Microstructure evolution and mechanical properties of diffusion bonding Al5(TiZrHfNb)95 refractory high entropy alloy to Ti2AlNb alloy

In this study, the refractory high entropy alloy Al-5(TiZrHfNb)(95) was diffusion bonded (DB) with the intermetallic compound (IMC) Ti2AlNb under 5 MPa at 970-1100 degrees C in vacuum. The microstructure and element distribution were analyzed using a scanning electron microscope (SEM) and mechanical properties were evaluated by nanoindentation and shear tests. The typical microstructure of DB joint was Ti2AlNb substrate/solid solution/scattered Al3Zr5/solid solution/Al-5(TiZrHfNb)(95) substrate, which the discrete brittle Al3Zr5 phase was arranged along a straight line in the bonding surface, and gradually elongated and dissolved as the bonding temperature increased from 970 degrees C to 1100 degrees C. The formation of Al3Zr5 was attributed to the lowest Gibbs energy of all the possible IMCs in the range of the welding temperature. In the joining region, the IMC Al3Zr5 had the highest hardness (9.01 GPa) and modulus (143.37 GPa) but the lowest elastic recovery (20.37%), thus, it tended to cause stress concentration at the phase interface and induced crack initiation, which had a detrimental effect on the shear strength of joints. The shear strength of DB joints improved with the increase of bonding temperature and reached the highest (427 MPa) at 1100 degrees C owing to is lowest Al3Zr5 content. As the bonding temperature increased, the fracture of the joint transformed from intergranular brittle to ductile.