# Effect of Heat Treatment on Some Titanium Alloys Used as Biomaterials
## 3 Results and Discussions
### Optical Microscopy Analysis  
Figures 5 and 6 show the structure of Ti-Mo-Zr-Ta alloys with grain-specific aspects of titanium alloys. Optical microscopy images for Ti15 Mo7 Zr5 Ta, Ti15 Mo7 Zr10 Ta, and Ti15 Mo7 Zr15 Ta highlight the lamellar dendrites inside the \(\beta\)-type grains. Ti20 Mo7 Zr5 Ta, Ti20 Mo7 Zr10 Ta, and Ti20 Mo7 Zr15 Ta alloys show a dendritic structure with irregular grain boundaries. These acicular and coarse structures are specific to \(\beta\) alloys [24,25]. Figure 4: Extended heat and pressure diagrams generated in MATLAB. Figure 5: _Cont_. Figure 5: Microstructure analysis of Ti15 Mo/Zr15 Ta alloys with different percentage of Si added at a magnifying power of 70x: (**a**) Ti15 Mo/Zr15 Ta, (**b**) Ti15 Mo/Zr15 Ta0.5 Si, (**c**) Ti15 Mo/Zr15 Ta0.75 Si, (**d**) Ti15 Mo/Zr15 Ta1 Si and of 280x: (**e**) Ti15 Mo/Zr15 Ta, (**f**) Ti15 Mo/Zr15 Ta0.5 Si, (**g**) Ti15 Mo/Zr15 Ta0.75 Si, (**h**) Ti15 Mo/Zr15 Ta1 Si. Figure 6: _Cont._  
At 882 \({}^{\circ}\)C, titanium undergoes an allotropic transformation, which allows the metal to transit from an \(\alpha\)-phase hexagonal close-packed structure to a \(\beta\)-phase body-centered cubic structure [26]. Due to molybdenum and tantalum stabilizers, studies have shown that \(\beta\)-phase alloys have the advantage of increased mechanical strength and an elastic modulus similar to human bone, both of which are important aspects of the long-term use of biomaterials in the medical field [27]. Thermal treatments are a sequence of stages that consist of the heating, maintenance, and cooling of some metal alloys to obtain certain structures that ensure the desired set of physico-chemical characteristics without changing the state of aggregation of the material [28; 29; 30]. Compared to other conventional biomaterials, Ti-Mo alloys containing various biocompatible components such as zirconium and tantalum have superior mechanical qualities such as high tensile strength and a significantly lower modulus of elasticity close to that of human bone. When the \(\alpha\) alloying elements are introduced into the titanium, the temperature at which the phase transition occurs changes [31; 32]. The temperature range in which an \(\alpha\) phase rises when pure titanium is alloyed with \(\beta\) stabilizing elements (Mo, Ta, Si), whereas the alloy with elements expands the \(\beta\) phase domain. Other elements, such as zirconium, have a neutral effect on the temperature domains where the two phases coexist [33; 34; 35]. The images obtained by optical microscopy highlight a biphasic structure consisting of a high proportion of \(\beta\) solid solution, in which intergranular lamellar structures specific to \(\alpha^{\prime\prime}\) orthorhombic martensite appear (Figures 5 and 6). The structure of the Ti-Mo-Zr-Ta alloys is similar to the microstructure of the commercial Ti-6 Al-4V alloys. Typically, the Ti-6 Al-4V microstructure contains \(\alpha\)-type very fine acicular needles, \(\alpha+\beta\) lamellar structures, and \(\beta\)-type grains [36; 37]. In general, the microstructure of titanium alloys consists of a lamellar structure of \(\alpha\) and \(\beta\). However, the \(\alpha\) morphology could change with heat treatment or depending on the amount of \(\beta\) elements added. The lamellar microstructure, produced on cooling from the phase field, and the equiaxial microstructure, resulting from the recrystallization process, are the two extreme situations of phase arrangements, and they both affect the microstructure of titanium alloys. Much investigation has gone into how phase size and arrangement affect mechanical properties. With the increase of the volume fraction of the \(\beta\) phase, the strength of the alloy will increase, and the properties will be close to the human body [38].