Study of crack tip deformation in FeAl and NiAl crystals with optical interference microscopy and atomic force microscopy

Abstract The elastic and plastic deformation at crack tips in stoichiometric single crystals of FeAl and NiAl were investigated by atomic force microscopy and optical interference microscopy. Crack propagation and plastic deformation were observed during in-situ loading in an atomic force microscope. Soft oriented NiAl crystals with and without 0.14 wt% Fe doping were investigated, and the plastic deformation in front of the crack tip is analysed by slip steps on the surface. Doping with Fe leads to a more ductile behaviour. Optical interference microscopy is an excellent technique to measure the elastic deformation around loaded crack tips on a large lateral scale. The elastic constants cij of both materials were determined with ultrasonic techniques. Because of the strong anisotropy of these intermetallic compounds the elastic out-of-plane displacement field is asymmetric. The elastic displacements measured at crack tips in FeAl and NiAl bend specimens are significantly different, which is interpreted as arising from the differences in the elastic constants. To determine the stress intensity factor from the elastic displacements around crack tips the stress state near the intersection of a crack tip and a free surface is analysed. Analytical calculations of the stress field around cracks for isotropic solids by Benthem are compared with finite-element simulations of the stress field on the surface of four-point bend specimens.