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 thermal treatments. Fracture toughness behavior was found to be sensitive to thermal exposures at moderate to high temperatures as well as subsequent cooling rates. The nature of the crack propagation was also found to be sensitive to the loading rate. In order to follow up on a previous suggestion by the present authors that the sensitivitity to cooling rate was a result of solute-dislocation interactions, compression tests were conducted over the range 20-600 degrees C at several strain rates on commercially pure and high purity materials. Evidence of dynamic strain aging observed at elevated temperature in commercially pure material included serrated yielding, anomalous increases in yield and now stresses and negative rate dependence. The activation energy for the onset of serrated yielding was found to fit well with those typically observed for the diffusion of interstitial elements in b.c.c. transition metals. Preliminary tests on high purity material show no serrated yielding under similar conditions. Taken together, the current and previous results imply that the brittle behavior typically observed in commercially pure single crystals of NiAl may be a result of static strain age embrittlement.