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The UK's Advanced Gas-cooled Reactor (AGR) nuclear plants, operated by EDF Energy, are using a CO2 based primary circuit gas coolant. The boilers of these reactors have numerous stainless steel (SS) components, which can experience premature cracking due to the severely aggressive environments. Stainless steel components operate in a carburising environment under the creep regime, and thus undergo carburisation over time, which in turn alters the mechanical properties at the surface of the components. Carburised material is more susceptible to premature creep cracking due to the embrittlement of the outer surface, therefore a necessity of new and fundamental understanding of the environmentally assisted degradation and failure mechanisms is required in order to extend the lifetime of the AGRs safely and economically. This study focuses on the use of laboratory oxidation experiments under creep to investigate the interaction of the carburising environment in combination with creep deformation. Parent, cold worked (CW) and cracked SS material, as well as pre-carburised material (used in in-situ high resolution digital image correlation (HRDIC)) were used to further investigate the deformation properties of the carburised layer. Oxidation experiments under creep with parallel and tapered gauge specimens, revealed that carburisation was present on material that was subjected to applied stress whereas oxidation was observed on samples that underwent significant plastic deformation at high temperature. Cold word showed a beneficial effect, as both oxidation and carburisation as well as the total creep elongation were reduced. Overall, surface preparation, cold work and coolant chemistry can be carefully chosen to control the level of oxidation and carburisation, ensuring the safe operation of austenitic stainless steel components in AGR plants. Experiments with cracked specimens highlighted the effects of the carburising environment on oxidation and cracking behaviour of Type 316H SS and in-situ HRDIC experiments with carburised material explored the cracking behaviour of the carburised layer, although further work needs to be performed to investigate its response to deformation. Overall, those findings can be used to increase the service life of these components by extrapolating the materials' behaviour for long exposures, but more investigative work needs to be performed at more prolonged exposure times since the 500 hour exposure time used in this study may not be totally representative of long term plant conditions.

Austenitic alloys are commonly used in the nuclear industry due to their high corrosion resistance and excellent mechanical properties. De- spite boasting an impressive service history, it is possible for critical structural components made from these alloys to undergo stress cor- rosion cracking (SCC) under pressurised water reactors (PWR) condi- tions. As the primary method of material degradation, SCC has been studied extensively for several decades, and many factors have been shown to affect SCC susceptibility, these include cold-work, tempera- ture and water chemistry. To develop a better mechanistic understand of SCC further research is required. This body of research evaluates the effect of hydrogen on mechanical deformation in SCC. Localised and high-resolution testing has been employed successfully to characterise key processes in SCC crack propagation. For this reason, nanoindenta- tion, transmission Kikuchi diffraction (TKD), NanoSIMS and Thermal desorption spectrscopy (TDS) have been used to characterise material deformation on a length-scale relevant for SCC. The main objective of this thesis was to correlate the SCC crack growth rate's (CGR) dependency on nickel content, first observed by Coriou, with both the localised mechanical deformation and hydrogen up-take at PWR temperatures. To satisfy this objective three alloys have been systematically tested in every chapter, these include SS316L (12%- Ni), A800 (32%-Ni) and A600 (72%-Ni) as they represent commercially available alloys with low, intermediate and high nickel contents. Room temperature micromechanical testing has been used to compare the mechanical properties of austenitic alloys on a length-scale that is relevant to SCC. The mechanical properties of all austenitic alloys are highly comparable at room temperature, and show no deviation with changing nickel content. High-magnification cross-sectional analysis shows that the deformation mechanisms in A800 differ from SS316L and A600 at room temperature. A800 does not undergo Σ3 defor- mation twinning, but instead deforms by dislocation glide. This is unexpected as the stacking fault energy of A800 is an intermediate of the other alloys. Deformation is a main SCC mechanism, and the ob- served alloy deformation correlates with Arioka's reported SCC CGR in PWRs, i.e the slowest CGR undergoes no twinning. As the me- chanical properties of alloys with a 25-40% nickel content do not differ significantly from other austenitic alloys, it is thought that their corro- sion resistance is weighted towards diffusion-based SCC mechanisms. Such an explanation is consistent with findings at hotter temperatures, where diffusion-based mechanisms are more active. High temperature micromechanical testing supports these observations, and can be used to better understand the individual mechanism weights depending on alloy composition and temperature. NanoSIMS and TDS were used to quantitatively and qualitatively ex- plore the hydrogen trapping in austenitic alloys exposed to pure heavy water at PWR temperatures. Hydrogen is found trapped by various microstructural features, such as carbides, grain boundaries and ox- ides by NanoSIMS. The total hydrogen content of SS316L and A800 is five-times greater than that of A600, which correlates with the en- thalpy of diffusion in the alloys. These results show an inverse trend with the SCC failure reported by Coriou, and a weaker inverse correla- tion to Arioka's results. The weaker correlation with Arioka's results is due to a change in the corrosive medium from pure water, used in this study and by Coriou, to simulated primary PWR water. It is suggested that the inverse relationship could be due to the elastic shielding pro- vided by the hydrogen enhancing dislocation motion, and preventing the stress intensity factor reaching a critical values that will initiate crack propagation.

