A CsCl-type phase in electrodeposited AlMn alloys

Abstract

Since the work of Goedecke and Koester it has been known that the central part of the Al-Mn phase diagram is occupied at room temperature by the [gamma][sub 2] phase, while at high temperature two other phases, [gamma] and [gamma][sub 1], have been reported in the same compositional range. A more recent study has confirmed the existence of these phases. The low-temperature phase has a rhombohedrally distorted [gamma]-brass lattice of the Al[sub 8]Cr[sub 5]-type. The structures of the high-temperature phases have not been determined since they could not be retained in alloys quenched to room temperature due to their high transformation rates. Only recently, the structure of the high-temperature [gamma]-phase was studied in-situ by Ellner using x-ray diffraction. According to this study, [gamma] has a bcc structure (W-type) with a lattice parameter of 0.3063 nm. The structure of the second high-temperature [gamma]1 phase has not been determined directly due to technical problems associated with its higher formation temperature; however, it has been suggested to be of the cubic [gamma]-brass type. The previous study of electrodeposited Al-Mn alloys with compositions ranging up to 50 atomic pct Mn revealed a phase exhibiting an x-ray diffraction pattern which was consistent with a CsCl-typemore » structure with a lattice parameter of 0.299 nm. The compositions of the alloys containing this phase ranged from 36 to 43 atomic pct Mn. At lower Mn concentrations, the electrodeposits contained the cubic [gamma]-brass phase described earlier while at higher Mn concentrations, metastable [tau] (CuAu-type) was observed. In this paper the authors confirm the presence of a CsCl-type phase in electrodeposited Al-Mn alloys and associate it with the [gamma]-phase reported by Ellner.« less