Thermal stability and structural changes during heat treatment of nanostructured Al2024 alloy

Thermal stability and structural changes during isothermal heat treatment of nanostructured Al2024 alloy prepared by mechanical milling (MM) were investigated. Al2024 powders were subjected to high-energy milling for various times to produce nanostructured alloy. Nanostructured Al2024 alloy was subsequently annealed at 150–550 °C for 1–3 h under argon atmosphere. The as-milled and annealed powders were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that after 30 h of milling, an Al–Cu–Mg supersaturated solid solution with a grain size of 30 nm was obtained. This structure was then isothermally heat-treated at various temperatures for different times. The CuAl2 and CuMgAl2 precipitates formed after heat treatment at T < 350 °C. In contrast to CuAl2 phase, CuMgAl2 precipitates disappeared on XRD traces taken after annealing at temperatures higher than 350 °C. Investigation of grain growth kinetics showed that nanostructured Al2024 had high thermal stability so that Al grain size remained in nanosized scale (about 70 nm) even after heating at 550 °C for 3 h. The value of grain growth exponent and activation energy at different annealing temperatures were obtained and discussed in terms of solute and second phase drag effects.