HIGH-TEMPERATURE DEFORMATION PROPERTIES OF NIAL SINGLE-CRYSTALS

The high-temperature deformation properties of stoichiometric NiAl sin gle crystals have been studied in the temperature range from 850 degre es C and 1200 degrees C. We have established a basic data set for and have explored the high-temperature deformation characteristics of this intermetallic compound. The results provide a basis for determining t he controlling mechanisms of high-temperature deformation. Constant st ress tension creep and constant stress or constant strain rate compres sion experiments were conducted on crystals oriented with loading axes along the ''hard,'' [001] orientation, where no driving force exists for glide of b = [001] dislocations,and along various ''soft'' orienta tions, [223], [111], and [110], where deformation can occur by the gli de of these dislocations. In addition to these monotonic tests, high-t emperature deformation transients were studied using stress relaxation , strain rate change, and stress change experiments. These transient d eformation experiments were conducted in an effort to further elucidat e the mechanisms that control high-temperature deformation of this mat erial. The steady-state deformation properties of these differently or iented single crystals can be characterized by creep activation energi es that all coincide, within experimental error, with the activation e nergy for diffusion of Ni in NiAl, 308 +/- 10 kJ/mol. The stress depen dence of steady-state deformation can be characterized with stress exp onents that range from about 9 at 850 degrees C to about 4 at 1200 deg rees C. At all temperatures and stresses, the soft oriented crystals c reep about two orders of magnitude faster than the hard oriented cryst als at the same stresses and temperatures. Soft oriented crystals load ed along [223] and [111] axes tested in both tension creep and constan t stress or constant strain rate compression are found to deform at th e steady-state rate from the very beginning of the deformation experim ent. Crystals with these orientations exhibit virtually no evidence of strain hardening. Transients associated with stress changes suggest t hat deformation is limited primarily by the mobility of dislocations a nd not by dislocation interactions. These characteristics of deformati on are consistent with the operation of easy b = [001] glide processes in these crystals. Crystals loaded along [110] exhibit small deformat ion transients which indicate both sluggish dislocation motion and som e substructure formation. We speculate that cross-slip of dislocations from {110} to {010} planes is responsible for this effect. Deformatio n in hard oriented crystals provides evidence for both mobility and su bstructure controlled deformation. Creep in hard oriented crystals is characterized by a dramatic sigmoidal transient suggesting very low di slocation mobility. However, the strain hardening observed in monotoni c tests and the transient responses suggest that deformation is also l imited by a dislocation substructure that forms during deformation. Th ese findings support the conclusion, explored fully in a forthcoming a rticle, that creep deformation in the hard orientation is controlled b y the motion and interaction of b = [101] dislocations.