Cleavage Phenomena and Topographic Features

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Abstract
Cleavage fractures have been widely studied in body-centered-cubic and certain hexagonal-close-packed metals and alloys as well as ionic crystals. It is shown that the theoretical treatments due to Stroh, Cottrell, and Petch have successfully accounted for the effects of yield strength, grain size, and mode of deformation on crack nucleation and propagation. The results of these studies are summarized, and their application to the prediction of the ductile-to-brittle transition temperature is described. In addition, theoretical studies which have led to the prediction of cleavage planes and to an explanation of the influence of dispersed particles are reviewed. Fractographic studies have established topographic features characteristic of cleavage fracture; namely, steps and river patterns, cleavage tongues, and herringbone patterns. Examples of these features are presented, and factors associated with their formation are described. It is shown that steps and river patterns are formed by secondary cleavage, slip, or plastic fracture and are influenced by crystallographic imperfections and microstructural discontinuities. Tongues and herringbone patterns are shown to be associated with deformation twins formed ahead of the advancing crack. In addition, a discussion of “cleavage-like” features observed in quenched and tempered steels, face-centered-cubic materials, and in certain environmentally induced fractures is presented.