Microstructures, tensile properties, and strengthening mechanisms of novel Al-Mg alloys with high Mg content

This study investigated the microstructures, mechanical properties, tensile deformations, and strengthening mechanisms of the Al-7-mass%-Mg (7Mg) and Al-9-mass%-Mg (9Mg) alloys developed from the Mg+Al2Ca master alloy. The microstructures of both as-extruded alloys comprised an Al matrix and eutectic Mg2Al3, Al2Ca (C15), and Al6(Mn,Fe) phases. The 7Mg alloy mainly consisted of elongated grains surrounded by small equiaxed grains, whereas the 9Mg alloy contained only fine equiaxed grains. Higher Mg content resulted in higher fractions of evenly distributed C15 phases. Tensile tests revealed yield strengths and maximum-tensile strengths of 234 and 429 MPa, respectively, for the 7Mg alloy, and these values increased to 276 and 479 MPa, respectively, for 9% Mg content. The characteristic serrated flow in the 7Mg alloy started at approximately 10% of the engineering strain. In contrast, in the 9Mg alloy, the development of the serrated flow from the beginning of tensile deformation, immediately after the yield point, is considered to be facilitated by the fine equiaxed grains and higher Mg content and number densities of obstacles (owing to the formation of secondary phases). Solid-solution and grain-boundary strengthening mainly contributed to the overall yield strength of the aluminum-magnesium (Al-Mg)-based alloys. Three microstructural factors, higher Mg solid solubility, smaller grain size, and a higher fraction of secondary phases, were responsible for the higher yield strength of the alloy with higher Mg content. Theoretical calculations based on conventional strength-prediction models predicted yield-strength values almost identical to those obtained experimentally, demonstrating the potential of the conventional models to accurately predict the yield strengths of Al-Mg alloys with high Mg content.