Thermal Spike Phase Research Articles

Irradiation effects in Ag-Fe bilayers: Ion-beam mixing, recrystallization, and surface roughening.

Ag/Fe bilayers deposited onto ${\mathrm{SiO}}_{2}$/Si substrates were irradiated at 20, 77, and 300 K with 300--750 keV Ar and Xe ions in order to study the ion-beam-induced mixing and phase formation in this thermally immiscible system. A combination of Rutherford backscattering spectroscopy (RBS), channeling, conversion electron M\ossbauer spectroscopy (CEMS), and scanning tunneling microscopy (STM) was used to analyze the atomic transport at the interface and the resulting microstructure and morphology changes of the samples. In the CEMS measurements, a 13 nm thin $^{57}\mathrm{Fe}$ marker layer at the Ag/Fe interface was introduced in order to enhance the sensitivity to alterations of the interfacial composition. From the small amount of Ag atoms found to be dissolved in Fe and from the sharpness of the element profiles at the interface, we derived a very small mixing efficiency, which is significantly smaller than the prediction of the ballistic model. Since ballistic mixing is expected in any case, we argue that demixing and phase separation occur in the relaxation stage or thermal spike phase of the collision cascade, as a consequence of the positive heat of mixing. On the other hand, ion irradiation induces a large surface roughening of the Ag top layer as proven by STM. This effect is obviously due to recrystallization of Ag, which results in grain growth and texture formation along the direction of the impinging ion, as demonstrated by RBS/channeling measurements. \textcopyright{} 1996 The American Physical Society.

The Effect of Temperature on Defect Production by Displacement Cascades in alpha;-IRON

AbstractMolecular dynamics (MD) simulations have been used to study the number and arrangement of defects produced by displacement cascades as functions of irradiation temperature, Tirr, in α-iron. The continuum treatment of heat conduction was used to adjust the temperature of the MD boundary atoms throughout the cascade process. This new hybrid model has been applied to cascades of either 2 or 5 keV at 100K, 400K, 600K and 900K. The number of Frenkel pairs decreases by about 20–30% as Tir increases from 100K to 900K, due to the increase in the lifetime of the thermal-spike phase. The same effect also brings about an increase in the proportion of selfinterstitial atoms that form clusters.

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 ...

Significant differences in defect accumulation behaviour between fcc and bcc crystals under cascade damage conditions

Various aspects of irradiation-induced damage production and accumulation in fcc and bcc metals and alloys are examined. Results on the evolution of defect morphology in individual cascades, the global evolution of planar (clusters/loops) and three-dimensional defect agglomerates (voids) and the temperature dependence of defect accumulation in the form of loops and voids in fcc and bcc metals are compiled and compared. The comparison demonstrates that the damage accumulation in the form of clusters/loops and voids and their dependence on irradiation temperature are dramatically different in fcc and bcc metals. These differences may arise because of significant differences in the number of surviving defects (in cascades) and intracascade clustering of vacancies and self-interstitial atoms (SIAs) that get established already during the thermal spike phase of cascades. The implications of these differences are discussed. It is suggested that theoretical treatments of the damage accumulation in fcc and bcc metals must explicitly include the details of intracascade clustering of vacancies and SIAs, for example, the ratio of clustered to non-clustered vacancies and SIAs.

Response of helium bubbles in gold to displacement-cascade damage.

Evolution of He bubbles in Au has been followed with in situ electron microscopy during 400 keV Ar ion irradiation at 500 K. He bubbles were produced by implantation at 500 K of 3 keV He ions into prethinned Au samples that were then annealed to 670 K. During Ar irradiation, bubbles undergo athermal migration, coalescence, and disintegration. The bubbles after annealing were underpressurized, but Ar irradiation brought the bubbles to equilibrium pressure after a dose of 0.6 displacements per atom (dpa). Bubble behavior can be understood on the basis of their interactions with adjacent cascades. We propose that a discrete bubble jump is induced by the melt zone formed during the thermal spike phase of a contiguous cascade. The rate of bubble migration is modeled with the assumption that the jump distance is determined by the bubble and cascade volumes.

