DEFORMATION TWINNING AT LOW-TEMPERATURES IN A HF-V-NB CUBIC LAVES PHASE

Deformation twinning in the C15 cubic laves phase of the Hf-V-Nb alloy system is analyzed at temperatures between 77 K and room temperature by conventional and high-resolution TEM. In order to reduce the stacki ng fault energy (SFE), alloy compositions are chosen such that the cub ic laves phase composition is close to a region of C14 phase stability . A high density of stacking defects of intrinsic character in the as- homogenized alloy confirms that the SFE has been reduced. Twinning is observed at all temperatures placing a low upper limit on any thermall y activated deformation process. Twin clusters are observed which are narrow and have a similar distribution to the stacking defects in the undeformed alloy. The structure of twins and stacking defects is solve d via HREM combined with image calculations. Finally, the mechanism of twinning is discussed and the use of alloying to control the SFE or t o refine the microstructure in order to increase the low temperature d uctility in these intermetallic compounds with complex cubic crystal s tructures is proposed. (C) 1998 Acta Metallurgica Inc.