Dissimilar metal welding (DMW) between thick section low alloyed and stainless steels is essential in pressurised water reactor (PWR) construction. This study explored the potential of narrow gap laser welding (NGLW) to improve the quality and manufacturability of these welds. It concentrates on DMW joints located in the primary cooling circuit, where austenitic stainless steel pipes are connected to forged low alloy pressure vessel nozzles. Current welding processes for this application are manual metal arc (MMA) and narrow gap gas tungsten arc (NG-GTA) welding, which are slow and inefficient. They require large amounts of filler material to be deposited and generate considerable residual stresses. The residual stresses contribute to stress corrosion cracking (SCC), which has been found a major issue to the longevity and reliability of PWR's. NGLW has the potential to reduce the amount of filler material required and has been shown to reduce the detrimental residual stresses. In this study NGLW was applied for welding SA508 Gr3 Cl2 low alloyed steel with AISI 316L austenitic stainless steel using Inconel Alloy 52 filler metal up to 40 mm thickness. This thesis is the first time that dissimilar metal NGLW has been reported. The process characteristics are discussed. The resulting welds were subjected to industry-standard radiographical approval according to ASME IX. Hardness mapping and microstructural analysis were carried out. Tensile and impact toughness tests were executed. Residual stresses were mapped using the contour method. Welding equipment was developed. The unusually narrow welding groove required special shielding gas and wire feed nozzles. Real-time weld monitoring systems using two different approaches were developed. An appropriate restraint system for the high distortion forces caused by the thick section welding was designed. The microstructural and hardness properties were found to be sound and impact toughness requirements were fulfilled in the as welded condition. This was caused by an effective multi-pass tempering. Lack of fusion (LoF) defects were found to limit the repeatability of the welds. Two main mechanisms were recognised: oxidation induced LoF and undercut related LoF. The filler material used was found to be prone to oxidation. Solutions to overcome the issues found are suggested.

