J.O. Stiegler

Correlation of neutron and heavy-ion damage: I. The influence of dose rate and injected helium on swelling in pure nickel☆

Abstract Displacement damage structures in pure nickel at the 1 dpa level are compared for two widely disparate damage rates, 10 −7 dpa/s for neutron irradiations and 3 X 10 −3 dpa/s for self-ion bombardments over a range of temperatures spanning those for void formation. Peak swelling at about 0.7% is found at 400° and 600°C, respectively. At equivalent swelling temperatures, voids in the ion-bombarded material are larger and fewer than those from neutron irradiation, especially at temperatures above the peak swelling temperature. Additions of 20 appm He, matching that generated in the neutron irradiations, were made to the ion-bombarded nickel either prior to ion bombardment (preinjection) or during ion bombardment (simultaneous injection). This helium caused increased swelling at the upper and lower temperature extremes. Simultaneously implanted helium did not otherwise significantly affect microstructures, whereas preinjected helium increased the dislocation density and caused more but smaller voids over the full temperature range of swelling.

Defect structure of neutron irradiated boron carbide

Abstract Boron carbide irradiated in thermal and fast reactor neutron spectra at 500 to 840°C up to 81% 10B burnup has been examined by transmission electron microscopy. Helium bubbles formed by agglomeration of helium from the 10B (n,α)7 Li reaction constitute the only visible form of irradiation-induced damage. The helium bubbles tend to have a disc-like morphology with the plane of the disc parallel to (111). Strong lattice strain fields around the bubbles evidenced a lack of thermal equilibrium. Bubble nucleation was influenced markedly by grown-in dislocations and stacking faults. Bubble coalescence, either by impingement or enhanced by bubble stress fields frequently gave rise to both trans- and intergranular microcracking. The significance of these observations on gas release and swelling of boron carbide are discussed briefly.

The effect of minor constituents on swelling in stainless steel

Abstract The small quantities of solute interstitial elements in stainless steel (C, N and possibly Si) reduce the swelling under neutron irradiation ( ∼ 2 × 10 22 neutrons/cm 2 ) by more than an order of magnitude between 500 and 600° C over high purity material. The solute interstitials reduce both the numbers and sizes of irradiation-caused voids. Current swelling models ignore — of necessity — this gross effect. Several possible mechanisms are suggested to account for the effect.

The effects of large fast-neutron fluences on the structure of stainless steel☆

Abstract The changes in microstructure which occur when type 304L stainless steel is irradiated at temperatures between 370 and 472 °C and fast-neutron fluences between 0.8 and1.4 × 1022n/cm2 have been investigated by electron microscopy. The fuel cladding from an Experimental Breeder Reactor-II driver fuel element served as the experimental material. Voids and dislocation loops formed by the precipitation of irradiation-produced vacancies and interstitials were observed in each specimen. As the irradiation temperature was increased the density of each type of defect decreased and the average size increased. Upon postirradiation annealing at 600 °C, the dislocation loops converted to a dislocation network and the smallest voids ( diameters A ) began to disappear. As the annealing temperature was increased the dislocation and void densities both decreased. After annealing 1 h at 900 °C, the voids were completely removed and the dislocation density was reduced to that of an unirradiated annealed specimen.

Austenitic stainless steels with improved resistance to radiation-induced swelling☆

Stainless steel specimens were bombarded with Ni ions at elevated temperatures to simulate fast neutron damage. Results show that type 316 stainless steel with additions of silicon and titanium exhibits low swelling over the entire swelling temperature range under high-dose nickel-ion bombardment. Neutron irradiation data on commercial alloys and ion data on high-purity alloys also indicate that the most effective suppression of swelling is achieved with combinations of silicon and titanium. It is suggested that suppression of swelling by alloying with silicon and titanium may be effective over a range of nickel and chromium base composition levels and will provide the basis for the development of low-swelling alloys that are technologically similar to type 316 stainless steel. It should be noted that the influence of silicon and titanium on swelling is likely to depend strongly on the concentrations of other elements such as carbon, oxygen, and nitrogen and on the extent to which silicon and titanium partition to various phases.

The effect of irradiation temperature on strength and microstructure of stainless steel

Abstract The effects of irradiation at temperatures between 93 and 454° C upon the room-temperature mechanical properties and electron microstructuros of AISI type 304 stainless steel have been determined. Irradiation at temperatures between 93 and 300° C produced a high density of defect clusters on the order of 100 A in dia. which wore responsible for an increased yield stress. After irradiation at 371°C practically no defect clusters were observed and the yield stress was lower by a factor of two. At irradiation temperatures of 371° C and higher, precipitates formed within the grains. Deformation (10% by rolling) in a specimen containing the defect clusters was concentrated in very narrow slip bands, while in a specimen containing precipitate particles, the deformation was homogeneous.

High-neutron fluence damage in an aluminum alloy☆

A cold-drawn Al/1 wt % Ni alloy irradiated to 1 × 1022 fast neutrons/cm2 (E > 0.821 MeV) and > 2 × 1022 thermal neutrons/cm2 at 60 °C in an aqueous environment contained polyhedral voids up to 550 A diameter in concentrations of approximately 1015cm3, and a fine solid precipitate of transmutation-produced silicon. Post-irradiation annealing for 1 h at temperatures above 220 °C caused the voids to disappear; cold pressing the specimens through a 65% reduction in thickness also eliminated most of the voids. The irradiation-induced precipitate coarsened markedly with increasing annealing temperature. Gas bubbles were observed after a 400 °C anneal and are believed to be responsible for a large increase in recrystallization temperature. Growth of the bubbles on annealing at temperatures of 530 °C and above caused significant swelling. Irradiation-induced strengthening decreased with increasing tensile test temperature and disappeared at about 300 °C. Post-irradiation annealing also decreased the strength. Ductility was reduced by irradiation, particularly over the test temperature range ≈ 100 to ≈ 260 °C.

Transmission electron microscopy of omega phase in a Zr-15 % Nb alloy

Abstract Transmission electron microscopy has been used to follow the formation and reversion of the metastable ω phase in a Zr-15 % Nb alloy. In a fully aged specimen the ω phase is shown to consist of noncoherent platelets approximately 1000 A in diameter and approximately 200 A thick lying on {111} planes of the retained β phase. The platelets form a regular three-dimensional network having an average spacing of approximately 1000 A. The retained β-phase regions contain a line structure similar to that observed in specimens aged at lower temperatures when the ω phase is coherent. Aging of the foils in the electron microscope resulted in partial reversion of the noncoherent platelets and formation of the coherent structure. Possible mechanisms for the formation of the two types of ω phase are discussed along with an explanation of the changes in mechanical properties which accompany their formation.

Point defect interactions and growth of dislocation loops

A direct method of determining the effective measure of the interaction of point defects with dislocation loops is proposed. A system of partial differential rate equations for loop growth is formulated and is solved by a numerical method. An in-situ HVEM irradiation experiment was conducted to study growth kinetics of dislocation loops in Ni at 450/sup 0/C. It is found from systematic variations of the vacancy migration energy, E/sub v//sup m/, and the preference factor of dislocation loops for self-interstitials, delta/sub i/, that delta/sub i/ = 0.06 and E/sub v//sup m/ = 1.2 eV together yield reasonable agreement between the theoretical model and the experimental observations.