Microstructure and properties of gradient nitrided layer on Ti6Al4V alloys

The vacuum electromagnetic induction nitriding technology was applied to prepare a gradient nitrided layer on the surface of a Ti6Al4V alloy, which possesses TiN and α-Ti (N) phases. Moreover, transmission electron microscopy was conducted to confirm the presence of numerous high-density stacking faults caused by TiN and Ti2N phases distributed on the surface of the alloy, along with a large number of basal stacking faults inside. A high-density stacking fault led to serious distortion of lattice fringes. Lattice and numerous edge dislocations caused by defects were observed in the subsurface layer. For the surface layer, the Vickers hardness reached HV0.25 1211.30 and the residual compressive stress increased, while the nano-hardness increased to 14.07 from 5.31 GPa in the substrate. The micrometre scratch test results indicated that the plasticity and hardness of the nitrided layer changed in a gradient. The 50-μm effective hardened layer depth and surface compressive stress of the Ti6Al4V alloy were enhanced by the stacking faults.