Effect of active heterogeneous nucleation particles on the grain refining efficiency in an Mg–10 wt.% Y cast alloy

Addition of pure Al into the melt of an Mg–10 wt.% Y cast alloy leads to in situ formation of Al2Y particles. Part of these particles can act as heterogeneous nucleation sites and reduce the grain size. Such particles are termed active nucleation particles. In this paper, grain refining efficiency of the Al2Y particles was experimentally studied in terms of the number density and size of the total and active Al2Y particles. The results show that the as-cast grain size is closely related to the number density of the active particles ρa that is controlled by the size of active particles, dap, at different Al addition levels. The finest grains were achieved at 2 wt.% Al addition level, which corresponds to the average dap of 6–6.5 μm with ρa of 260–290 /mm2. The active particle size effect was discussed based on the competitive processes between heterogeneous nucleation and crystal growth, both of which are associated with the curvature undercooling.