Precipitation in rapidly solidified Al-Mn alloys
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INTRODUCTION
I T has been known for many years that solid solutions containing far more than the limiting equilibrium concentration of manganese in aluminum can be obtained by rapid solidification, t A phase designated G, usually considered to be metastable and with composition corresponding to A1L2Mn, appears during the annealing of these supersaturated solutions, and at temperatures below about 500 ~ the G phase can persist for long times in samples which also contain the A16Mn phase. 2 Nes, et al., 3 have found two additional phases in AI-I.8 wt pct Mn annealed at 460 ~ One phase was designated G" and it has a hexagonal lattice with c/a -- 1.04, and the other phase G' has a simple cubic structure; both phases were metastable. The A1-Mn system is of special interest because of the possibility that even concentrated alloys can freeze without microsegregation, as predicted by the theory of morphological stability,4 and because of the very slow rate at which phase equilibrium is established. Unfortunately, none of the previous studies of precipitation in this system has covered the wide range of solid solution compositions which is attainable by rapid solidification. Extensive reviews 5'6"7covering many aspects of the study of rapidly solidified aluminum alloys point out that a solid solubility extension has been observed in almost every system which has been investigated. For alloying elements such as Fe or Ni, which have extremely low maximum equilibrium solid solubilities in AI, the observed solubilities after rapid solidification lie far beyond any reasonable metastable extension of the solidus and thus indicate a non-equilibrium reaction at the solid-liquid interface. In contrast, manganese has a relatively high solubility in aluminum and if the formation of competing phases can be avoided, large extensions of the solubility limit would be expected even without any departure from equilibrium at the solid-liquid interface. When an alloy solidifies by extraction of heat through the solid, as is normally the case in melt spinning, a microsegregation-free solid can be produced if the solid-
liquid interface remains planar. If the planar interface is morphologically unstable, however, a cellular (or dendritic) microsegregation pattern can develop. On the basis of interface equilibrium, the theory of morphological stability predicts an absolute stability regime in which plane-front microsegregation-free solidification of an alloy is stabilized by surface energy effects at sufficiently high solidification velocities. Such microsegregation-free solidification in the absolute stability regime has recently been demonstrated in Ag-Cu alloys, a In the aluminummanganese system the velocity required to produce this effect is predicted to be relatively low because of the high value of the solute partition coefficient. 9 By this mechanism, rapid solidification processing could produce rather concentrated AI-Mn alloys without microsegregation. For example, a planar solid/liquid interface is predicted to be stable in an al
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