Effect of Common Impurity Elements on Grain Refinement of Magnesium Alloys

There has been much confusion evident in the literature in terms of the influence of impurity elements on grain refinement of magnesium alloys. This thesis addresses how impurity elements such as iron, manganese, carbon and beryllium affect grain refinement in magnesium alloys. The thesis starts with an investigation into the effect of the uptake of iron on grain refinement of Mg-Zr alloys. The highly detrimental influence of the uptake of iron on grain refinement in Mg-Zr alloys has been confirmed. The gradual loss of grain refinement of Mg-Zr alloys partly arises from the consumption of Zr by the formation of Fe2Zr via the reaction between soluble Zr and Fe picked up from mild steel crucibles. (Settling of undissolved Zr particles also partly attributes to the gradual loss of grain refinement.) The morphological evolution of Zr-rich cores from circular to rosette-like has been reported here for the first time. In contrast to the detrimental effect in Mg-Zr alloys, a positive effect of iron has been observed in grain refinement of Mg-Al based alloys. The addition of iron in the form of anhydrous FeCl3 produces significant grain refinement of high-purity Mg-Al alloys. Obvious grain refinement was also achieved through the uptake of iron from steel crucible surfaces; however, the addition of Fe powder in the form of an ALTABTM Fe75 powder compact (75%Fe, 15%Al and 10% Na-free flux) did not give rise to grain refinement. The results obtained from both the grain refinement tests conducted in aluminium titanite crucibles and an ultra-low carbon 316L stainless crucible indicate that the grain refinement of Mg-Al alloys by iron inoculation has little to do with the Al4C3 hypothesis. The nucleant particles have been clarified to be Fe- and Al-rich intermetallics. The effect of manganese on the grain refinement of high purity Mg-Al based alloys and commercially available AZ31 alloys has been investigated using an Al-60%Mn master alloy splatter at 730 aC in aluminium titanite crucibles. Grain refinement was readily achievable in these alloys. Electron microprobe analyses revealed that prior to the addition of extra manganese the majority of the intermetallic particles found in AZ31 are of the Al8Mn5 type. However, after the addition of extra manganese in the range of 0.1% to 1.0%, the predominant group of intermetallic particles changed to the metastable AlMn type. This leads to a hypothesis that the metastable AlMn intermetallic particles are more effective than Al8Mn5 as nucleation sites for magnesium grains. The hypothesis was supported by the observation that a long period of holding at 730 aC led to an increase in grain size, due probably to the transformation of the metastable AlMn to the stable Al8Mn5. Native grain refinement in magnesium alloys has been clarified. Based on the fact that native grain refinement is an exclusive feature of high purity Mg-Al alloys, it is hypothesized that Al4C3 particles act as nucleation centres. This is also the mechanism of carbon grain refinement of Mg-Al alloys. A trace of beryllium leads to dramatic grain coarsening in Mg-Al alloys at normal cooling rates. Apart from Mg-Al alloys, a trace of beryllium also causes considerable grain coarsening in Mg-Zn, Mg-Ca, Mg-Ce, Mg-Nd and also hinders grain refinement of magnesium alloys by Zr. Modelling grain refinement to predict the final grain size has been made on the basis of understanding of existing models. The modified model has resolved a fundamental gap in the relative grain size model using a more universal expression of solute concentration in the liquid.