Effect of cooling rate on microstructure and mechanical properties of AlMo0.5NbTa0.5TiZr refractory high-entropy alloy prepared by laser metal deposition

Abstract

AlMo_0.5NbTa_0.5TiZr is a novel lightweight refractory high-entropy alloy (RHEA) with a unique B2/BCC dual-phase structure. It exhibits excellent high-temperature mechanical properties while maintaining a low density, making it highly promising for aerospace applications. In this study, AlMo_0.5NbTa_0.5TiZr RHEAs are prepared by laser metal deposition (LMD). The effect of cooling rates at different temperatures on the microstructure and mechanical properties of LMD-formed AlMo_0.5NbTa_0.5TiZr after homogenization is investigated for the first time. This study highlights the formation of basket-weave structure, the competitive precipitation behavior of Al-Zr phases, and the strengthening mechanism. The LMD-formed specimens consist of columnar and cellular dendrites, which transform into equiaxed grains with a basket-weave structure after homogenization. At 1200 °C, a high density of Al₃Zr₄ point-like phases form, increasing as the cooling rate decreases. In the 1400 °C specimens, numerous blocky precipitates appear, and the size gradually increases with the decrease of the cooling rate. Additionally, the basket-weave structure is only formed in the 1400 °C furnace-cooled specimen, indicating that high cooling rate or low heat treatment temperature will inhibit the formation of the basket-weave structure. The 1400 °C air-cooled specimen exhibits a higher microhardness of 668 HV and compressive strength of 2314.7 MPa due to grain refinement and the precipitation of the hard Al₄Zr₅ strengthening phase. The specimen treated at 1200 °C exhibits a significant decrease in microhardness and compressive strength, due to the disappearance of the B2 phase caused by the precipitation of point-like Al₃Zr₄ phases. This study provides a foundation for optimizing the microstructure regulation of AlMo_0.5NbTa_0.5TiZr RHEAs.