Economic feasibility study of total energy system options for the Massachusetts Institute of Technology

Includes bibliographical references (leaf 39)Prepared for the MIT Physical Plant Dep

A Systematic Study of Radiation-Induced Segregation in Ferritic–Martensitic Alloys

A systematic approach to measuring radiation-induced segregation (RIS) was used on four ferritic–martensitic (F–M) alloys: T91, HCM12A, HT9, and a Fe–9Cr model alloy, irradiated with 2.0 MeV protons over a range of doses (1–10 dpa) and temperatures (300–700°C). The experimental conditions are established so as to isolate the dependence of RIS on the experimental parameters: temperature, dose and bulk composition. RIS is measured at prior austenite grain boundaries (PAGBs) using the STEM/EDX technique. Chromium is found to enrich at PAGBs in all conditions with the exception being T91 irradiated to 3 dpa at 700°C. The magnitude of enrichment is small (\u3c2 at%). Minor elements Si, Ni, and Cu also enrich consistently. A bell-shaped temperature dependence of RIS is observed in all elements. The amount of Cr enrichment decreases as a function of increasing bulk Cr concentration. Lastly, it is found that the 9Cr model alloy reaches a steady-state Cr RIS behavior at approximately 7 dpa, while the T91 reaches what may be a steady state near 3 dpa, then the amount of enrichment decreases at 10 dpa

Radiation-Induced Segregation and Phase Stability in Candidate Alloys for the Advanced Burner Reactor

Major accomplishments of this project were the following: 1) Radiation induced depletion of Cr occurs in alloy D9, in agreement with that observed in austenitic alloys. 2) In F-M alloys, Cr enriches at PAG grain boundaries at low dose (<7 dpa) and at intermediate temperature (400°C) and the magnitude of the enrichment decreases with temperature. 3) Cr enrichment decreases with dose, remaining enriched in alloy T91 up to 10 dpa, but changing to depletion above 3 dpa in HT9 and HCM12A. 4) Cr has a higher diffusivity than Fe by a vacancy mechanism and the corresponding atomic flux of Cr is larger than Fe in the opposite direction to the vacancy flux. 5) Cr concentration at grain boundaries decreases as a result of vacancy transport during electron or proton irradiation, consistent with Inverse Kirkendall models. 6) Inclusion of other point defect sinks into the KLMC simulation of vacancy-mediated diffusion only influences the results in the low temperature, recombination dominated regime, but does not change the conclusion that Cr depletes as a result of vacancy transport to the sink. 7) Cr segregation behavior is independent of Frenkel pair versus cascade production, as simulated for electron versus proton irradiation conditions, for the temperatures investigated. 8) The amount of Cr depletion at a simulated planar boundary with vacancy-mediated diffusion reaches an apparent saturation value by about 1 dpa, with the precise saturation concentration dependent on the ratio of Cr to Fe diffusivity. 9) Cr diffuses faster than Fe by an interstitial transport mechanism, and the corresponding atomic flux of Cr is much larger than Fe in the same direction as the interstitial flux. 10) Observed experimental and computational results show that the radiation induced segregation behavior of Cr is consistent with an Inverse Kirkendall mechanism

Ion beam modification of metals: Compositional and microstructural changes

Ion implantation has become a highly developed tool for modifying the structure and properties of metals and alloys. In addition to direct implantation, a variety of other ion beam techniques such as ion beam mixing, ion beam assisted deposition and plasma source ion implantation have been used increasingly in recent years. The modifications constitute compositional and microstructural changes in the surface of the metal. This leads to alterations in physical properties (transport, optical, corrosion, oxidation), as well as mechanical properties (strength, hardness, wear resistance, fatigue resistance). The compositional changes brought about by ion bombardment are classified into recoil implantation, cascade mixing, radiation-enhanced diffusion, radiation-induced segregation, Gibbsian adsorption and sputtering which combine to produce an often complicated compositional variation within the implanted layer and often, well beyond. Microstructurally, the phases present are often altered from what is expected from equilibrium thermodynamics giving rise to order-disorder transformations, metastable (crystalline, amorphous or quasicrystalline) phase formation and growth, as well as densification, grain growth, formation of a preferred texture and the formation of a high density dislocation network. All these effects need to be understood before one can determine the effect of ion bombardment on the physical and mechanical properties of metals. This paper reviews the literature in terms of the compositional and microstructural changes induced by ion bombardment, whether by direct implantation, ion beam mixing or other forms of ion irradiation. The topics are introduced as well as reviewed, making this a more pedogogical approach as opposed to one which treats only recent developments. The aim is to provide the tools needed to understand the consequent changes in physical and mechanical properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28153/1/0000605.pd

Localized Deformation as a Primary Cause of Irradiation Assisted Stress Corrosion Cracking

The objective of this project is to determine whether deformation mode is a primary factor in the mechanism of irradiation assisted intergranular stress corrosion cracking of austenitic alloys in light watert reactor core components. Deformation mode will be controlled by both the stacking fault energy of the alloy and the degree of irradiation. In order to establish that localized deformation is a major factor in IASCC, the stacking fault energies of the alloys selected for study must be measured. Second, it is completely unknown how dose and SFE trade-off in terms of promoting localized deformation. Finally, it must be established that it is the localized deformation, and not some other factor that drives IASCC

