Migration Of Point Defects Research Articles

Clustering of Point Defects Under Electron Irradiation in Dilute Iron Alloys and an Iron Manganese Nickel Alloy

AbstractIn low copper steels for nuclear reactor pressure vessel, point defect clustering seems to play an important role in hardening. In order to study the hardening component which results from the clustering of freely migrating point defects, we irradiated, in a high voltage electron microscope, Fe, the alloys Fe0.13%Cu and Fe0.014%P, alloys and the alloy Fel.5%Mn0.8%Ni0.1%Cu0.01%P, the composition of which is close to the matrix of pressure vessel steels. We studied the nucleation of dislocation loops and their growth velocity. We find out that copper and phosphorus have no effect on the vacancy migration energy but that this parameter decreased significantly in the complex alloy. The main point is certainly that loops are nucleated in the complex model alloy up to 500°C while no loop appears above 300°C in Fe and in FeCu. FeP shows an intermediate behavior.

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.

Effects of concurrent irradiation with ions and electrons on the formation process of defect clusters in covalent and ionic crystals

In order to understand concurrent effects of damage cascades, isolated point defects and ionization on the formation process of defect clusters in covelent and ionic crystals, in-situ observations of Ge, Si and MgAl 2O 4 crystals have been performed under dual-beam irradiation with ions and electrons in the HVEM accelerator facility at Kyushu University. Damage cascades in the covalent crystals show up their contrast through overlap or help from other damage cascades. Simultaneous electron irradiation eliminates damage cascades in the covalent crystals through the irradiation-induced and -enhanced migration of point defects. In the ionic crystal, on the other hand, no damage cascades show up their contrast, but interstitial loops are formed through the nucleation and growth process. Effects of concurrent irradiation with ions and electrons on this process are based on defect reactions among damage cascades and isolated point defects. Homogeneous ionizing radiation plays no significant role on the nucleation and growth process of defect clusters.

Microstructure evolution during irradiation

Microstructural evolution in metals and alloys during energetic particle irradiation is reviewed with emphasis placed on the underlying defect reaction processes. The microstructures produced by electron irradiation with a high-voltage electron microscope, fusion-neutron irradiation with a D-T fusion neutron source, fission-neutron irradiation with reactors and ion irradiation with accelerators are compared and contrasted. The section on the damage produced by electron irradiation starts with a description of the development of the defect structure due to primary damage, and then extends to include the measurement of point defect properties, and the detection of a variety of point defect processes such as radiation-induced diffusion, the stochastic fluctuation of point defect reactions, and the instability of small interstitial clusters. The section on neutron irradiation begins with the detection and analysis of defects produced by cascades and subcascades and leads to an analysis of the recoil energy spectrum. The accumulation mode of defects is categorized in terms of the competition between defect formation by cascades and the interaction of these defects with freely migrating point defects. A direct comparison is made of the damage produced by fission- and fusion-neutron irradiation, to find a difference of more than 10 times depending on the defect processes involved. A wide variation of defects produced by mono-energetic recoil is detected by heavy-ion irradiation and the role of freely migrating interstitials is examined as a function of permanent sink strength.

Effect of concurrent irradiation with electrons on ion-induced amorphization in silicon

The effect of concurrent irradiation with electrons on ion-induced amorphization has been investigated under irradiation with high energy ions and fast electrons in the HVEM-TANDEM Facility at Argonne National Laboratory. The simultaneous irradiation with a focused electron beam prevents or retards the ion-induced amorphization, forming a distinct interface between crystalline and amorphous regions. The position of the interface has been converted to the critical electron flux, which increases with increasing energy deposition density and flux of ions and energy of electrons. It is concluded that amorphous embryos are essentially formed through overlap of subcascades, and that the prevention of ion-induced amorphization is mainly caused by athermal migration of point defects.

