytotoxic, while aluminum has been associated with neurological disorders (such as Alzheimer’s disease) when used long term [7]. Thus, researchers have been looking for alloys without these elements and with a low elastic modulus. These new alloys have been produced with the addition of molybdenum, zirconium, tantalum, niobium, and manganese, which are elements that do not present cytotoxic reactions with organisms [8,9,10,11,12,13].

Gaining a reduction in the elastic modulus (or Young’s modulus) without impairing the mechanical or chemical properties of titanium is a significant challenge in the development of novel titanium alloys [14], since the alloys that are currently used have elastic moduli 3 to 4 times higher than cortical human bone moduli [15]. The presence of a significant difference between the value of the elastic modulus of an implant and bone can cause local pain to the patient and increase the implant’s failure rate. A material with a high elastic modulus will cause the local mechanical load to be supported by the implant and not by the bone, causing the bone to lose its density and become brittle [16]. This poor stress distribution is called the “stress shielding” effect.

New Ti-Mo-Mn alloys have been produced by arc-furnace melting, with only the β-phase in the form of equiaxial grains and with no cytotoxic effects on fibroblast cells over short culture periods [9]. These alloys have a hardness higher than that of CP-Ti and the increase in hardness decreases the incidence of wear in the implant material. The elastic modulus of these alloys have been shown to be below that of commercial alloys. The excellent mechanical properties associated with cytotoxicity tests are adequate for possible biomedical applications.

Heat treatment is understood as the act of heating a material to a specific temperature and then cooling after some time under established conditions to obtain particular material properties [17]. The main conditions that must be considered are heating, the temperature residence time, cooling, and the heating atmosphere. The purpose of heat treatment is to obtain changes in the structure and microstructure of a material and its mechanical characteristics, such as changes in ductility, mechanical resistance, and hardness [18,19,20,21].

Usually, homogenization heat treatment is performed after the melting of an alloy. This treatment reduces the imperfections acquired by the melting furnace’s cooling gradient and eliminates the possible segregation through the alloy elements’ atomic diffusion, thus homogenizing the samples [22]. After this treatment, it is customary to perform thermomechanical processing to obtain a regular format to perform adequate characterization [23]. Two other heat treatments are usually performed in the processing of titanium alloys. The solution used in [24], with rapid cooling in water, improves a material’s mechanical resistance. Another treatment, annealing [25], removes internal stresses caused by mechanical processing, changing the alloy’s mechanical properties.

This paper aims to analyze the influence of some thermomechanical treatments such as homogenization, hot-rolling, solution, and annealing in terms of structure, microstructure, and some selected mechanical properties (Vickers microhardness, Young’s modulus) of Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys.

2. Materials and Methods

Ti-5Mn-10Mo (wt.%) and Ti-5Mn-15Mo (wt.%) alloys were melted in an arc furnace. The details of samples’ preparation and the chemical, structural, microstructural, and cytotoxicity characterization of the cast alloys are described in Lourenço et al. [9].

After melting, the alloys underwent a homogenization heat treatment to relieve internal stresses, reduce the imperfections caused by the melting furnace’s cooling gradient, and eliminate possible segregates. This also made the alloys more ductile, preventing damage to the sample’s rolling equipment and microcracks when submitted to the hot-rolling process [26]. The treatment was carried out in a vacuum of 10⁻⁵ Torr for 24 h at 1000 °C, with slow cooling taking place at a rate of 5 °C/min.

The hot-rolling process was then conducted to obtain a regular shape for Young’s modulus measurements. The samples were heated in a tubular furnace at approximately 1000 °C and then passed through the hot-rolling equipment. This process was repeated, achieving an around 1 mm reduction in thickness with each pass. A final thickness of 4 mm was reached.

To study the influence of heat treatments on the structure, microstructure, and selected mechanical properties of the alloys, two treatments were carried out after hot-rolling: annealing and solution. The annealing heat treatment was performed to remove internal stresses caused by the hot-rolling mechanical processing, which can cause changes in an alloy’s mechanical properties. The first treatment was carried out in a 10⁻⁵ Torr vacuum with a heating rate of 10 °C/min at 900 °C for three hours, followed by a controlled cooling of 10 °C/min. With rapid cooling in water, the solution heat treatment was carried out to improve the material’s mechanical strength. This second treatment was carried out in a 10⁻⁵ Torr vacuum with a heating rate of 10 °C/min at 900 °C for 3 h, followed by rapid cooling in water.

After each heat treatment, the alloys were characterized by X-ray measurements using a MiniFlex600 diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.54056 Å). The data were collected using the powder method and the fixed-time mode, with steps of 0.04°, ranging from 20° to 100°, 2θ step sizes, and a 10°/min collection time. Microstructural analysis was performed with an Olympus BX51M model optical microscope (Olympus, Tokyo, Japan) and a EVO LS15 model scanning electron microscope (Carl Zeiss, Oberkochen, Germany).

Mechanical characterization was performed with microhardness measurements. The samples were indented five times in different regions. A force of 0.245 N, mass of 25 g, and duration of 60s was used, along with a HMV-2 microdurometer (Shimadzu, Tokyo, Japan). The technical standards for this test were followed [27,28]. The impulse excitation dynamic method was used for the modulus of elasticity measurements, performed on the Sonelastic^® (ATCP Physical Engineering, São Carlos, Brazil) equipment.

3. Results and Discussion

The X-ray diffractograms for the Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys are shown in Figure 1 and Figure 2, respectively. It can be seen that only the peaks corresponding to the β phase of titanium are presented for all processing conditions [29]. The phase composition of the alloys was not changed by the thermomechanical processes. The two alloying elements (manganese and molybdenum) added to titanium are considered β-stabilizations and do not allow the precipitation of other phases [30]. By the equivalent molybdenum theory, it is necessary to use at least 10 wt.% of molybdenum in order to obtain β phase predominance at room temperature [31]. X-ray diffractograms corroborate this theory. Other studies have also shown only the presence of β phase for Ti-Mo alloys with a molybdenum content between 15 and 20 wt.% [32,33,34].

