Effect of high dilution on the in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coating on zirconium alloy substrate by laser cladding

Highlights: • High dilution effect on the surface modification of transition metal was explored. • ZrB₂ is thermodynamically more readily formed than ZrC in laser cladding process. • The microstructure evolution shows strong dependence on the elemental distribution. - Abstract: High dilution of transition metals was employed as a new idea for in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coatings by high power diode laser (HPDL) cladding Ni–Cr–B–Si powders on zirconium substrate. Microstructure, phase composition, the mechanism of in situ synthesis reinforcement and the microhardness of coatings were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and micro-sclerometer. The results reveal that the morphologies and phase constituents are related to the content of alloying elements in powders. In low alloy coatings, the matrix was mainly composed of intermetallic compounds including NiZr and Ni₁₀Zr₇, while the reinforcements consisted of Zr₅Si₄, β-ZiSi, α-ZrSi and ZrC. At the top of high alloy coatings, the matrix was partially comprised of Zr-based amorphous phase with the reinforcements containing ZrB₂. It is thermodynamically favorable for ZrB₂ ceramic reinforcement to form compared to ZrC phase. The microstructure evolution was dependent on the contribution of the high dilution zirconium alloy substrate to the in situ reinforcement synthesis. The microhardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.