D.N. Braski

The effect of tensile stress on the growth of helium bubbles in an austenitic stainless steel

Abstract Foils of the ternary alloy, Fe-17 wt% Cr-17 wt% Ni, were cyclotron-injected with ~160 at ppm helium and annealed at 1023 K for times up to 1.74 Ms (482 h). Some foils were subjected to tensile stresses which ranged from 9.8 to 27.5 MPa during annealing while others were unstressed. Helium bubbles grew in unstressed specimens at different rates depending on their location in the microstructure; the largest bubbles were found at triple grain junctions, smaller ones on the grain boundaries, and the smallest in the grain matrices. Bubble size in the matrix was approximately proportional to t 1 4 for annealing times greater than 2.88 × 104s (8 h). Applied tensile stress accelerated the growth of bubbles, especially at triple grain junctions and grain boundaries. The enhanced growth in these areas appear to be associated with grain-boundary sliding. The number of bubbles in the matrix and on grain boundaries generally decreased with annealing time, suggesting that a migration-coalescence mechanism was also operating.

Improved swelling resistance for PCA austenitic stainless steel under HFIR irradiation through microstructural control

Abstract Six microstructural variants of Prime Candidate Alloy (PCA) were evaluated for swelling resistance during HFIR irradiation, together with several heats of type 316 stainless steel (316). Swelling was negligible in all the steels at 300°C after ~44 dpa. At 500 to 600°C 25%-cold-worked PCA showed better void swelling resistance than type 316 at ~44 dpa. There was less swelling variability among alloys at 400°C, but again 25%-cold-worked PCA was the best. Microstructurally, swelling resistance correlated with development of fine, stable bubbles whereas high swelling was due to coarser distributions of bubbles becoming unstable and converting to voids (bias-driven cavities).

The effect of neutron irradiation on vanadium alloys

Abstract Neutron-irradiated vanadium alloys were evaluated for their susceptibility to irradiation hardening, helium embrittlement, swelling, and residual radioactivity, and the results were compared with those for the austenitic and ferritic stainless steels. The VANSTAR-7 and V-15Cr-5Ti alloys showed the greatest hardening between 400 and 600°C while V-3Ti-Si and V-20Ti had lower values that were comparable to those of ferritic steels. The V-15Cr-5Ti and VANSTAR-7 alloys were susceptible to helium embrittlement caused by the combination of weakened grain boundaries and irradiation-hardened grain matrices. Specimen fractures were entirely intergranular in the more severe instances of embrittlement. The V-3Ti-1Si and V-20Ti alloys were most resistant to helium embrittlement. Except for VANSTAR-7 irradiated to 40 dpa at 520°C, all of the vanadium alloys exhibited low swelling that was similar to the ferritic steels. Swelling was greater in specimens that were pre-implanted with helium using the tritium trick. The radioactive decay characteristics of vanadium alloys will simplify waste disposal for spent reactor components, compared to the requirements for conventional ferrous alloys.

Accelerated neutron embrittlement of ferritic steels at low fluence: Flux and spectrum effects

Abstract The results of Charpy V-notch surveillance testing of ferritic steels from the High Flux Isotope Reactor pressure vessel revealed significant radiation-induced embrittlement in A212 grade B, A350 grade LF3, and A105 grade II steels. The steels were irradiated at about 50°C for about 17.5 effective full-power years at a neutron flux (E > 1MeV) of 1012 to 1013 n m−2 s−1 to fluences of 1021 to 1022 n m−2. These fluences are only about one-tenth those required to cause the same embrittlement in the higher flux (~1017 n m−2 s−1) environments of test reactors. The findings suggest that the degree of embrittlement per unit fast fluence is increased at low neutron flux. Changes in neutron energy spectrum may be involved, too. Potential mechanisms for effects of neutron flux and neutron spectrum on embrittlement are discussed.

Microstructural design of PCA austenitic stainless steel for improved resistance to helium embrittlement under HFIR irradiation

Abstract Several variants of Prime Candidate Alloy (PCA) with different preirradiation thermal-mechanical treatments were irradiated in HFIR and were evaluated for embrittlement resistance via disk-bend tensile testing. Comparison tests were made on two heats of 20%-cold-worked type 316 stainless steel. None of the alloys were brittle after irradiation at 300 to 400°C to ~44 dpa and helium levels of 3000 to ~3600 at. ppm. However, all were quite brittle after similar exposure at 600°C. Embrittlement varied with alloy and pretreatment for irradiation to 44 dpa at 500°C and to 22 dpa at 600°C. Better relative embrittlement resistance among PCA variants was found in alloys which contained prior grain boundary MC carbide particles that remained stable under irradiation.

