Microstructure and mechanical properties of a biomedical beta-type titanium alloy subjected to severe plastic deformation after aging treatment


YILMAZER H. , Niinomi M., Nakai M., Hieda J., Akahori T., Todaka Y.

2nd International Symposium on Advanced Synthesis and Processing Technology for Materials (ASPT2011)/8th Materials Science School for Young Scientists (KINKEN-WAKATE2011), Sendai, Japan, 1 - 02 December 2011, vol.508, pp.152-154 identifier identifier

Abstract

Strengthening by grain refinement and increasing dislocation density through high-pressure torsion (HPT), which is an attractive technique to fabricate ultrafine grained and nanostructured metallic materials, is expected to provide beta-type Ti-29Nb-13Ta-4.6Zr (TNTZ) higher mechanical strength while maintaining low Young's modulus because they keep the original beta phase. However, the ductility shows reverse trend. Greater strength with enhanced ductility can be achieved by controlling precipitated phases through HPT processing after aging treatment. Aged TNTZ subjected to HPT processing at high N exhibits a homogeneous microstructure with ultrafine elongated grains having a high dislocation density and consequently non-equilibrium boundaries and distorted subgrains with non-uniform shapes and nanostructured intergranular precipitates of alpha phases. Therefore, the effect of HPT processing on the microstructure and mechanical hardness of TNTZ after aging treatment was systematically investigated in this study. TNTZ, which was subjected to aging treatment at 723 K for 259.2 ks in vacuum followed by water quenching, subjected to HPT processing at rotation numbers (N) of 1 to 20 under a pressure of around 1.25 GPa at room temperature. The microstructure of TNTZ(AT) consisted of precipitated needle-like omega phases in beta grains. However, TNTZ(AHPT) at N >= 10 comprises very fine alpha and small amount omega phases in ultrafine beta grains. Furthermore, the hardness of every TNTZ(AHPT) was totally much greater than that of TNTZ(AT). The hardness increased from the center to peripheral region of TNTZ(AHPT). In addition, the tensile strength of every TNTZ(AHPT) was greater than that of TNTZ(AT). The tensile strength of TNIZ(AHPT) increased, but the elongation decreased with increasing N and then both of them saturated at N >= 10.