Molecular dynamics computer simulations of displacement cascades in metals

MD simulations of displacement cascades have become important in the study of atomic-scale processes in radiation damage. Here, we review recent advances and discuss results on a variety of processes in pure metals and alloys. The MD procedures, and the many-body interatomic potentials on which most recent simulations are based, are described, and then the general features of cascades in metals are reviewed. The ways in which the cascade state during the thermal-spike phase can be investigated are presented, and it is found that a liquid-like core is generated for cascade energies above 1 to 2 keV. This is shown to have important consequences for atomic mixing, disordering in ordered alloys, and vacancy clustering. Frenkel-pair production efficiency in the primary damage state at the end of the cascade process is found to be well below the NRT theoretical value in all the metals and alloys modelled to date. Clustering of interstitials is a persistent feature of cascade simulations for all pure metals. The mechanisms underlying these results are discussed, and it is observed that the simulations are in generally good agreement with what has been found by experiment.

An Examination of the Factors Which Determine the Thermal Spike Lifetime of a Displacement Cascade

AbstractThe thermal spike phase of a displacement cascade is thought to be of fundamental importance to a number of issues in radiation effects. For example, the formation of stable vacancy clusters and the efficiency of ion mixing are thought to be directly controlled by the duration of the liquid-like state during the thermal spike. A number of factors have been proposed to be of significance to the lifetime of the thermal spike. For example, materials which have a high melting temperature, or which exhibit strong electron-phonon coupling, which provides an efficient method of energy transport to the surrounding material, are expected to have short lifetimes. We have examined these factors in detail and demonstrated that, in both elemental metals and alloys, electron-phonon coupling provides the most consistent explanation for variations in the thermal spike lifetime and, hence, the efficiency of ion mixing and the number of stable vacancy defect clusters formed.

A molecular dynamics study of displacement cascades in α-iron

The mechanisms of defect production in displacement cascades in α-iron have been investigated by computer simulation. Cascades of up to 5 keV in energy have been simulated by molecular dynamics in crystals with atomic interactions described by a many-body potential. The effects of lattice temperature have been studied by using block temperatures of either 100 or 600 K. 80 cascades have been modelled overall. The morphology of cascades during the collisional phase changes at about 1–2 keV, due to the collective nature of atomic displacements at higher energy. This transition is reflected in the relaxation time during the subsequent recombination phase, and it also decreases the efficiency factor for defect production. This factor is similar in size to that obtained from recent modelling of copper, an fcc metal. Although the cascade zone contains a large number of displaced atoms, true melting was not observed in α-Fe, and vacancy clustering did not occur in the thermal spike phase. Interstitial clustering has been analysed, and found to be less pronounced than in copper. One large cluster was observed to grow by interstitial movement during the thermal spike, and visual analysis has shown that it formed a perfect dislocation loop: it was not nucleated by the Eyre-Bullough mechanism, however. Statistics on the cascade parameters are presented, and comparisons with work on other crystal structures are drawn where possible.

A model for the formation mechanism of depleted zones with a high concentration of vacancies in displacement cascades in metals

Abstract The mechanism of redistribution of vacancies in the depleted zone of a displacement cascade in copper has been investigated by molecular dynamics simulations. A simplified model of the thermal spike was used, for which the calculation cell was cooled down along one axis after previously introducing into it vacancies and kinetic energy. The time evolution of the temperature profile and the vacancy concentration distribution was calculated, and it was found that in a nonuniform temperature field, vacancies are swept to the centre of the thermal spike as a result of the advance of the solid-liquid interface. This effect leads to the development of a zone where the average concentration of vacancies is several times larger than that produced before the onset of the thermal spike phase. The redistribution of density of the surrounding matrix which is realized in this way is analogous to ‘collapse’ of a vacancy platelet and the consequent formation of a vacancy loop, a feature which is characteristic o...

Influence of dilute impurities on the evolution of defect cascades in nickel

We have employed transmission electron microscopy to study the formation of vacancy-type dislocation loops from defect cascades produced by a 50 keV Kr + ion irradiation of Ni and its dilute alloys with Si and Al. An unusual and reproducible dependence of the probability of loop formation and loop size distributions on solute content was found. The results are explained by impurity caused changes in both the collisional and thermal spike (molten zone) phase of the defect cascade. Specific mechanisms include disruption of focussons by impurities to change the energy density profile within the collision cascade, interruption of replacement collision sequences, and interstitial trapping to reduce recombination during the thermal spike phase. Applying these mechanisms to a statistical distribution of the spatial extent of cascades and to the molten zones, all the experimental observations can be explained.

