Impurity effects on heterogeneous nucleation

Heterogeneous nucleation is important in controlling the undercooling during solidification and therefore the formation and microstructure of equilibrium and metastable phases. This paper describes recent experimental results concerned with the effect of impurities on heterogeneous nucleation during solidification, using the entrained droplet and sedimentation techniques. The materials discussed include: Al-(Pb,Cd)-(Ge,Si); Al-Si-(P,Na,Sr); Al-(Si,Fe)-(Ti,V,Zr); and X-(TiAl3,ZrAl3). (For a system referred to as A-B-C, a catalyst A nucleates solidification of B mediated by the presence of impurity C; and X refers to an unknown impurity). Analysis of recent results demonstrates that heterogeneous nucleation is highly sensitive to the presence of ppm levels of potent impurities, often causing dramatic changes in undercooling and solidified microstructure.