Phase Transformation of Nickel‐Rich Phases in Al–Si Alloy Under Thermal Exposure

Abstract

Al–11.5Si–xCu–yNi–Mg alloys with different Cu and Ni contents are prepared. The microstructure of the alloy and phase transformation of Al3CuNi are investigated by X‐ray diffraction (XRD), optical microscope, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy‐dispersive spectrometry (EDS). The nickel‐rich phase in Al–11.5Si–3Cu–2Ni–Mg alloy is only Al3CuNi phase. With the increase in Cu and Ni content, the Al3Ni phase and the Al7Cu4Ni phase appear, respectively. The alloys are subjected to thermal exposure treatment at 350 and 425 °C, respectively, and hardness tests are performed on alloys of different exposure times. The Brinell hardness of the alloy decreases with the extension of thermal exposure time. It is found that the microstructure of nickel‐rich phases dissolved and granulated after thermal exposure, and the decrease in the Brinell hardness of the alloy is related to the transformation of nickel‐rich phases. The changes of Al3CuNi during thermal exposure are studied by means of in situ follow‐up observation. A large number of serrated new phases are deposited on the smooth Al3CuNi phase interface during thermal exposure. In addition, the Al3CuNi phase break and granulates with time. TEM analysis shows that the new jagged phase formed at the interface of Al3CuNi phase is Al7Cu4Ni phase.