Phase transformations in the Al-Li-Zr system

Phase transformations in an Al-2.3Li-1.1Zr (wt%) alloy have been studied using electron microscopy techniques. The L12-ordered phase, or α', which is a stable precipitate at temperatures below the solidus, is described as Al3(Zr,Li) with varying Zr/Li ratios depending upon precipitation mechanism. Discontinuously precipitated filaments of α' have a Zr/Li atom ratio of about unity, whereas normal nucleation-and-growth α' has Zr/Li ratios of approximately 4:1. This compositional analysis was accomplished using transmission electron microscopy (TEM) image calculation techniques. The α' is typically perfectly coherent with the aluminum matrix and serves as a preferred nucleation site for Al3Li, or δ', precipitation when the alloy is aged at 1900C. The δ', also L12-ordered, nucleates in the variation which results in the minor sublattice being continuous across the α'/δ' interface. The fine α' distribution is extremely stable for extended periods at 4500C although it appears that a transformation to an equilibrium tetragonal phase is initiated during extended heat treatment