Computational Materials Science and Engineering of Materials in Nuclear Reactors: Microstructure and Atomistic Simulations
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee
Program Organizers: Dilpuneet Aidhy, Clemson University; Michael Tonks, University Of Florida; Mahmood Mamivand; Giovanni Bonny, Belgian Nuclear Research Center

Tuesday 2:00 PM
February 25, 2020
Room: Theater A-9
Location: San Diego Convention Ctr

Session Chair: Izabela Szlufarska, University of Wisconsin; Simon Middleburgh, Bangor University


2:00 PM  Invited
Exploration of Fundamental Radiation Effects Phenomena in Materials: Steven Zinkle1; Ling Wang1; Yan-Ru Lin1; Yajie Zhao1; Arunodaya Bhattacharya1; 1University of Tennessee
    Three examples of recent fundamental experimental radiation effects studies that help validate and inform computational modeling predictions will be presented. 1) Molecular dynamics simulations predict that small interstitial clusters in FCC and BCC metals will exhibit rapid 1-dimensional (1D) glide, as opposed to random-walk 3D diffusion historically assumed. By monitoring depth-dependent loss of precipitate coherency in ion irradiated metals and the extent of peak void swelling zones next to grain boundary denuded zones, we have verified the occurrence of 1D glide but this effect disappears for sink strengths above ~1014/m2. 2) Cr-rich alpha prime precipitation in high-purity Fe-Cr alloys is observed to occur over a wide range of irradiation temperatures (300-450C) and dose rates (up to 0.001 dpa/s), in contrast to early ion irradiation reports of no precipitation. 3) The effects of He on cavity swelling in ion-irradiated Fe-Cr alloys up to ~10 appm He/dpa at 400-550C will be summarized.

2:40 PM  
First Principle Studies of Effects of Solute Segregation on Grain Boundary Strength in Ni-based X-750 Alloy: Ziqi Xiao1; Axel Seoane1; Xian-Ming Bai1; Lingfeng He2; 1Virginia Tech; 2Idaho National Laboratory
    Nickel-based alloys such as X-750 alloy are important structural materials in nuclear reactors. Radiation can cause segregation of solute elements to grain boundaries (GBs), which can modify the GB cohesive strength and may lead to intergranular cracking. In this work, density functional theory studies are conducted to understand how GB character and different solute elements affect the GB strength in Ni-based X-750 alloy. Four different types of GBs are studied. In each GB, the effects of seven different solute elements are investigated. The results show that in general the strength of twin GBs are less affected than other GBs. Solute elements can either reduce, enhance, or has little effect on GB strength. The bonding types and charge transfer between solute elements and Ni atoms at GBs are further analyzed to understand why they cause different effects on GB strength.

3:00 PM  
Molecular Dynamics Simulations of Phosphorus Migration in a Grain Boundary of α-iron: Ken-Ichi Ebihara1; Tomoaki Suzudo1; 1Jaea
    Phosphorus(P) causes steels the grain boundary(GB) embrittlement, which is considered to influence the ductile-brittle transition in reactor pressure vessel steels. In order to develop a rate theory model for calculating GB P segregation based on atomistic processes, so far, we have evaluated the diffusion coefficient of P migration due to dragging by vacancies and self-interstitial atoms and the influence of thermal GB fluctuation and strain around GBs to the P migration. However, the atomistic process that P atoms de-trap from GBs, which is essential to the rate theory model, is still unclear. In this study, we simulated the migration of a P atom in the region of a Σ 3 symmetrical tilt GB using molecular dynamics and evaluated the migration barrier energy. From the results, we found that P atoms can migrate through gaps between iron atoms inside the GB region. In addition, the influence of vacancies is discussed.