Re-processed oxide fuel product from the Thermal Oxide Reprocessing Plant (THORP) is stored in Type 316L stainless steel, using a design of several nested cans, with the outer can providing the safety case containment barrier. The research reported in this PhD thesis aims to support the safety case related to these storage cans, by identifying and characterising susceptible microstructure sites and associated material surface conditions. The overarching goal of this project is to understand the propensity of THORP storage cans towards localised corrosion and Environment Assisted Cracking (EAC) in HCl and chloride-bearing atmospheric environments. The investigation focused on two possible corrosion cases: (1) understanding the effect of surface finishing on material performance in chloride-containing atmospheric environments, and (2) characterising the effects of the HCl aqueous solutions inside the can, with potential formation of HCl vapour. Microstructure investigations were carried out on surface-treated type 316L coupon specimens. The application of aqua blasting resulted in a deformed near-surface microstructure, containing compressive residual stresses to a depth of 100-120 micrometres. Subsequent laser engraving produced a recrystallized surface layer with tensile residual stresses reaching to a depth of 200 micrometres. Changes of surface roughness topography were accompanied by the development of a thick oxide/hydroxide film after laser engraving. Atmospheric exposure revealed similar corrosion attack for all samples, with laser engraving exhibiting the lowest number of corrosion sites, but with the largest average depth of attack. In addition, laser engraving led to atmospheric-induced stress corrosion cracking (AISCC) within two weeks of exposure to 386 ug/cm2 MgCl2-laden droplet deposits, with crack growth rates similar to ground U-bend samples. Strategies to reduce the likelihood of AISCC of laser-engraved components are discussed. The influence of HCl concentration and exposure temperature on the corrosion type and rate of annealed and cold rolled type 316L stainless steel has also been investigated. Cold rolling of up to 20 % reduction was introduced, with potentio-dynamic polarization measurements conducted in 0.01 - 3 M HCl aqueous solution. Results are compared to microstructures immersed under open circuit conditions, and to HCl-laden droplet deposits at temperatures up to 80C. Corrosion type diagrams are introduced to describe the transition between uniform corrosion, mixed-mode uniform with pitting corrosion, and pitting corrosion only, as a function of temperature, HCl concentration, and cold deformation. SCC tests of type 316L stainless steel have been carried out at 110C, by exposing U-Bend samples to HCl-laden droplets and HCl vapour. The humidity of the environment was controlled using defined volume fractions of H2O in a sealed environmental chamber. HCl-laden droplets with chloride deposition densities exceeding 1.5 ug/cm2 led to SCC after 90 minutes of exposure, whereas no corrosion attack was observed for samples with exposure to 0.15 ug/cm2 HCl. Increasing HCl concentrations resulted in fewer, but longer cracks, reaching up-to several hundreds of micrometres in length. HCl vapour exposure was carried out by adding various volumes of HCl solution in a beaker to the sealed test chambers. These HCl vapour tests confirmed a change of corrosion type with HCl concentration, from pitting corrosion with SCC, to the occurrence of uniform corrosion.

Environmentally-assisted cracking (EAC) phenomena affect the in-service behaviour of austenitic stainless steels in nuclear power plants. EAC includes such degradation phenomena as Stress Corrosion Cracking (SCC) and Corrosion Fatigue (CF). Factors affecting EAC include the material type, microstructure, environment, and stress. This is an important degradation issue for both current and Gen III+ light water reactors, particularly as nuclear power plant lifetimes are extended ( > 60 years). Thus, it is important to understand the behaviour of the alloys used in light water reactors, and phenomena such as SCC to avoid failures. Although there is no agreement on the mechanism(s) of SCC, the importance of localized electrochemical reactions at the material surface is widely recognised. Considerable research has been performed on SCC and CF crack growth, but the initiation phenomena are not fully understood. In this project, novel in situ analytical TEM techniques have been developed and applied to explore localised reactions in Type 304 austenitic stainless steel. In situ transmission electron microscopy has become an increasingly important and dynamic research area in materials science with the advent of unique microscope platforms and a range of specialized in situ specimen holders. In metals research, the ability to image and perform X-ray energy dispersive spectroscopy (XED) analyses of metals in liquids are particularly important for detailed study of the metal-environment interactions with specific microstructural features. To further facilitate such studies a special hybrid specimen preparation technique involving electropolishing and FIB extraction has been developed in this thesis to enable metal specimens to be examined in the liquid cell TEM specimen holder using both distilled H2O and H2SO4 solutions. Furthermore, a novel electrode configuration has been designed to permit the localized electrochemical measurement of electron-transparent specimens in the TEM. These novel approaches have been benchmarked by extensive ex situ experiments, including both conventional electrochemical measurements and microcell measurements. The results are discussed in terms of validation of in situ test data as well as the role of the electron beam in the experiments. In situ liquid cell TEM experiments have also explored the localized dissolution of MnS inclusions in H2O, and correlated the behaviour with ex situ experiments. Based on the research performed in this thesis, in situ liquid cell and in situ electrochemical cell experiments can be used to study nanoscale reactions pertaining to corrosion and localized dissolution leading to "precursor" events for subsequent EAC phenomena.

