Abstract
Cast-Al alloys that include a high amount of the second element in their matrix have comparatively high strength but low ductility because of the high volume fraction of strengthening phases or undesirable inclusions. Al–Zn alloys that have more than 30 wt% Zn have a tensile strength below 300 MPa, with elongation under 5% in the as-cast state. However, we found that after substitution of 2% Zn by Cu, the tensile strength of as-cast Al–Zn–Cu alloys was 25% higher and ductility was four times higher than for the corresponding Al–35% Zn alloy. Additionally, for the Al–43% Zn alloy with 2% Cu after 1 h solution treatment at 400 °C and water quenching, the tensile strength unexpectedly reached values close to 600 MPa. For the Al–33% Zn alloy with 2% Cu, the tensile strength was 500 MPa with 8% ductility. The unusual trends of the mechanical properties of Al–Zn alloys with Cu addition observed during processing from casting to the subsequent solution treatment were attributed to the precipitation of Zn in the Al matrix. The interface energy between the Zn particles and the Al matrix decreased when using a solution of Cu in Zn.
Highlights
Structural metals including Fe, Cu- and Al-base alloys require high strength to sustain the desired shape and high ductility for easy forming
The heterogeneously nucleated particles were located at the grain boundary (GB) (Fig. 4a and b); lamellar precipitates covered the GBs in the Al–33Zn–2Cu alloy
The results of this study show that Cu addition reduces the interface energy between the Zn phase and Al matrix, which causes a drastic decrease in precipitate size and encourages discontinuous precipitation at GBs during solidification or solution treatment
Summary
Structural metals including Fe-, Cu- and Al-base alloys require high strength to sustain the desired shape and high ductility for easy forming. The Cu addition to Al–Zn alloys definitely increased strength or simultaneously improved both strength and ductility at certain compositions during casting and solution treatment (Fig. 1). This unusual behaviour of the Al–Zn alloy with Cu is important for cast Al alloy fabrication because it means that an economical production process consisting only of casting and heat treatment could achieve good mechanical properties. Based on the relationship between microstructure and mechanical properties, we used density functional theory (DFT) calculations in this study to clarify why adding Cu to an Al–Zn alloy changed the precipitation structure and simultaneously increased strength and ductility during the solidification stage after casting and subsequent solution treatment
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