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 theβ→αtransformation in Ti-6 Al-4 V alloy to investigate the growth mechanism ofαlamellae of various morphologies from undercooledβmatrix.Theαcolony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters ofαandβphases with the phase field governing equations.The simulations show thatαlamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature.αlamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900℃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 ofαlamellae,and the equilibrium shape ofαlamellae changes from rod to plate as the interface energy anisotropy ratio k_(x):k_(y) vary from 0.1:0.4 to 0.1:0.8 under 820℃.Experiments were conducted to study theαlamellar side branches in Ti-6 Al-4 V(Ti-6.01 Al-3.98 V,wt.%)and Ti-4211(Ti-4.02 A1-2.52 V-1.54 Mo-1.03 Fe,wt.%)alloys with lamellar micro structure.Electron backscatter diffraction(EBSD)re sults show thatαlamellar side branches and their related lamellae share the same orientation.The predicted temperature range forαlamellar side branches fo rmation under various interfacial energy anisotropy is consistent with experimental results.