The stress corrosion cracking (SCC) of components which are fabricated from austenitic stainless steel has been observed in the primary circuit of pressurised water reactors (PWR). In recent years it has become an increasing concern that cold work can induce susceptibility to SCC in these materials, even when exposed to good-quality flowing coolant. Laboratory studies which were launched in response to this observation have confirmed that SCC susceptibility is enhanced by cold work. The intention of this study is therefore to investigate the link between the effects of cold work on the material and the susceptibility to SCC. The investigation has been conducted on a grade 304 austenitic stainless steel. Characterisation of the microstructure and mechanical properties has been carried out in the annealed condition, and following cold rolling to a reduction in thickness of 20 %. The cold rolled material has then been subjected to SCC tests in simulated PWR primary circuit coolant. Two types of test were utilised: slow strain rate tests (SSRTs) were carried out in order to investigate the initiation of cracks from a smooth surface and constant load tests using pre-cracked specimens were used to investigate the crack propagation behaviour. In both types of test the SCC produced was predominantly intergranular. The SSRTs revealed that the most susceptible grain boundaries separated grains which had dissimilar deformation microstructures (one grain deformed heavily by planar bands, the other more homogenously). It was also observed that initiation could occur on a grain boundary which is adjacent to an annealing twin. In both microstructural configurations the susceptibility is likely to be due to the deformation incompatibility across the failed boundary, possible indicating that shear at the boundary is important for the initiation of cracking. The crack propagation behaviour of the rolled material was particularly anisotropic; regardless of the loading direction (specimens were manufactured to allow loading along the rolling, transverse and normal plate directions) cracking was observed to occur parallel to the rolling-transverse plane. The origin of this behaviour was explored in terms of preferential alignment of the deformation microstructure and the anisotropic mechanical properties of the rolled plate. Limited transgranular cracking was also observed, which occurred along oxidised deformation bands. The results overall indicate that heterogeneous deformation between different regions of the material, and preferential alignment of the deformation microstructure are important with respect to the SCC susceptibility of the rolled material.

Surface treatments used to improve the life of a material known as peening are already extensively used in industry. The main aim of peening is to introduce compressive resiudal stress to the surface and subsurface of a metallic material, however literature also includes a number of microstructural and mechanical effects that peening introduces to a material when the compressive residual stress is established. The aim of this dissertation is compare and contrast the mechanical and microstructural effects of a current industrial peening method called shot peening, with three new increasingly competitive surface treatments. These are laser shock peening, ultrasonic impact treatment and water jet cavitation peening. The surface finish, and changes in microstructure, hardness depth profile, residual stress depth profile and plastic work depth profile of the four surface treatments are analysed. The effect of the peening parameters on the material is also determined, such as length of time of treatment, shot size, step size, direction of treatment, and irradiance per centimetre squared. The effect of peening on the residual stress depth profile of a gas tungsten eight pass grooved weld is also determined. Welding is a known region of early failure of material, with one of the factors affecting this being the introduction of tensile residual stress to the surface and near surface of the weld. An analysis to determine if peening the welded region alters the residual stress was carried out. In all experiments in this dissertation, the material that was used was austenitic stainless steel, as this material is highly used, especially within the nuclear industry. The results of this dissertation show that different peening types and peenign parameters produce a variety of surface, microstructural and mechanical effects to austenitic stainless steel. Peening of an aaustenitic stainless steel welded region results in teh near surface tensile residual stress to alter to ccompressive residual stress.

This thesis investigates the heat transfer involved in the heating of wire in an Electrically-Heated Tubular Strand Annealing Furnace. The fundamental concepts of the furnace are explained, followed by the rationale behind the basic design of the furnace. This is followed by an experimental investigation of a furnace which is actually used in production at a local wire factory. The overall efficiency of this furnace in heating wire is found to be 69%, as compared with the manufacturer's estimate of 78%. As a consequence of this work, a prototype furnace rig has been designed and set up in the Thermodynamics laboratory, where tests have been carried out on the heating of 2,3 and 4 mm diameter, Austenitic stainless steel, over a temperature range of 20-1000°C. The results from these experiments are then presented, and the prototype furnace is found to be able to anneal Stainless Steel wire at a maximum rate of 5.4 Kg per hour, with an overall efficiency of 60% The results of the experiments have been analyzed and are subsequently discussed, and the wire temperature profiles corrected for thermocouple installation error. A model is developed to predict the heating of wire due to radiant heat transfer in the prototype furnace. This model is used to predict the temperature profiles of 2,3 and 4 mm diameter, Austenitic Stainless Steel wire being heated in the furnace, with the three heater zones at a set temperature of 1000°C. The results from these simulations suggest that over all the tests presented, the heat transferred to the wire by radiation provides approximately 65% of the energy which is required to heat the wire at the experimentally determined rate. A second prediction model is then developed which considers the heat transfer to the wire from the process tube by convection, in addition to radiation. The wire temperature profiles predicted using this model are determined to be within approximately 5% of the experimental results. A more complex model is discussed and has been developed, leading to the final model of a multi-tube furnace, and this is used to predict the wire temperature profiles for the multitube furnace on which the experimental work was carried out. These are found to be within approximately 10% of the experimental results. This model is then developed into a design tool, for use by the manufacturers to predict the performance of a given furnace design.

