D. L. Damcott

Microchemistry and microstructure of proton-irradiated austenitic alloys : toward an understanding of irradiation effects in LWR core components

Abstract Over 1200 measurements of grain boundary composition and microstructure have been made on 14 different austenitic Fe–Cr–Ni alloys following proton irradiation in the temperature range 200–600°C and in the dose range 0.1–3.0 dpa. From these data, a greater understanding of radiation induced segregation (RIS) and microstructure development has been gained. Grain boundary composition measurements revealed that Cr depletes at grain boundaries, Ni enriches and Fe can either enrich or deplete depending on alloy composition. Analysis of temperature and composition dependence of RIS revealed that the magnitude and direction of grain boundary segregation depends on alloy composition because the values of migration enthalpy of the alloy constituents are not the same, and diffusivities of the alloy constituents are composition-dependent. The dose dependence of segregation revealed ordering in Ni-base alloys and temperature dependence was used to show that RIS is consistent with a vacancy exchange mechanism. The dependence of segregation on composition is consistent with all known, relevant neutron data. RIS was found to be related to the development of the dislocation and void microstructures. Alloys in which the microstructure develops slower with dose also show slower changes in RIS. Similarly, it was shown that the dependence of swelling on composition is the same for neutron, ion and proton irradiation and all can be explained by the effect of RIS on defect diffusivity at the void nuclei. This paper illustrates the value of conducting carefully chosen irradiation experiments over several, well-controlled variables to elucidate the mechanisms underlying the microchemical and microstructural changes.

Effects of proton irradiation on the microstructure and microchemistry of type 304L stainless steel

Abstract A research program has been undertaken to determine the origins of irradiation-assisted stress corrosion cracking (IASCC) in austenitic alloys in light water reactors, and the effect of impurities on IASCC susceptibility. Controlled purity alloys of 304L stainless steel were irradiated with protons at 400°C to a dose of 1 dpa and analyzed via Auger electron spectroscopy (AES) and scanning transmission electron microscopy (STEM). The alloys investigated were an ultra-high purity (UHP) alloy and UHP alloys containing phosphorus (UHP + P), sulfur (UHP + S), or silicon (UHP + Si). Microstructural and microchemical changes were quantified and compared with literature results for other irradiating species. Following irradiation, the alloys showed dislocation loop formation and growth, “black dot” loops, and a change in the nature of the dislocation network. AES and STEM microchemical analysis of the alloys revealed Cr depletion of up to 6 at% and Ni enrichment of up to 6.6 at% at the grain boundaries of the alloys, with more segregation observed in the alloys containing impurities than in the UHP alloy. Significant grain boundary enrichment of P and Si in the UHP + P and UHP + Si alloys, respectively, was also observed. The results of the analyses of proton-irradiated samples are shown to compare favorably with previous studies on samples irradiated with neutrons at or near LWR conditions.

Quantitative analysis of radiation-induced grain-boundary segregation measurements

Radiation-induced and precipitation-induced grain-boundary segregation profiles are routinely measured by scanning-transmission electron microscopy using energy-dispersive X-ray spectroscopy (STEM-EDS). However, radiation-induced grain-boundary segregation (RI9 profiles achieved at low and moderate temperatures are e.xceedingly narrow, typically less than 10 nm full width at half maximum. Since the instrumental spatial resolution can be a significant fraction of this value, the determination of grain boundary compositions poses a formidable challenge. STEM-EDS and Auger electron spectroscopy @ES) measurements are reported, performed on controlled-purity alloys of type 304L stainless steel irradiated with 3.4 MeV protons to 1 displacement per atom at 400°C. Because of statistical noise and the practical lower limit on the step size in STEM, deconvolution of the measured data does not yield physical results. An alternative analysis of STEM data is presented. Numerical calculations of RIS profiles are convoluted with the instrumental broadening function and modified iteratively to fit the data, yielding a “best estimate” profile. This “best estimate” is convoluted with the Auger intensity profile to yield a simulated AES measurement, which is compared with the actual AES measurement to provide an independent test of the validity of the “best estimate”. For impurities with a narrow segregation profile and an Auger electron escape depth of one monolayer, a combination of STEM and AES data allows a determination of the width of the segregated layer. It is found that, in an ultrahigh-purity alloy doped with P, the latter is essentially contained in a single monolayer.

Irradiation assisted stress corrosion cracking of controlled purity 304L stainless steels

Abstract The effect of chromium, phosphorus, silicon and sulfur on the stress corrosion cracking of 304L stainless steel in CERT tests in high purity water or argon at 288°C following irradiation with 3.4 MeV protons at 400°C to 1 dpa, has been investigated using ultrahigh purity alloys (UHP) with controlled impurity additions. Grain boundary segregation of phosphorus or silicon due to proton irradiation was quantified using both Auger electron spectroscopy and scanning transmission electron microscopy, and the alloys with impurity element additions were observed to have greater grain boundary chromium depletion and nickel enrichment than the UHP alloy. The UHP alloy suffered severe cracking in CERT tests in water. Less cracking was found after CERT test of irradiated UHP+Por UHP+Si alloys, despite greater chromium depletion. This suggests a mitigating effect of phosphorus and silicon at grain boundaries. No cracking was found in argon tests, eliminating a purely mechanical embrittlement mechanism, but not eliminating a contribution from radiation hardening. Implanted hydrogen was not a factor in the intergranular cracking found.

Proton-induced grain boundary segregation in stainless steel

Abstract A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from...

A radiation effects facility using a 1.7 MV tandem accelerator

Abstract A facility has been established at the Michigan Ion Beam Laboratory for the study of radiation effects on materials. The capabilities include a broad range of materials (metals, ceramics and polymers), radiation damage rates (10−8 to 10−3 dpa/s) and irradiation temperatures (−196°C to 600°C). The key to the utility of this facility is the control of irradiation dose, dose uniformity, and sample temperature during irradiation. Temperature stability is maintained by simultaneous heating and cooling of the sample stage, and use of a liquid metal interface (for metal samples). The temperature of individual samples in the irradiated area is measured via an infrared pyrometer and thermocouples. Temperature uniformity is confirmed by the pyrometer, while dose uniformity is provided by a split aperture. A total of eight input channels transmit temperature and beam current signals to a 486DX computer to provide feedback to the operator and to record the irradiation history at a frequency of 1 point per second. Continuous irradiations lasting up to 120 hours have been successfully conducted.