FATIGUE DAMAGE ACCUMULATION IN NI3AL AND NIAL

The accumulation of fatigue damage during strain-controlled cycling (l ow-cycle fatigue) of single crystals of NiAl and very coarse-grained p olycrystals of Ni3Al+W in air at room temperature is described. Surfac e topography was studied for all test materials by scanning electron m icroscopy and, in the case of Ni3Al+W polycrystals, also by profilomet ry. The development of persistent slip bands and an extrusion-intrusio n morphology were recorded, as were transmission electron microscopy o bservations of dislocation substructures. Point-defect clusters, dipol es and other evidence of damage due to cycling were noted in both inte rmetallics. The density of dipoles was non-uniform and was modulated o n the slip plane in NiAl, unlike Ni3Al+W, which displayed a homogeneou s distribution. Computer simulation of the microstructure in NiAl has been carried out, based upon the mechanism of dipole diffusion in a st ress gradient.