Radiation-induced Microstructural Evolution Research Articles

Primary radiation damage in tungsten-based high-entropy alloy: Interatomic potential and collision cascade simulations

Tungsten (W)-based high-entropy alloys (HEAs) have shown promising properties as nuclear fusion materials. Characterizing the primary damage is a critical step in describing and revealing the irradiation-induced damage and radiation resistance mechanism. However, there are a limited number of studies on collision cascades and primary radiation damage in W-based HEAs, owing to the large amount of calculations involved and a lack of appropriate interatomic interaction potentials. In this work, we developed a semi-empirical interatomic potential for W–Ta–Cr–V. By using the developed potential, molecular dynamics simulations of collision cascades in W-based HEAs were performed to assess the primary damage due to irradiation. Based on experimental samples, we reported defect production in W38Ta36Cr15V11 and compared it to pure W for primary knock-on atom with energies ranging from 1 keV to 100 keV. Our findings showed that the number of FPs at the thermal spike and the number of surviving FPs at the end of the cascade in W38Ta36Cr15V11 are more than those in pure W, mainly due to the lower threshold displacement energy, melting temperature, and formation energy of point defects. Collision cascades in the W38Ta36Cr15V11 are less likely to result in the formation of dislocation loops compared to pure W. After collision cascade, in W38Ta36Cr15V11, the concentrations of Cr and V atoms in defects is significantly higher than their corresponding concentrations in the system, showing an aggregation tendency. The current collision cascade results provide insights into the primary damage of W-based HEA system under irradiation and should provide reliable guidance for describing the primary damage source terms needed in the kinetic models used to simulate radiation-induced microstructural evolution.

Capture efficiency and bias from the defect dynamics near grain boundaries in BCC Fe using mesoscale simulations

The capture efficiency describes the capability of a sink, such as a grain boundary (GB), dislocation, and void, to absorb point defects (PDs). The bias defines the difference in capture efficiency between the absorption of a vacancy and dumbbell at a sink. Complete kinetic information on PDs, including diffusion barriers and diffusion orientations, as well as accurate saddle points, are needed to determine the capture efficiency and bias at a sink accurately, which is computationally demanding. In the present study, the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) method was used to investigate the defect dynamics of PDs near different types of grain boundaries (GBs) (with both 〈100〉 and 〈110〉 families) accurately in body-centered cubic (BCC) iron (Fe). The capture efficiency, sink strength, and bias factor of different types of GBs were determined in Fe, which, different from traditional rate theory estimation, showed a distinct capture efficiency, sink strength, and bias in different GBs. The results demonstrate a strong positive correlation between the capture efficiency and the GB strain width, instead of the GB misorientation, GB energy, or GB-PD binding energy, which have been investigated previously. This work provides valuable insight into the radiation-induced microstructural evolution of GBs.

Inferring radiation-induced microstructural evolution in single-crystal niobium through changes in thermal transport

Ion beams enable accelerated radiation exposure experiments, but traditional destructive post-irradiation examination techniques remain time-consuming. Here, measurements of thermal diffusivity are used to monitor microstructural changes in single crystal niobium irradiated with Si3+ ions up to 1.9 dpa (5.1 × 1015 ions/cm2). Changes in thermal transport are correlated to defect generation and clustering through dose-microstructure relationships. These measurements are made using transient grating spectroscopy (TGS), a multi-modal, non-destructive, rapid characterization technique. This work demonstrates that in this low-dose point defect clustering regime, direct measurements of thermal properties are a powerful tool for understanding irradiation-induced microstructure evolution.

