The role of solutes in grain refinement of hypoeutectic magnesium and aluminum alloys

The effect of solutes on grain formation has been studied over a century but is still under debating, simply because it is a very complex topic. This article focuses on the effect of dissolved solute on the growth, fragmentation of a dendrite and the resultant grain size. Experimental data on grain size in magnesium and aluminum alloys with various solute concentrations are collected and analyzed using phase diagram variables including Q, P, and ΔT. The physical meaning of each phase diagram variable is discussed. Curve fitting of the recently proposed two-parameter models with experimental data suggests that there is a clear correlation between the measured grain size and the solidification range of the alloy over the entire range of the hypoeutectic composition. Such a trend of grain size vs. ΔT is closely related to the grain refining mechanisms operating under relevant experimental conditions. A critical review of the grain refining mechanisms indicates that the mechanisms associated with the dendrite fragmentation are operating under the conditions where convection is substantial in the melt during its solidification.