Mechanical properties and optimum layer thickness in an amorphous Ni–P/coarse-grained Ni bi-layered structure

In the present study, we systematically investigate the mechanical behavior of a bi-layered structure consisting of a coarse-grained (CG) Ni substrate plus an amorphous Ni–P surface layer. It is found that a desirable combination of high strength and high ductility can be attained by tailoring the thickness of Ni–P layer. An optimal layer thickness of ~6.5 μm is discovered, where the specimen having a bi-layered structure exhibits a yield strength of 236 MPa and an uniform elongation of 32.6%, which are 1.13 and 1.11 times of those in the corresponding coarse-grained Ni. Experimental observations and finite element modeling (FEM) simulations reveal that this optimum bi-layered structure under tension induces large stress and strain gradients in the CG Ni substrate, resulting in the significant back stress and deformation twinning. Therefore, the plastic deformation can be more effectively accommodated, leading to the observed extra uniform elongation in the bi-layered structure.