Role of Intermetallics in Creep Behavior of Squeezed Cast Ca‐ and Sr‐Modified AZ91 Magnesium Alloy

Magnesium (Mg) alloys, particularly AZ91, have gained significant attention due to their castability and lightweight properties. The present study investigates the effects of individual strontium (Sr) and combined calcium (Ca)–strontium (Sr) addition on the microstructure evolution and creep performance of AZ91 alloy. Alloys are prepared via squeeze‐casting and subjected to tensile creep tests. Scanning electron microscopy and X‐ray diffraction studies reveal Al2Ca and Al4Sr phases at the expense of the β‐Mg17Al12 phase. Further, thermodynamic analysis shows these phases have higher dissolution compared to the β‐Mg17Al12 phase. The AZ91 alloy with the combined addition of Ca and Sr demonstrates a much lower steady‐state creep rate compared to individual Ca‐ or Sr‐modified AZ91 alloys. Further analysis of the creep data indicates that dislocation climb‐controlled power‐law creep governs in both Sr‐ and Ca–Sr‐modified AZ91 alloy. Microstructural investigation on the postrupture sample indicates that needle‐shaped Al4Sr acts as a crack/cavity initiator, while a skeleton shape‐interconnected network consisting of Al2Ca and/or Al4Sr precipitates is effective in retarding crack propagation. The higher thermal stability of Al2Ca and Al4Sr phases, along with the grain size refinement, is found to be the major factors contributing to improved creep performance in Sr–Ca‐modified AZ91 alloy. The creep resistance of the AZ91 alloy is significantly enhanced when alloying with combined Ca and Sr and its processing via squeeze‐casting route. This enhancement is primarily due to the precipitation of thermally stable Al–Ca and Al–Sr intermetallic phases, which suppress the formation of the thermally unstable β‐Mg17Al12 phase in the AZ91–Ca–Sr alloy. The underlying creep deformation mechanism in the study is dislocation climb controlled by lattice diffusion.