Mechanical compatibility of titanium implants in hard tissues

This review addresses the recent development of high performance β Ti alloys for biomedical applications. To develop low Young's modulus Ti alloys having high strength for bone implant applications, Young's modulus and tensile strength of β Ti-Nb-Sn alloys consisting of minimal cytotoxic elements have been investigated as functions of alloy composition and microstructure. Based on the observation that precipitation of athermal and isothermal ω particles in β Ti alloys increases Young's modulus significantly, Sn is added to Ti-Nb alloys to suppress or retard ω transformation. It is found that Young's modulus of β Ti-Nb binary and Ti-Nb-Sn ternary alloys possesses a minimum around the composition where athermal ω transformation is almost completely suppressed by quenching from the beta phase region at high temperature. At the composition very weak ω reflection or diffuse scattering is observed by TEM. Referring to the theoretical calculation of β phase stability, the relationship between Young's modulus and the stability of β Ti-Nb-Sn alloys was investigated. Quenched, metastable β Ti-Nb-Sn alloys exhibit low Young's modulus by optimizing alloy composition for β phase to be metastable without athermal ω transformation. Cold rolling after further quenching decreases Young's modulus in the rolling direction; this decrease is explained by preferred texture evolution in stress-induced α″ martensite. Reverse martensitic transformation from α″ to β on heating evolves fine grains containing a high density of dislocations and α precipitates, which results in high tensile strength. Low Young's modulus of 40-50 GPa and high strength over 1 GPa are simultaneously achieved in β Ti-Nb-Sn alloys by appropriate combination of alloy composition, cold rolling and heat treatment.