Microstructural design of magnesium alloys for elevated temperature performance

摘要

Magnesium alloys are promising for automotive and aerospace applications requiring lightweight structural metals due to their high specific strength. Weight reductions through material substitution significantly improve fuel efficiency and reduce greenhouse gas emissions. Challenges to widespread integration of Mg alloys primarily result from their limited ductility and elevated temperature strength.;This research presents a microstructurally-driven systems design approach to Mg alloy development for elevated temperature applications. The alloy properties that were targeted included creep resistance, elevated temperature strength, room temperature ductility, and material cost. To enable microstructural predictions during the design process, computational thermodynamics was utilized with a newly developed atomic mobility database for HCP-Mg. The mobilities for Mg self-diffusion, as well as Al, Ag, Sn, and Zn solute diffusion in HCP-Mg were optimized from available diffusion literature using DICTRA. The optimized mobility database was then validated using experimental diffusion couples.;To limit dislocation creep mechanisms in the first design iteration, a microstructure consisting of Al solutes in solid solution and a fine dispersion of Mg2Sn precipitates was targeted. The development of strength and diffusion models informed by thermodynamic predictions of phase equilibria led to the selection of an optimum Mg-1.9at%Sn-1.5at%Al (TA) alloy for elevated temperature performance. This alloy was cast, solution treated based upon DICTRA homogenization simulations, and then aged. While the tensile and creep properties were competitive with conventional Mg alloys, the TA mechanical performance was ultimately limited because of abnormal grain growth that occurred during solution treatment and the basal Mg2Sn particle morphology.;For the second design iteration, insoluble Mg2Si intermetallic particles were added to the TA alloy to provide enhanced grain boundary pinning during heat treatment and creep deformation. An optimal Mg-1.9at%Sn-1.5at%Al-1.0at%Si (TAS) alloy was cast, solution treated, and aged. The high aspect ratio Mg 2Si particles were found to effectively limit grain growth during solution treatment. Tension testing revealed no statistical difference between the TA and TAS due to the Mg2Si location at the HCPMg grain boundaries. The TAS alloy, however, exhibited approximately an order of magnitude decrease in the minimum creep rate compared to TA because the Mg2Si particles hindered grain boundary motion during deformation.