The microstructures and mechanical properties of martensite Ti and TiN phases in a Ti6Al4V laser-assisted nitriding layer

A fiber laser assisted the preparation of a nitride layer on the Ti6Al4V surface in a pure nitrogen atmosphere. Through SEM, XRD, and XPS analyses, the nitride layer is found to be mainly composed of dendritic TiN phase and an interdendritic alpha '-Ti phase. Not only the true crystal structure of the two main phases (TiN and alpha '-Ti) were directly characterized using TEM, but also the mechanical properties were tested by micro-indentation and nanoindentation methods for the first time.It is found that the nano-hardness of the TiN phase is 2609 HV, and the elastic modulus is 208 GPa. During the TiN dendritic fracture process, the primary dendrites undergo near ductile fracture along the growth direction of the secondary dendrites, and microcracks, bifurcations, deflection, etc. occur to form a crack-toughening mechanism. The nano-hardness of alpha '-Ti is 632 HV, and the elastic modulus is 142.8 GPa. The entanglement of two sets of parallel dislocations in alpha '-Ti crystals forms subgrain boundaries, which exert a positive influence on work hardening and fine-grain strengthening.