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

The mechanical behavior of individual grain boundaries in boron-free N i3Al was investigated using miniaturized disk-bend tests performed on specimens 3 mm in diameter and nominally 200 mum thick. A directionall y-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 f ifteen disk specimens were extracted from it such that one long grain boundary extended across the diameter of each disk. Electron channelin g patterns were used to determine the relative orientation of the grai ns on either side of the boundary. Low-angle boundaries are so strong they do not fracture, whereas high-angle boundaries always fracture, a s indicated by a load drop in the curve of load vs displacement. Consi derable plastic deformation of the individual grains occurs in all tes ts regardless of grain boundary character. Slip traces were observed o n many of the grain-boundary fracture surfaces, providing some evidenc e for slip transmission across high-angle boundaries. The relative fra cture strengths of the high-angle boundaries were estimated by extrapo lating the elastic portion of the load-displacement curves to the disp lacement at fracture, yielding values ranging from about 2 to 4 GPa, w ith an average of 3.06 +/- 0.71 GPa. These are roughly an order of mag nitude smaller than the fracture strengths of special boundaries predi cted by computer simulations. No correlation was found between the fra cture stresses and the relative orientations of the high-angle boundar ies, as defined by the coincidence site lattice model.