2018 Volume 104 Issue 4 Pages 235-241
To investigate basic characteristics of the phase transformation that strongly affects mechanical properties in a high-strength β-rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, microstructures of the specimens continuously cooled from the β region at various cooling rates were examined.
Vickers hardness at room temperature sharply increased with increase of cooling rates in the specimens cooled at cooling rates lower than 20°C/s, reached the maximum value of 477 HV at 20°C/s and decreased with increase of cooling rates higher than 50°C/s.
At the cooling rates higher than 50°C/s, the α″ phase (orthorhombic martensite phase) laths of 0.1 to 0.5 μm in width were formed. Meanwhile, the grain boundary α phase and the side-plate α phase were formed in the specimens cooled to room temperature at 20°C/s. In the specimens cooled at 5 to 50°C/s, the black plate type of the α phase was formed. TEM observation revealed that the black plates have hcp crystalline structure and very fine (0.05 to 0.2 μm in width) compared with the α″ phase in the specimens cooled at 50°C/s or higher cooling rates. In addition, the extremely fine acicular hcp phase of 50 nm or less in width was also formed at the area where the black plates were not formed. It is considered that the above fine microstructures led to quite high Vickers hardness of around 477 HV.
The formulated continuous cooling transformation diagram indicated that Ti-5Al-2Fe-3Mo is excellent in hardenability and thermal processability, suggesting that the alloy has a lot of advantages industrially.