Effect of alloying elements on defect evolution in Ni-20X binary alloys

The effect of alloying elements on radiation-induced microstructural evolution in Ni and Ni-20X (X = Fe, Cr, Mn and Pd) binary alloys was investigated using ion irradiation and cross-sectional transmission electron microscopy. The three-dimensional migration mode is identified to be the dominating migration mechanism for interstitial clusters in these binary alloys, contrary to the one-dimensional mode that dominates in the well-studied dilute alloys. The results reveal that: (1) the average size of defect clusters decreases as the solute atomic volume size factor increases. Smaller void size in Ni-20Cr is attributed to faster vacancy mobility in the near surface region, and weaker vacancy binding energy beyond the irradiation peak than Ni-20Fe. The smaller voids observed in Ni-20Mn and Ni-20Pd beyond the damage peak are due to the stronger Mn/Pd-vacancy binding effect of largely oversized solute atoms. (2) Oversized solutes can act as strong trapping sites for interstitials. The larger the solute atomic volume factor, the stronger the trapping force. This leads to a more significantly sluggish interstitial migration and smaller dislocation loop size. The average dislocation loop size in Ni-20Fe was four times larger than Ni-20Pd (atomic volume factor being 10.6% and 41.3%) but an order of magnitude lower in density. The smaller dislocation loop size in Ni-20Cr is attributed to stronger interstitial binding of Cr-Ni. Overall, the alloying effect on defects is more significant in concentrated binary alloys than in dilute binary alloys, due to the concentration difference of alloying atoms and the interstitial dominant migration mechanisms in the main irradiated region.

How to apply the phase field method to model radiation damage

The phase field (PF) method provides a valuable means of predicting radiation induced microstructure evolution of domains ranging roughly from 100 nm to 100 μm in size and for lengths of time ranging from microseconds to years, depending on the rate of diffusion. In this work we summarize how to apply the PF method to modeling radiation damage. We begin by summarizing the PF method, and discuss the numerical solution of the PF equations. We then summarize approaches for representing the free energy of defects within a material. Next, three PF approaches for representing extended defects are discussed: the Wheeler, Boettinger, and McFadden model, the Kim, Kim, Suzuki model, and the Grand Potential model. We use a simple 1D void growth problem to compare the predictions of these models, as well as their computational expense. We end by describing how defect generation, recombination and annihilation at sinks can be described by the PF method, as well as discussing modifications required to model nucleation.

Radiation response of oxide-dispersion-strengthened alloy MA956 after self-ion irradiation

We studied the radiation-induced microstructural evolution of an oxide-dispersion-strengthened (ODS) ferritic alloy, MA956, to 180dpa using 3.5MeV Fe2+ ions. Post-irradiation examination showed that voids formed rather early and almost exclusively at the particle-matrix interfaces. Surprisingly, voids formed even in the injected interstitial zone. Comparisons with studies on other ODS alloys with smaller and largely coherent dispersoids irradiated at similar conditions revealed that the larger and not completely coherent oxide particles in MA956 serve as defect collectors which promote nucleation of voids at their interface. The interface configuration, which is related to particle type, crystal structure and size, is one of the important factors determining the defect-sink properties of particle-matrix interfaces.

The co-evolution of microstructure features in self-ion irradiated HT9 at very high damage levels

Understanding the void swelling and phase evolution of reactor structural materials at very high damage levels is essential to maintaining safety and longevity of components in Gen IV fast reactors. A combination of ion irradiation and modeling was utilized to understand the microstructure evolution of ferritic-martensitic alloy HT9 at high dpa. Self-ion irradiation experiments were performed on alloy HT9 to determine the co-evolution of voids, dislocations and precipitates up to 650 dpa at 460 °C. Modeling of microstructure evolution was conducted using the modified Radiation Induced Microstructure Evolution (RIME) model, which utilizes a mean field rate theory approach with grouped cluster dynamics. Irradiations were performed with 5 MeV raster-scanned Fe2+ ions on samples pre-implanted with 10 atom parts per million He. The swelling, dislocation and precipitate evolution at very high dpa was determined using Analytical Electron Microscopy in Scanning Transmission Electron Microscopy (STEM) mode. Experimental results were then interpreted using the RIME model. A microstructure consisting only of dislocations and voids is insufficient to account for the swelling evolution observed experimentally at high damage levels in a complicated microstructure such as irradiated alloy HT9. G phase was found to have a minimal effect on either void or dislocation evolution. M2X played two roles; a variable biased sink for defects, and as a vehicle for removal of carbon from solution, thus promoting void growth. When accounting for all microstructure interactions, swelling at high damage levels is a dynamic process that continues to respond to other changes in the microstructure as long as they occur.

