Microstructure evolution in metastable β titanium alloy subjected to deep cryogenic treatment

Abstract Deep cryogenic treatment (DCT) has been an effective method for modification of materials. It is usually combined with the traditional heat treatment of steels. However, the effect of its combination with the heat treatment of metastable β titanium alloy still remains unknown. Microstructure evolution in TB8 metastable β titanium alloy subjected to DCT was investigated experimentally. DCT was conducted on annealed TB8 alloy (RC) directly, prior to aging treatment (SCA) and after aging treatment (SAC), respectively. Tensile properties and microhardness were tested to evaluate the effect of DCT on mechanical properties. The results showed that RC and SAC both have no influence on the tensile properties and microhardness of TB8 alloy, while strengthening effects were obtained by SCA compared to the conventional solution and aging treatment (SA). The microstructure of SCA and SA specimens was detected by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Electron Back Scattering Diffraction (EBSD) and Transmission Electron Microscope (TEM). Refinement and homogenization of α precipitates were induced by the additive of DCT. High content of α laths in SCA specimen distributed vertically with each other, and some of α precipitates grew up into herring-bone morphological pattern. The volume fraction of α phase was increased from 23% in SA specimen to 27% in SCA specimen and more nano-scale α precipitates were observed in SCA specimen, which had great contribution to the improvement in strength and microhardness. The conduction of DCT after solution provided more nucleation sites for α phase through increasing dislocation density and the possible ω precipitates.