Metallic alloys are important structural and cladding materials for current and future reactors. Understanding radiation-induced damage on metallic alloys is important for maintaining the safety of nuclear reactors. This dissertation mainly focuses on radiation-induced primary damage in iron-based metallic. Systematic molecular dynamics simulations were conducted to study the alloying element effects on the primary damage in Fe-based alloys, including defect production and dislocation loop transformations, and their connections with defect thermodynamics.
First, effects of alloying elements on the primary damage in three Fe-based ferritic alloy systems were studied, with a particular focus on the production behaviors of solute interstitials. The production behaviors of solute interstitials include over-production or under-production, compared with their solute concentration in the Fe matrix. The three alloy systems are: (1) a Fe-Cr alloy system; (2) a Fe-Cu alloy system; and (3) an ideal but artificial Fe-Cr alloy system, which is used as a reference system. It is found that the number ratio of solute interstitials to the total interstitials is distinct in these alloys. The solute interstitials are over-produced in the Fe-Cr systems but under-produced in the Fe-Cu system, compared with solute composition in the alloys. The defect formation energies in both dilute and concentrated alloys, interstitial-solute binding energies, liquid diffusivities of Fe and solute atoms, and heat of mixing have been calculated for both Fe-Cr and Fe-Cu alloys. Among these factors, our analysis shows that the relative thermodynamic stability between Fe self-interstitials and solute interstitials plays the most important role on the production behaviors of solute interstitials.
Next, to obtain a correlation that can quantitatively estimate the solute interstitial fraction in the Fe-based alloys, molecular dynamics simulations were conducted to simulate the cascade damage in a series of "artificial" Fe-Cr alloys with tunable binding energies between a substitutional solute (Cr) atom and a Fe self-interstitial atom (SIA). To achieve this, the Fe-Cr cross pair interaction in the interatomic potential was modified by multiplying a scaling factor so that the solute-SIA binding energy varies linearly from positive to negative values. It is found that the solute interstitial fraction has a strong correlation with the solute-SIA binding energy, and the correlation can be approximately described by a Fermi-Dirac-Distribution-like equation. The independent defect production results reported in literature are found to align well with this correlation. The correlation may be used to estimate the solute interstitial fraction in a wide range of Fe-based alloys simply based on the solute-SIA binding energy, without conducting laborious cascade simulations.
Furthermore, primary damage was further investigated in Fe-tungsten (W) alloys to investigate the atomic size effect. The large difference in atomic size between Fe and W can introduce both global volume expansion and local lattice distortion in the Fe matrix. In order to understand how oversized W influences the defect production behaviors in Fe-based alloys, molecular dynamics simulations were conducted to study the primary damage in three systems at 300 K: (a) unstrained pure Fe, (b) Fe-5at.%W alloy, and (c) strained pure Fe with the same volume expansion as the Fe-5%W. The investigation of defect production behaviors include the production of Frenkel pairs, and cluster formation preference. Based on the total number of Frenkel pairs, it indicates that the global volume expansion introduced by oversized W and external strain can lead to enhanced defect production. Meanwhile, the defect cluster analysis in all three systems indicates that the local lattice distortion induced by oversized W can significantly influence the morphologies and size distributions of defect structures. Defect formation energies were calculated to interpret the different defect production behaviors in these systems.
