HETEROGENEOUS NUCLEATION OF AL2CU IN AL-CU EUTECTIC LIQUID DROPLETS EMBEDDED IN AN AL MATRIX

The heterogeneous nucleation of Al2Cu in Al-Al2Cu eutectic liquid droplets embedded in an Al matrix has been studied by a combination of optical microscopy, transmission electron microscopy and differential scanning calorimetry. Nucleation of A12CU is stimulated catalytically by the surrounding matrix at a temperature approx. 25-degrees-C below the eutectic temperature. With increasing cooling rate, the solidification onset, peak and end temperatures decrease and the peak height and width of the solidification exotherm increase. The contact angle at the Al-Al2Cu liquid triple point is calculated to be 24.6-degrees from the variation of exothermic peak width with cooling rate, but the corresponding calculated value of the number of potential catalytic nucleation sites is physically unrealistic.