Changes in the subboundary mesh size with creep strain in 304 stainless steel
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Changes in the Subboundary Mesh Size with Creep Strain in 304 Stainless Steel M. E. KASSNER, J. W. ELMER, and C. J. ECHER
Fig. 3 - - S E M fractographs of irradiated 20 pct CW316 specimens after testing to failure at 625 ~ (a) is from a constant load 1 : 1 Cs-Te coated specimen which failed after 5.5 min at 0.91LF; (b) is from an uncoated (control) specimen which failed after 95.5 min at a constant load of 0.91LF (the thin surface coating is believed to be residue from a cleaning solution).
The authors acknowledge and appreciate the support of the United States Department of Energy; the work was carfled out under contract DE-AT03-76SFI031.
METALLURGICALTRANSACTIONS A
Several recent investigations have suggested that the average spacing of dislocations that compose the subgrain boundaries may influence the creep resistance of a material. The nature of the influence varies with each study: the spacing has been proposed to affect the effectiveness of subgrain boundaries as obstacles to gliding dislocations,' determine the distance that edge dislocation segments must climb and annihilate to allow subsequent dislocation glide, 2 and influence the stress necessary to emit dislocations from subgrain boundaries. 3:,5 If the rate-controlling process for creep is influenced by the mesh size or dislocation spacing (d), then certain trends might be expected: the average spacing of dislocations composing the subgrain boundaries would decrease with increasing primary (Stage I) creep strain, and the spacing would be constant over secondary or M.E. KASSNER is on leave from the Lawrence Livermore National Laboratory, Livermore, CA 94550, as Adjunct Professor, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943. J. W. ELMER is on leave from the Lawrence Livermore National Laboratory as Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. C.J. ECHER is Consultant to the Lawrence Livermore National Laboratory. Manuscript submitted March 4, 1986.
U.S. GOVERNMENTWORK NOT PROTECTED BY U.S. COPYRIGHT
VOLUME 17A, NOVEMBER 1986--2093
steady-state (Stage II) creep. Although there have been only a few studies that have determined the variation in the spacing (d) with strain, 3'6'7several investigations have examined the changes in the subgrain misorientation angle (0) with strain for a variety of metals and alloys (AI, Fe, Cu, Sn, and a ferritic stainless steel). 8-~3 These studies provide some insight into the spacing-vs-strain behavior, since it is expected that 0 and d are related. The studies suggest that d decreases with strain during primary creep. Therefore, hardening might be associated with a decrease in the spacing of subgrain boundary dislocations. The results for steady-state creep were less conclusive, since only a few data points were reported at strains beyond that required to attain steady state. This is probably due partly to the limited steady-state strain that can be achieved by tensile deformation. Whereas so
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