Transient Deformation of Single Crystal NiAl at High Temperatures
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TRANSIENT DEFORMATION OF SINGLE CRYSTAL NiAI AT HIGH TEMPERATURES Keith R. Forbes and William D Nix Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 Abstract The deformation transients associated with changes in stress and strain rate have been studied as a means of determining the controlling deformation mechanisms in NiAl. Stress change experiments in tension, and strain rate change tests in compression have been performed on single crystals of 0 NiAl at temperatures between 850 and 1200 C. The orientation dependence of these transients was and a soft [223] orientation. Strain rate change [001] orientation in the hard studied by testing experiments suggest increased contribution from structure-controlled mechanisms in hard oriented crystals. Stress change experiments in samples tested along the hard orientation produce transients that are characteristic of the evolution of a stable dislocation substructure. In soft oriented crystals, however, stress change transients suggest that deformation is not significantly affected by the formation of a dislocation substructure. These results are consistent with observations of dislocation substructure formation and strain hardening in hard oriented crystals. Introduction The high temperature strength of single crystal NiAl has been found to be strongly orientation 1 dependent , 2, 3. Soft orientations deform by the glide of b= dislocations. In crystals deformed along the hard [001] orientation, easy b= glide is prohibited and deformation occurs by b= glide4 . The core structure of b= dislocations has been observed to be more compact than that of b= dislocations; thus b= should have a higher mobility in NiA15 . The low mobility of b=< 110> dislocations can account, at least in part, for the increased strength of hard oriented crystals. The creep curves of soft oriented crystals are characterized by immediate steady state creep with no significant primary creep transient. Strain hardening is minimal in both tension creep and constant strain rate compression. However, creep of hard oriented crystals is characterized by significant strain hardening. Dislocation networks, forming subgrains and tangles, are observed in NiAl single crystals deformed along the hard orientation but are not common in soft oriented crystals 6 . This suggests that dislocation substructure additionally strengthens hard oriented crystals. Deformation in soft oriented crystals is limited by the mobility b= dislocations. However, deformation in the hard orientation is controlled by the extensive dislocation substructure that is formed. This investigation focuses on the difference in the controlling deformation mechanism in a soft [223] orientation and the hard [001] orientation on the basis of insight provided from deformation transients. The preparation of the tension and compresion samples used in these tests is explained elsewhere 4 . In compression tests, samples were deformed between two flat alumina platens, onto which extensometery was attached in order to me
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