Observations on a structural transition in aluminium alloys hardened by rapid solidification

Abstract

Binary alloys of Al with Fe and other first series transition elements have been rapidly solidified by splat cooling and surface pool freezing. Many alloys showed structural transitions on etching polished normal sections of splat-cooled samples. The most striking effect was shown by hypereutectic AlFe alloys which showed a hardened surface layer (zone A) of limited etching response and a dark etching and less hardened central layer (zone B). Either zone could extend through the splat thickness, the extent of zone A increasing with decreasing Fe content and improved thermal contact between splat foil and substrate, implying that a higher cooling rate was necessary to form zone A than zone B. Frozen surface pools showed substantially a single zone, A, tending to be replaced by B as Fe content increased. X-Ray diffraction showed zone A to give rise to α-Al line broadening and shift indicating fine structure and Fe supersaturation respectively, but no second phase was detectable. For zone B, the α-Al effects were absent but a metastable phase FeAl 6 was detected, displacing the equilibrium phase FeAl 3 for alloys containing between 4% and 20% Fe. Electron microscopy showed zone A to contain a primary “dendritic” structure of α-Al solid solution with a dendrite arm spacing of 300 Å for Al-8% Fe, and zone B to comprise developing primary FeAl 6 in an α-Al dendritic background coarser than in zone A. These observations are related to those of Scheil and Masuda on coupled region formation in AlFe alloys. It is argued that any differential in cooling rates required to give a zone A to zone B transition by splat cooling involves solidification by layers during deposition.