Effect of Zr Addition and Heat Treatment on Microstructure and Mechanical Properties of Suction Cast Al-Si-Cu Alloys

The role of Zr addition and a three-stage heat treatment to suction cast Al-(6-8)Si-Cu (at%) alloys are studied, emphasizing the microstructure, its stability, and high-temperature tensile properties. The addition of 0.15 at% Zr to the alloys refines the microstructure. The suction casting resulted in the high supersaturation of Zr in the dendrite interiors. When aged directly at 450 ℃, it leads to fine, spherical, coherent L12 order Al3Zr precipitates of size 9-11 nm in the α-Al matrix. After further ageing at 190 ℃ following a brief solutionizing at 500 ℃, the alloys develop a fine-scale microstructure achieved through heterogeneous nucleation of θʹ plates on Al3Zr precipitates formed during initial ageing. These plates are finer in size (~58-67 nm in length and ~4 nm in thickness) than the plates formed in Al-6Si-Cu alloy aged at 190 °C after normal solutionizing (~234 nm in length and ~9.3 nm in thickness). The complex microstructure with a finer length scale rationalizes the high yield strength of the Zr modified alloys (306-320 MPa at room temperature), which is 1.7 times higher than the corresponding Al-6Si-Cu alloy. EDS composition analysis shows the partitioning of Zr and Si in θ' plates that impart long-term thermal stability and coarsening resistance at 250 °C. It leads to a yield strength of ~140 MPa under tension at 250 °C. The coarsening of θ' plates was ledge controlled in Al-(6-8)Si-Cu-Zr alloys.