MICROSTRUCTURE DEVELOPMENT IN UNDERCOOLED AL-BE POWDERS

The development of solidification microstructure in aluminum-rich Al-Be alloy powders was studied as a function of melt undercooling, cooling rate. powder size and beryllium content. Auger electron spectroscopy indicates that BeO partially displaces Al2O3 from the powder surface. The BeO-Al2O3 coating reduces the attainable undercooling at beryllium contents above about 4 at.% Be. Powders containing Al-9at.%Be when cooled at 0.5-degrees-C s-1 contain facetted primary beryllium and a cellular aluminum matrix. Increasing the cooling rate to 25-degrees-C s-1 reduces the extent of facetting of the primary beryllium phase, and at a cooling rate of 500-degrees-C s-1 multiple primary beryllium dendrites are observed. In Al-4at.%Be powders cooled at 500-degrees-C s-1, primary beryllium formation is avoided and solidification commences with a cellular aluminum and intercellular beryllium structure. The aluminum cells evolve to a dendritic network during recalescence. Following recalescence the dendritic aluminum and interdendritic beryllium structure is replaced by a coupled eutectic morphology which completes solidification. The microstructure development is analyzed in terms of the solidification pathways possible based on a skewed coupled eutectic growth zone and composite stable-metastable phase diagrams including a metastable liquid miscibility gap.