Cascade Region Research Articles

A model of disordered zone formation in Cu3Au under cascade-producing irradiation

Abstract A model to describe the disordering of ordered Cu3Au under irradiation is proposed. For the thermal spike phase of a displacement cascade, the processes of heat evolution and conduction in the cascade region are modelled by solving the thermal conduction equation by a discretization method for a medium that can melt and solidify under appropriate conditions. The model considers disordering to result from cascade core melting, with the final disordered zone corresponding to the largest molten zone achieved. The initial conditions for this treatment are obtained by simulation of cascades by the MARLOWE code. The contrast of disordered zones imaged in a superlattice dark-field reflection and projected on the plane parallel to the surface of a thin foil was calculated. The average size of images from hundreds of cascades created by incident Cu+ ions were calculated for different ion energies and compared with experimental transmission electron microscopy data. The model is in reasonable quantitative ...

Ion-induced energy propagating front and migration of point defects in metals

A Molecular Dynamics calculation, using the Embedded Atom Method, shows very clearly the emission and propagation of an energy front from a collision cascade induced by an ion impact in a metal. Additionally, this front is capable of producing change of site of interstitial atoms, situated far from the cascade region. This property entitles the phenomenon to be a candidate for a mechanism of Ion Beam Mixing, not considered in the previous works. The calculation was done at zero temperature.

Animated molecular dynamics simulations of energetic mixed cluster impacts

Molecular dynamics and computer animation techniques have been used to investigate atom ejection and damage caused by the impact of energetic, uniformly mixed Al 32Au 31 clusters on 12-layer, 3750-atom Au(100) targets at total cluster energies of 5, 10, 50, 100, and 200 keV. At all incident energies we find that a significant fraction of the aluminum is ejected from the cluster on impact. At the two lowest bombarding energies damage to the target is limited generally to the primary impact zone, and crystallinity is retained throughout most of the the target. At the higher bombarding energies significant damage occurs throughout the central region of the target, and crystallinity is lost throughout the entire target. In contrast to collision cascades initiated by single ions, those initiated by energetic clusters are observed to be highly nonlinear (essentially all the atoms in the cascade region are in motion) and to propagate through the target with relatively uniform wavefronts.

Production bias driven defect accumulation in the transient regime

Abstract Consequences of the nature of the damage production in multidisplacement cascades are analyzed. Considerations of intracascade clustering of self-interstitial atoms (SIAs) and vacancies during the cooling-down phase of a cascade and of the difference between thermal stabilities of SIA and vacancy clusters give rise to the concept of production bias. Within this concept, the fraction of vacancies and SIAs escaping the cascade region is taken to be the effective rates of defect production. The fraction of vacancies in excess of SIAs has been calculated and used for determining the temporal dependence of void swelling purely in terms of production bias. It is proposed that small SIA clusters/loops may perform one-dimensional glide motion and may get transported to dislocations and grain boundaries. This is considered to be a viable mechanism for the removal of a fraction of SIA population. The resulting vacancy supersaturation would drive void nucleation, growth and void swelling. The swelling behaviour in the transient regime predicted by the production bias is found to be in reasonable agreement with the experimental results.

Kinetics of radiation-induced disordering of A3B intermetallic compounds: A molecular-dynamics-simulation study.

Intermetallic compounds are susceptible to disordering when exposed to irradiation with energetic particles. However, the atomistic mechanisms of this process are not fully understood. We present molecular-dynamics studies of the kinetics of disordering induced by 5-keV energetic displacement cascades in ${\mathrm{Cu}}_{3}$Au and ${\mathrm{Ni}}_{3}$Al. Our results show that the underlying crystalline structure is lost in the local region of the cascade core. However, kinetic constraints imposed by the fast quenching rate of the cascade region prevent a complete loss of chemical short-range order from occurring. After the cascade energy dissipates, the Bragg-Williams long-range-order parameter in the central region is found to have a value near zero. The shape of the interface between the cascade-induced disordered region and the surrounding ordered lattice is shown to depend on the ordering energy of the irradiated compound.

