Structure of Magnesium–Lithium β-Phase Alloys

Abstract

DURING no work magnesium–lithium–silver alloys, Hume-Rothery, Raynor and Butchers¹ examined X-ray powder photographs from binary magnesium–lithium alloys containing the body-centred cubic β-phase, or a mixture of this phase with the close-packed hexagonal magnesium-rich α-phase. Diffraction patterns from (α + β) alloys showed, in addition to fundamental reflexions, extra lines which were provisionally identified as derived from an Fe₃Al type of superlattice in the β-structure. The extra lines persisted on annealing at temperatures up to 300° C. and were also present in diffraction patterns of thin rods quenched from 420° C., but were not observed in photographs from homogeneous β-alloys. It was suggested that the β-phase develops an Fe₃Al type of superlattice very near to the (α + β)/β boundary. The existence of such a structure in a system of two components of such similar atomic size and electrochemical characteristics is surprising, but no further examination of the phenomenon was possible. More recently, however, the original diffraction patterns became available for re-examination; inconsistencies in the development of the suspected superlattice at various temperatures were noted, while identification of the extra lines with Fe₃Al-type ordering appeared uncertain. The appropriate region of the binary diagram has therefore been re-investigated.