Effect of growth rate on the microstructural transition and microhardness of directionally solidified Ni–11.8 wt% Si hypereutectic alloy

Abstract

Ni–11.8 wt% Si hypereutectic alloys were directionally solidified under growth rates of 0.5–15 μm/s. The microstructures of Ni–11.8 wt% Si alloys were characterized by field-emission scanning electron microscopy–energy dispersive spectrometry, and the phase composition was identified by X-ray diffraction. The alloys exhibited three types of microstructures: rods, wavy lamellae, and straight lamellae. The alloys grown at 4–12 μm/s exhibited a completely lamellar structure that consisted of α-Ni and β-Ni3Si phases. During lamellar–rod transition, coarse lamellae first underwent zigzag instability to form wavy lamellae and then transformed into rods. An interface deflection parameter was defined based on the deviation of the interface shape from a flat interface and reached the maximum value under maximum lamellar spacing. Moreover, zigzag instability resulted in a macroscopically flat liquid-solid interface. Out of the three microstructures, the rod-like structure exhibited the highest microhardness, followed by straight lamellae and then wavy lamellae. The difference in microhardness was related to the volume fraction of α-Ni phase and microstructural type. The lamellar microstructure more effectively improved the microhardness of eutectic materials than the rod-like microstructure.