Low-Temperature Superplasticity of Ultra-Fine-Grained Ti-6Al-2Sn-4Zr- 2Mo-0.1Si Alloy

摘要

This study aimed to achieve low-temperature superplasticity of Ti- 6Al-2Sn-4Zr-2Mo-0.1Si alloy utilizing dynamic globularization, and to elucidate the deformation mechanisms in the context of inelastic-deformation theory. The as-received microstructure with equiaxed-alpha grain/particle size of 13microneters was refined to 2.2 micronmeters by dynamic globularization at 775 C, which was confirmed by electron back-scattered diffraction (EBSD) analysis. Uniaxial tension tests were carried out for both coarse (starting material) and fine (dynamically globularized materials) grained materials at strain rate range of 10-4 approximately 10-2 s-1 and temperature range of 650 to 750 C. The total elongation of fine grained microstructure (382-826 %) was considerably enhanced as compared to that of coarse grained microstructure (189-286 %) at 10-4 s-1. With respect to the microstructural evolution, the dynamic coarsening rate of alpha phase during deformation was approximately 12 times faster than that of static coarsening, and both static and dynamic coarsening rate for Ti-6Al-2Sn-4Zr-2Mo- 0.1Si alloy were approximately 2 to 4 times lower than those for Ti-6Al-4V alloy, which was attributed to lower diffusivity of rate-limiting solute at the similar test temperature. It was found that the mechanism of the low-temperature superplasticity of Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy was grain boundary sliding (GBS) accommodated by dislocation motion with both stress exponent (n) and grain size exponent (p) values of approximately 2. When the alpha grain/particle size was considered to be an effective grain size, the apparent activation energy for low temperature superplasticity of Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy (approximately 169 kJ/mol) was close to that of Ti-6Al-4V alloy (approximately 160 kJ/mol).