Failure Process Observation and Failure Mechanism of Ni-Coated TC4 Alloy by Gradually Changed Curvature Bending Method

To observe the failure process of coating/substrate materials, a gradually changed curvature bending (GCCB) method is proposed, and the relevant equipment is developed. Before GCCB experiment, Ni-coated Ti6Al4V (TC4) samples were prepared. The effect of vacuum heat treatment on the microstructure at Ti/Ni interface is discussed. With the increase in heat treatment time, intermetallic compounds of TiNi3, TiNi, and Ti2Ni start to form. Kirkendall void forms in Ni coating at first, and then the pore number decreases due to interdiffusion. Based on the interdiffusion of Ni into TC4 substrate, beta-Ti phase forms at the surface region. The whole failure process of Ni-coated TC4 sample by GCCB experiment is observed and analyzed. With the increase in bending curvature, the fracture order of each layer is TiNi3 > Ti2Ni > TiNi. With the increase in heat treatment time, the bonding property of the coating first increases and then decreases, and the failure mode also changes. The failure occurs at TiNi3 layer when the heat treatment time is 60 min. When the heat treatment time is 120 min, the bonding property of the coating is the highest and surface vertical crack occurs. When treated 180 min, failure occurs at Ti2Ni layer.