Structural stability of the Laves phase Cr2Ta in a two-phase Cr-Cr2Ta alloy

Detailed microstructural analysis of a two-phase alloy of composition Cr-9.8 at.% Ta and lying in the Cr-Cr2Ta region of the Cr-Ta binary system confirmed that the existing phase diagram is inaccurate; in the cast and annealed condition (1273 K/24 h), blocky primary Cr2Ta precipitates were observed although the phase diagram indicates the eutectic composition to be similar to 13 at.% Ta. The eutectic structure is composed of Cr solid solution and the Laves phase Cr2Ta; the morphology is primarily lamellar although the rod morphology was occasionally observed. The Laves phase eutectic microconstituent exhibits the C14 (2H) hexagonal structure with a low stacking fault (basal faults) density and an average composition corresponding to 28.5 at.% Ta. After a prolonged high-temperature anneal (1573 K/168 h), the morphology breaks down to form discrete particles of Cr2Ta; the C14, C36 and C15 structures were all recognized in this annealed condition, often more than one form being present in a single precipitate. The C15 structure was not twinned but contained some stacking faults on the {111} planes. Composition measurements confirmed that these structural transformations were accompanied by composition changes, the precipitates becoming more Ta-rich as they transitioned from the C14 via the C36 to the C15 phase. These observations are coupled with the results from earlier studies to present a discussion on factors that influence the stability and C14/C36/C15 transformation kinetics. (C) 2000 Acta Metallurgica mr. Published by Elsevier Science Ltd. All rights reserved.