Micromechanisms of fracture in NiAl studied by in situ straining experiments in an HVEM

The room temperature brittleness of NiAl constitutes a major problem for te chnical applications. In order to investigate the micromechanisms of fractu re in NiAl, we have carried out in situ tensile straining experiments on st oichiometric NiAl single crystals in a high-voltage electron microscope. Ac cording to our observations, crack propagation always involves dislocation activity around the crack tip, even in the hard orientation at room tempera ture. The Burgers vectors and the typical arrangements of the dislocations, as well as the extension of the corresponding plastic zone vary with the l oading direction and the orientation of the microcrack versus potential gli de systems. We observe that local concentrations of slip leads to irregular deviation of the cleavage plane from the {1 1 0} facets one usually observ es at the macroscopic level. The results of our experiments help to underst and why the mode I fracture toughness of NiAl is significantly larger for [ 1 0 0] loading directions than for non-[1 0 0] directions. (C) 1999 Elsevie r Science Ltd. All rights reserved.