Abstract
A quantitative model was applied to analyse the grain refinement potency of bismuth in magnesium. The efficiency of the grain refiner was identified through the analysis of grain size vs inverse of the growth restriction factor (Q) plots, with a comparison of the Zr solute under similar experimental conditions. It was concluded that ‘Q’ could be a suitable predictor of the relative grain size in the Mg-Bi system containing potent nucleant particles. The cooling rate analysis reinforced the efficiency of bismuth grain refinement with the formation of Mg-Bi-O phases at 0.4 wt% Bi in magnesium.
Highlights
The potency of the nucleant particles along with the segregating power of solute is critical in determining the final grain size of the alloys [1, 2, 3, 4]
Earlier work [11] proposed an increase in yield strength and creep resistance of AZ91 alloy containing bismuth (Bi), which was attributed to the formation of Mg3Bi2 intermetallic phase
Whilst the actual ∆Tn value of the Bi containing nucleant is not known, it can be seen that the trend in the average grain size with bismuth addition is similar to the trends in relative grain size (RGS) and is pointing towards a lower value of ∆Tn
Summary
The potency of the nucleant particles along with the segregating power of solute is critical in determining the final grain size of the alloys [1, 2, 3, 4]. The parameter Q is referred to as the growth restricting factor and is used as a measure of the effects of a solute on the grain refinement in the absence of solute interactions [3]. Earlier work [11] proposed an increase in yield strength and creep resistance of AZ91 alloy containing bismuth (Bi), which was attributed to the formation of Mg3Bi2 intermetallic phase. The focus of each of these research papers has been on the role of Bi solute in high Al (≥ 5 wt %) containing magnesium (Mg) alloys, while our recent study [14] has shown effective grain refinement even in commercial purity (CP) Mg and AZ31 directchill cast billets at significantly lower addition rates of Bi solute. Current research aims to understand the potency of this grain refinement effect of Bi through application of a quantitative model [3], which was originally developed to predict the grain size of aluminium castings inoculated with Al-Ti-B master alloy
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