Post-irradiation annealing (PIA) was conducted on a 304L stainless steel irradiated to 5.9 dpa in the Barsebäck-1 boiling water reactor (BWR), to investigate its effect on the irradiated microstructure and the mitigation of irradiation-assisted stress corrosion cracking (IASCC) susceptibility. Evolution of irradiated microstructure including faulted dislocation loops, radiation-induced solute clusters and radiation-induced segregation (RIS) at the grain boundary was investigated in the as-irradiated condition and following thermal annealing at 500°C and 550°C for times up to 20 h. IASCC susceptibility was measured for the as-irradiated and four PIA conditions (500°C: 1 h and 550°C: 1, 5, and 20 h) via interrupted constant extension rate tensile and four-point bend experiments under simulated BWR normal water chemistry (NWC) conditions. While the faulted dislocation loop size did not change significantly after annealing up to 550°C: 5 h, the loop number density decreased steadily with increased annealing treatments, and faulted loops were fully removed after annealing at 550°C: 20 h. The average size of Ni-Si and Al-Cu clusters increased while the number density decreased with increased annealing. RIS of various elements progressively decreased with extent of annealing but were not fully removed by 550°C: 20 h. The annealing treatments were also observed to progressively reduce IASCC susceptibility, as measured by the final intergranular fracture fraction (tensile) and crack length per unit area (four-point bend), with full removal of IASCC susceptibility being observed following annealing at 550°C: 1 h for tensile specimens and 500°C: 1h for four-point bend specimens. Among the microstructure and mechanical property parameters measured as a function of PIA, the changes in the average dislocation channel spacing was observed to most closely relate to the mitigation of IASCC; decreasing by ~25% and ~40% from the as-irradiated condition after annealing at 500°C: 1 h and 550°C: 1 h, respectively. The mitigation of IASCC susceptibility correlated well with the decrease in the average dislocation channel spacing and is consistent with a process in which crack initiation is controlled in part by the high normal stress at discontinuous dislocation channel-grain boundary intersections.

