Microstructure, mechanical properties and texture evolution of AZ31 alloy containing trace levels of strontium

The effect of low levels of Sr (0.01, 0.03, 0.05 wt.%) on the microstructure, mechanical properties and texture of AZ31 magnesium alloy has been investigated. Thermodynamic modeling has been used to study the effect of Sr on phase precipitation at different temperatures. Cooling curve analysis reveals a decrease in solidification superheat with the addition of 0.03 wt.% Sr to AZ31. The as-cast microstructures of the alloys have been studied using optical microscopy (OM) and electron probe micro analysis (EPMA). Results show the refining effect of Sr on the grain size and on the β-Mg17Al12 precipitates through growth poisoning and inoculation, respectively. To investigate the mechanical properties of the alloys, as-cast samples were compression tested at elevated temperatures. Hot compression peak stress (σmax) and critical strain before recrystallization (εc) initially drop but then increase. σmax first decreases due to the depletion of Al from solid solution and then increases when the amount of the Al–Sr precipitates reach a significant amount. εc drops due to the acceleration of dynamic recrystallization kinetics as a result of grain refinement. It increases with increasing Sr when the concentration Sr in solid solution is increased leading to dislocation pinning and retardation of recrystallization. X-ray texture measurements on the hot compressed and extruded samples show a decrease in maximum intensity of the basal pole figures with increasing Sr as a result of reduced twining and the changes in Al and Sr concentrations in solid solution.

Research Highlights
► Microstructures/precipitates of AZ31+trace levels of Sr were examined by OM and EPMA.
► Thermodynamic calculations & cooling curve analysis were used to predict ppt formation.
► It has been shown that Sr refines the grains and Mg17Al12 precipitates.
► Mechanical properties & texture were studied using hot compression & X-ray diffraction.
► Different micro-mechanisms were responsible for mechanical and texture changes.