Phase transformation and age-hardening of hexagonal α′ martensite in Ti-12 mass%V-2 mass%Al alloys studied by transmission electron microscopy

Microstructure and phase decomposition of α′ martensites in Ti-12 mass%V-2 mass%Al alloys have been studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The as-quenched alloy showed an acicular structure of the hexagonal α′ martensite. Significant age-hardening was observed in the alloys after aging at 300 and 400 °C. A hardness, as high as 377 Hv, was obtained for the alloy after aging at 400 °C for 24 h. The age-hardening can be attributed to fine β precipitates nucleated inside primary α′-martensite plates. These fine acicular precipitates, about 50 nm in length, were identified using high-angle annular dark-field (HAADF) STEM as bright contrasts, indicating a vanadium-rich composition of the β precipitates. As the aging temperature increases, partitioning of vanadium becomes prominent, indicating phase decomposition towards the equilibrium α and β phases. Decomposition of the α′ martensite and formation of α/β two-phase structure were observed after aging at 500 °C for 24 h, which resulted in an abrupt decay of hardness. Plate-like three-dimensional shapes of large β precipitates have been revealed by HAADF-STEM electron tomography.