Finally, radiation can produce not only point defects but also both <100> and ½<111> type dislocation loops in pure Fe and Fe-Cr alloys. However, contradictory experimental results have been reported on how the Cr concentration affects the ratio of <100> to ½<111> dislocation loops. In this section, molecular dynamics simulations were conducted to study how Cr concentration affects the formation probability of <100> dislocation loops from overlapping cascades on a pre-existing ½<111> dislocation loop in a series of Fe-Cr alloys with 0 – 15%Cr at 300 K. Our atomistic modeling directly demonstrates that the ratio of <100> to ½<111> dislocation loops decreases with the increasing Cr concentration, which is consistent with many experimental observations. Next, independent molecular statics calculations show that the formation energies of both <100> and ½<111> dislocation loops increase with the increasing of Cr content. However, the former has a much faster increase rate than the latter, indicating that the formation of <100> loops becomes energetically more and more unfavorable than ½<111> loops as the Cr content increases. The results provide a thermodynamics-based explanation for why Cr suppresses the formation of <100> dislocation loops in Fe-Cr alloys, which can be applied to all <100> loop formation mechanisms proposed in literature. The possible effects of other alloying elements on the formation probability of <100> loops in Fe-based alloys are also discussed. / Doctor of Philosophy / Metallic alloys are important structural and cladding materials for current and future nuclear reactors. The understanding of radiation-induced damage in metallic alloys is important for the safe operation of nuclear reactors. This dissertation mainly focuses on radiation-induced primary damage in iron-based ferritic alloys. Systematic molecular dynamics simulations were conducted to study how different alloying elements influence the primary damage behaviors in iron-based alloys, including defect production behaviors and dislocation loop transformations. The relations between defect production and defect thermodynamics are also studied.
First, molecular dynamics simulations were conducted to study the effects of alloying elements on the primary damage behavior in three Fe-based ferritic alloy systems (Fe-Cr, Fe-Cu, and ideal Fe-Cr), with a particular focus on the production behaviors of solute interstitials. It is found that the number ratio of solute interstitials to the total interstitials has distinct behavior in these alloys. In the Fe-Cr alloys, the ratio of Cr interstitials is much higher than the Cr concentration in the Fe-Cr alloys. By contrast, in the Fe-Cu alloys Cu interstitials are barely produced. In the ideal alloy system, the fraction of solute interstitials is close to the solute concentration in the alloys. Among all the factors we have investigated, it is found the relative thermodynamic stability between Fe self-interstitials and solute interstitials plays the most important role on affecting the production behaviors of solute interstitials.
Next, to obtain a quantitative correlation that can predict the solute interstitial fraction in the Fe-based alloys, molecular dynamics simulations were conducted to simulate the cascade damage in a series of "artificial" Fe-Cr alloys with tunable binding energies between a substitutional solute (Cr) atom and a Fe self-interstitial atom (SIA). It is found that the solute interstitial fraction has a strong correlation with the solute-SIA binding energy, and the correlation can be approximately described by an analytical equation. The correlation may be used to estimate the solute interstitial fraction in a wide range of Fe-based alloys simply based on the solute-SIA binding energy, without conducting laborious cascade simulations.
Furthermore, primary damage was further investigated in iron-tungsten (Fe-W) alloys. W is about 10.5% larger in atomic radius or 34.8% larger in atomic volume than Fe. The oversize W can introduce both global volume expansion and local lattice distortion in the Fe matrix. Through molecular dynamics simulations in a series of model systems for comparison, it is found that oversized W can lead to enhanced defect production. In addition, it is found that oversized W can significantly influence the morphologies and size distributions of defect clusters.
Finally, molecular dynamics simulations were conducted to study how Cr concentration affects the formation probability of <100> and ½<111> dislocation loops in a series of Fe-Cr alloys. Our results demonstrate that the ratio of <100> to ½<111> dislocation loops decreases with the increasing Cr concentration, which is consistent with many experimental observations. The formation energies of both <100> and ½<111> dislocation loops indicate that the formation of <100> loops becomes energetically more and more unfavorable than ½<111> loops as the Cr content increases. The results provide a thermodynamics-based explanation for why Cr suppresses the formation of <100> dislocation loops in Fe-Cr alloys.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106670 |
| Date | 26 May 2020 |
| Creators | Zhang, Yaxuan |
| Contributors | Materials Science and Engineering, Bai, Xianming, Reynolds, William T., Zhang, Jinsuo, Murayama, Mitsuhiro |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.1517 seconds