Design perspectives for creep-resistant magnesium die-casting alloys

Abstract

The microstructure of die-cast magnesium alloys is highly non-uniform, which leads to a non-uniform distribution of the solidus/homologous temperature in the α(Mg) phase and a non-uniform distribution of deformation stresses and strains in the specimen during creep testing. Experimental observations suggest that significant creep deformation occurs in the α(Mg) phase in and adjacent to the eutectic regions while deformation in the primary α(Mg) dendrites is less pronounced. This article addresses the effect of the non-uniform as-cast microstructure on the creep resistance of die-cast magnesium alloys. Computational thermodynamic simulations were carried out to determine solute segregation, solidus temperature, and the corresponding homologous temperature distribution in the α(Mg) phase. Transmission electron microscopy studies provided evidence of non-uniform creep deformation in the creep-tested specimens. The results suggest that the creep resistance of magnesium alloys is determined by the weakest aggregate and/or phase in the alloy, viz., the α(Mg) phase in and adjacent to the eutectic regions. Microstructural design efforts that increase the homologous temperature or reinforce the eutectic α(Mg) phase hold significant promise for increasing the creep resistance of magnesium alloys.

Acknowledgements

Research sponsored by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technology, Lightweight Vehicle Materials Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC. The authors thank J.A Horton and R.W. Swindeman for reviewing the article and M.L. Atchley for preparing the manuscript. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.