Effect of Al2O3 nano-particles on the microstructural stability of AZ31 Mg alloy after equal channel angular pressing

The microstructure and thermal stability of the equal channel angularly pressed (ECAPed) AZ31 magnesium alloy and its composites, reinforced with 0.5, 1, and 2 wt.% of Al2O3 nano-particles, were investigated. The alumina nano-particles with an average diameter of 100 nm were added to AZ31 by a stir-casting method. After extruding, the materials were ECAPed at 500 K for 4 passes using route Bc, in which each sample was rotated 90° around its longitudinal axis between the passages. Textural studies revealed that nano-particles increased the intensity of the maximum orientations of the basal and prismatic planes in both extruded and ECAPed conditions, without changing their final positions. The microstructural stability of the ECAPed AZ31 and AZ31–1% Al2O3 was examined by isochronal and isothermal annealing regimes in the temperature range of 473–740 K. The measured activation energies for isochronal grain growth showed three different values, depending on the temperature range investigated. In the low- and high-temperature ranges of 490–530 K and 700–740 K, the respective activation energies were 74.1 and 94.4 kJ/mol. These energies are respectively 0.8Qgb and 1.03Qgb, where Qgb is the activation for grain boundary diffusion. For the intermediate-temperature range of 530–700 K, however, the unexpectedly low activation energy of Q = 0.21Qgb was obtained. The same trend was observed for the particle-containing material with higher activation energies. To consider the pinning force of nano-particles, the Burkeʹs model of grain growth during isothermal annealing was applied. Accordingly, two grain-growth regimes of the low-temperature region (<630 K) and the high-temperature region (>630 K) were identified. For the low-temperature region, the low activation energy of 32.1 kJ/mol may correspond to the energy for the reordering of grain boundaries in the fine-grained composite material. The activation energy of 110.9 kJ/mol found for temperatures higher than 630 K, lies between that for grain boundary diffusion and lattice diffusion of magnesium. The role of nano-particles in grain boundary pinning was also verified by the Hall–Petch plot.