DETERMINING FRACTURE FACET CRYSTALLOGRAPHY USING ELECTRON BACKSCATTERPATTERNS AND QUANTITATIVE TILT FRACTOGRAPHY

A methodology is presented to characterize the crystallography of indi vidual fracture surface facets. Electron backscatter patterns (EBSP's) from a metallographic section through a facet identify grain orientat ion, and quantitative tilt fractography identifies facet orientation; these results are combined to establish fracture facet crystallography . For this technique, facet electropolishing is not required, the face t alignment procedure is accurate and quick, and the method can be gen eralized to different microstructures, test environments, or facet ori entations. Method accuracy is illustrated for 25 to 50 mum fatigue cra ck facets in an unrecrystallized Al-Li-Cu alloy (AA2090) that has 5 mu m thick subgrains in elongated grains that are 10 to 200 mum thick. Th e fine subgrain structure and tortuous fatigue crack profile precludes the use of other diffraction techniques for determining AA2090 facet crystallography. EBSP and tilt fractography results demonstrate that v acuum fatigue cracks in AA2090 are nearly parallel to local {111} plan es.