Radiation effects on time-dependent deformation: Creep and growth
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INTRODUCTION
TIME-dependent deformation of stressed materials during irradiation is defined as "irradiation creep" if volume is conserved. Similarly, irradiation growth occurs at constant volume but in the absence of an externally applied stress. Irradiation creep depends on material microstructure, stress, and temperature. For thermal creep, deformation maps have been developed to describe the creep dependence on temperature and stress, t1'2,3] For irradiation creep, deformation maps have been proposed schematically t41 but have not been developed because of the difficulty in obtaining sufficient experimental data to establish mechanism parameter boundaries. In spite of these experimental and theoretical limits, much has been learned about the time-dependent deformation of materials during irradiation. I4-71 The environmental factors that affect time-dependent deformation mechanisms are summarized in Figure 1. Four regions of interaction between stress, temperature, and radiation are identified: (1) thermal creep; (2) irradiation growth, swelling, and precipitation; (3) athermal processes; and (4) irradiation creep and volumetric creep. In this review, regions 2 and 4 are of principal concern. All contributions to deformation, however, must be identified to establish mechanistic causes for irradiation creep. Thermal creep (region 1) is deformation induced by stress and controlled by thermally activated processes. During irradiation, thermal creep can be either enhanced or suppressed compared to deformation rates measured without irradiation. The term "in-reactor creep" is sometimes used to describe radiation-modified thermal creep. Basically, radiation can enhance thermally activated processes such as atomic diffusion and it can reduce dislocation motion by creating glide obstacles. The in-reactor creep rate reflects both radiation-enhanced dislocation E.P. SIMONEN, Senior Research Scientist, is with Pacific Northwest Laboratory, Richland, WA 99352. This paper is based on a presentation made in the symposium "Irradiation-Enhanced Materials Science and Engineering" presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. METALLURGICAL TRANSACTIONS A
climb and radiation-reduced dislocation glide. An excellent discussion of radiation effects on thermal creep rates is contained in a review by Nichols. t41 Climb-glide thermal creep rates are expected to be influenced by irradiation, but thermal creep mechanisms that depend on a net transport of atoms from one sink to another, such as Nabarro-Herring creep, are not expected to be influenced by irradiation. I41 The interpretation of irradiation growth measurements tSl in region 2 is complicated by residual stresses in polycrystalline metals, t91 Experiments on single crystals, however, have revealed that shape change can occur during irradiation without the application of stress. 00] Irradiation gr
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