Deformation Mechanisms in Ferritic/martensitic Steels Irradiated in HFIR
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Deformation Mechanisms in Ferritic/martensitic Steels Irradiated in HFIR Naoyuki Hashimoto1*, Steven J. Zinkle1, Ronald L. Klueh1, Arthur F. Rowcliffe1, and Kiyoyuki Shiba2 1 Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN 37831-6151, USA 2 Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-1195, Japan ABSTRACT A reduced activation ferritic/martensitic steel, F82H (IEA heat), developed for fusion energy applications was irradiated at 300 and 500°C to 5 dpa in the High Flux Isotope Reactor (HFIR). Changes in yield strength, deformation mode, and strain-hardening capacity were seen, with the magnitude of the changes dependent on irradiation temperature. Irradiation at 300°C led to a significant loss of strain-hardening capacity with a large change in yield strength. There was a tendency for a reduction in strain rate to cause a decrease in yield strength and elongation. Irradiation at 500°C had little effect on strength, but a reduction in strain rate caused a decrease in ductility. In order to determine the contributions of different microstructural features to strength and to deformation mode, transmission electron microscopy (TEM) specimens were prepared from the gage sections of the tested (strained) flat tensile specimens and examined; fracture surfaces were examined by scanning electron microscopy (SEM). The fracture surfaces showed a martensitic mixed quasi-cleavage and ductile-dimple fracture in the center at both irradiation temperatures. The microstructure in the necked region irradiated at 300°C showed defect free bands, which may be dislocation channels. This suggests that dislocation channeling could be the dominant deformation mechanism in martensitic steels irradiated at 300°C, resulting in the loss of strain-hardening capacity. INTRODUCTION For structural applications in fusion energy systems the ferritic-martensitic steels have several advantages based upon their resistance to void swelling, good thermal stress resistance, and well-established commercial production and fabrication technologies. Ferritic-martensitic steels, however, undergo radiation-induced hardening during neutron irradiation at temperatures up to 400°C, with a transition to fluence-dependent radiation softening at temperatures above 400-450°C. Radiation-hardening is often accompanied by a reduction in strain hardening capacity and uniform elongation, an increase in the temperature delineating the transition from quasi-cleavage to ductile fracture, and increased propensity for brittle failure under certain combinations of temperature and loading conditions. In order to determine the contributions of different microstructural features to strength and to deformation mode, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed on tested (strained) tensile specimens of irradiated reduced activation F82H (IEA heat).
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Table 1. Chemical composition of F82H IEA (wt.%) (Balance Fe). Cr 82H (IEA heat) 7.71
W 1.95
V 0.16
Ta 0.02
Mn 0.16
Al 0.003
C 0.090
B 0.0002
Si 0.
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