Thermal behaviour and microstructure evolution of new ternary eutectic alloy in Al-Cu-Si-Ni system
Qing Cai, Changming Fang, C.L. Mendis, I.T.H. Chang, B. Cantor
Abstract
Eutectic alloys were fabricated from the quaternary Al-Cu-Si-Ni system via arc melting and suction casting. An invariant ternary eutectic reaction (α-Al+Si+θ-Al2(CuNi)) was found in the quaternary alloy system with a composition of Al67.2Cu24Si8Ni0.8 (wt%). The dissolution of Ni (∼1.7 at%) into tetragonal θ-Al2Cu takes place during this ternary eutectic reaction. Density functional theory (DFT) calculations show that the configurational entropy stabilises this level of randomly substituted Ni with Cu sites in the θ-Al2Cu lattice at high temperatures. The as-solidified eutectic microstructure exhibits a lamellar θ-Al2(CuNi) phase showing fragmented lamellar morphology with a lamellar thickness of 130 ± 30 nm and Si exhibits fibrous morphology with a fibre diameter below 100 nm. The thermal stability of the Al-Cu-Si-Ni eutectic alloy after post-solidification annealing was investigated, and the thermal stability of the ternary eutectic microstructure is better than the corresponding Al33Cu (wt%) binary eutectic microstructure. It was found that Ni solution in θ-Al2(CuNi) phase contributes to the thermal stability of this ternary eutectic microstructure and β2-Al3(CuNi)2 (β1-(Cu2.9Ni0.1)Al type, Fm-3 m) phase can transform from θ-Al2(CuNi) phase after annealing at different temperatures. The Al-Cu-Si-Ni eutectic alloy has excellent as-cast hardness together with thermal stability. It is potentially valuable for the design of new aluminium alloys for serving at elevated temperatures.