Microstructure-strength correlations in Al-Si-Cu alloys micro-alloyed with Zr

Abstract

The paper presents the microstructure-strength correlation of copper mould cast thin section of Al-(6–8)Si-Cu (at%). The efficacy of small Zr addition (0.15at%) and three-stage heat treatment on these alloys for achieving superior high-temperature mechanical properties were evaluated. The addition of Zr refines the alloy microstructure, exhibits a feather-like eutectic morphology in the interdendritic spaces and Zr supersaturation in the dendrites. Direct ageing at 450 ℃ yields coherent, spherical, L12 order Al3Zr precipitates of size 9–11 nm in the α-Al matrix. A characteristic composite microstructure of fine θʹ plates (∼58–67 nm in length and ∼4 nm in thickness) on these Al3Zr precipitates could be developed on ageing at 190 ℃ following a brief solution treatment at 500 ℃. The plates are finer in size when compared to that formed in alloys without Zr for similar heat treatment (Al-6Si-Cu:∼234 nm in length and ∼9.3 nm in thickness). Composition analysis has established Zr and Si partitioning in θ′ plates that impart long-term thermal stability and coarsening resistance at 250 °C. The observed complexity of the 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. At 250 °C, the observed tensile yield strength is ∼140 MPa. A detailed analysis with available models using quantitative microstructural parameters enable rationalization of our observations.