Micro-, Meso-, and Macrotecture and Fatigue Crack Roughness in Al-Li 2090 T8E4l

摘要

The use of synchrotron polychromatic x-ray microbeams in the transmission geometry is described for mapping grain orientation as a function of position and for relating this microexture to the formation of large asperities on fatigue crack surfaces in Al-Li 2090 T8E41. In common with the centers of rolled plates of many aluminum alloys, Al-Li 2090 T8E41 has a sharp average texture or macrotexture different from that in the outer portions of the plate. The geometry of large asperities in Al-Li 2090 has been related to this macrotexture, and the resulting roughenss-induced creack closure is recognized to be responsible for the very low crack propagation rates in certain plate orientations. This report focuses on why asperities form at certai npositions and why the crack remains relatively planar elsewhere. The microtexture (i.e., the grain-to-grain orientation variation0 seems to be organized into a specific type of mesotexture: multiple adjacent gains have nearly identical orientaions and form substantial volumes of near-single-crystal material. Transitions between differently oriented near-single-crystal volumes or between a near-single- crystal region and more randomly oriented grains appear to bound asperities.