Effect of Mn addition on one-dimensional migration of dislocation loops in body-centered cubic Fe

Elucidation of the one-dimensional (1-D) motion of dislocation loops is important for describing the microstructural development of materials under irradiation. In this study, the effect of Mn on radiation-induced microstructure evolution in body-centered cubic Fe was experimentally investigated by focusing on the migration of dislocation loops. Pure Fe and Fe–1.4Mn alloy were irradiated with Fe3+ ions to introduce dislocation loops. In pure Fe, inhomogeneous distribution of loops in the vicinity of the residual dislocation was observed. However, in Fe–1.4Mn, isolated dislocation loops were homogeneously distributed in a high number density. In situ transmission electron microscopy during annealing revealed that 1-D motion of dislocation loops occurred in pure Fe at 623K, while 1-D motion of dislocation loops occurred minimally in Fe–1.4Mn annealed at temperatures below 773K. These results indicate that 1-D motion of dislocation loops play a key role in producing the differences in the microstructures between pure Fe and Fe–1.4Mn. In pure Fe, dislocation loops were mobile and trapped in the strain field of a dislocation, leading to the formation of loop decoration of dislocations. However, in Fe–1.4Mn, dislocation loops were less mobile and dislocation loops were homogeneously formed in high density in the matrix. The migration of dislocation loops by Mn solute is strongly suggested as one of the key mechanisms of microstructure development in irradiated Fe–Mn alloy.

The effect of oversized solute additions on the microstructure of 316SS irradiated with 5 MeV Ni ++ ions or 3.2 MeV protons

The effect of the oversized hafnium or platinum (0.3 at.%) solutes on the evolution of irradiated microstructure in 316SS was investigated. Irradiations were conducted with 5 MeV Ni-ions at 500 °C to doses up to 50 dpa or with 3.2 MeV protons at 400 °C to a dose of 2.5 dpa, and previous studies demonstrated that these irradiations are capable of producing similar irradiated microstructure and microchemistry relevant to LWR cores. Microstructures of 316SS, 316SS + 0.3 at.% Pt and 316SS + 0.3 at.% Hf were characterized using transmission electron microscopy. The addition of Hf showed a strong effect in suppressing radiation-induced microstructure evolution; no voids were observed at doses up to 50 dpa for Ni-ion irradiation and 2.5 dpa for proton irradiation. The mean diameter of the Frank loops in the Hf-doped samples is about 40% smaller than loops in 316SS. The microstructural examinations from both types of particle irradiation revealed that for 0.3 at.% Pt addition there is no beneficial effect on irradiated microstructure. The mechanisms for the role of oversize solute additions on the microstructure evolution are discussed.

New scenario for the accumulation and release of radiation damage in rock salt and related materials

Rock salt might be a promising geological medium for a radioactive waste repository. However, we have observed that even a basically stable compound such as NaCl may become unstable after heavy irradiation. As a result of the irradiation, dislocations, Na and Cl 2 precipitates and large voids are produced followed ultimately by sudden explosion-driven fracture of the material. We present a new concept of the radiation-induced micro-structural evolution, which explains the phenomena observed in heavily irradiated NaCl samples. This concept can be a prototype of a more general assessment of radiation effects in crystalline radwaste and repository materials, which is necessary for the evaluation of critical effects and maximization of the safety of repositories.

Structural Alterations in Titanate Pyrochlores Induced by Ion Irradiation: Xray Photoelectron Spectrum Interpretation

ABSTRACTTitanate pyrochlores are candidate host materials for the disposition of plutonium from dismantled nuclear weapons. These pyrochlores also have potential applications as solid electrolytes and oxygen gas sensors. The radiation-induced microstructural evolution of titanate pyrochlores has been characterized using x-ray photoelectron spectroscopy (XPS). XPS analysis of the Ti 2p and O 1s binding energy shifts of REE2Ti2O7 (REE: rare earth) surface layers before and after irradiation shows that the primary manifestations of amorphization are distortions or changes in the coordination number of the titanium polyhedra. A model based on glass network structure was developed, and predictions of the relative susceptibilities for amorphization of the titanate pyrochlores are obtained that are consistent with the experimental results.

Critical parameters controlling mechanical stability of NaCl under irradiation

A new concept of the radiation-induced microstructural evolution in ionic crystals has been verified experimentally by phenomena observed in heavily irradiated NaCl, such as the formation of large vacancy voids followed by a sudden explosion-like fracture of crystalline samples. The new concept can be a prototype of an adequate description of the long-term behavior of important insulating materials in intense radiation fields, which can be employed for an evaluation of the critical effects expected under conditions of storage of high level nuclear waste and development of radiation resistant materials. In this paper, we present some new data on the microstructural parameters of materials doped with different impurities and summarize the critical microstructural, irradiation and material parameters that control mechanical stability of rock salt materials under irradiation.

