Rapid solidification processing of magnesium-lithium alloys
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J. MESCHTER and J. E. O'NEAL are Scientists, McDonnell Douglas Research Laboratories, St. Louis, MO 63166. Manuscript submitted April 22, 1983. METALLURGICAL TRANSACTIONS A
to microstructural examination. Analyzed compositions of the three alloys (wt pct) were Mg-8.66Li, Mg-8.35Li1.08Si, and Mg-8.50Li-0.55Ce, respectively. Most of the Li loss was by vaporization from the melt, and cerium may have been lost to the melt by reaction with the crucible. Optical and scanning electron micrographs of typical roller quenched flakes are shown in Figures l(b) through 1(d). Alpha plate thicknesses and plate spacings obtained by quantitative metallography are 1.8 ---0.6 and 4.4 ---2.1 /xm for Mg-8.66Li, 0.70 +-0.33 and 1.2 +-0.5 /zm for Mg8.35Li-1.08Si, and 3.6 -+1.4 and 3.4 -+1.4/zm for Mg8.5Li-0.55Ce flakes, respectively. Roller quenching thus produces microstructural refinement by a factor of 10 to 30 compared with conventional processing. Dispersoids in as-quenched Mg-8.35Li-l.08Si and Mg8.5Li-0.55Ce flakes are shown in Figures 2(a) and 2(b). Dispersoids in both systems are introduced during rapid solidification and are typically 50 nm diameter in Mg-Li-Si flakes and 20 nm diameter in Mg-Li-Ce flakes. The dispersoids decorate subgrain boundaries within each phase, but appear to be evenly distributed between the two phases. The dispersoids.in the Mg-Li-Si alloy have been identified by X-ray diffraction as the equilibrium Mg2Si phase, while no diffraction lines attributable to dispersoids in the Mg-LiCe flakes were observed. Lattice parameters of the a and/3 phases in all alloys were essentially identical, suggesting that Li was not diverted to the dispersoids, nor were significant amounts of Si or Ce incorporated into the matrix. Samples of the flakes were encapsulated in evacuated quartz capsules and annealed for two hours at 100 to 400 ~ to simulate thermal treatments involved in particulate preconsolidation and extrusion or forging. Typical microstructures of annealed flakes are shown in Figure 3, and dispersoid distributions in annealed flakes are shown in Figures 2(c) and 2(d). No coarsening of the a + /3 structure occurred at annealing temperatures up to 200 ~ At 300 ~ the Mg-Li flakes exhibited slight coarsening, Mg-Li-Si flakes were unaffected, and Mg-Li-Ce flakes showed extensive coarsening (compare Figure 3 with Figure 1). At 400 ~ all three alloys had coarsened to a nearly equiaxed grain structure. The Mg2Si dispersoids in Mg-8.35Li-1.08Si flakes spheroidize, but coarsen only slightly to - 7 0 nm diameter, after annealing at 300 ~ for two hours (Figure 2(c)). A similar slight coarsening occurs in Mg-8.5Li-0.55Ce at 200 ~ (Figure 2(d)), while annealing at 300 ~ for two hours results in partial dissolution of the dispersoids. Average dispersoid diameters for these alloys determined by quantitative metallography are 70 +- 30 nm for Mg-8.35Li1.08Si after annealing 300 ~ hours, and 50 --- 20 nm for Mg-8.5Li-0.55Ce after annealing 200 ~ hours. Based on grain size and dispersoid coarsening observations, consolidation of
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