THE MICROSTRUCTURE OF RAPIDLY SOLIDIFIED AL-8FE-4NI-1MO ALLOY POWDERS

The microstructure of rapidly solidified A1-8wt%Fe-4wt%Ni-1wt%Mo alloy powder particles was studied by analytical electron microscopy, optic al microscopy and X-ray diffraction analysis. Powder particles below 2 00 mu m in diameter contained only the alpha-Al and tau [Al-9(Fe,Ni)(2 )] phases. The smaller powder particles started solidification with mi crocellular structures while the larger ones were composed of cellular and/or eutectic structures in which primary tau particles had acted a s nucleants of the (alpha + tau) eutectic. Specific orientation relati onships between alpha-Al and tau were observed in powder particles of different sizes. The predictions of predominant nucleation maps were i n good agreement with the nucleation of the alpha-Al and tau phases. T he predicted nucleation undercoolings were higher than those estimated from the solidification microstructures of the powder particles. Valu es of the contact angles between nucleant and melt representing the ch ange in nucleation mechanisms from homogeneous to heterogeneous nuclea tion have been estimated. Cellular and eutectic spacings have been rel ated to calculated solidification front velocities.