Effect of dilute solute additions on loop formation from heavy-ion produced displacement cascades in Ni

Abstract Transmission electron microscopy has been employed to study the formation of vacancy dislocation loops from defect cascades produced by 50 ke V Kr+ ion irradiations of Ni and its dilute alloys with Si and Al. An unusual and reproducible dependence of loop collapse probabilities and loop size distributions on solute content was found. The temperature dependence at 30 and 300 K and ion dose dependence of these loop collapse probabilities were measured. The results are explained by impurity-caused changes in both the collisional phase and thermal spike (molten zone) phase of the defect cascade. Specific mechanisms include disruption of focusons by impurities to change the energy density profile within the collison cascade, disruption of replacement collision sequences to place the self-interstitials closer to the molten zone, and interstitial trapping to reduce recombination during the thermal spike phase.

Production bias and void swelling in the transient regime under cascade damage conditions

Abstract Molecular dynamics (MD) studies of collision cascades have firmly established that, in addition to clusters of vacancies, clusters of self-interstitial atoms (SIAs) are formed within the cascade volume during the thermal spike phase of a cascade. These clusters are formed in a segregated fashion such that the vacancy-rich core is surrounded by SIA clusters. At temperatures above stage V the vacancies evaporate from the vacancy cluster and diffuse into the medium whereas the SIA clusters remain thermally stable at temperatures even above the peak swelling temperature. This asymmetry in the production of free and mobile vacancies and SIAs gives rise to a production bias. Some of the vacancies evaporating from the vacancy-rich core annihilate at the SIA clusters surrounding it and others escape into the medium. It is this escaping fraction of vacancies which determines the strength of the production bias. Diffusion calculations have been performed to estimate the magnitude of this escaping fraction ...

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 ...

Considerations of recoil effects in microstructural evolution

The cascade processes are discussed which potentially give rise to a dependence of microstructural evolution on the recoil spectra set up by a flux of fast particles. It is emphasised that the thermal spike phase of cascade development is central to these processes. Recent results from Molecular Dynamics simulations are presented to highlight the potential importance of interstitial clustering in cascades and the impact of cascades on pre-existing features of the microstructure. A qualitative discussion is presented of the likely importance on microstructural evolution at low and high doses of the different cascade processes.

Molecular dynamics simulation of displacement cascades in Cu and Ni: Thermal spike behavior

Molecular dynamics simulations of energetic displacement cascades in Cu and Ni were performed with primary-knock-on-atom (PKA) energies up to 5 keV. The interatomic forces were represented by the Gibson II (Cu) and the Johnson-Erginsoy (Ni) potentials. Our results indicate that the primary state of damage produced by displacement cascades is controlled basically by two phenomena: replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike. The thermal-spike phase is of longer duration and has a more marked effect in Cu than in Ni. Results for atomic mixing, defect production, and defect clustering are presented and compared with experiment. Simulations of “heat spikes” in these metals suggest a model for “cascade collapse” based on the regrowth kinetics of the molten cascade core.

Displacement cascade collapse at low temperatures in Cu 3 Au

A systematic study by transmission electron microscopy has been made of factors affecting the collapse of displacement cascades in ordered Cu 3 Au. Thin foils of ordered Cu 3 Au were bombarded with Ar + , Cu + and Kr + ions of energies 50 keV and 100 keV to nominal doses of 10 15 ions m -2 both at room temperature and at 30 K. The damage induced at low temperature was observed in situ and after subsequent warming to room temperature. The imaging techniques employed allowed disordered zones produced at individual cascade regions, together with their associated dislocation loops, to be identified and characterized separately, and so permitted defect-yield values to be measured accurately independently of the ion dose. Collapse of displacement cascade to vacancy dislocation loops was observed to occur with moderately high probability under all irradiation conditions at low temperature, with no subsequent increase in the number of loops on warming to room temperature. Collapse, however, occurred with significantly greater probability in corresponding irradiations performed at room temperature. An increase in collapse probability with ion mass was also found, although no dependence on ion energy was observed. Detailed correlations between collapse and the sizes and shapes of disordered zones at cascade sites are reported. The observations are shown to be consistent with cascade collapse resulting from systematic vacancy migration during the thermal spike phase of the cascade. The disordering of cascade peripheries, probably by point-defect migration, is also considered to occur during this phase.