D. Klingensmith

On the effect of dose rate on irradiation hardening of RPV steels

The effect of dose rate (DR), or neutron flux (ϕ), on irradiation hardening (Δσy) and embrittlement of reactor pressure vessel (RPV) steels is a key unresolved issue. We report a rigorous evaluation of DR effects based on a very large Δσy database we developed for RPV steels with a wide range of compositions, including a set of split-melt alloys with controlled and systematic variation in Cu, Ni and Mn content. The steels were irradiated at 290°C in three ϕ-regimes to a wide range of overlapping fluences (ϕt). The contribution of copper-rich precipitates (CRPs) to Δσy increases up to a plateau hardening that is a strong function of the alloy Cu, Ni and Mn content, but is relatively independent of DR. However, the pre-plateau region is shifted to higher ϕt with increasing DR. The shift can be approximately accounted for by defining an effective fluence (ϕte) as ϕte ≈ ϕt(ϕr /ϕ)1/2, where ϕr is a reference flux. The ϕ −1/2 scaling is consistent with a vacancy plus self-interstitial-atom (SIA) recombination rate controlling mechanism. The Δσy data are analysed with a combined model describing: (a) the excess vacancy concentration under irradiation as a function of DR, including the effect of solute vacancy traps on recombination; (b) the corresponding radiation enhanced Cu diffusion (RED) coefficient (D*); (c) the resulting accelerated growth of CRPs; and (d) the contribution of CRPs to Δσy. Recombination is shown to increase with higher alloy Ni and Mn content, consistent with a solute–vacancy trapping mechanism. In spite of high recombination rates, however, RED is extremely efficient, with the D* ranging up to a factor of 60 or more times higher than predicted by simple rate theory models. Various explanations of the high diffusion rates are discussed, including large vacancy–solute binding energies that control the vacancy concentrations and jump frequencies near solutes in a way that can enhance both diffusion and recombination.

Some recent innovations in small specimen testing

Abstract New innovative small specimen test techniques are described. Finite element simulations show that combinations of cone indentation pile-up geometry and load–penetration depth relations can be used to determine both the yield stress and strain-hardening behavior of a material. Techniques for pre-cracking and testing sub-miniaturized fracture toughness bend bars, with dimensions of 1.65×1.65×9 mm3, or less, are described. The corresponding toughness–temperature curves have a very steep transition slope, primarily due to rapid loss of constraint, which has advantages in some experiments to characterize the effects of specified irradiation variables. As one example of using composite specimens, an approach to evaluating helium effects is proposed, involving diffusion bonding small wires of a 54Fe-based ferritic–martensitic alloy to a surrounding fracture specimen composed of an elemental Fe-based alloy. Finally, we briefly outline some potential approaches to multipurpose specimens and test automation.

Neutron irradiation effects on magnetic minor hysteresis loops in nuclear reactor pressure vessel steels

Magnetic minor hysteresis loops have been measured on A533B-type nuclear reactor pressure vessel steels with various combinations of Cu and Ni contents after neutron irradiation to a fluence up to 3.32 × 1019 n cm−2. A strong compositional dependence of minor-loop properties, which are indicators of internal stress, was found. The properties of high-Cu and high-Ni steel show a large increase in the low fluence regime below 0.4 × 1019 n cm−2, followed by a slow decrease, while those for low-Cu or low-Ni steel show a sudden decrease. The changes are roughly in linear proportion to the yield strength changes. The results were explained from the viewpoint of the formation and growth of Cu-rich precipitates and/or fine scale defects in the matrix and along pre-existing dislocations.

