Effects of Heat Treatment on Microstructure and Hardness of Graphene Reinforced Ti-6Al-4V Alloy

Graphene reinforced Ti-6Al-4V alloy was prepared by spark plasma sintering technique with graphene as reinforcing phase. The effects of different annealing, solution and aging temperature on the microstructure and hardness of the alloy were studied. The results show that Ti-6Al-4V alloy reinforced by graphene after holding at 750 ℃ for 1 h, air cooling to room temperature, a small number of dispersed second phase particles are observed, and a small number of α’ phase is separated from β phase. The hardness of the alloy decreases by 2.95%. After holding at 930 ℃ for 1 h, air-cooled to 550 ℃for 4 h, and then air-cooled to room temperature, the alloy can obtain more uniform α +α’ double state microstructure, and there are a few α’ phase decomposed α+β phase and scattered second phase particles with slightly increased hardness. During solution at different temperatures and aging process, the transformation from equiaxed α phase to duplex α+α’ microstructure mainly occurs and part of the α’ phase decomposes into α+β phases, which are dispersed at the boundaries of the β phase, contributing to precipitation hardening. As the solution temperature increases, more α’ phase decomposes. When the solution temperature is 930 ℃, the maximum hardness of the alloy is 542.6 HV, which increases about 6.32% compared to the sintered state. When the alloy is solution treated at 930 ℃ and aged at different temperatures, the hardness of the alloy increases firstly and then decreases with the increase of aging temperature. When the aging temperature increases from 500 ℃ to 550 ℃, the α’ phase decomposes to dispersive α+β microstructure with precipitation hardening effect, the hardness of the alloy increases by about 1.19%. When the aging temperature increases from 550 ℃ to 650 ℃, the driving force for the decomposition of the α’ phase increases, α’ phase merges up into plate or sheet, β phase merges and grows up, which distributes unevenly, weakening the dispersion precipitation hard effect of the α+β mixed microstructure, and the hardness of the alloy significantly decreases.