Gradient microstructure and interfacial strength of CoCrFeMnNi high-entropy alloy in solid-state ultrasonic welding

This study investigated the microstructure gradient and interfacial strength of CoCrFeMnNi high-entropy alloy bonded by ultrasonic welding (USW). It was found that bonding occurred after welding for 1.0 s and bonding strength reached about 1500 N (30 MPa of shear strength) after welding for 7.0 s. Such interfacial bonding was featured with the ultrafine microstructure with a grain size of 100-200 nm, which resulted from discontinuous dynamic recrystallization induced by severe plastic strain and a peak temperature of 511 degrees C. Strain and temperature due to severe plastic deformation in the ultrasonic sliding decreased with increasing distance from the interface. This contributed to a microstructural gradient from discontinuous dynamic recrystallization to continuous dynamic recrystallization and finally dislocation cells. This gradient microstructure was confined to areas within about 2 mu m from the interface and showed no solidification defects or transformation of detrimental phases.