The Structure Stability And Elastic Properties Of β Type Biomedical Ti Alloys From First-principles Calculations

The traditional Ti alloys have been successfully applied as biomedical implant materials due to their superior biocompatibilty,mechanical property, shape memory property and superelasticity. However, the high elastic modulus of the traditional Ti alloys may cause the"stress shielding''when they are impanted in human body. Moreover Ni, V and Al are cytotoxic, which may cause the adverse tissue reaction. Therfore, developing newβTi alloys with lowering the elastic moduli and promoting the safety of the alloys are a hot topic in this research field. In this thesis, the elastic characteristic and phase stability of the novelβTi alloys were investigated by the calculations from first-principles based on density functional theory, and the principle to design the newβTi alloys with a low modulus is suggested. The main conclusions are as follows:(1) The structural stability ofβTi-Nb, Ti-Mo and Ti-Ta alloys increased with Nb, Mo and Ta content increasing. Theβphase appear in Ti-Nb, Ti-Mo and Ti-Ta alloys when the Nb, Mo and Ta content are 8.08%, 3.7% and 12.5% (in atom pesent), respectively. The strength to stabilizeβphase of Nb, Mo and Ta is in the sequence of Mo>Nb>Ta. The elastic modulus ofβTi-Nb, Ti-Mo and Ti-Ta alloys first decrease and then increase with the increase of Nb, Mo and Ta content and the lowest value are achieved to be 37.5GPa,30.8GPa and 37.9GPa when the Nb, Mo and Ta content are about 25%, respectively. Moreover, the critical valence electronic number for realizing the lowest elastic modulus inβTi-Nb, Ti-Mo and Ti-Ta alloys are about 4.25 forβTi-Nb and Ti-Ta, and 4.5 forβTi-Mo.(2) The phase stability ofα,βandα" of Ti6Mo2 alloys followed the order ofα">β>α, andαof Ti6Mo2 alloys wasn′t exist at 0K. The elastic modulus E ofα,βandα" of Ti6Mo2 alloys followed the order ofα>α">β.(3) The effects of Mo, Sn, Ta and Zr elements on the structure stability and elastic properties of Ti₁₂Nb₄ were different. Mo and Ta can stabilize theβphase and increase the elastic modulus; Sn has less effect on the structure stability ofβphase. Zr can decrease theβphase stablility. Sn and Zr have little effect on the elastic modulus of theβ-phase Ti₁₂Nb₄.