Grain Coarsening of Cast Magnesium Alloys at High Cooling Rate: A New Observation

Most studies in the area of grain refinement have always taken for granted that higher cooling rate results in finer grains. However, when microstructural variation of the as-cast Mg with cooling rate was investigated using a specially designed V-shaped copper mold, the results were different. Although fast cooling during solidification led to microstructural refining in pure Mg, grain coarsening was observed at a higher cooling rate in Mg alloys that were inoculation treated with 1.0wt pctZr and 1.4wt pctCaO, and in the Mg-Al binary alloys. It is considered that the grain coarsening at higher cooling rate was attributed to the smaller constitutional undercooling zone formed at fast cooling due to the high temperature gradient in the three Mg alloys. These results can help in redefining the role of cooling rate in the grain refinement process.