Ni-Rich Phases in Al-12%Si-4%Cu-1.2%Mn-x%Ni Heat-Resistant Alloys and Effect of Ni-Alloying on Tensile Mechanical Properties

In this study, formation of Ni-rich phases in Al-12 wt.%Si-4 wt.%Cu-1.2 wt.%Mn-xwt.%Ni (x = 0.8, 2.0, 2.4, 3.4) alloys and their effect on high temperature strength were investigated by microstructure characterization and tensile test. Three types of Ni-rich phases are observed: δ-Al3CuNi, γ-Al7Cu4Ni and e-Al3Ni. The amount of Ni-rich phases is increased with Ni content in the alloy. Ni-rich phases in Ni1-Ni2 alloys (x = 0.8 and 2.0, respectively) are in the form of δ-Al3CuNi and γ-Al7Cu4Ni, and however, a great amount of rod-like or needle-like e-Al3Ni phase is observed in Ni3-Ni4 alloy (x = 2.4 and 3.4, respectively). Microstructure observation of the samples after solutionizing at 510 °C for 5 h indicates that these Ni-rich phases have good thermal stability. Ni-alloying in Al-12 wt.%Si-4 wt.%Cu-1.2 wt.%Mn alloy decreases the strength at room temperature, but proper addition of Ni can remarkably improve the strength at 350 °C and considerably and efficiently slow down the softening effect of material at high temperature stage. 0.8 wt.% Ni addition results in a considerable increase in UTS at 350 °C from 65 MPa (without Ni addition) to 97 MPa. During tensile test at room temperature, both Mn-rich phase dendrites and Ni-rich compounds are the crack originating sites of cleavage fracture. However, during tensile test at 350 °C, the Mn-rich dendrites are the sites for fracture, but Ni-rich compounds are not.