Crystallographic orientation evolution during the development of tri-modal microstructure in the hot working of TA15 titanium alloy

The crystallographic orientation evolution and its dependence on processing parameters during the development of tri-modal microstructure of titanium alloy were studied by the thermal-mechanical processing tests and electron backscatter diffraction (EBSD) examination. It is found that the development of tri-modal microstructure undergoes two stages: firstly, bimodal microstructure consisting of equiaxed alpha (alpha(p)) and transformed beta matrix (beta(t): a mix of secondary alpha phase (alpha(s)) and beta phase) is formed after first-stage near-beta forging; then, the tri-modal microstructure consisting of alpha(p), lamellar alpha phase (alpha(l)) and beta(t) are obtained after the following heat treatment. The equiaxed alpha in final tri-modal microstructure does not follow the Burgers orientation relationship (OR) with beta phase. Its crystallographic orientation is hardly influenced by the hot processing parameters. The lamellar alpha in tri-modal microstructure is right the undissolved secondary alpha of bimodal microstructure (obtained after first step) during the heating process of the second step. Both of them keep the Burgers OR with beta phase, however, the dissolution of secondary alpha present selectivity to some extent making the variant selection degree of lamellar a greater than that of secondary alpha. The variant selection degree of lamellar alpha in tri-modal microstructure decreases with increasing the cooling rate, deformation degree and strain rate of near-beta forging. The secondary a in tri-modal microstructure is precipitated from beta phase and obeys the Burgers OR with beta phase during the cooling process of the second step. The existing lamellar alpha plays a strengthening role in the variant selection during its precipitation. While the cooling rate, deformation degree and strain rate of near-b forging show limited effect on the probabilities of each type of misorientation and variant selection degree of secondary alpha. (C) 2018 Elsevier B.V. All rights reserved.