Barothermal Analysis and Processing, Plastic Deformation, Microstructure, and Properties of Al–Zn Binary Alloys

Abstract

Al–4 wt % Zn and Al–9 wt % Zn binary alloys have been characterized by differential scanning calorimetry (DSC) at atmospheric pressure and differential barothermal analysis (DBA) at ~100 MPa near their melting point. The solidus temperature evaluated from the DSC data for the alloys has been shown to slightly exceed that in the equilibrium Al–Zn phase diagram, which is accounted for by nonequilibrium experimental conditions. The solidus temperatures of the Al–4Zn and Al–9Zn alloys evaluated from DBA heating curves obtained at 100 MPa exceeded those in the equilibrium Al–Zn phase diagram by 18 and 16°C, respectively, and their liquidus temperatures were 25 and 11°C higher than the phase diagram values. Barothermal processing (BTP) at a temperature of ~0.8t_s in a 100 MPa/460°C/3 h cycle caused partial decomposition of the solid solution of Zn in Al in the alloys, leading to the formation of Zn particles in an aluminum matrix. The number density of Zn particles in the Al–9Zn alloy was 76% higher. We propose a thermodynamic model for the external pressure-driven decomposition of the Al〈Zn〉 solid solution and demonstrate that the Zn particles in the aluminum matrix have a mainly exponential size distribution, with equivalent particle diameters in the range 0.11–0.3 μm. Zinc in the as-cast Al–4Zn alloy has been shown to be completely dissolved, whereas the number density of zinc particles in the as-cast Al–9Zn alloy was 3.0 × 10¹¹ cm^–3. After BTP, the number density of precipitates resulting from the partial decomposition of the Al〈Zn〉 solid solution reaches 2.5 × 10¹¹ and 4.4 × 10¹¹ cm^–3 in the Al–4Zn and Al–9Zn alloys, respectively. We have measured the microhardness of the alloys in different states. The highest values, 383 ± 7 and 607 ± 28 MPa, were obtained for the cold-deformed Al–4Zn and Al–9Zn, respectively. The yield stress, ultimate strength, and relative elongation of the Al–4Zn alloy after BTP and cold deformation were determined to be 106.5 MPa, 122.8 MPa, and 14.8%, and those of the Al–9Zn alloy were 174.9 MPa, 183 MPa, and 13.2%, respectively, with the possibility of improving these properties by artificial aging. The results of this study demonstrate the feasibility of controlling the microstructure and properties of zinc-containing aluminum-based alloys by BTP (hot isostatic pressing).