Spinodal decomposition of Al-rich Al-Li alloys

Abstract

The microstructure of a series of as-quenched and aged Al-Li base alloys has been studied by high resolution transmission electron microscopy (HREM) and by X-ray diffraction. In alloys which had been ice brine quenched and room temperature aged for 168 h the HREM studies showed positive evidence for spinodally decomposed Al-rich δ′ Al 3Li (Ll 2 structure) with modulated order/disorder regions of varying lithium content. The 100 and 110 superlattice reflections associated with the ordered regions were detected by X-ray diffraction, but were weak and highly broadened due to the order modulation associated with the spinodal. Appropriate X-ray intensity measurements allowed a determination to be made of the overall degree of long-range order, which was found to be about half the maximum possible for each alloy had they been fully ordered. Further HREM studies on under, and overaged, samples indicated that the ordered δ′Al 3Li precipitates which develop in a disordered matrix in Al-Li alloys arise from lithium enrichment of the ordered regions and further lithium depletion of the disordered regions in the as-quenched spinodally decomposed alloys, rather than by nucleation from a supersaturated solid solution. These results are in agreement with the latest theoretical ideas on this topic, and suggest that Al-Li base alloys are intermetallic compounds rather than primary solid solutions. This view is, of course, consistent with the fact that the elastic moduli of Al-Li base alloys are significantly higher than that of pure aluminum.