Atomic Displacement Cascades Research Articles

The role of cascade energy and temperature in primary defect formation in iron

Molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascades in iron with energies up to 50 keV (corresponding to a primary knock-on atom (PKA) energy of 79 keV) at 100 K, up to 20 keV at 600 K, and up to 10 keV at 900 K. The cascade damage production has been characterized in terms of several parameters: the number of surviving point defects, the fraction and type of surviving point defects found in clusters, and the size distributions of the in-cascade point defect clusters. A sufficient number of simulations have been completed at each condition to evaluate the statistical variation in these primary damage parameters as a function of irradiation temperature and cascade energy. The energy dependence of stable defect formation can be conveniently separated into three regimes with the number of defects in each regime correlated by a simple power law with a characteristic exponent. The primary effects of cascade energy on defect formation at high energies are explained in terms of subcascade formation. Only a modest effect of temperature is observed on defect survival, while irradiation temperature increases lead to a slight increase in the in-cascade interstitial clustering fraction and a decrease in the vacancy clustering fraction. Cascade energy has little effect on the in-cascade clustering fractions above about 5 keV. However, there is a systematic change in the cluster size distribution, with higher energy cascades producing larger clusters. The loosely coupled nature of the in-cascade vacancy clusters persists at higher energies.

Field ion microscopy of ion-implanted alloys

In this review we describe research in which field ion microscopy is used for the first time for a precision investigation of the structural and phase changes that occur in the surface layers of ordered and aging alloys as a result of implantation with ions of different gases. It is established that when ordered and aging solid solutions are irradiated with gas ions with an energy of 15–40 keV and doses of 1013–1018 ion/cm2 a structural phase transition occurs in the surface volume of the alloys. Radiationally disordered zones in the case of order-disorder transitions and ordered zones in the case of the reverse transition are detected and their dimensions are found. The atomic structure of the defects formed due to ion implantation are studied, these include radiationally disordered regions, dislocation configurations, dislocation loops and barriers, and also complexes of these defects localised in small volumes, and the segregation of atoms of one of the components. The structure of these defects in regions of single cascades of atomic displacements, their dimensions and mutual arrangement and their crystal-geometric characteristics are determined.

Response of a Material: From Single Ions to Experimental Times and Fluences

The preceding article by Davies treated the behavior of single ions as they slow down in a solid, colliding with atoms of the solid and often transferring enough energy to initiate atomic displacement cascades. For a 200 keV As ion with a speed ~108 cm/s and a range in Si of ~ 2 × 10−5 cm, the cascade is completed in less than 10−12 s. The small volume excited in the cascade relaxes over 15 decades or more in time in a series of increasingly deliberate steps. The experimental evidence for most of these decades is indirect. No probes with sufficient spatial and time resolution exist to examine the spatial and temporal evolution of collision cascades created by individual incident ions. The first few decades in time have been followed in metals by molecular dynamics (MD) calculations and also approximated by continuum theory. The results of MD calculations for 5 keV cascades in Ni are shown in Figure 1. The behavior is in-terpreted as indicative of local “melting,” consistent with the concept of a “thermal spike” in the material. Extension of MD calculations to times of nanoseconds and longer is still prohibitively expensive in computer time, and application to covalently bonded systems, where the interatomic forces are much more complex than in metals, has not yet been reported.As Davies has noted, the atoms of the cascade rapidly lose their high vibrational excitation to atoms in the surrounding material, and in only a few picoseconds the system cools. This constitutes a very rapid thermal quench—slower only than the quench associated with deposition of individual atoms (or small atomic clusters) on a cold substrate. The quench may produce a small volume of material with a different phase than its surrounding. For example, a small volume of “hot” Si atoms may be highly disordered and may quench to produce a small amorphous Si region in a crystalline Si matrix.

Production of transmutants and displacement effect in the EURAC spallation neutron source for testing of fusion materials

Two aspects have been criticized when using spallation reactions for fusion material testing: different final damage structures produced by high-energy recoils, and adequacy of the production of gaseous and solid transmutants. Using an original procedure to compute the displacement efficiency it is concluded that no new damage structures can be expected from the high-energy recoils at low temperature. An interaction displacement efficiency representing the interference of subcascades is defined, and it has been found independent of the recoil energy. Computed atomic displacement cascades for recoils of 200 keV and 1 MeV support this conclusion. The rate of He and H production is well compared with fusion, and the contribution from high-energy reactions is remarked. Solid transmutant generation from high-energy spallation neutrons and fusion are compared. A small contribution in the production of important elements is found, the proton flux effect being less significant than that of the neutron flux.

Structural relaxation during annealing and neutron or electron irradiation, in glassy Ti50Be40Zr10

Abstract We have studied the effects of thermal annealing and both electron and neutron irradiation on the state of short-range order in melt-spun Ti50Be40Zr10 (Metglas 2204), using electrical resistivity measurements. A reversible resistivity variation is observed during thermal cycling with no flux, akin to the one we evidenced recently in two binary all-metal amorphous alloys. It is similarly interpreted in terms of thermodynamical variations of the (metastable) equilibrium degree of chemical short-range order. On the other hand, the resistance of specimens exposed to a fast electron flux at 28 K is but little affected. However it is progressively decreased during post-irradiation isochronal anneals at subambient temperatures with a main, broad stage located between 120 K and 220 K. Interestingly the resistivity evolution related to structural relaxation which takes place above room temperature, in the unirradiated condition, is not markedly influenced by the irradiation. In contrast, irradiation with reactor neutrons at 300 K results in a strong resistivity increase, which is suggested to be reflection of chemical short-range ordering, presumably within the atomic displacement cascades.

Computer simulation of atomic-displacement cascades in solids in the binary-collision approximation

A comprehensive computer program has been developed for the simulation of atomic-displacement cascades in a variety of crystalline solids, using the binary-collision approximation to contruct the projectile trajectories. The atomic scattering is governed by the Moli\'ere potential.Impact-parameter-dependent inelastic losses are included using Firsov's theory. Thermal vibrations of the target atoms and crystal surfaces may be included. Permanent displacement of lattice atoms may be based on either an energy-threshold criterion or a Frenkel-pair-separation criterion. An extensive series of calculations has been made for cascades in the simple metals Cu, Fe, and Au, to test the effects on the results of many of the model parameters. When a displacement-threshold energy is used, the number of Frenkel pairs is found to be a linear function of that part of the primary recoil energy which remains as the kinetic energy of atoms. This result is independent of target temperature, of the presence or absence of inelastic energy losses, and of various details of the model. In contrast, when a separation criterion is used, the number of defects increases less rapidly than linearly. This effect is caused by increased recombination in the highly disturbed tracks of the energetic recoils. Agreement between theoretical and experimental estimates of the radiation damage produced by neutron irradiation of Cu is substantially improved in the latter model.

Computer simulation in radiation damage studies

The application of atomistic computer simulation to problems involving radiation damage and lattice defects in crystalline solids is discussed. The different topics include static defect configurational simulations, molecular dynamics of thermal motion, low energy atomic collision events, high energy atomic displacement cascades, and the thermal annealing of concentrations of defects in a region of lattice. The methods described are evaluated from the point of view of the assumed physical models, and their capabilities and limitations are indicated. Emphasis is on techniques used rather than on results obtained.