Effect of cycling heat treatment on the microstructure, phase, and compression behaviour of directed energy deposited Ti-Mo alloys

In this study, the effect of triple-cycling heat treatment on the microstructure, phase, and compression behaviour of directed energy deposited (DED) Ti-7Mo alloy was investigated with a focus on a non-equilibrium to equilibrium microstructure transition. As a result of thermal accumulation, in situ cycling, and rapid solidification, the as-deposited sample presents a continuous gradient microstructure with α-Ti in the top region and α+β in the bottom region. After the triple-cycling heat treatment, the α+β Ti at the bottom region, which is non-equilibrium, changes to a state of equilibrium near α-Ti. Meanwhile, the microstructure becomes more uniform throughout the entire sample. The morphology of the α-Ti phase changes from acicular to a short rode-like shape with increases in the number of dimensions. In terms of the mechanical properties, both the microhardness and compression properties were improved, particularly with respect to the fracture characteristics. The heat-treated sample possesses a much higher ductility than the brittle fractural behaviour. This work provides new insights into the microstructure and property optimisation and homogenisation of DED-processed Ti-based components with cycling heat treatment.