Austenitic stainless steel, hard-facing alloys have recently been developed as a replacement to the cobalt-based Stellite alloys traditionally used for valve seat coatings in the nuclear power industry. NitroMaxx, a new nitrogen-saturated stainless steel, has proven capable of overcoming the performance degradation issues seen in other austenitic stainless steels at high temperatures (~350°C). NitroMaxx’s excellent galling-resistance has, in part, been attributed to a stacking fault energy (SFE) modification brought about by the addition of interstitial nitrogen. To better understand the matrix-specific mechanical properties that processing parameters affect and to develop a cost and time efficient screening tool for future alloy development, nanoindentation and post-indentation analysis was performed. Nanoindentation was used to obtain both composite and matrix-specific mechanical properties. The indents were subjected to post-indentation topographical analysis with an optical profilometer and an atomic force microscope (AFM). Scanning electron microscopy coupled with electron backscatter diffraction (EBSD) provided crystallographic information and further deformation characterization. The composite mechanical properties measured using room temperature nanoindentation did not show any processing parameter dependence, but did accurately predict the poor wear resistance of 316L stainless steel. The matrix-specific nanoindentation, on the other hand, found a strong Hmatrix, Ematrix, and (H/E) matrix heat treatment temperature dependence. As the heat treatment temperature increased, both Hmatrix and (H/E) matrix increased, while Ematrix decreased. The increase in predicted wear resistance with increasing heat treatment temperature was attributed to the increased amount of interstitial nitrogen in the FCC matrix. Variations in the amount of melt nitrogen seemed to have a negligible effect on the measured matrix properties. Similarly, optical profilometry measurements indicated that the distribution of the plastic zone under the indenter depends upon the heat treatment temperature. The pile-up geometries suggested that the plastic zone expands to greater depths in NitroMaxx samples heat treated at 1200°C, relative to samples heat treated at 1100°C and 316L. This is significant because for materials with similar mechanical properties, deeper plastic zone penetration depths indicate increased amounts of work-hardening. AFM measurements showed that NOREM 02 and NitroMaxx (0.40 wt%N) - 1200°C recovered 34% and 39% of the total strain from indentation, respectively, as measured by the hf/hmax ratio. The AFM results proved that a large portion of the plastic flow during indentation occurred on the samples’ surfaces. Also, small, discrete steps were found within the large pile-up around the indents. The average step width was about 50% less in NitroMaxx, relative to NOREM 02, for a given step height. This may be the result of improved work-hardenability in NitroMaxx from the addition of nitrogen. SEM imaging provided persuasive evidence of planar slip and low SFE values in both the NitroMaxx and NOREM 02 matrix-phases. EBSD measured an average grain diameter of 3.0 – 4.5µm in both NitroMaxx (0.40 wt%N) -1200°C and NOREM 02 and neither alloy showed signs of texturing. Grain reference misorientation deviation (GROD) maps measured strains in NitroMaxx (0.40 wtN) - 1200°C that were 8.5% - 9.9% smaller than those in NOREM 02. This difference closely matched that seen between the samples’ measured Hmatrix values, suggesting a correlation. Also, the strain was more localized in the NitroMaxx sample. It was concluded that the NitroMaxx sample work-hardening more effectively than the NOREM 02 sample and that the heat treatment temperature, not melt nitrogen content, is the key processing parameter in determining room temperature, matrix-specific wear resistance.

The loss of nitrogen from conduction-mode laser welds on high-nitrogen austenitic stainless steel was examined through numerical simulation. The transport and absorption and desorption mechanisms for nitrogen in a weld pool are not well understood. A three dimensional simulation using the Volume of Fluid (VOF) technique was developed based on mathematical and physical models to separate the well understood effects of conduction-mode laser weld pool development and the less understood chemistry effects of nitrogen loss and gain. Nitrogen macroparticles tracked the transport and desorption of nitrogen from the transient weld pool formed by a moving Gaussian laser heat input for two cases with different laser-power densities. Simulation results showed an inhomogeneous distribution of macroparticles in the molten weld pool due to non-uniform flow patterns. However, the final nitrogen distribution was homogeneous in the solidified region at the trailing edge of the weld pool. The simulation showed approximately a 25-32% loss of nitrogen which is comparable with experimental conduction-mode laser welds on Nitronic-40 and literature, which reports 22-30% loss of nitrogen. The penetration depth and width of the weld pool predicted in the simulation was comparable to the penetration depth and width seen in macrographs of conduction-mode laser welds.

The mechanical behavior of hydrogen-charged 316L stainless steel has been studied in samples containing sharp fatigue-induced pre-cracks, and compared with material that had been carburized at low temperatures following pre-cracking. The hydrogen charging involved exposure to high pressure (~138 MPa) H2 at 573 K for 24 days.Non-treated samples either stayed elastic in low stress intensity range or exhibited plasticity at the crack tip at high range; no hydrogen-induced cracking could be observed. On the carburized sample, however, the main crack extended during loading. The variation of crack depth along the crack was verified and reveals that the crack becomes shallower as it approached the newly extended crack tip. The threshold stress intensity factor for the hardened case, Kth, is calculated based on this observation. Plastic zone evolution at the crack tip suggests the presence of hydrogen facilitates the motion of dislocations at the room temperature.

