The solidification behaviour of Bi particles embedded in an Al matrix

This paper reports heterogeneous nucleation behaviour of embedded nano- and micron-scaled Bi particles in an aluminium matrix synthesized by rapid solidification and chill casting. It is shown that only a fraction of the nanoparticles that have a truncated octahedron shape show orientation relation with the matrix. The solidification of the embedded Bi melt was found to take place over a wide temperature range. The micron sized particles do not show any solidification peak while a broad shallow peak can be observed for nano-scaled particles. The observed broad differential scanning calorimetry exotherm has been deconvoluted into two peaks, one sharp and the other broad in shape. The exotherms are also simulated using classical theory of heterogeneous nucleation using a parametric approach and marched with the experimental one. It is shown that the broad peak can be best explained by a spread of contact angles during nucleation while the sharp peak can be fitted with a single contact angle. The latter is most likely associated with particles showing orientation relation and represents nucleation of Bi on the 111 plane of aluminium. Analysis of the present result emphasizes the important role of the local microscopic structures of the liquid-solid interface in promoting the catalytic nucleation efficiency of the Al matrix.