Low Young's modulus of cold groove-rolled β Ti–Nb–Sn alloys for orthopedic applications

Abstract

The effects of composition and cold groove-rolling on Young's modulus were investigated to achieve low Young's modulus in β Ti–Nb–Sn ternary alloys for orthopedic applications, using Ti-(27.5–37.5)%Nb-(2.5–11.25)%Sn ternary alloys. Optical microscopy, transmission electron microscopy, and X-ray diffractometry revealed that the constituent phases of quenched and rolled alloys were classified into three regions of β, β(ω)+α” and α” in the Ti–Nb–Sn ternary phase diagram. The β phase was observed to stabilize by the addition of Sn, and the least stable β alloys existed along a line connecting Ti-37.5Nb-2.5Sn and Ti-27.5Nb–11Sn. Low Young's moduli of approximately 50 GPa were achieved by the least stable, quenched β Ti–Nb–Sn alloys, and these further decreased by cold groove-rolling, reducing the cross section by 75%. The lowest Young's modulus of 36 GPa was realized for the cold groove-rolled Ti–35Nb-3.75Sn alloy, in which the textures of [010]α” of deformation-induced α” martensite and [011]β of the β matrix developed preferentially, parallel to the rolling direction. The lattice deformation strain along the [010]α” axis accompanied by β to α” martensitic transformation was observed to depend on the alloy composition and was maximized in Ti–35Nb-3.75Sn. Thus, the low Young's modulus of the least stable β Ti–Nb–Sn alloys obtained after cold groove-rolling was attributed to the development of [010]α” and [011]β textures along the rolling direction, and the lowest Young's modulus of the cold groove-rolled Ti–35Nb-3.75Sn alloy was due to a large lattice deformation strain along the [010]α” axis accompanied by β to α” transformation.