Anomalous Hardening in Model Alloys and Steels Thermally Aged at 290°C and 350°C: Implications to Low Flux Irradiation Embrittlement

Age hardening at 290 C and 350 C in five simple ferritic alloys with systematic variations in Cu and Mn and four A533B-type split melt steels with systematic variations in Cu and Ni was evaluated for times up to 7200h. Significant yield stress increases were found in alloys containing high copper and nickel. The hardening is consistent with formation of copper-rich precipitates. However, the precipitation kinetics are accelerated relative to estimates based on extrapolations of the high temperature copper diffusion coefficient. Further the observed high sensitivity to both copper and nickel content is not understood. The rapid thermal kinetics suggest that embrittlement due to age hardening could occur in sensitive pressure vessel steels during extended service. Further, high thermal diffusion rates must be considered in models of the effect of flux on hardening and embrittlement. Anomalously high diffusion would also introduce a new rate-dependent regime at very low fluxes. Hence, the peak irradiation hardening from copper-rich precipitates may occur at much lower fluences than predicted by current data correlations.

Effect of Neutron Flux on Magnetic Hysteresis in Neutron-Irradiated Pressure Vessel Steels

The effect of neutron flux on magnetic minor hysteresis loops has been investigated on nuclear reactor pressure vessel steels, which were irradiated to a fluence of 3.3 × 1019 n/cm2. A minor-loop coefficient, which is an indicator of internal stress, exhibits a local maximum at a fluence of ~1 × 1019 n/cm2, whose position shifts to a low-fluence regime with decreasing neutron flux. Introducing an effective fluence, used to correct the flux effect of irradiation hardening, the data obtained by different flux were found to almost fall on single curve for some alloys. This implies that the flux effect on magnetic property is dominated by efficiency of radiation-enhanced diffusion of solute atoms, such as Cu, as in the case of irradiation hardening.

FCRD Milestone Report: M21AF050901

The objective of this study was to perform mechanical testing on large scale heats of the advanced ODS 14YWT alloy to investigate the effects of processing parameters on mechanical properties. Mechanical properties tests were conducted on two heats of the advanced ODS 14YWT ferritic alloy: the 14YWT-SM11 was produced by extrusion at ORNL and OW4 was produced by HIP at UCSB. The 14YWT-SM11 showed very high tensile strength compared to OW4, but showed less ductility as a result. The fracture toughness transition temperature of 14YWT-SM11 was determined in two orientations and showed T{sub 0} = 48 C in the favorably strong L-T direction while shifting by 63 C to T{sub 0} = 111 C in the weaker T-L direction. The fracture toughness transition temperature for OW4 was not determined but appeared to be within the range observed for 14YWT-SM11. The fracture toughness of 14YWT-SM11 at room temperature was 86.8 MPa{radical}m and 93.1 MPa{radical}m, which was much higher than that of OW4 (27.4 MPa{radical}m). The strain rate jump tests conducted on OW4 indicated that the creep properties were similar to MA957 at 750 C.

Correlating Radiation Exposure with Embrittlement: Comparative Studies of Electron- and Neutron-Irradiated Pressure Vessel Alloys

Comparative experiments using high energy (10 MeV) electrons and test reactor neutrons have been undertaken to understand the role that primary damage state has on hardening (embrittlement) induced by irradiation at 300 C. Electrons produce displacement damage primarily by low energy atomic recoils, while fast neutrons produce displacements from considerably higher energy recoils. Comparison of changes resulting from neutron irradiation, in which nascent point defect clusters can form in dense cascades, with electron irradiation, where cascade formation is minimized, can provide insight into the role that the in-cascade point defect clusters have on the mechanisms of embrittlement. Tensile property changes induced by 10 MeV electrons or test reactor neutron irradiations of unalloyed iron and an Fe-O.9 wt.% Cu-1.0 wt.% Mn alloy were examined in the damage range of 9.0 x 10{sup {minus}5} dpa to 1.5 x 10{sup {minus}2} dpa. The results show the ternary alloy experienced substantially greater embrittlement in both the electron and neutron irradiate samples relative to unalloyed iron. Despite their disparate nature of defect production similar embrittlement trends with increasing radiation damage were observed for electrons and neutrons in both the ternary and unalloyed iron.