THE SOLIDIFICATION BEHAVIOR 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 fract ion of the nanoparticles that have a truncated octahedron shape show o rientation relation with the matrix. The solidification of the embedde d Bi melt was found to take place over a wide temperature range. The m icron sized particles do not show any solidification peak while a broa d shallow peak can be observed for nano-scaled particles. The observed broad differential scanning calorimetry exotherm has been deconvolute d into two peaks, one sharp and the other broad in shape. The exotherm s are also simulated using classical theory of heterogeneous nucleatio n using a parametric approach and marched with the experimental one. I t is shown that the broad peak can be best explained by a spread of co ntact angles during nucleation while the sharp peak can be fitted with a single contact angle. The latter is most likely associated with par ticles 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 th e Al matrix.