EFFECT OF GE ON THE HETEROGENEOUS NUCLEATION OF CD SOLIDIFICATION BY AL

Heterogeneous nucleation of solidification of Cd has been studied in melt spun Al-2 wt% Cd-0.5 wt% Ge by using differential scanning calorimetry and transmission electron microscopy. The microstructure of the melt spun alloy consists of small faceted Cd particles embedded in an Al(Ge) matrix. The Cd particles exhibit an orientation relationship of {111}Al(Ge)//{0001}Cd and [110]Al(Ge)//[1120BAR]Cd with the surrounding Al(Ge) matrix, and have a truncated octahedral shape bounded by {111}Al(Ge) and {001}Al(Ge) planes, with four variants of the particle shape depending upon which of the four {111}Al(Ge) planes is parallel to {0001}Cd. On cooling from above the equilibrium Cd melting temperature, solidification of the Cd particles is nucleated heterogeneously on the {111} facets of the Al(Ge) matrix at an undercooling of 65 K, 9 K larger than that required on the {111} facets of pure Al in the binary alloy Al-4.5 wt% Cd. The contact angle at the {0001}Cd-liquid Cd-{111}Al(Ge) triple point is 50-degrees compared with 43-degrees in the binary alloy. The catalytic efficiency of Al for heterogeneous nucleation of Cd solidification reduces with an addition of 0.5 wt% Ge, even though the lattice mismatch across {111}Al(Ge) and {0001}Cd decreases. Chemical interactions between the Al, Cd, and Ge atoms dominate over structural details of the {111}Al(Ge)-{0001}Cd interface in controlling the heterogeneous nucleation of Cd solidification by Al(Ge).