Electron microscope in situ annealing study of voids induced by irradiation in aluminum

Abstract The shrinkage of irradiation-induced voids in 8001 aluminum alloy and pure aluminum (Cominco Grade 69) was investigated by annealing thin-foil specimens in a high-vacuum electron microscope. The general shrinkage characteristics of both materials were quite similar, but the shrinkage curves for individual voids showed considerable variation. Most of the voids exhibited an initial high rate of shrinkage. Both effects may reflect the influence of unresolvable local vacancy sinks.

Modeling the effect of creep on the growth of helium bubbles in metals during annealing

Abstract The kinetics of helium bubble growth during annealing of unstressed and stressed metals is modeled. Growth in the matrix, on grain boundaries as well as on triple grain junctions is considered. A coalescence model is developed for the description of helium bubble growth by volume and surface diffusion. The dependence of growth on the location of the bubbles in the microstructure is discussed. An applied tensile stress has been observed to accelerate bubble growth on grain boundaries and on triple grain junctions [1]. A theoretical model of the influence of stress-induced grain boundary sliding on bubble growth on triple grain junctions in a stressed metal due to bubble sweeping by moving dislocations during creep is suggested. The time-dependent bubble size distribution function is discussed for different mechanisms and paths of bubble migration. The time dependences of the number density and mean radius of bubbles in unstressed metal is derived. The present theoretical model is compared with experimental TEM results for an Fe 17Cr 17Ni alloy which was cyclotron-injected with 160 appm helium and annealed at 1023 K for times up to 1.74 Ms (482 h), due to Braski [J. Nucl. Mater. 83 (1979) 265].

Modification of the grain boundary microstructure of the austenitic PCA stainless steel to improve helium embrittlement resistance

Abstract Grain boundary MC precipitation was produced by a modified thermal-mechanical pretreatment in 25% cold-worked (CW) austenitic prime candidate alloy (PCA) stainless steel prior to HFIR irradiation. Postirradiation tensile results and fracture analysis showed that the modified material (B3) resisted helium embrittlement better than either solution annealed (SA) or 25% CW PCA irradiated at 500 to 600°C to ∼ 21 dpa and 1370 appm He. PCA SA and 25% CW were not embrittled at 300 to 400°C. Grain boundary MC survives in PCA-B3 during HFIR irradiation at 500°C but dissolves at 600°C; it does not form in either SA or 25% CW PCA during similar irradiation. The grain boundary MC appears to play an important role in the helium embrittlement resistance of PCA-B3.

Effect of preinjected helium on the response of V-20Ti pressurized tubes to neutron irradiation☆

Abstract Vanadium-20% titanium tubes, pressurized to stresses of 34 and 39 MPa, were irradiated in the Experimental Breeder Reactor (EBR-II) at 700°C to a fluence of 3.9 × 10 26 n/m 2 , corresponding to a displacement damage level of 22 dpa. Sections of the tubes were injected with 15 appm He prior to irradiation to determine the effect of helium on the microstructural and creep response of this alloy to irradiation. It was found that helium promoted cavity formation, primarily within existing precipitates, but total swelling remained low. Under some conditions, an apparent enhanced creep deformation due to the presence of helium was found. The results suggest that the increase in creep deformation in the presence of helium may be very sensitive to stress.

The resistance of (Fe, Ni)3V long-range-ordered alloys to neutron and ion irradiation☆

Abstract A series of (Fe, Ni) 3 V long-range-ordered alloys were irradiated with neutrons in the Oak Ridge Research Reactor (ORR) and with 4 MeV Ni ions at temperatures above 250°C. The displacement damage levels for the two irradiations were 3.8 and 70 dpa, and the helium levels were 29 and 560 at. ppm, respectively. Irradiation in ORR generally increased the yield strength and lowered the ductility of an LRO alloy, but produced relatively little swelling. The LRO alloys retained their long-range order after ion irradiation below the critical ordering temperature, T c , and exhibited low swelling. Above T c the alloys were disordered and showed greater swelling. Adjustment of alloy composition to prevent sigma phase formation reduced swelling.