Evolution of microstructural and mechanical properties of an industrial 2017 Aluminium alloy processed by equal channel angular extrusion

The evolution of mechanical properties, microstructure and their stability after different thermal treatments of an industrial Al-4wt% Cu alloy processed by Equal Channel Angular Extrusion (ECAE) were studied by Hardness measurements, tensile test, X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) techniques. A strong increase in the flow-stress and in the Vickers Hardness of the alloy was observed after ECAE. An important conclusion is that the strength level achieved by ECAE is higher than that obtained by ageing. X-ray diffraction analysis showed that the microstructure after ECAE consisted of low-sized crystallites (≈ 100 nm) and high values of root mean squared strain (rms-strain), almost 1%. In Transmission Electron Microscopy (TEM), it was found that sub-micrometric grains are formed by severe plastic deformation. The study of thermal stability showed that for N = 1 and N = 3, the alloy does not recover after annealing at 150°C for one hour. A substantial recovery is observed only at 250°C. At lower annealing temperatures, an increase in hardness is remarked, due to a reprecipitation enhanced by ECAE. The full recrystallized alloy exhibits a coherency length as low as 110 nm, revealing the effects of precipitates in dragging grain growth.