Effects of Ni-P coating on mechanical properties of Al0.3CoCrFeNi high-entropy alloys

The Al0.3CoCrFeNi high-entropy alloys were successfully coated with the Ni-P amorphous films through electroless plating. The room-temperature mechanical behavior under uniaxial tension was systematically investigated. There are good adhesive strengths between the films and substrates. And the surface hardness of the plating for 5 h is around 381HV with dramatically increasing by 121%, as compared to the uncoated substrate of 172HV. As a result, the yield strength and ultimate tensile strength (UTS) of coated samples were significantly increased as the film thickness rises. Specially, the specimen plating for 5 h, the yield strength (sigma(y)) is 400 MPa with a 62% improvement, and UTS is 732 MPa, increased by 16% in comparison to the substrate (sigma(y )= 246 MPa and UTS = 630 MPa). It can be attributed to the appearance of the repulsive image stress that will counteract the shear applied stress for the generation and propagation of dislocations during the elastic-deformation stage. Owing to the back stress inhibiting local plasticity and promoting the nucleation of cleavage cracks in the substrate, the cracks originated from the brittle film propagate into the substrate, resulting in the ductility losses. A deformation map is proposed to account for the transition of deformation modes from shear banding to cracking. These findings shed light on the fact that the surface coating is an effective means to enhance the mechanical properties of HEAs.