Mn–Al phases in Mg–Al–Mn–C master alloys and the related grain refinement effect on AZ31 alloy

Grain refinement can enhance the strength and ductility of Mg–Al alloys, thus expanding their application range. Bulk grain refiners for Mg–Al alloys are yet to be developed. This study investigates the precipitation, phase transformation, growth, and morphology of Mn–Al phases in Mg–Al–Mn–C alloys and the related grain refining effect on the AZ31 alloy using scanning electron microscopy and transmission electron microscopy. Primary ε-MnAl–C precipitates first and transforms to the transformed τ-MnAl–C later during solidification, and primary γ2(Al8Mn5) precipitates on ε/τ-MnAl–C or independently. Under slow cooling, the transformed τ-MnAl–C are branched plates and covered by the primary γ2(Al8Mn5), whereas under high cooling, the transformed τ-MnAl–C are branched dendrites, and a majority of them are uncovered. The orientation relationship between the transformed τ-MnAl–C phase and attached primary γ2(Al8Mn5) phase is [1¯ 10]τ-MnAl–C ‖ [10 12¯]γ2 and (11 1¯)τ-MnAl–C at 2° to (10 1¯ 1)γ2. Our newly developed bulk grain refiner, Mg–Al–Mn–C master alloy, effectively refines the grain size of the commercial AZ31 alloy from 465 ± 34 μm to 154 ± 18 μm with considerably good fading resistance, attributable to the released branched dendrites with the majority-uncovered transformed τ-MnAl–C.