Howard L. Heinisch

On the structure of irradiation-induced collision cascades in metals as a function of recoil energy and crystal structure

Abstract The binary collision code MARLOWE is used to study cascade sizes, defect densities and subcascade configurations in f.c.c., b.c.c. and h.c.p. metals as a function of recoil energy up to 1 MeV. The threshold energies for the formation of subcascades are determined using a definition based on the defect configuration after the collisional phase of the cascade. A computational method was devised for identifying subcascades, and it was used to determine the number and spacing of subcascades as a function of recoil energy. The results are presented to illustrate the effect of recoil energy, atomic mass density and crystal structure on cascade volume and vacancy density after the collisional phase of the cascade, and on the number of subcascades and the subcascade spacing. The cascade morphology in the collisional phase sets the initial conditions for defect production and clustering during the development of each cascade, as well as influencing the global evolution of the microstructure. Comparisons w...

Atomistic simulation of helium-defect interaction in alpha-iron

Molecular dynamics (MD) methods are utilized to study the formation of vacancy clusters created by displacement cascades in α‐Fe containing different concentrations of substitutional He atoms. Primary knock-on atom energies, Ep, from 500eV to 20keV are considered at a temperature of 100K, and the results are compared with those performed in pure α‐Fe. There are distinct differences in the number and size of vacancy clusters within displacement cascades with and without substitutional helium atoms. It is found that many large vacancy clusters can be formed within cascade cores in α‐Fe with helium atoms, in contrast to a few small vacancy clusters observed in pure α‐Fe. The number and size of helium/vacancy clusters generally increase with increasing helium concentration and PKA energy. One of the striking results is that the number of self-interstitial atoms (SIAs) and the size of interstitial clusters are much smaller than those in pure α‐Fe.

Low activation materials

Abstract Low or reduced activation materials are currently being developed and evaluated as structural materials for fusion energy systems. The goal of developing low activation materials is to provide fusion energy systems with a competitive edge over fission energy systems where high level waste issues abound. The primary low activation materials being developed by the international fusion materials community are: (1) ferritic/martensitic steels, (2) vanadium alloys and (3) SiC/SiC composites. These three materials offer a range of temperature and coolant design options and would likely be the optimum choices even without a low activation criteria. However, there are a number of activation, safety and disposal issues that must be solved to achieve an optimum blanket design.

Kinetic Monte Carlo simulations of void lattice formation during irradiation

Over the last decade, molecular dynamics simulations of displacement cascades have revealed that glissile clusters of self-interstitial crowdions are formed directly in cascades and that they migrate one-dimensionally along close-packed directions with extremely low activation energies. Occasionally, under various conditions, a crowdion cluster can change its Burgers vector and glide along a different close-packed direction. The recently developed production bias model (PBM) of microstructure evolution under irradiation has been structured specifically to take into account the unique properties of the vacancy and interstitial clusters produced in the cascades. Atomic-scale kinetic Monte Carlo (KMC) simulations have played a useful role in understanding the defect reaction kinetics of one-dimensionally migrating crowdion clusters as a function of the frequency of direction changes. This has made it possible to incorporate the migration properties of crowdion clusters and changes in reaction kinetics into the PBM. In the present paper we utilize similar KMC simulations to investigate the significant role that crowdion clusters can play in the formation and stability of void lattices. The creation of stable void lattices, starting from a random distribution of voids, is simulated by a KMC model in which vacancies migrate three-dimensionally and self-interstitial atom (SIA) clusters migrate one-dimensionally, interrupted by directional changes. The necessity of both one-dimensional migration and Burgers vectors changes of SIA clusters for the production of stable void lattices is demonstrated, and the effects of the frequency of Burgers vector changes are described.

Radiation hardening in neutron-irradiated polycrystalline copper: Barrier strength of defect clusters

Defect cluster formation in 14-MeV neutron irradiated polycrystalline copper has been observed by transmission electron microscopy (TEM) and correlated with the increase in yield stress. The measurements indicate that the radiation hardening component of the yield strength in polycrystals is not directly additive to the unirradiated yield strength. A transitional behavior was observed for radiation hardening at low fluences, which produces an anomalous variation of the defect cluster barrier strength with fluence. The behavior is attributed to the effect of grain boundaries on slip band transmission. An upper limit for the room temperature barrier strength of defect clusters in neutron-irradiated copper was determined to be α = 0.23.