3:20 PM  
The Effect of Minor Additives on Radiation Induced Segregation in Austenitic Steel Alloys: Yongfeng Zhang1; Anus Manzoor1; Dilpuneet Aidhy2; Miao Song3; Xiaoyuan Lou4; lingfeng He1; 1Idaho National Laboratory; 2University of Wyoming ; 3University of Michigan ; 4Auburn University
    Radiation induced segregation (RIS) such as Cr depletion at grain boundaries can facilitate stress corrosion cracking in stainless steels. It has been shown experimentally that additives that trap point defects can effectively mitigate Cr depletion. However, such mitigation may just be temporary at low irradiation doses, as suggested by other experiments. To help understand the experimental results and select beneficial additives, a lattice kinetic Monte Carlo model is developed and parameterized by first principle calculations for RIS in austenitic FeNiCr alloys. It’s shown that additives that strongly bind with vacancies, such as Hf, Y and Zr, can indeed delay Cr depletion. However, these additives themselves are subject to radiation induced precipitation, losing the mitigating effect at high irradiation doses. This explains the inconsistency in previous experiments. Such precipitation is kinetically driven and can occur even in under-saturated alloys. The model and the results will help select additives for mitigating RIS.

3:40 PM Break

4:00 PM  Invited
DFT Calculations for Modeling Point Defect and Fission Gas Behavior in Nuclear Fuels: David Andersson1; 1Los Alamos National Laboratory
    The properties of point defects in nuclear fuels are important for fuel performance, since they influence fission gas behaviour, creep, densification, thermal conductivity and properties relevant for manufacturing, such as sintering and grain growth. As part of a continuing effort to develop physics-based predictive fuel performance models we have applied DFT calculations to better understand point defect and fission gas behavior in several different nuclear fuel types, including standard UO2, doped-UO2 and U3Si2. In order to predict concentrations and diffusion rates of point defects and Xe for both in-pile and out-of-pile conditions, the results from DFT calculations are coupled to thermodynamic and kinetic models based on, for example, phase-field and cluster dynamics methodologies, and also complemented by empirical potential simulations. In this talk we will highlight the DFT methodology for UO2 and U3Si2 fuels as well as the approach taken to upscale these results to engineering scale simulations.

4:40 PM  
A First-principles Investigation on the Co-segregation Energetics of Chromium-helium at Grain Boundaries in α-Fe: Sainyam Nagar1; Pulkit Garg1; Nitin Muthegowda1; Mehul Bhatia1; Ilaksh Adlakha1; Kiran Solanki1; 1Arizona State University
    Mitigating the radiation damage of structural materials during nuclear applications is critical to extend the lifetime of nuclear reactors. The mechanical properties of structural alloys such-as ferritic/martensitic steels are affected by the presence of impurities (hydrogen/helium). Thus, the co-segregation energetics of Cr-He at Σ3(111), Σ9(114) and Σ11(113) GBs in α-Fe were investigated using first-principles calculations. In the absence of Cr, maximum of four He atoms segregated favorably at interstitial tetrahedral sites at α-Fe GBs; however, presence of Cr reduced segregation tendency of He atoms at GBs. The suppressing effect of Cr on He segregation was further examined using density of states and charge transfer calculations. Furthermore, presence of Cr atom increased the energy barrier for He migration thereby reducing the mobility of He atoms along the GBs. Thus, Cr addition suppresses He segregation at α-Fe GBs and reduces the deteriorating effects of He on ferritic steels for nuclear reactor applications.

5:00 PM  
DFT+U Point Defect Calculations of Uranium Mononitride: Bryant Jerome1; Dilpuneet Aidhy1; 1University of Wyoming
    DFT+U is used to capture the antiferromagnetic ground state of UN. However, the +U correction leads to possibilities of convergence to metastable states. We provide a comprehensive analysis of ground and metastable states and the resulting defect properties, from convergent supercells calculated using occupation matrix control (OMC), quasi-annealing (QA), and U-ramping. We preform each of these calculations using both the Dudarev and Liechtenstein rotationally-invariant forms of DFT+U. We find that QA and OMC are adequate to reach the ground state, whereas U-ramping tends to converge to metastable states. All three methods predict the N and U interstitials to be the most and least favorable defects respectively. The defect formation energies using the Dudarev form are 1.76 eV (U vacancy), 9.38 eV (U interstitial), 3.86 eV (N vacancy), and 0.94 eV (N interstitial). The results also show that there are significant deviations in defect formation energies across various metastable states.