MECHANICAL-PROPERTIES OF INDIVIDUAL GRAIN-BOUNDARIES IN NI3AL USING A MINIATURIZED DISK-BEND TEST

The mechanical behavior of individual grain boundaries in boron-free Ni3Al was investigated using miniaturized disk-bend tests performed on specimens 3 mm in diameter and nominally 200 mum thick. A directionally-solidified ingot containing 24 at.% Al was annealed between 1300 and 1350-degrees-C to produce an average grain size well over 1 mm, and fifteen disk specimens were extracted from it such that one long grain boundary extended across the diameter of each disk. Electron channeling patterns were used to determine the relative orientation of the grains on either side of the boundary. Low-angle boundaries are so strong they do not fracture, whereas high-angle boundaries always fracture, as indicated by a load drop in the curve of load vs displacement. Considerable plastic deformation of the individual grains occurs in all tests regardless of grain boundary character. Slip traces were observed on many of the grain-boundary fracture surfaces, providing some evidence for slip transmission across high-angle boundaries. The relative fracture strengths of the high-angle boundaries were estimated by extrapolating the elastic portion of the load-displacement curves to the displacement at fracture, yielding values ranging from about 2 to 4 GPa, with an average of 3.06 +/- 0.71 GPa. These are roughly an order of magnitude smaller than the fracture strengths of special boundaries predicted by computer simulations. No correlation was found between the fracture stresses and the relative orientations of the high-angle boundaries, as defined by the coincidence site lattice model.