Formation mechanism of α lamellae during β→α transformation in polycrystalline dual-phase Ti alloys

Phase field simulations incorporating contributions from chemical free energy and anisotropic interfacial energy are presented for the beta ->alpha transformation in Ti-6Al-4 V alloy to investigate the growth mechanism of alpha lamellae of various morphologies from undercooled beta matrix. The alpha colony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters of alpha and beta phases with the phase field governing equations. The simulations show that alpha lamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature. alpha lamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900 degrees C provided that the interfacial energy anisotropy ratio k(x): k(y) was set as 0.1: 0.6. Besides, higher interfacial energy anisotropy can accelerate the formation of alpha lamellae, and the equilibrium shape of alpha lamellae changes from rod to plate as the interface energy anisotropy ratio kx: ky vary from 0.1: 0.4 to 0.1: 0.8 under 820. C. Experiments were conducted to study the alpha lamellar side branches in Ti-6Al-4 V (Ti-6.01Al-3.98 V, wt.%) and Ti-4211 (Ti-4.02Al-2.52V1.54Mo-1.03Fe, wt.%) alloys with lamellar microstructure. Electron backscatter diffraction (EBSD) results show that alpha lamellar side branches and their related lamellae share the same orientation. The predicted temperature range for alpha lamellar side branches formation under various interfacial energy anisotropy is consistent with experimental results. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.