Atomic force microscopy investigations of loaded crack tips in NiAl

Scanning probe microscopy has great advantages over other high-resolution techniques like transmission electron microscopy in that no extensive specimen preparation, such as thinning, is required. This is important for examinations of crack tips, since thin film effects obscure the deformation behavior of cracks. For the first time, the concurrent processes of crack tip blunting by dislocation emission and unstable crack propagation were studied systematically with the atomic force microscope. Brittle cracks were initiated in NiAl single crystals. NiAl serves as a model alloy for the mechanical behavior of intermetallic compounds. A small bending device was constructed in which the specimens were loaded stepwise to measure the displacement fields of the crack tip in situ at different load levels. With this loading device, cracks were propagated in small steps of a few micrometers. From the atomic force microscope images, dislocation distributions were obtained as functions of the applied load. Nearly radial symmetric elastic deformation fields were observed at the crack tip with a maximum depth of 46 nm. In addition, material parameters such as the fracture toughness K-IC were calculated from the crack tip opening at the onset of brittle crack growth. The measured value of 1.5 MPa root m compares favorably with results from standard fracture tests. (C) 1996 American Vacuum Society.