초록
The mechanism of microstructure and hardness changes during mill annealing of Ti-6Al-4V alloy was investigated. The annealing heat treatments were performed at $675{\sim}795^{\circ}C$ in vacuum for 2 hours, followed by air cooling. The microstructure was observed by using an optical microscope and X-ray diffraction, and hardness was measured by using a Rockwell hardness tester and micro Vickers hardness tester. The average grain size becomes smaller at $675^{\circ}C$ to $735^{\circ}C$ due to the formation of new grains rather than grain growth, but becomes larger at $735^{\circ}C$ to $795^{\circ}C$ due to growth of the already-formed grains rather than formation of new grains. The mill annealing temperature becomes higher, the ${\beta}$ phase fraction decreases and ${\alpha}$ phase fraction increases at room temperature. This is because the higher annealing temperature, the smaller amount of V present in the ${\beta}$ phase, and thus the ${\beta}$ to ${\alpha}$ transformation occurs more easily when cooled to room temperature. As the mill annealing temperature increases, the hardness value tends to decrease, mainly due to resolution of defects such as dislocations from $675^{\circ}C$ to $735^{\circ}C$ and due to grain growth from $735^{\circ}C$ to $795^{\circ}C$, respectively.