New formulation of the modeling of radiation-induced microstructure evolution in alkali halides

Recent experimental results on heavily irradiated natural and synthetic NaCl crystals give a strong evidence for the formation of large vacancy voids, which cannot be explained by the Jain—Lidiard model used up to date for description of metal colloids and dislocation loops formed during earlier stages of irradiation. We propose a new mechanism of dislocation climb, which involves production of cation vacancies (with a trapped hole) as a results of absorption of extra H centers. We consider an additional set of reactions between experimentally observed extended defects (dislocations, metal colloids, gas bubbles and voids) and point defects. The latter include F and H centers that are the primary defects produced by irradiation, and cation vacancies that are secondary defects produced in the process of dislocation climb. We show that highly overpressurized bubbles of fluid halogen are strongly biased for absorption of H centers, which makes them grow via punching out interstitial dislocation loops. The loops grow and produce cation vacancies that are subsequently trapped at the surfaces of extended defects, such as the incoherent colloids, and recombine with extra F centers giving rise to the void formation. Elastic interaction between extended defects and point defects is shown to play a major role, since it determines the bias factors of extended defects, which is a major driving force of the microstructural evolution under irradiatioin.

Persistence of microstructural evolution in irradiated metals and its consequences at high radiation exposure

A review is presented of a very general aspect of the response of all metals subjected to displacive irradiation. This aspect is referred to as «persistence» and describes the tendency of both radiation-induced microstructural evolution and the associated changes in material properties or dimensional stability to evolve to saturation states that resist further change upon continued irradiation. It is shown that new persistent states can develop on a longer time frame associated with the late-term loss of existing microstructural components or the gain of new components, especially when transmutation and/or segregation occurs. The persistent states are often dependent on the irradiation conditions, and if these are changed, the material usually adjusts to form the persistent state characteristic of the new conditions, with the memory of the former state often lost, and sometimes leaving no visible record of the former state in the new microstructure. Depending on the microstructural components involved, the transition toward the new persistent state can occur quickly or very slowly.

Modeling of the radiation-induced microstructural evolution in ionic crystals

Results of experimental and theoretical investigations are presented on heavily irradiated natural and synthetic NaCl crystals in the temperature range where anion defects are mobile. They give a strong evidence for the formation of vacancy voids, which cannot be explained by the Jain–Lidiard model used up to date for description of metal colloids and dislocation loops formed in ionic crystals during earlier stages of irradiation. We consider an additional set of reactions between experimentally observed extended defects (metal colloids, gas bubbles and voids) and point defects. The latter include F and H centers that are the primary defects produced by irradiation, and cation vacancies (with a trapped hole) that are secondary defects, produced in the process of dislocation climb due to absorption of extra H centers. We show that highly overpressurized bubbles of fluid halogen are strongly biased for absorption of H centers, which makes them grow via punching out interstitial dislocation loops. The loops grow and produce cation vacancies that are subsequently trapped at the incoherent colloids together with extra F centers giving rise to the colloid–void transition. Elastic interaction between extended defects and point defects is shown to play a major role, since it determines the bias factors of extended defects, which is a major driving force of the microstructural evolution under irradiation. A quantitative comparison of the new model for radiation damage in NaCl with experimental data is presented. Mean sizes and volume fractions of all types of observed defects are calculated. It is shown that voids formed due to agglomeration of F centers and cation vacancies can grow to the dimensions exceeding the mean distance between colloids and bubbles, eventually absorbing them, hence, bringing the halogen gas and metal to a back reaction. Impurities play a major role in the void development with increasing irradiation dose, which strongly affects the radiation stability of NaCl.

