An electron-microscopical identification of Al6Fe precipitates in an Al-0.03wt%Fe alloy.

Abstract

Is is well known that a small amount of iron in aluminum markedly retards its recrystallization 1) and brings about R-orientation in annealing textures of cold rolled alloys 2), which is generally attributed to the fact that iron precipitates during annealing and interacts with progress of recrystallization 1), 3), 4).Iron in aluminum supersaturated by solid quenching 4), 5) to such an extent as about 0.04wt% is possible to precipitate even below 300°C in a cold worked matrix, while precipitation kinetics in an undeformed matrix follows a C-curve with a nose at about 470°C and iron does not precipitate practically below 300°C. Electron microscopical observations have revealed that iron in an undeformed matrix precipitates between 300°C and 500°C as flat needles 4), 6) in the Al directions with the {110}Al habit planes and that iron in a deformed matrix precipitates on subboundaries as irregular spheres 1), 3), 4). The spherical precipitates 3) and the born-like precipitates 7) formed by longer annealing above 500°C have been tentatively identified by electron diffraction as a stable compound, Al3Fe, though some ambiguity was left. The flat needle precipitates are, however, not yet reported to be identified directly either by electron or X-ray diffraction. A measurement 5) of Mossbauer effect in an Al-0.05wt%Fe alloy showed that only Al3Fe was present in an undeformed matrix aged between 370°C and 520°C, while a metastable compound, Al6Fe, was prominant in a cold rolled matrix annealed between 270° and 420°C. The correspondence of the electron microscopical observations to the results of the Mossbauer effect measurement seems to have not been discussed yet.In the case of aging of highly supersaturated alloys (several wt%Fe) formed by splat quenching 8), 9), 16), precipitation of Al6Fe and Al3Fe is reported to occur below 400°C and above 450°C respectively, and formation of the former phase at an earlier stage followed by growth of the latter phase is observed at intermediate temperatures.The Al6Fe phase is also formed during unequilibrium solidification of Al-Fe alloys, where two more Al-Fe compounds, a tetragonal phase in an Al-0.5wt%Fe alloy 10) and a monoclinic phase in an Al-0.5wt%Fe-0.2wt%Si alloy 11), were found together with Al3Fe and Al6Fe. Wheather these two metastable phases concern with the aging process of solid quenched Al-Fe alloys is yet unknown.Recent investigations 12), 13) of recrystallization textures of cold rolled and annealed Al-Fe alloys have shown that relative amounts of R-orientation to cube-orientation can be remarkably varied by final annealing temperatures. One of the reasons is certainly dependence of precipitating quantity in a period of recrystallization on annealing temperatures 4), but the possibility that precipitating phases may be also changed by annealing temperatures must be pointed out when the results of the Mossbauer effect measurement and of the splat quenched alloys are taken into consideration 13).An electron-microscopical work is consequently being done to identify precipitates formed during recrystallization annealing in a cold rolled Al-Fe alloy.The 99.998% purity Al-0.03wt%Fe alloy was melted and cast in vacuo, solution treated at 630°C, cold worked, and intermediately annealed for 30s at 550°C in a salt bath. A specimen was further cold rolled to 0.2mm thickness by the reduction of 95% and finally annealed for 16h at 280°C in the salt bath. Photo 1 shows precipitates which are very likely to have been formed initially on subboundaries and left in a recrystallized grain after migration of recrystallization front.