Production bias: a proposed modification of the driving force for void swelling under cascade damage conditions

A new concept of point defect production as the main driving force for void swelling under cascade damage conditions is proposed. This concept takes into account the recombination and formation of immobile clusters and loops of vacancies and interstitials in the cascade region. The lifetimes of the clusters and loops due to desolution are strong functions of the temperature, as well as their vacancy and interstitial nature. The resulting biased production of free point defects from the internal sources is shown to be strong driving force for void swelling. The characteristics of void swelling due to production bias are described and compared with experimental results. We conclude that the production bias concept provides a good description of void swelling under cascade damage conditions.

Molecular dynamics studies of defect production and clustering in energetic displacement cascades in copper

We review recent molecular dynamics studies of displacement cascades in copper at energies from near threshold to 25 keV where nascent subcascades develop. We discuss the structure of the cascade region and present results on the dynamical evolution and lifetime of the cascades. We show that the defect production efficiency in cascades follows a downward trend with energy, in good quantitative agreement with experiments. The cluster size distributions from the simulations show a strong recoil energy dependence. At low energy, loosely correlated vacancy distributions are generally observed. However, as the energy and hence the lifetime of the cascade increase, large vacancy clusters appear. Evidence is presented for the athermal collapse of a 25 keV cascade to a vacancy dislocation loop. Interstitial production mechanisms are identified. At all energies studied replacement collision sequences are observed. Moreover, vacancy-interstitial separation mechanisms based on ballistic clustering and dislocation loop punching from cascades are presented.

Production bias due to clustering of point defects in irradiation-induced cascades

Abstract In the conventional rate-theory treatment of irradiation-induced dimensional changes based on the concept of dislocation bias, it is implicitly assumed that interstitials and vacancies are produced continuously and uniformly in the form of Frenkel pairs. This implies that all produced point defects are free and are available for annihilation at sinks. This approach may not be appropriate, however, for cases where the damage is produced in the form of cascades. In the cascades both vacancies and interstitials are produced in a highly segregated fashion in that the distributions of vacancies and interstitials are separated from each other in space. This spatial segregation is maintained even after the end of the thermal spike phase during which vacancies condense in the central region of the cascades whereas the high concentration of highly mobile interstitials form clusters at a certain distance from the cascade centre. The consideration of interstitial clustering within the cascade region yields ...

Atom—atom collision cascades in non-uniformly stressed metals

Abstract Atom–atom collision cascade development in α-Fe under different non-uniform stress fields is studied by a computer simulation method. It is shown that such fields can significantly change a cascade region structure. The depleted zone becomes more compact; the distance between it and that where interstitials are stopped increases; the number of stable defects produced is less than that in an unloaded crystal. These effects can be caused by changes in the atom–atom replacement chain development in the low-energy cascade branches and threshold displacement energy. Special investigations of the variations in the displacement threshold energy E d under non-uniform stresses have been carried out. It is shown that averaged in all directions the E d change is near to 9%. The present results are compared with cascade development and E d calculations in α-Fe under a uniform load.

Kinetics of the Diffuse Processes within a Cascade Region in the Sub-Threshold Stages of F.C.C. and H.C.P. Metals

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The mechanism of nucleation of vacancy loops with burgers vectors 〈100〉 in BCC metals

The mechanism of nucleation of unfaulted vacancy loops (VL) with Burgers vector b = 〈100〉 in bcc-metals is suggested. The mechanism takes into account the experimental fact that the nucleation of such loops takes place exclusively in the cascade region of damage. In substantiating the mechanism, use is made of data on simulation of the kinetics of vacancy clusters (VC) nucleation in depleted cascade zones in alpha-iron. When the VC contains a plane vacancy fragment {100} of or above a critical size in the depleted zone, the formation of an unfaulted loop nucleus takes place via the Eyre-Bullough shear reaction. In this case, the shift direction and, correspondingly, the Burgers loop vector depends on the configuration of vacancies in the VC surrounding the loop nucleus. Computer simulation has shown that the shear leading to the VL formation with b = 〈100〉 is the most probable for a dense three-dimensional VC which has over 20–35% of vacancies not in the original vacancy platelet of {100} critical size. With nucleation the VC is simply an isolated platelet of N v > 46 vacancies and the spontaneous unfaulting is always realized, resulting in the formation of an unfaulted loop with b = 1 2 〈111〉 .

