Role of Localized Deformation in Irradiation-Assisted Stress Corrosion Cracking Initiation
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INTRODUCTION
AUSTENITIC steels are commonly used for structural components of light water reactors (LWRs), and are among the candidate materials for Generation IV reactors. Under normal conditions, these steels exhibit a high corrosion resistance; however, an increase in cracking susceptibility has occurred after exposure to irradiation.[1] In the high-temperature water environments of LWRs, austenitic steels suffer from irradiationassisted stress corrosion cracking (IASCC). This is a growing concern as structural materials are experiencing higher doses of irradiation in LWRs operating under extended licenses. As Generation IV reactors are developed, it is essential that the materials selected for their structural components are able to withstand the harsh environmental conditions and higher irradiation doses expected in those reactors. Despite decades of research on IASCC, a clear understanding of the mechanism has not been established. Recent results have shown that a strong correlation exists between localized deformation and IASCC.[2–4] Deformation in irradiated metals becomes localized to narrow channels in which defects are cleared by passing dislocations, resulting in a cascade of dislocations down these cleared channels, inducing a high level of strain in a localized volume. When the dislocation channels intersect a free surface, a surface step is formed.
ELAINE A. WEST and MICHAEL D. McMURTREY, Graduate Student Research Assistants, ZHIJIE JIAO, Assistant Research Scientist, and GARY S. WAS, Professor, are with the University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] Manuscript submitted March 15, 2011. Article published online July 20, 2011 136—VOLUME 43A, JANUARY 2012
Understanding the relationship between localized deformation and IASCC is not straightforward. Several characteristics of localized deformation may be related to cracking susceptibility, such as the properties of the channels themselves (width, height, and spacing), and the interaction of the channel with the grain boundary (degree to which slip is continuous across the boundary). The Taylor and Schmid factors of the grains adjacent to boundaries are also indicators of deformation propensity. The Schmid factor describes the ease with which deformation occurs on an individual slip system given a specified tensile direction. Slip occurs at a lower applied tensile stress on a slip system with a higher Schmid factor than a slip system with a lower Schmid factor. Face-centered cubic materials have 12 slip systems, each of which has a specific Schmid factor. The Schmid factor of a grain is the highest of those 12 Schmid factors. The Taylor factor describes the amount of shear strain required in an individual grain, assuming five active slip systems, to match the overall macroscopic strain of the polycrystal. A grain with a high Taylor factor must undergo more shear strain to accommodate the macroscopic strain than a grain with a low Taylor factor. Several studies have found correlations between IASCC and the Taylor and Schmid
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