Effect of δ-Al3CuNi phase and thermal exposure on microstructure and mechanical properties of Al-Si-Cu-Ni alloys

Abstract

The effect of δ-Al3CuNi phase and thermal exposure on microstructure and mechanical properties of Al-Si-Cu-Ni alloy is investigated by optical microscopy, scanning electron microscope, and tensile test. The alloys with different volume fractions of δ-Al3CuNi phase are designed according to the thermodynamic calculation, which is confirmed by the microstructure characterization. The mean diameter of primary Si increases while the eutectic Si decreases with increasing the volume fraction of δ-Al3CuNi phase. The eutectic Si is coarsening during thermal exposure at 350 °C. Both strip-shape and fishbone-shape δ-Al3CuNi phase are observed in the as-cast alloys. In addition, the fishbone-shape δ-Al3CuNi phase is composed of many independent short rods. Parts of the thin short rods of δ-Al3CuNi phase change into γ-Al7Cu4Ni phase after thermal exposure at 350 °C for 200 h. The ultimate tensile strength and yield strength increase while the elongation decreases with increasing the volume fraction of δ-Al3CuNi phase. Comparing with the as-cast alloys, the ultimate tensile strength and yield strength are reduced after thermal exposure at 350 °C for 200 h, but the elongation of the alloys is increased. The failure mechanism of the alloys tested at 25 °C is quasi-cleavage fracture and ductile fracture is the dominant fracture mechanism tested at 350 °C.