Deformation of a HF-V-NB C15 Laves Phase
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DEFORMATION OF A HF-V-NB C15 LAVES PHASE Fuming Chu and David P. Pope Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. ABSTRACT We have performed compression tests on Hfl 4 V64Nb 22 at both room temperature and elevated temperatures with different strain rates to study the deformation modes of the ternary C15 Laves phase based on HfV2 . At room temperature the alloy exhibits fair ductility and is insensitive to strain rate. TEM observation revealed that the major deformation modes at room temperature are {111} mechanical twinning. Compression tests up to 1000°C, combined with SEM and TEM examinations, revealed that the mechanical properties can be divided into three temperature regions: at low temperatures the alloy is fairly ductile due to the formation of deformation twins, in the intermediate temperature region the alloy exhibits a ductility minimum, and at high temperatures (>0. 6 5Tm) the alloy shows large plastic deformations via 1/2 dislocation slip. A new deformation twinning mechanism is proposed based on synchroshear mechanism. INTRODUCTION Due to growing interest in intermetallic compounds as potential high temperature structural materials, there have been many investigations carried out on various compounds, but mostly on structures that are ordered forms of fcc, bcc or hcp; however, most intermetallics have more complex structures and Laves phases constitute the single largest group. Laves phases have either the cubic C15(MgCu 2), hexagonal C14(MgZn 2) or mixed C36(MgNi 2) structure. Generally speaking, they have high melting temperatures and fairly low specific gravities (1), and therefore could be attractive candidates for high temperature structural applications if the deformability could be increased at. low temperatures. Inoue et al (2-3) have shown that C15 Hf-V-Nb alloy can be deformed by large amounts at room temperature and subsequent work by Livingston and Hall (4) and Chu and Pope (5) has shown that twinning is the main deformation mode. We report here the compression test results obtained at different temperatures and strain rates, TEM and SEM observations of deformation modes for different temperature regions and a new mechanism for C15 mechanical twinning based on the synchroshear mechanism. EXPERIMENTAL PROCEDURES Based on the Hf-V-Nb phase diagram (6), a C15 alloy of composition HflaV64Nb 22 was chosen for this study. The arc-melted buttons were made using elemental Hf, V, and Nb with nominal purities 99.99%, 99.6% and 99.7%, respectively. The buttons were turned over and remelted five times to ensure Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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homogeneity. The buttons were homogenized at 1200'C for 48 hours and then argon quenched. Compression specimens of dimension 3x3x5 mm were cut using EDM and mechanically polished. Before mechanical testing and SEM observation polished sections of these samples were examined in an optical microscope after etching with a reagent consisting 30 ml HNO 3, 30 ml
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