Effect of heat treatments on the microstructural evolution and tribological behaviors of a NiCrSiB/Y2O3 alloy fabricated by laser additive manufacturing

NiCrSiB + xY(2)O(3) (x = 0, 0.5, 1, 2, and 5 wt%) alloy samples were fabricated on mild steel via laser additive manufacturing (LAM). The microstructural evolution and mechanical properties of the NiCrSiB + 0.5 wt% Y2O3 alloy were studied systematically in the as-deposited condition and after three different types of post-heat treatment conditions. The scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction results showed that the structures of the as-deposited samples mainly comprised a dendritic matrix of gamma-Ni, a continuous interdendritic eutectic of gamma + Ni3B, discrete Cr-rich carbides (Cr23C6 and Cr7C3) and a brittle phase (Ni3Si2). With the addition of Y2O3, the volume fraction of the interdendritic eutectic was approximately 19.2, 32.9, 34.5, 34.1 and 26.9%. Y2O3 played an important role in increasing the volume fraction of the eutectic phase and refining the grain size. After the NiCrSiB+0.5 wt% Y2O3 alloy samples were subjected to solution (HT1), solution + aging (HT2) and annealing (HT3) heat treatments, the microstructures evolved considerably. After HT1 and HT2, a carbide (Cr7C3) and brittle phase (Ni3Si2) precipitated on the boundaries of the eutectic structure and between the blocky phases, respectively. After HT3, the grains were refined, and dot-shaped or rod-shaped hard phases (Cr7C3) were uniformly dispersed along the grain boundaries. The microhardness (up to 514.1 HV), wear resistance and microwear properties of the NiCrSiB alloy specimens subjected to HT3 were further improved under the positive action of an appropriate amount of Y2O3. After the heat treatments, the wear mechanism of these samples was mainly adhesive wear.