Effects of heating rate on the alloy element partitioning and mechanical properties in equiaxed α+β Ti-6Al-4V alloy

doi:10.1016/j.jmst.2021.02.061

Abstract

Abstract:

The effects of heating rate on the alloy element partitioning and mechanical properties during the phase transformation of α→β in Ti-6Al-4 V alloy under solution treatment have been investigated by the experiments and phase field simulations, which reveal the evolutions of microstructure and compositions at the non-equilibrium state and well verify the experimental results. The specific results indicate that the compositions measured through electron probe micro-analysis (EPMA) under a lower heating rate are close to the equilibrium ones corresponding to the solution temperature. Heating up to the target solution temperature, as the heating rate increases, the Al content decreases and V increases in the primary α (α_p) grain with a larger size, the volume fraction of α_p increases and the composition gradient between α_p and β phases gets steeper. The interrelated relationship among the diffusion, compositions, solution temperature and free energy of the system has been discussed in detail. Moreover, increasing the heating rate (~20.0 K/min) may help to improve the mechanical properties of the alloy by mainly adjusting the α_p/β_t volume fractions, α_p particle size and secondary α (α_s) size during the process of heating up to the solution temperature. These results may shed some light on the optimization of the knowledge-based heat treatment route.

Key words: Heating rate, Alloying element partitioning, Microstructure morphology, Mechanical properties

Jinhu Zhang, Hongtao Ju, Haisheng Xu, Liang Yang, Zhichao Meng, Chen Liu, Ping Sun, Jianke Qiu, Chunguang Bai, Dongsheng Xu. Effects of heating rate on the alloy element partitioning and mechanical properties in equiaxed α+β Ti-6Al-4V alloy[J]. J. Mater. Sci. Technol., 2021, 94: 1-9.

Figures/Tables 11

Table 1 Chemical compositions of as-received Ti-6Al-4 V alloy (wt%).

Ti	Al	V	Fe	O	H	N
Bal.	6.12	4.2	0.06	0.15	0.0014	0.0056

Fig. 1. The schematic of solution treatment heated up to (a) 880, (b) 920 and (c) 960 oC under different heating rates of 1.0, 5.0 and 20.0 K/min respectively.

Fig. 2. Based on the relationship between conductivity vs. temperature (a), the thermal permeability of the sample is simulated using finite element method (FEM) from 750 to 960 °C under different heating rates of (b) 1.0, (c) 5.0 and (d) 20.0 K/min respectively.

Fig. 3. The microstructure morphology vs. heating rates/solution temperature after water quenching.

Fig. 4. The microstructure morphology vs. heating rates/solution temperature after air cooling.

Fig. 5. The compositions of Al and V (wt%) under different heating rates (1.0, 5.0 and 20.0 K/min), water quenching from (a) 880, (b) 920 and (c) 960 oC respectively.

Fig. 6. The microstructure morphology evolution simulations under different heating rates heated up to T = 920 °C. (a) the original microstructure, the one at the upper right corner in (a) is characterized by the experiment. (b-d) the microstructures under different heating rates of 1.0, 5.0 and 20.0 K/min, corresponding to the reduced time τ=68,000, 13,600, 3400 respectively. It should be pointed out that the red regions indicate αp phase, and the blue ones indicate β phase.

Fig. 7. The alloy element partitioning during α→β transformation under different heating rates heated up to T = 920 °C, (a, b, c) show the distributions of Al element, (d, e, f) show the distributions of V element, the reduced time τ=68,000 (a, d), 13,600 (b, e), 3400 (c, f) respectively.

Fig. 8. The atom mobilities of Al and V between β (order parameter η=0.0) and α (order parameter η=1.0) phases under T = 880, 920, 960 °C respectively.

Fig. 9. The Al (a) and V (b) concentration profiles between αp and β phases with the reduced time τ from 0 to 68,000, 13,600 and 3400 corresponding to 1.0, 5.0 and 20.0 K/min respectively at T = 920 °C. The light blue shaded areas indicate the αp/β interface.

Fig. 10. The yield and tensile strength of Ti-6Al-4 V alloy under different heating rates of 1.0, 5.0 and 20.0 K/min raising to (a) 880, (b) 920 and (c) 960 °C, respectively.

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