Martensite evolution in additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures

Based on classical nucleation and growth theory, the phase evolution of martensite α′ phase of additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V-0.7Nb alloy under different annealing temperatures (600–960 °C) was systematically investigated. The results show that: Owing to insufficient diffusion of alloy elements during laser deposition, numerous β-stabilizing elements are retained in α′ phase and the microstructure is made up of 95.4% α′ phase and 4.6% β phase. According to TEM, EBSD, SEM/EDX results, the α′ phase can be divided into α′ phase containing more β-stabilizing elements (α′β), α′ phase containing more Al (α′Al). Under annealing condition, martensite decomposition tends to occur at the α′/α' interfaces and the matrix of α′β phase. In the matrix of α′β phase, fine adjacent α′ phase precipitates at β phase because a lot of β-stabilizing elements are consumed by precipitated β phase, thus improving the precipitation of adjacent α′ phase. The decomposition of α′ phase is controlled by diffusion of alloy elements. As the temperature increases, the diffusion of alloy elements and degree of α′ martensitic decomposition become more sufficient. When the Al content in the α′ phase reaches that in equilibrium α phase, the lattice parameters of α′ phase increase from a = 2.97 Å, c = 4.72 Å to a = 3.04 Å, c = 4.77 Å and meanwhile the α′ martensitic decomposition is finished.