Grain size dependent tensile behavior of Mg-3%Al alloy at elevated temperatures

In this study, we experimentally investigate the effect of grain size on the tensile properties of Mg-3%Al alloys at elevated temperatures. Bulk nanostructured Mg-3%Al alloys were prepared by mechanical alloying followed by conventional sintering and extrusion. The microstructure and grain size were investigated by XRD, field emission scanning electron microscope (FESEM) and optical microscope. The formation of second phase particles was observed in nanostructured powder and the extruded bulk samples. Tensile tests were performed at elevated temperatures up to 250 °C and the yield strength, tensile strength and elongation to failure were measured for microcrystalline and nanocrystalline samples. Remarkably, the strain to failure was observed to be non-monotonic with temperature with the failure strain increasing up to a critical temperature and decreasing thereafter. This critical temperature was found to be strongly dependent on grain size. This phenomenon is attributed to a competition between uniform elongation and necking deformation. The latter dominates at higher temperatures due to decreased strain hardening, particularly in the fine grained samples.