Microstructural evolution, strengthening and thermal stability of an ultrafine-grained Al–Cu–Mg alloy

To gain insight into the origin of the ultra-high strength of ultrafine-grained (UFG) alloys, the solute clustering, precipitation phenomena, and microstructural evolutions were studied in an UFG Al-4.63Cu-1.51 Mg (wt.%) alloy (AA2024) processed by high-pressure torsion (HPT). The thermal analysis was performed using differential scanning calorimetry. The microstructures, internal microstrains and hardness following heating at a constant rate were characterised at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM) and atom probe tomography (APT). The microhardness of the HPT processed sample initially increases following heating to 140 °C, and then remains unchanged on further heating to 210 °C. As the temperature increases up to 210 °C, the crystallite size calculated from XRD line broadening remains about 60–70 nm, while the dislocation densities remain in excess of 2 × 10^14 m^2. A multimechanistic model is established to describe the strengthening due to grain refinement, dislocation accumulation, solid solution, precipitation, solute clusters and their segregation. The analysis reveals that solute clusters and lattice defects are key factors in HPT-induced strengthening of alloys, and illustrates the interactions between alloying elements, dislocations and grain boundaries enhance strength and stabilize ultrafine microstructures. Furthermore, for an HPT sample heated beyond 210 °C, the formation of nano-precipitates also contributes to hardness increment. The multimechanistic model for hardness contribution indicates the short-range order strengthening due to cluster-defect complexes is the dominant mechanism, which accounts for more than 40% of overall hardness.