The Morphology of Tensile Failure in Tantalum
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IN 1959, before the introduction of scanning electron microscopy, Puttick authored ‘‘Ductile Failure in Metals,’’ a first detailed look at the process of ductile tearing in copper, iron, and aluminum, using cross-sectional optical metallography.[1] The summary paragraph from this classic text reminds the current reader how far our understanding has come in just over five decades: To sum up: there are two stages in the formation of a neck in a tensile test. During the first stage the material strain hardens; any holes present are enlarged by the hydrostatic tensions developed in the interior of the specimen and may lead to tensile fracture. When the strain hardening is exhausted localized slip begins on planes of maximum shear stress, and holes may grow into shear fractures on these planes. If fracture does not supervene during either stage, complete slipping-off finally ensues
The proliferation of scanning electron microscopes in the late 1960s permitted first investigations on the morphology of the fracture surface associated with ductile tearing, such as provided by Bauer and Wilsdorf in 1973.[2] These studies revealed the common ductile dimple morphology that is associated with void coalescence. Figure 1 shows classic images of the ductile failure process at different length scales and with BRAD L. BOYCE, PING LU, JAY D. CARROLL, and CHRISTOPHER R. WEINBERGER, Research Staff, are with Materials Science and Engineering Center, Sandia National Laboratories, P.O. Box 5800, MS0889, Albuquerque, NM 87185-0889. Contact e-mail: [email protected] BLYTHE G. CLARK, Research Staff, is with Physical, Chemical, and Nano Sciences Center, Sandia National Laboratories, P.O. Box 5800, MS 1423, Albuquerque, NM 87185. Manuscript submitted June 27, 2012. Article published online June 5, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
different imaging techniques. While most of the historic studies associated cavity formation with decohesion at particles or inclusions,[3–5] similar voids were also found in pure metals.[2,6] Two early reviews of ductile failure processes provide additional details on early observations: a 1968 review by Rosenfield[7] and a 1979 review by Goods and Brown[6]. Shortly thereafter, Wilsdorf provided a seminal review which focused on microstructural aspects of ductile failure.[8] These observations have led to extensive development of models for void growth and coalescence, with classic contributions by McClintock[9], Rice and Tracy[10], Gurson[11], and Needleman and Tveergard[12]. The development of void nucleation models are considerably more limited, with nearly all of the models focusing on nucleation at a second-phase particle interface. Yet, a detailed mechanistic description of the void nucleation process in single-phase ductile metals remains elusive. For a much more detailed description of models for void nucleation and growth, the reader is referred to recent review chapters.[13,14] Still to this day, the mechanical descriptions of void nucleation, growth, and coalescence are based largely on
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