The existing forms of Zr in Mg-Zn-Zr magnesium alloys and its grain refinement mechanism

Abstract

The formation of the Zr-containing phases during the solidification of Mg-Zn-Zr alloy is simulated, the structure of the Zr-containing compounds and the solid solution amount of Zr in α-Mg solid solution as well as the grain size of the as-cast alloy are measured. Based on this, the existing form of Zr element in Mg-Zn-Zr alloy and its action mechanism of grain refinement are discussed. The results show that either α-Zr or α-Mg solid solution precipitates first from melt of Mg-Zn-Zr alloy at the beginning of solidification due to the different Zr content, then some Zn-Zr phases with different Zn/Zr atomic ratios form in the alloy melt during solidification. The Zr element in Mg-Zn-Zr alloy mainly exists in the form of compounds, its solid solution amount in α-Mg is not large and is inhibited by Zn element. The Zr in Mg-Zn-Zr alloy has the effect to refine grain, the reason is that the α-Zr phase and α-Mg formed by peritectic reaction or α-Mg formed from Zr atomic clusters can be used as the nucleus of subsequent α-Mg crystals. In addition, the Zr-containing compound formed during solidification can prevent the growth of grains, the enrichment of Zr in the crystallization front can also promote nucleation by component undercooling.