SOLIDIFICATION OF HIGH-PRESSURE GAS ATOMIZED AL-CR ALLOY POWDERS .1. DEPENDENCE OF NUCLEATION TEMPERATURE ON NUCLEANT POTENCY

A study of Al13Cr2 intermetallic particles in gas atomised Al-Cr alloy powders has been made by optical and electron microscopy for different alloy compositions (< 5wt%Cr) and powder sizes (< 200-mu-m). The fraction of powder particles free of primary intermetallics has been determined as a function of powder particle size and alloy composition. The data is consistent with heterogeneous nucleation of the intermetallic phase. Nucleant concentrations of the order of 10(15) to 10(17) m-3 have been calculated. A nucleation map has been constructed giving the powder particle sizes for intermetallic-free cellular solidification as a function of Cr alloy content. The dependence of nucleation temperature on nucleant potency has been determined as a function of the contact angle theta between nucleant and melt. Values of theta representing the change in nucleation mechanism from heterogeneous to homogeneous nucleation have been calculated for each alloy.