Fatigue-Induced Nanoscale Patterns and Microstructures

Abstract

Dislocation patterns as produced by monotonic loading of polycrystalline aluminum or fatigue loading of a low-alloy ferrltic steel have been evaluated. A proposed theoretical model Involves an accounting procedure based upon dislocations emanated at grain boundary sources being stored in dislocation patterns as a series of subcells. Near equilibrium, the scale parameters are roughly 100 µm grains being divided into 1µ subcells which contain dislocations spaced at about 10 nm Intervals within the cell walls. Pattern generation and dislocation spacings are dictated by the quasi-static state of equilibrium during the workhardening phase of the loading. Experimental nanometer-scale probes, used in the fatigue study, consisted of three forms of electron microscopy. The first involved direct imaging of intersecting slip at the free surface of electropolished samples, using channeling contrast from back-scattered electrons (BSE). The second involved the use of selected-area electron channeling patterns (SACP’s) to evaluate crystallography and defect densities, while the third directly evaluated cell sizes and dislocation spacings with transmission electron microscopy of thin films (TEM). BSE, SACP and TEM probes determined dislocations to be packed into cell walls with a wave length of 0.78 µm and an interwall spacing of 33nm, consistent with the theoretical model. BSE channeling contrast revealed elliptical patterns to develop within single grains and SACP’s verified that these were sharp valleys with a crystallographic misorientation of 17 degrees across the valley. One of those was shown to lead to fatigue crack nucleation in a transgranular mode.