Figure 3 and Figure 4 show the optical micrographs (200×) and the micrographs obtained by SEM (2000×) for Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys, respectively, in all processing conditions. In the heat-treated condition after melting (Figure 3a,e), grains with regular shapes can be seen, indicating that the alloys were homogenized. These also increased the grains’ size concerning the as-cast condition [9]. This increase in the grain size occurred due to the homogenization heat treatment, where more energy was provided to the material. After hot-rolling (Figure 3b,f), it was observed that there was a decrease in the grain size. The grains became irregular, with a preferential orientation (texturized). This orientation occurs preferentially in the mechanical deformation direction. The mechanical process causes this effect at high temperatures without a controlled atmosphere and cooling rate. The solution heat treatment (Figure 3c,g) promoted the new growth of the grains, while the annealing treatment (Figure 3d,h) promoted the new organization of the grains, recrystallizing the material. The same morphology can be observed in Figure 4 for the Ti-5Mn-15Mo. Dark regions and spots resembling pitting corrosion can be observed. Corrosion of this type occurs after the chemical etching of polished surfaces, favored by internal stresses on the surface, creating regions where the chemical kinetics of the reaction are accelerated. The predominance of grains of the β phase, with the absence of acicular structures from α’ and α’’ phases, can also be seen [35].

The microhardness results for the alloys used in this study are shown in Figure 5 for all processing conditions. For the Ti-5Mn-10Mo alloy, there was an increase in microhardness in the homogenized state compared to the as-cast material [9]. This increase occurs because the homogenization heat treatment stabilizes the β phase, contributing to a strengthened solid solution and increasing the material’s hardness [36,37]. In the hot-rolled condition, the hardness remained high due to the internal stresses caused by mechanical deformation in the process [38]. After solution heat treatment, there was a decrease in the hardness value because, due to the rapid cooling, the instability of the β phase increased, decreasing the alloy’s hardness [14]. Annealing reduced the internal stresses resulting from the hot-rolling process, decreasing the hardness of the alloy. For the Ti-5Mn-15Mo alloy, no significant variation was seen between the processes [39]. Comparing the alloys produced by them under all conditions, it can be seen that as the concentration of molybdenum increased, the microhardness decreased. This relationship is inversely proportional due to solid solution hardening, since the lattice parameter of the alloy with 10% Mo was 3.247470 Å and that of the alloy with 15% Mo was 3.243753 Å. These values were calculated using the Law of Bragg. All samples showed values above CP-Ti due to hardening by solid solution [35]; as there were more substitutional elements than the pure element, the hardness increased. This effect occurred due to the variation in the lattice parameter in the unit cell, causing distortions in the crystalline structure, making the dislocation motion difficult [40]. Comparing the alloys to each other, it can be seen that the composition of Ti-5Mn-10Mo was more sensitive to thermomechanical treatments than that of Ti-5Mn-15Mo. As manganese was fixed in the alloys (melting point of 1246 °C) and the amount of titanium (melting point of 1668 °C) and molybdenum (melting point of 2623 °C) changed [41], the melting point of Ti-5Mn-10Mo became lower than that of Ti-5Mn-15Mo; consequently, more variation in hardness occurred. Similar values were obtained by Gabriel et al. [42,43,44] for Ti-10Mo-Nb and Ti-12Mo-3Nb. Similar behavior was observed by Sandu et al. [45] using TiMoSi. As the Mo content increased from 0.5 to 1.0 wt.%, the hardness decreased from 233 to 189 HV.

The elastic modulus (Young’s modulus) is a property that depends on the bond strength between the atoms of a material. Young’s modulus is related to the crystalline structure and interatomic spacing of atoms, which can be affected by adding alloy elements, heat treatments, and plastic deformation [46]. The closer the bone elastic modulus value is, the better a material will be for biomedical applications [1]. A comparison of the Young’s moduli of the Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys under all processing conditions studied in this paper with those of some commercial alloys is presented in Figure 6. All the Ti-5Mn-10Mo and Ti-5Mn-15Mo alloy values were below those of some alloys that are already commercially used as biomaterials. The Young’s modulus values decreased compared to CP-Ti due to the addition of the alloying elements, manganese and molybdenum, which are β-stabilizing agents, therefore presenting suitable values for application as biomaterials [3]. Young’s modulus is related to a material’s stiffness [47]. The larger the Young’s modulus is, the more rigid the material is and the lower the elastic deformation will be. The Young’s modulus decreased in the solubilized condition because, with rapid cooling, the alloy atoms’ interatomic bond strength decreased [48]. There was an increase in Young’s modulus in the annealed condition because, due to the slow cooling, there was time for the atoms to reorganize and thus the interatomic bond strength between them increased [48].

4. Conclusions

This paper produced Ti-5Mn-10Mo and Ti-5Mn-15Mo alloys by arc melting and subjected them to homogenization heat treatment, a hot-rolling process, and solution and annealing heat treatments. Based on the presented results, we conclude that the structure and microstructure of the studied alloys showed the only characteristics of the β phase. These alloys were intended to be applied as biomaterials with a good combination of hardness and elastic modulus values. The materials’ hardness remained higher than that of CP-Ti; this increase in hardness will lead to a decrease in material wear. The Young’s modulus of the alloys was below that of commercial alloys. Among the studied alloys, the composition of Ti-5Mn-10Mo was more sensitive to heat treatments than that of Ti-5Mn-15Mo.