Fracture initiation/propagation parameters for duplex TiAl grain boundaries based on twinning, slip, crystal orientation, and boundary misorientation

Abstract

The role of mechanical twinning in microcrack nucleation and crack propagation in a near-γ TiAl alloy was characterized using 4-point bend tests and analyzed with advanced electron microscopy techniques. Smooth bend specimens were used to examine microcrack nucleation prior to fracture, and notched bend specimens were used to examine crack propagation. A combination of selected area channeling patterns and electron backscattered diffraction pattern maps were used to identify grain orientations in regions of microstructure surrounding microcracks or growing cracks. Microcracks were observed, where twins on highly stressed twinning systems intersected certain grain boundaries. An anisotropic elastic analysis of a tri-crystal showed that local stresses near boundaries can vary by as much as 45%, which may enhance microcracking in the presence of deformation defects. To determine why some twin boundary intersections resulted in cracking, but not others, a fracture initiation parameter was developed to identify how efficiently slip can be transferred across the boundary. Cracks were found to be more prevalent when this parameter is large, which occurs when the twin Burgers vector is closely aligned with the direction of maximum tensile stress and when the twin Burgers vector is somewhat aligned with other slip systems that could accommodate the twin strain in the neighboring grain. The orientation of the grain boundary plane has little effect. On the basis of a fracture initiation parameter, it is possible to predict the path of cracks knowing only the primary tensile stress axis and spatially resolved grain orientations.