Achieving recrystallization of prior β in Ti-6Al-4V alloy by electroshock

Abstract

Fine grain strengthening is an important approach to improve mechanical properties in titanium alloys. The martensite (α′) of Ti-6Al-4V alloy generally shows fine acicular grain and has high strength. However, the grain size of martensite is influenced by the prior β grain, thereby affecting the overall strength-plasticity combination of the alloy. Generally, the prior β grains grow rapidly above β-transus temperature (∼1000 ℃). It is difficult to refine the prior β grain through traditional heat treatment alone if it coarsened. In this work, we found that electroshock treatment could directly recrystallize the coarse prior β grain in α+β titanium alloy without plastic deformation process. EBSD and TEM analyses found that the prior β grain size reduced from 44 µm to 29 µm and the remained textures had been eliminated after electroshock, therefore the tensile strength and elongation were simultaneously improved from 1196 MPa and 4.7 % to 1259 MPa and 10.4 %. The residual subgrain boundaries within prior β grain, electric current-induced extra driving force and extremely high heating rate (106 ℃/s) are the main reasons for the occurrence of recrystallization as well as no grain growth of prior β. These findings may be used for repairing the mechanical properties of coarse overheated microstructure or eliminating the anisotropy of columnar grain that produced in additive manufactured α+β titanium alloy without deformation processing.