A powder x-ray diffraction study of a solution treated and ice brine quenched Al–14.25 at.% Li alloy

Abstract

An x-ray diffractogram was generated from a powder sample of solution treated and ice brine quenched Al–14.25 at.% Li alloy. The Bragg reflections obtained were characteristic of a very-nearly fully ordered Al-rich L12 phase based on δ'Al3Li together with two very weak reflections associated with δAlLi. All the lines were significantly broadened due to particle size effects. The average particle size associated with the 100 L12 superlattice line was found to be 4.2 (3) nm and with the fundamental lines, 26.8 nm. A simple structure factor calculation indicated the volume fraction of ordered phase to be around 0.77, assuming that the lack of maximum order was due to the presence of disordered fcc AlLi solid solution. These results suggest that the microstructure of this as-quenched alloy comprises ordered regions of about 4 nm in size in a sea of disordered matrix with a very small amount of δAlLi present. This conclusion is in excellent agreement with recent small angle x-ray and transmission electron microscope studies on similar alloys and suggests that AlLi alloys which are ostensibly disordered at high temperatures go through a disorder-order transformation and then decompose into regions of order and disorder which are associated with a composition spinodal.