The effect of minor addition of insoluble elements on transformation kinetics in amorphous Al alloys

Nanocrystalline metallic materials based on partial devitrification of amorphous aluminum alloys show an attractive combination of high strength and low density. A key feature concerning the improved mechanical properties is the high number density of Al nanocrystals (1022-1023 m−3) that precipitate within the amorphous precursor structure upon low temperature annealing. For Al87Y7Fe5Pb, the melt-spun ribbons consisted of an amorphous matrix with a dispersion of Pb nanoparticles (10 nm diameter). HRTEM images of the Pb-Al interface revealed a good wetting behavior between the Al and the Pb nanoparticles. Isothermal annealing for Al87Y7Fe5Pb showed no transient stage even though the crystallization onset, Tx, was at a much lower temperature (247 °C) compared with Al88Y7Fe5 (267 °C). For Al(88−x)Y7Fe5Inx (x = 0.5, 1.0, 1.5, 2.0), the DSC results indicated that Tx continuously decreased from 232 °C to 220 °C as the indium level increases from 0.5 at.% to 2.0 at.%. Under STEM, the image showed a coherent interface between Al and In particles. In the analysis of the transformation kinetics, the addition of minor elements can effectively promote additional nucleation of Al nanocrystals by providing heterogeneous nucleation sites. These developments offer new opportunities for the control of nanoscale microstructures.