The Loss of Dissolved Zirconium in Zirconium-Refined Magnesium Alloys after Remelting

Abstract

Magnesium–rare earth–zirconium (Mg-RE-Zr) materials are a high value-added class of speciality alloys used chiefly for aerospace applications, where the use of zirconium ensures the critically needed structural uniformity and consistency in performance through potent grain refinement. However, the manufacture of these alloy components has proved to be problematic and volatile. A viable approach preferred by manufacturers is to simply remelt fully grain-refined prealloyed ingot materials and cast. Unfortunately, the dissolved Zr content in these alloys decreases appreciably after remelting, leading to significantly increased grain sizes and inhomogeneity. This work investigates the mechanisms responsible for the loss of dissolved Zr in Zr-refined magnesium alloys after remelting. It was revealed that the phenomenon stems from the unique Zr-rich cores existing in these alloys. Precipitation of zirconium from these supersaturated cores during heating reduces the dissolved Zr content in the α-Mg phase. On the other hand, once precipitated out, the zirconium precipitates show very limited dissolution in molten magnesium due to their discontinuous dissolution behavior in the absence of agitation. The loss of dissolved Zr was found to be reversible by agitation when remelted in an iron-free environment while irreversible in steel crucibles due to the interference from iron.