Nonlinear effect on surface charging by diatomic ion bombardment

Surface charging by ion bombardment is investigated using a quasistable charging condition in which one can evaluate the degree of charging by an intensity of secondary ions. It is found that a nonlinear effect on the charging occurred under monoatomic and diatomic ion bombardments. Namely, O+2 -ion irradiation enhances the charging in comparison with O+-ion irradiation, although atomic dose and energy/atom are kept the same in both ion bombarbments. Since the results also show that the surface charging is suppressed when the energy of incident ions is higher, the nonlinear effect by diatomic ion irradiation is attributed not to an increase of energy density, but to a dense cascade in the overlapping region of cascades caused by two constituents of the incoming diatom.

Atomic mechanism of primary defect heterogeneous nucleation under radiation in fcc metals with a large stacking fault energy

Abstract Evolution of the defect structure of the cascade region (CR) formed under irradiation in fcc materials, both with a large stacking fault energy (SFE) and with a small one, has been studied by use of the molecular dynamics technique (MDT). It is shown that when a 1 keV cascade is simulated under identical conditions a tangle of vacancy dislocations is formed in the first case, and a depleted zone in the second case. The connections of these observations with irradiation creep and swelling of alloys, both with a large (on the base of Ni and Al) and a small (Ag, Cu, Au, stainless steel, etc.) SFE, is discussed. A comparison with experimental results is also made.

Silicon amorphization model in the process of ion implantation

When bombarded with light ions, the region of primary collision is not considered as amorphous, and the attained amorphous state in the irradiated layer is the result of the diffusion-coagulation processes with participation of vacancies originating in different collision cascades. From our viewpoint, however, the phenomenon of amorphization has the same nature for ions of different masses. It starts with decay of oversaturated solid state solution of vacancies. Note that, as a result of the interaction of the bombarding particle with the silicon atoms and subsequent spatial separation of vacancies and interstitials, the collision cascade region can be considered as the region of local vacancy oversaturation

X-ray emission from inner shell (2p) excited sputtered Al-atoms/ions resulting from Ar + and Ne + impact on solid Al

The X-ray emission of innershell (2p) excited sputtered AI atoms/ions in the photon energy region ≈ 60–80 eV is extracted from measurements of the soft X-ray emission spectra of solid Al under 40 keV Ar + and Ne + impact, using a procedure which includes extrapolation towards vanishing atomic line radiation. Thus the basis of the extraction procedure is not the subtraction of the Al L 23 band, as it is given by proton or electron impact on solid Al. The shape of the Al L 23 emission band turning out from this procedure is distinctly different from the H + induced band, presumably due to high temperatures in the region of the collision cascades produced by Ar + impact. The ratio of X-ray emission from innershell (2p) excited sputtered Al atoms to emission from solid Al changes drastically from ≈ 0.1 at nearly normal ion incidence on the Al surface to ≈ 4 at nearly grazing incidence. The 6 observed atomic lines show a strong dependence on the incidence angle of impinging Ar + ions which differs distinctly from the usual behavior of sputtered particles.

The model of nucleation of vacancy clusters in the cascade range in the stage of the thermal spike

Abstract The calculation model of nucleation of large vacancy clusters in the cascade damage zone formed by means of some mechanisms of spatial vacancy rearrangements in the cooling stage of the energetic cascades has been proposed. Using the Monte-Carlo method, the model describes the clustering of monovacancies, divacancies and larger vacancy clusters migrating in the nonhomogeneous temperature field of the thermal spike (TS). An anisotropic character of the defect jumps resulted from temperature gradients in the TS has been considered. The model involves the possibility of the defect rearrangements due to melting and recrystallization of the depleted zone. In terms of the model the nucleation of vacancy clusters in the isolated cascade region of the 30-keV cascade in copper has been investigated and the size distribution of clusters has been obtained.

