In situ high-entropy solid solution and ceramic particles co-reinforced Ni-based composites with outstanding strength-ductility synergy and good pitting resistance

In present work, a novel Ni-based composite co-reinforced by in-situ solid solution and ceramic phase through a decomposition of metastable dual-phase (CoCrFeNiMn)90Al10 high-entropy alloy (HEA) was fabricated by spark plasma sintering (SPS). The SPS-ed composites with high relative density are composed of Al-rich oxide particles, γ* nanoprecipitates and Ni matrix, all of which reveal the face-centered cubic (FCC) structures with almost identical lattice constant. The maximum microhardness of composite is 321 HV, which is over 2 times larger than that of the SPS-ed pure Ni. It is worth noting that the composite with 20 vol.% HEA addition exhbits high ultimate tensile strength of 580 MPa, while maintaining excellent fracture strain of 32.2%. The notable improvement in mechanical properties is mainly attributed to the twin/stacking faults strengthening and precipitation strengthening, as well as grain-boundary strengthening. A coherent boundary between γ* phase and Ni matrix and a hierarchical precipitate spatial distribution of nano-γ* and micron-oxides in Ni matrix have been obtained. Combined with FCC characteristics of matrix and reinforcements, these scenarios effectively promote the coordinated deformation of composites during tensile testing, which are beneficial to the improvement of plasticity. In combined with the significantly broadened passive region and low passive current density, the SPS-ed composites present the good pitting resistance in seawater.