Size-effects in textural strengthening of hierarchical magnesium nano-composites

In a recent work, we demonstrated the efficacy of a hierarchical Mg nano-composite concept where the reinforcement is itself a composite (Al+Al2O3) at a finer scale, referred to as level-I composite (Habibi et al., 2010 [1]). In this work, we systematically investigate the influence of the Al2O3 reinforcement size dp and its overall volume fraction (v.f.) on the microstructural characteristics and the quasi-static tensile and compressive responses. We consider three different Al2O3 sizes with dilute fp thereby giving different hierarchical configurations (H-configs). All H-configs exhibit enhanced tensile and compressive responses compared to pure Mg. The compressive strengthening is nearly independent of the hierarchical configuration. On the other hand, the level of strengthening in quasi-static tension varies with H-config. The average grain sizes in all the H-configs are found to be nearly the same and therefore, it does not account for the strength variation. Microstructural analysis indicates that the variation in the strengthening for H-configs arises from at least two sources: (a) level-I size, dI which shows a weak dependence on dp, and (b) systematic dependence of the as-extruded textures on the level-I size and v.f. The hierarchical configurations exhibit stronger prismatic texture and weaker basal texture with decreasing size and increasing v.f. of the level-I phase. The underlying mechanism is not clearly understood, but we suggest that for the micron/submicron sized dp the level-I serves as a more effective sources for recrystallization compared to nanoscaled dp, thereby producing weaker textures in the former compared to the latter.