Historically it has been very difficult to harden the surface of stainless steels without the loss of corrosion resistance. A novel low temperature carburization process developed by Swagelok Co. which greatly improves the wear resistance of stainless steel has accomplished this feat. This project has studied the friction and wear behavior of low temperature carburized AISI 316L; 1.) Sliding against hard materials, 2.) Sliding under a range of contact stresses and sliding speeds, and 3.) The relationship of wear resistance to the properties of the surface hardened region. It was found that low temperature carburization provided an immense improvement in wear resistance when sliding against hard materials, and this improvement is maintained under stresses and sliding speeds that are too severe for non-treated stainless steels. Furthermore, the improvement in wear resistance was found to be heavily dependent on the hardness and thickness of the case hardened region.

The objective of this thesis is to quantify the effect of oversized solutes on radiation-induced segregation in austenitic stainless steels and to determine the mechanism of this effect. Zr or Hf additions to stainless steels demonstrated a reduction in radiation-induced segregation of Cr and Ni at the grain boundary after proton irradiation at 400°C and 500°C to low doses, but the solute effect disappeared at higher doses. Rate theory modeling of RIS incorporated a solute-vacancy trapping mechanism to predict solute effects on RIS. The model showed that RIS is most sensitive to solute-vacancy binding energy. First principles calculations determined a binding energy of 1.08 eV for Zr and 0.71 eV for Hf. Model and experiment agreed in showing suppression of Cr depletion at 3 dpa at 400°C and 1 dpa at 500°C, and experimental results were consistent with the model in showing greater effectiveness of Zr relative to Hf due to a larger binding energy. The dislocation loop microstructure was measured at 400°C, 3 and 7 dpa, and showed a significant decrease in loop density and total loop line length in oversized solute alloys relative to reference alloys. Loop microstructure results were consistent with RIS results by confirming enhanced recombination of point defects by solute-vacancy trapping. Increases in RIS with dose indicated a loss of solute effectiveness, which was consistent with an observed increase in loop line length from 3 to 7 dpa. The loss of solute effectiveness at high dose is attributed to a loss of oversized solute from the matrix due to coarsening of carbide precipitates. X-ray diffraction identified a microstructure with ZrC or HfC precipitates prior to irradiation. Precipitate coarsening was identified as the most likely mechanism for the loss of solute effectiveness on RIS by the following: 1) diffusion analysis suggested significant solute diffusion by the vacancy flux to precipitates on time scales of proton irradiations, and 2) atom probe measurements confirmed the loss of oversized solute in solution as a function of irradiation dose. RIS measurements and subsequent analysis were consistent with the solute-vacancy trapping process as the mechanism for enhanced recombination and RIS suppression.

Nano/Submicron austenitic stainless steels increasingly attracted attention over last few years with their potential to revolutionize traditional materials design in many applications via fine structural control to tailor the engineering properties. At these nano/submicron grain scales, the fraction of atoms at the grain boundaries is of the same order as those inside the grains, and thus grain boundary strengthening can be significant (1000 MPa for 0.5 μm grain size). To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90% cold rolled and subsequently annealed at temperatures varying from 600 to 900ºC for a dwelling time of 30 minutes. The effect of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation is also determined herein. The analysis of the as-cold rolled microstructure reveals that a 90% cold reduction produces a combination of the lath type and dislocation cell type martensitic structure. For the annealed samples, the average austenite grain size increases from 285 nm at 600ºC to 5.85 μm at 900ºC. On the other hand, the amount of reverted austenite exhibits a maximum at 750ºC, where austenite grains with an average grain size of 1.7 μm compose 95% of the microstructure. Annealing temperatures above 750ºC show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ nanoscale carbides form within the grains and at the grain boundaries.

The feasibility of using Laves phase precipitation as a major strengthening constituent in a work-hardenable austenitic Fe-Cr-Ni-Mo stainless steel has been demonstrated. Thermodynamic and kinetic analyses showed that among the three intermetallic phases: sigma (ς), chi (χ) and Laves (η) that may form in the system, increased Mo content promoted most extensively Laves phase formation in a cold-worked structure. By altering the AISI 316L base alloy composition by the addition of up to 4 wt.% Mo favorable precipitation reactions were induced to occur before the onset of recrystallization. This precipitation of Laves phase produced a fine, well distributed particles in an austenitic matrix. Overall three groups of materials were investigated and compared based upon their response to aging at 704°C (1300°F) after having been cold worked by extrusion to a true strain of 1.08.