Microstructure and nano-scale mechanical behavior of Mg-Al and Mg-Al-Ca alloys

The microstructure of magnesium alloys containing aluminum (AM50) and calcium (AC51 and AC52) cast by high-pressure die casting (DC) or permanent mold casting (PM) were characterized by SEM and TEM, and the influence of these microstructural features on the local mechanical properties was investigated. The presence of (Al, Mg)(2)Ca phase was identified in PM AC51 and AC52 alloys, which has a high melting temperature, was thermally more stable than the Mg17Al12 phase. A nanoindentation technique using a Berkovich indenter was used to characterize the nanoscale mechanical behavior of the alpha-Mg grains of the alloys at room temperature. The results of nanoindentation testing show that the hardness and composite modulus of the alloys vary with penetration depth. Ca addition to the alloy gives rise to an increase in the hardness and modulus, which can be attributed to solid solution strengthening. A regression analysis of the experimental nanoindentation data reveals a linear correlation between the contact stiffness and the penetration depth for all the tested alloys. The established correlations were used to calculate the continuous hardness and the composite modulus. Both the experimental and calculated data show that hardness values exhibit peak load dependence, i.e. indentation size effect (ISE) on all the tested alloys, while composite modulus was insensitive to peak load. Upon reaching a peak value of hardness at the beginning of the indentation test, the hardness of all the tested alloys decreases considerably by six times from around 1.2 to 0.2 GPa with increasing the penetration depth during the course of testing. However, the composite modulus of the alloy is much less dependent on the penetration depth only changing from 35 to 25 GPa during the tests. The comparison of the calculated results with experimental measurements indicates that they are in good agreement. Crown Copyright (C) 2007 Published by Elsevier B.V. All rights reserved.