Stochastic annealing simulation of intracascade defect interactions

Abstract Atomic scale computer simulations are used to investigate the intracascade evolution of the defect populations produced in cascades in copper over macroscopic time scales. Starting with cascades generated using molecular dynamics, the diffusive transport and interactions of the defects are followed for hundreds of seconds in stochastic annealing simulations. The temperature dependencies of annihilation, clustering and free defect production are determined for individual cascades, especially including the effects of the subcascade structure of high energy cascades. The subcascade structure is simulated by closely spaced groups of lower energy MD cascades. The simulation results illustrate the strong influence of the defect configuration existing in the primary damage state on subsequent intracascade evolution. Other significant factors affecting the evolution of the defect distribution are the large differences in mobility and stability of vacancy and interstitial defects and the rapid one-dimensional diffusion of small, glissile interstitial clusters produced directly in cascades. Annealing simulations are also performed on high-energy, subcascade-producing cascades generated with the binary collision approximation and calibrated to MD results.

Stochastic annealing simulation of differential defect production in high energy cascades

Abstract Recent molecular dynamics (MD) studies have confirmed that significant clustering of both vacancies and self-interstitial atoms (SIAs) takes place by the end of the quenching stage of a cascade, and that small interstitial clusters are glissile, with migration energies on the order of 0.1 eV. The spatial segregation and clustering of the vacancies and SIAs give rise to a differential production of mobile vacancies and SIAs that has a strong temperature dependence. At temperatures above recovery Stage V, vacancies can evaporate from clusters, while large SIA clusters produced in the cascade remain stable, leading to a differential increase of mobile vacancies that represents a ‘production bias’ that may be responsible for void swelling. The stochastic annealing simulation code ALSOME is used to investigate quantitatively the differential production of mobile vacancy and SIA defects as a function of temperature for isolated 25 keV cascades in copper generated by MD simulations. The ALSOME code and cascade annealing simulations are described. The annealing simulations indicate that above Stage V, where the cascade vacancy clusters are unstable, nearly 80% of the post-quench vacancies escape the cascade volume, while about half of the post-quench SIAs remain in clusters. The results are sensitive to the relative fractions of SIAs that reside in small, highly mobile clusters and in large, sessile clusters, respectively, which may depend on the cascade energy.

Effects of interatomic potential on He bubble creation by cascades in α -iron

The effects of using different interatomic potentials in molecular dynamics (MD) simulations of the formation of He-vacancy clusters within displacement cascades in α-Fe are investigated using two sets of potentials. Simulations of cascades produced by primary knock-on atoms of energy Ep=1–20keV were performed in α-Fe containing a concentration of substitutional He atoms varying from 1to5at.% at an irradiation temperature of 100K. Although the effects of interatomic potentials on the nucleation of He-vacancy clusters induced by cascades are relatively small, the number and size of He-vacancy clusters produced are significantly different for the different potentials employed in this study. Thus, these differences may influence the microstructural evolution predicted in damage accumulation models that use the results from MD cascade simulations as input. The observed differences in postcascade configurations can be attributed mainly to the differences in the Fe–Fe and Fe–He potentials.

Effects of the neutron spectrum on mechanical property changes in low dose irradiations

Abstract Mechanical property measurements from comparative low dose irradiations of metals and alloys are reviewed. The emphasis is on recent experiments involving miniature tensile specimens irradiated at the Rotating Target Neutron Source II (RTNS-II) at Lawrence Livermore National Laboratory and the Omega West Reactor at Los Alamos National Laboratory. Pure metals, model alloys and structural alloys have been irradiated in RTNS-II from room temperature to 723 K to fluences as high as 8 × 10 22 n/m 2 . Where temperature effects are apparent, the effect of increasing the irradiation temperature is to delay the onset of irradiation hardening to higher fluences. Companion fission reactor irradiations have been more limited in scope, but comparative information has been obtained on tensile properties at irradiation temperatures of 363 K and 563 K. For most of the materials tested the differences in the neutron energy spectrum can be accounted for by using displacements per atom (dpa) as a correlation parameter. The exceptions are all pure metals: copper, niobium and vanadium. There is also evidence for rate effects in some of the data. The rate effects obscure the spectral effects and prevent examination of dpa as a damage correlation parameter.

Displacement damage cross sections for neutron-irradiated silicon carbide

Displacements per atom (DPA) is a widely used damage unit for displacement damage in nuclear materials. Calculating the DPA for SiC irradiated in a particular facility requires a knowledge of the neutron spectrum as well as specific information about displacement damage in that material. In recent years significant improvements in displacement damage information for SiC have been generated, especially the energy required to displace an atom in an irradiation event and the models used to describe electronic and nuclear stopping. Using this information, numerical solutions for the displacement functions in SiC have been determined from coupled integro-differential equations for displacements in polyatomic materials and applied in calculations of spectral-averaged displacement cross sections for SiC. This procedure has been used to generate spectrally averaged displacement cross sections for SiC in a number of reactors used for radiation damage testing of fusion materials, as well as the ARIES-IV conceptual fusion device.