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

doi:10.1016/j.jmst.2020.02.093

Abstract

Abstract:

Phase field simulations incorporating contributions from chemical free energy and anisotropic interfacial energy are presented for the β→α transformation in Ti-6Al-4 V alloy to investigate the growth mechanism of α lamellae of various morphologies from undercooled β matrix. The α colony close to realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters of α and β phases with the phase field governing equations. The simulations show that α lamellar side branches with feathery morphology can form under a certain combination of interfacial energy anisotropy and temperature. α lamellae tend to grow slowly at high heat treatment temperature and become wider and thicker as temperature increase from 800 to 900 °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 α lamellae, and the equilibrium shape of α 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 α lamellar side branches in Ti-6Al-4 V (Ti-6.01Al-3.98 V, wt.%) and Ti-4211 (Ti-4.02Al-2.52V-1.54Mo-1.03Fe, wt.%) alloys with lamellar microstructure. Electron backscatter diffraction (EBSD) results show that α lamellar side branches and their related lamellae share the same orientation. The predicted temperature range for α lamellar side branches formation under various interfacial energy anisotropy is consistent with experimental results.

Key words: α Lamellae, Side branches, Ti-6Al-4V, Interfacial energy anisotropy, Phase field model

Jia Sun, Min Qi, Jinhu Zhang, Xuexiong Li, Hao Wang, Yingjie Ma, Dongsheng Xu, Jiafeng Lei, Rui Yang. Formation mechanism of α lamellae during β→α transformation in polycrystalline dual-phase Ti alloys[J]. J. Mater. Sci. Technol., 2021, 71: 98-108.

Figures/Tables 12

Fig. 1. Schematic diagram showing the nucleation and growth of the α phase from grain boundary and into the grain when the alloy was cooled from β phase field [10].

Fig. 2. Schematic diagram of the rotation angle θ of α lamellae.

Fig. 3. Growth process of α lamellae under 820 °C with interfacial energy anisotropy being kx: ky = 0.1:0.6: (a) t = 50 s; (b) t = 500 s; (c) t = 1000s; (d) t = 1500s; (e) Optical micrographs of the microstructures of Ti-6Al-4 V (β annealed). Etched for ～10 s in 5 parts 70 pct HNO3, 10 parts 50 pct HF, and 85 parts of H2O [18].

Fig. 4. Microstructure at t = 1500s under different heat treatment temperatures with interfacial energy anisotropy kx: ky = 0.1:0.6: (a) 800 °C; (b) 820 °C; (c) 840 °C; (d) 860 °C; (e) 880 °C; (f) 900 °C; (g) evolution of volume fraction of α phase in Ti-6Al-4 V annealed under different temperatures.

Fig. 5. Microstructure of α lamellae under different interface energy anisotropy ratios at 820 °C: (a) kx:ky = 0.1:0.4; (b) kx:ky = 0.1:0.6; (c) kx:ky = 0.1:0.8 and (d) evolution of α phase fraction.

Fig. 6. Solute distribution at 820 °C under different interfacial energy anisotropy: (a) kx:ky = 0.1:0.4; (b) kx:ky = 0.1:0.6; (c) kx:ky = 0.1:0.8, 1-Al, 2-V.

Fig. 7. (a, b) Microstructures of Ti-6Al-4 V after β-annealing heat treatment; (c-f) Microstructure evolution of Ti-4211 during furnace cooling from 1050 °C. The microstructures were obtained by quenching from 870 °C, 830 °C, 650 °C to room temperature respectively.

Fig. 8. Microstructure of Ti-4211 and corresponding EBSD orientation micrograph of α phase quenched from 850 °C.

Fig. 9. (a) Simulation of α lamellae in Ti-6Al-4 V alloy under 820 °C with interfacial energy anisotropy kx: ky = 0.1: 0.6 and growth process of α lamellar side branches: (a-1) t = 200 s; (a-2) t = 500 s; (a-3) t = 800 s. (b) Schematics of the growth mechanism of α lamellar side branches.

Fig. 10. Zoomed in view of α sideplates under different heat treatment temperatures when interfacial energy anisotropy kx:ky = 0.1:0.6.

Fig. 11. Morphology of α lath under different interfacial energy anisotropy at 820 °C.

Fig. 12. The presence of α lamellar side branches under different heat treatment temperatures and interfacial energy anisotropy conditions.

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