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

摘要

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 Ni_(10)Zr_7, while the reinforcements consisted of Zr_5Si_4, 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 ZrB_2. It is thermodynamically favorable for ZrB_2 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 micro-hardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.