Microstructure and creep properties of MRI230D magnesium alloy

Abstract

The dependency of creep behaviour on the as-cast microstructure of MRI230D Mg alloy produced by ingot casting (permanent mould casting) and high pressure die casting was investigated. A detailed microstructural characterisation and study of creep properties have been accomplished. The alloy in both ingot-casting (IC) and high pressure die-casting (HPDC) conditions consists of á-Mg, C36 ((Mg,Al)2Ca) and Al-Mn rich phases. Microstructural study revealed finer grain size, closely spaced network and increased volume fraction of secondary eutectic phases for HPDC specimen when compared to that of the IC specimen. The grain size variation and denser network of eutectic phases was due to the considerable difference in cooling rate, high degree of undercooling associated with the faster cooling rate following HPDC facilitates the formation of a large number of nuclei in the liquid Mg alloy. The eutectic C36 phases were also observed to be coarser in the case of IC specimen owing to the slower solidification rate. Porosity was discovered significant in the HPDC specimen in comparison to the negligible extent in IC specimen. Following creep tests at 70 MPa stress level and temperature of 175°C and 200°C the HPDC specimens exhibited superior creep resistance as compared to the IC specimens. The significant creep resistance exhibited by the HPDC specimen was primarily due to the dispersion strengthening effect offered by hindering of dislocation movement at the grain boundaries. Stress dependence on the strain rates for the alloys cast by both the technologies were studied and deduced that the dominant creep mechanism was dislocation creep. The same was theortically confirmed from the deformation mechanism map established for Mg alloys. Post creep microstructural studies revealed increased precipitation of C36 phase at the grain boundaries. Larger amount of precipitation was observed in the HPDC specimens due to enhanced pre-existing eutectic phases in the same.