Effect of Yb on Microstructure and Mechanical Properties of Al-Cu-Mn Heat-Resistant Aluminum Alloys

The effect of Yb on the microstructure and tensile properties at room temperature and high temperature (350 °C) of as-cast and T6 heat-treated Al-6Cu-0.4Mn alloys was investigated. The results show that Yb can refine the α-Al primary grain and α-Al + Al₂Cu eutectic structure. The eutectic structure of the alloy containing 0.3 wt.%Yb has the best refining effect. After the T6 heat treatment, most of the network α-Al + Al₂Cu eutectic structure disappeared, and many dispersed (200-400 nm) θ'-Al₂Cu phases were precipitated in the matrix. With the addition of Yb, the amount of θ'-Al₂Cu phases increases, and the average size decreases from 260 nm in the base alloy to 176 nm in the Yb-containing alloy, which indicates that Yb can promote the precipitation and refinement of the θ'-Al₂Cu phase; after adding Yb, the high-temperature ultimate tensile strengths (UTS) of the as-cast and heat-treated alloy significantly increase. Both reached their highest when the Yb additions were 0.3 wt.%, reaching 95.5 MPa and 142.3 MPa, respectively, 13.66% and 17.71% higher than the base alloy. The thermal exposure test at 350 °C shows that Yb can improve the θ'-Al₂Cu phase's coarsening resistance. Analysis shows that the improvement of mechanical properties at room temperature is due to solid solution strengthening and grain refinement of Yb. The reason for improving mechanical properties at high temperatures is that adding Yb promotes the precipitation, refinement, and thermal stability of the θ'-Al₂Cu phase.