Hydrogen generation arising from the 59Ni(n, p) reaction and its impact on fission—fusion correlations

While the influence of transmutant helium on radiation-induced microstructural evolution has often been studied, there is a tendency to overlook the influence of concurrently-generated hydrogen. Recent studies suggest that the influence of hydrogen may be enhanced in the presence of large amounts of helium, especially at lower irradiation temperatures typical of projected ITER operation. This interaction may have an impact on potentially hydrogen-sensitive processes such as irradiation-assisted stress corrosion cracking (IASCC) of stainless steel. In nickel-bearing alloys, one of the major sources of helium in neutron spectra with a significant thermal neutron component is the two-step 58Ni(n, γ) 59Ni(n, α) 56Fe reaction. It has generally been overlooked, however, that the competing (n, p) reaction on 59Ni can dominate the hydrogen production process since about one hydrogen atom is generated for every six helium atoms. Inclusion of the damage energy deposited by the (n, p) reaction in the alloy matrix also requires a slight modification in the 59Ni(n, α) damage enhancement formula published earlier. The impact of the (n, p) reaction on both hydrogen generation and displacement rates is evaluated in this paper for a variety of neutron spectra often employed in fission—fusion correlations.

Why is magnesia spinel a radiation-resistant material?

Side-by-side irradiation of stoichiometric MgAl 2O 4 and α-Al 2O 3 in JOYO shows that the radiation-induced microstructural evolution to exposure of ≤ 6 dpa proceeds by very different paths in these two materials. The large difference in dislocation loop evolution appears to account for the ease of void swelling in α-Al 2O 3 and the strong resistance to void formation in MgAl 2O 4. Irradiation of MgAl 2O 4 to much higher exposure (22–230 dpa) in FFTF confirms the details of the dislocation evolution, which involves a progressive change in Burgers vector and habit plane as interstitial loops increase in size. Constraints unique to the MgAl 2O 4 crystal structure do not allow the formation of dislocation network structures that favor swelling.

Development of controlled temperature-cycle irradiation technique in JMTR

To investigate the effects of cyclic temperature changes during neutron irradiation upon radiation induced microstructure evolution and resulting property changes of materials is very important from both fundamental and engineering viewpoints. Therefore, a technique that allows us to do the controlled temperature-cycle irradiation was developed in the Japan Materials Testing Reactor (JMTR). The technique meets the following requirements: (1) the temperature-cycle irradiation is to be performed under three different conditions by changing lower and upper temperatures; 200⇓400°C, 300⇓400°C and 300⇓450°C; (2) the number and period of the temperature-cycles are to be six for 24-day full irradiation and approximately 44 h/44 h at the lower/upper temperatures; (3) the temperatures of each specimen assembly are to be maintained at the lower temperatures before start-up of the reactor and at the upper temperatures during shutdown until the complete absence of reactor power. In this paper, the details of the irradiation rig, successful results and several problems to be overcome for future improvement are presented.

Radiation Induced Microstructural Evolution in Reactor Pressure Vessel Steels

AbstractThe evolution of the fine scale microstructural features leading to irradiation embrittlement of reactor pressure vessel steels is described. Copper rich phases undergo accelerated precipitation from supersaturated solution due to radiation enhanced diffusion. In steels with significant trace quantities of copper the precipitates, characterized by high concentrations and small sizes, are the dominant embrittling feature. Precipitate concentrations, sizes, volume fractions and compositions are consistent with thermodynamic and kinetic models that rationalize the effects of a number of irradiation and metallurgical variables. Phosphide and carbonitride phases may also develop along with new manganese nickel rich precipitates, promoted by high nickel contents. These features may lead to severe embrittlement at high fluence even in low copper steels. While their detailed identity and characteristics are not known, defect cluster-solute complexes with a range of thermal stability are important both directly and indirectly; for example, in mediating flux and temperature effects. In conjunction with the application of state-of-the-art characterization methods, development of advanced modeling tools will be needed to address a number of outstanding issues.

Evolution of microstructure in face-centered cubic metals during irradiation

When fcc metals and alloys are irradiated at elevated temperatures, they tend to evolve toward saturation microstructures that are independent of the starting state of the metal and the early details of irradiation history. This leads to property changes and rates of dimensional change that also eventually become independent of the starting state. The evolution of microstructure in complex alloys, especially during the transient regime, is usually determined by the complex interaction of many microstructural and microchemical processes. The more complex the alloy, the more difficult it is to identify and define the separate influence of each participating mechanism. The use of irradiation studies conducted on simple metals or model alloys assists in understanding the behavior of alloys of engineering relevance. A review of such studies shows that a number of prevailing perceptions of radiation-induced microstructural evolution are not universally correct.