Abstract
The results of in situ straining of many thin micro-tensile specimens in a high voltage electron microscope provide both an overview and microstructural details of propagating cracks in 304 stainless steel. Sequences of highly magnified views of the crack tip show it to be a highly plastic region. Voids form in advance of the crack tip in most instances and these voids form crack precursors which link up with the crack tip to advance it. Sequences of electron micrographs of crack advance at magnifications ranging from 60x to 30 000x illustrate the process as one of crack growth, void (hole) growth and coalescence, crack growth etc. Plastic deformation associated with crack advance and fracture is highly localized within individual grains or grain portions (the grain size of the material was 15 μm). The crack-tip region and the edges of cracks are observed to have been partly or mostly transformed to α'-martensite (b.c.c.). Consequently, a propagating crack in 304 stainless steel creates a localized magnetic region as it propagates.
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