Microstructure and mechanical characterization of diffusion-bonded CoCrCuFeNi HEA/TC4 titanium alloy joints using a Ni interlayer

In order to enhance the mechanical properties of the CoCrCuFeNi high entropy alloy (HEA)/TC4 titanium alloy diffusion bonding joints, a Ni foil was chosen as an intermediate layer, resulting in the successful formation of sound joints. Concurrently, an investigation was conducted on the typical microstructure, the influence of temperature on both the microstructure and properties, as well as the mechanism of interfacial formation in the joints. The results indicated that the CoCrCuFeNi HEA/TC4 titanium alloy diffusion-bonded joint, with a Ni intermediate layer, exhibits a characteristic microstructure. This microstructure consists of several distinct layers, including the HEA diffusion layer, Ni(s,s) solid solution layer, TiNi3 layer, TiNi layer, Ti2Ni layer, and TC4 diffusion layer. The interfacial layers were primarily formed by the mutual diffusion of elements and the Ni-Ti diffusion reaction. The joint achieved at 792 degree celsius exhibits the highest shear strength of 230 MPa. This joint exhibits an interfacial structure characterized by a multilayer composition consisting of "HEA/HEA diffusion layer/Ni(s,s)/TiNi3/TiNi/TC4 diffusion layer/TC4". The crack mainly originated in the TiNi3 layer and then spread through multiple interfacial layers. When the bonding temperature exceeded 842 degree celsius, it resulted in the formation of a continuous layer of Ti2Ni compound. The generation of this layer reduced the shear strength of the joint. Consequently, the joint is prone to failure at this brittle layer once it is formed.