An examination of delta prime growth in an aluminum-lithium alloy by X-ray diffraction
Whitman, Clark E.
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A previously manufactured hot rolled sheet of A1-LiZr alloy of composition A1-2.5%Li-15%Zr, by weight, was solution treated and artificially aged at 194 C from C to 32 hours. X-ray diffraction analysis of the alloy was conducted to observe the growth characteristics of the delta prime precipitate (the precipitate hardening phase). Transmission electron microscopy was used to verify X-ray results. Significant superlattice intensity and line broadening occurred in the as-quenched sample. This observation supports a possible order/disorder reaction and a spinodal decomposition as opposed to the typical nucleation and precipitation reaction usually observed in a precipitation hardened alloy. The Scherrer equation was used to determine delta prime particle size from diffraction line broadening, and this size was found to coarsen following conventional Ostwald ripening theory once the initial effects of the spinodal had aged out. The delta phase (considered cause of low toughness) was observed in the as quenched sample and throughout the heat treatment. It was surmised that this may be the origin of he low-short transverse fracture toughness which is typical of some of these alloys.