Fatigue damage accumulation in Ni₃Al and NiAl

The accumulation of fatigue damage during strain-controlled cycling (low-cycle fatigue) of single crystals of NiAl and very coarse-grained polycrystals of Ni₃Al + W in air at room temperature is described. Surface topography was studied for all test materials by scanning electron microscopy and, in the case of Ni₃Al + W polycrystals, also by profilometry. The development of persistent slip bands and an extrusion-intrusion morphology were recorded, as were transmission electron microscopy observations of dislocation substructures. Point-defect clusters, dipoles and other evidence of damage due to cycling were noted in both intermetallics. The density of dipoles was non-uniform and was modulated on the slip plane in NiAl, unlike Ni₃Al + W, which displayed a homogeneous distribution. Computer simulation of the microstructure in NiAl has been carried out, based upon the mechanism of dipole diffusion in a stress gradient.