Ion beam induced damage in CaF 2

The change in lattice parameter and the induced damage are studied in single crystal CaF 2 bombarded by a 15 MeV Cl ion beam. The lattice parameter change (strain) and the damage for increasing ion beam dose (5 × 10 12/cm 2 to 7 × 10 15/cm 2) is observed via X-ray rocking curve analysis using a double-crystal diffractometer and X-ray reflection topography. The ion beam energy (range = ∼ 4.5 μm in CaF 2) is such that both the electronic region and the nuclear cascade region of energy loss show up in the diffraction signal. By kinematical X-ray diffraction theory analysis, the progress of strain/damage depth profile with increasing beam dose is shown explicitly. The increase in strain is nonlinear with beam dose for the dose range studied. For increasing beam dose, the strain level in the electronic energy loss region is fixed, while that in the nuclear collision loss region increases effectively until that region becomes completely amorphous.

Ion-induced phase formation in metal-metal and metal-silicon thin film structures

The phase formation in metals by using ion mixing techniques has been studied and compared with silicide formation. Upon irradiation of AlPt, AlNi and AlPd thin films, only the simplest intennetallic compounds of NiAl and PdAl were formed in crystalline structure, while the amorphous phase has been observed over a large range of composition. Ion mixing of AuCu bilayers resulted in the formation of substitutional solid solutions with no trace of ordered compounds. The formation of the ordered compound CuAu was achieved either by irradiation of bilayers with Ar ions at elevated substrate temperature or by irradiation of the mixed layers with He ions at relatively low temperature. In the AuAl system several crystal compounds existed in the as-deposited samples. These phases remained crystalline or transformed into other equilibrium compounds upon ion irradiation. The results suggest that the phase formation by ion mixing is dependent on the high quench rate in the collision cascade region and the atomic mobility at the irradiation temperature. The argument can be applied to silicide forming systems. With near-noble metals, the mixed atoms are mobile and form metallurgically distinct phases. With refractory metals, no equilibrium compounds are formed due to the lack of atomic mobility.

Ion-induced molecular ejection from D 2O ice

Studies of molecules ejected from water ice by fast ions provide insight into the electronic relaxation processes and subsequent chemistry occurring in ice at very low temperatures. The ion-induced ejection of D 2O, D 2, and O 2 molecules from thin films of D 2O ice has been measured as a function of the fluence of incident MeV ions at temperatures between 10 and 140 K. For a given beam current, the O 2 yield exhibits initial transients which are slow compared with the prompt ejection of D 2O. We interpret these results as due to the build-up of O 2 in the films following fragmentation of D 2O molecules by incident ions. The fragmens re-form into new molecular species which diffuse to and escape from the surface, aided by subsequent bombardment. The D 2 transient has a prompt component, which we postulate is due to rapid formation during electronic recombination near the surface. A slow component of the D 2 transient is also observed, which may arise through a two-step process similar to that of O 2. Time-of-flight energy spectra of the ejected D 2O molecules have also been measured. Incident ion masses and energies range from those for which nuclear elastic energy deposition dominates (50 keV argon) to those for which electronic energy deposition dominates (1.5 MeV helium). The spectra cannot be described by models typically employed for the sputtering of metals. For instance, the spectra do not have maxima characteristic of the sublimation energy of the solid. In addition, the sputtering yield in the high energy part of the ejection spectrum of D 2O is too large to arise from nuclear elastic energy deposition. It must result instead from relatively energetic non-radiative relaxation of electronic excitation. For incident MeV ions that deposit their energy predominantly in electronic excitation, the low energy part of the D 2O ejection spectrum is greatly enhanced, indicative of a weakly antibinding region formed along an incident particle track. Enhanced ion yields are also found in the collision cascade region which are attributed to electronic processes.

Change in pore size during relaxation of the thermal stage of a cascade

On the basis of the solution of the thermoelasticity problem, an analysis is given on the temperature and force fields in the cascade region in the presence of a pore and the change in the size of a small pore is estimated from the balance of point defects on it.