FRACTURE AND DEFORMATION OF NIAL SINGLE-CRYSTALS

The fracture behavior of commercially pure, stoichiometric NiAl single crystals is reviewed with particular attention to the influence of th ermal treatments. Fracture toughness behavior was found to be sensitiv e to thermal exposures at moderate to high temperatures as well as sub sequent cooling rates. The nature of the crack propagation was also fo und to be sensitive to the loading rate. In order to follow up on a pr evious suggestion by the present authors that the sensitivitity to coo ling rate was a result of solute-dislocation interactions, compression tests were conducted over the range 20-600 degrees C at several strai n rates on commercially pure and high purity materials. Evidence of dy namic strain aging observed at elevated temperature in commercially pu re material included serrated yielding, anomalous increases in yield a nd now stresses and negative rate dependence. The activation energy fo r the onset of serrated yielding was found to fit well with those typi cally observed for the diffusion of interstitial elements in b.c.c. tr ansition metals. Preliminary tests on high purity material show no ser rated yielding under similar conditions. Taken together, the current a nd previous results imply that the brittle behavior typically observed in commercially pure single crystals of NiAl may be a result of stati c strain age embrittlement.