Phase formation in ion‐irradiated and annealed Ni‐rich Ni‐Al thin films

Phase formation was studied in ion‐irradiated multilayer and coevaporated Ni‐20 at. % Al films supported by Cu, Mo, and Ni transmission electron microscopy (TEM) grids. Irradiation with either 700‐keV Xe or 1.7‐MeV Xe, to doses sufficient to homogenize the multilayers (≥7.5×1015 cm−2), resulted in the formation of metastable supersaturated γ and HCP phases in both film types. Post‐irradiation annealing of multilayers at 450 °C for 1 h transformed the metastable phases to a two‐phase γ+γ′ microstructure. In the absence of Cu, the formation of γ′ appeared to proceed by a traditional diffusional growth mechanism, resulting in small (<50 Å) γ′ precipitates in γ matrix grains. The presence of Cu caused the formation of a dual‐phase γ+γ′ structure (i.e., distinct, equal‐sized grains of γ and γ′) during post‐irradiation annealing. It is suggested that copper affected the nucleation of γ′ precipitates and increased the kinetics of growth resulting in the dual‐phase morphology. Strong irradiation‐induced textures were observed in the multilayers that were less pronounced in the coevaporated films. The texture in the multilayers was attributed to the presence of a slight as‐evaporated texture combined with the enhanced atomic mobility due to the heat‐of‐mixing released during irradiation. The irradiation‐induced texture appeared to be necessary for the formation of the dual‐phase structure since it likely provided high‐diffusivity paths for Cu to diffuse into the film from the TEM grid.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70874/2/JAPIAU-69-4-2021-1.pd

The heat‐of‐mixing effect on ion‐induced grain growth

Irradiation experiments were conducted on multilayer (ML) and coevaporated (CO) thin films in order to examine the role that the heat‐of‐mixing (ΔHmix) has in ion‐induced grain growth. Room‐temperature irradiations using 1.7‐MeV Xe ions were performed in the High Voltage Electron Microscope at Argonne National Laboratory. The ML films (Pt‐Ti, Pt‐V, Pt‐Ni, Au‐Co, and Ni‐Al) spanned a large range of calculated ΔHmix values. Comparison of grain growth rates between ML and CO films of a given alloy confirmed a heat‐of‐mixing effect. With the exception of the Pt‐V system, differences in grain growth rates between ML and CO films varied according to the sign of the calculated ΔHmix of the system. Substantial variations in growth rates among CO alloy films experiencing similar displacement damage demonstrated that a purely collisional approach is inadequate for describing ion‐induced grain growth. Therefore consideration must also be given to material‐specific properties, such as cohesive energy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70305/2/JAPIAU-70-3-1252-1.pd

Metastable phase formation by ion beam mixing

There are essentially four basic types of metastable alloys which may be formed through heavy ion irradiation of crystalline structures: amorphous phases with no long range order; crystalline phases with structures different from that of the stable intermetallic alloy; disordered crystalline phases with structures based on the same lattice as that of the stable intermetallic; and a quasicrystalline structure. With the exception of the quasicrystalline structure, all of these metastable structures are produced by ion beam mixing of nickel-aluminum alloys with 500 keV krypton ions. Ion beam mixing was performed on samples formed by alternate evaporation of layers of nickel and aluminum as well as on the intermetallic compounds at both 80 and 300 K. The structure resulting from ion beam mixing depended strongly on composition, and hence its formation was governed primarily by thermodynamic considerations. The thermodynamically favored state was determined analytically using the embedded atom method, and the model results are in qualitative agreement with observations of metastable phase formation. However, kinetic considerations are needed to explain the dependence of the final structures on initial structure and temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26736/1/0000287.pd

Automatic iterative fitting of Rutherford backscattering spectra from multielement samples

A computer code (BASF) has been constructed to perform automatic iterative fitting of Rutherford backscattering spectra using only the experimental spectrum and the parameter set defining the experiment. The code may be used to analyze samples containing anywhere from two to five elements. The code output consists of the total amount of each element present and a composition versus depth profile.The code's performance was verified on both computer generated and experimental backscattering spectra. Samples consisting of nickel substrates onto which layers of pure nickel and pure aluminum have been alternately evaporated in thicknesses of 130 and 100 A, respectively, were used to produce backscattering spectra. These spectra, when analyzed, demonstrated that the code was able to determine the total aluminum content to within 3% and the ratio of aluminum to nickel to within 1% of the thickness monitor readings taken during evaporation. The code has shown the ability to recognize sharp interfaces in well resolved spectra. The code performs equally well on slowly varying concentration profiles which are created during the annealing of layered samples. Limitations on the code and its use include the precise knowledge of the relevant experimental parameters used as input, and complete specification of all elements in the sample. The ultimate limits on the code's accuracy are the resolution of the spectrum and the accuracy of the computed stopping powers.This code provides a significant advantage over other spectrum fitting codes in that the process is fully automated and does not require constant user interaction. Further, it provides the capability of accurately determining concentration profiles in layered samples where the layer thickness is of the order 100 A.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25485/1/0000025.pd