Dislocation-grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission
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A.M. Minor and E.A. Stach National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720
J.W. Morris, Jr. Department of Materials Science and Engineering, University of California, Berkeley, California; and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (Received 1 July 2004; accepted 15 September 2004)
Dislocation–interface interactions in Fe–0.4 wt% C tempered martensitic steel were studied through in situ nanoindentation in a transmission electron microscope (TEM). Two types of boundaries were imaged in the dislocated martensitic structure: a low-angle (probable) lath boundary and a coherent, high-angle (probable) block boundary. In the case of a low-angle grain boundary, the dislocations induced by the indenter piled up against the boundary. As the indenter penetrated further, a critical stress appeared to have been reached, and a high density of dislocations was suddenly emitted on the far side of the grain boundary into the adjacent grain. In the case of the high-angle grain boundary, the numerous dislocations that were produced by the indentation were simply absorbed into the boundary, with no indication of pileup or the transmission of strain. This surprising observation is interpreted on the basis of the crystallography of the block boundary.
I. INTRODUCTION
Understanding the interaction of dislocations with grain boundaries is of fundamental importance to engineering the strength of polycrystalline materials. The flow stress of a material increases as the density of grain boundaries increases, usually in accordance with the well known Hall–Petch relation. A variety of models have been proposed to interpret this relation,1–5 and a number of experimental investigations have studied dislocation behavior in the vicinity of grain boundaries.6–10 However, most of these studies were performed on samples with extremely low dislocation densities so that the individual dislocations could be imaged. For instance, in situ straining transmission electron microscope (TEM) studies of dislocation activity at grain boundaries in steel have observed individual dislocations to pileup or pass through boundaries.9,10 These studies show interactions between pre-existing lattice dislocations and grain
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0474 3626
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 12, Dec 2004 Downloaded: 13 Mar 2015
boundaries and hence relate only to the initial stages of plastic deformation. The flow stress after yielding is strongly affected by dislocation interactions under conditions of high dislocation density, and deformation behavior in this regime must also be understood. Fe–C-based martensitic steels are among the most important high-strength materials. Their structures are often “dislocated lath martensite,” which has a high dislocation density in the as-quenched condition. The microstructure has a characteristic form that include
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