Microstructural evolution in low-dose neutron-irradiated Fe-16Ni-15Cr alloy

Reactor irradiation of the base alloy of modified austenitic stainless steel, a candidate alloy for fusion reactor application, was performed with improved temperature control. All three types of defects — vacancy clusters in the form of stacking-fault tetrahedra, interstitial-type dislocation loops and voids — are more pronounced in samples irradiated with conventional temperature control than those with improved control. They are attributed to the nucleation of those defects during the start-up of the reactor. From the irradiation of thin foils, the role of freely migrating interstitials was extracted. For bulk irradiation, the variations of the three types of defects mentioned above with irradiation temperature are thoroughly investigated. Reaction of the less freely migrating point defects in SUS than in pure metals is pointed out.

Effect of cascade localization induced bias effect and fluctuation of point defect reactions on defect structure evolution near planar sinks

Defect structure developments near planar sinks are analyzed as a competition of two effects. One is the cascade localization induced bias (CLIB) effect which leads vacancy-dominant point defect reactions during cascade damages. The other is a stochastic fluctuation effect of point defect reactions between defect clusters and freely migrating point defects. The sink geometry plays an important role in these effects. The strength of the two effects as a function of the distance to planar sinks is analyzed and the existence of a vacancy predominant volume near the sink is confirmed. The comparison is made between the analysis and experimental results.

〈100〉 dark line defect in II-VI blue-green light emitters

We have carried out a microstructural study of the 〈100〉 dark line defect that forms during degradation of II-VI blue-green light emitters. We find that these defects lie in or near the ZnCdSe quantum well and do not correspond to a readily observable dislocation network in transmission electron microscopy studies. We speculate that they may consist of point defects or small point defect complexes. We have also carried out estimates of point defect migration rates during device operation that can give rise to such degradation.

Reduction of post-implantation damage in semiconductors by weak magnetic fields

The experimental evidence of a reduction in the post-implantation damage by a factor of 2 and 1.5 in, respectively, Hg0.8Cd0.2Te and InSb caused by magnetic fields of H=0.7 kOe and 2 kOe at T=300 K is reported. This effect follows from the kinetic theory of atomic rate processes in solids linking them with the local electron transport in the submicrometre vicinity of the atoms, overcoming energy barriers. The magnetic field, which decreases the local electron transport, reduces exponentially rates of formation, migration and agglomeration of point defects and this decreases the post-implantation damage.

Cross-sectional transmission electron microscopy parallel to the active stripe of degraded buried heterostructure distributed feedback laser devices

We demonstrate for the first time that cross-sectional transmission electron microscopy can be applied parallel to the active stripe of a distributed feedback buried heterostructure laser diode to identify the cause for degradation during reliability qualification. A dominant defect mechanism is found to be the generation of dislocation loops at or near the grating/waveguide-layer interface which subsequently propagate up through the multilayer structure. The vertical segments of the loops tend to be trapped onto the {111} planes to form a V-shaped configuration as revealed in the (011) cross section. A dislocation reaction mechanism is proposed to explain the observed dislocation configuration. Furthermore, complicated dislocation loops are grown out of the segments of dislocation threading through the active region forming <100≳ dark line defects revealed by the electroluminescence. The defect growth mechanism is believed to be a condensation of point defects induced by the nonradiative recombination assisted point defect migration process similar to that previously observed in degraded channeled substrate buried heterostructure lasers. The nucleation of dislocations at or near the grating/waveguide layer interface is consistent with a high interfacial strain observed in the degraded devices.

Effects of Concurrent Irradiation with Ions and Electrons on Accumulation Process of Damage Cascades in Germanium and Silicon.

In-situ observation of Ge and Si has been performed under dual-beam irradiation with ions and electrons in the HVEM-accelerator facility at Kyushu University. Damage cascades show up their contrast and increase in number within a few seconds. The cascade contrast features accumulate with ion fluence, eventually leading to saturation. The saturated areal density decreases with increasing electron flux. Based on analyses using kinetic equations, it has been concluded that damage cascades show up their contrast through overlap or help from other damage cascades and that simultaneous electron irradiation eliminates damage cascades through the production of isolated point defects and irradiation-induced migration of point defects.

Irradiation-induced grain and lath boundary segregation in ferritic-martensitic steels

The temperature dependence of irradiation-induced nonequilibrium segregation of Si and P to grain boundaries has been modelled in bcc ferritic iron, taken to represent ferritic-martensitic steels. The relevant point defect properties required for analysis are documented and the approximations used to circumvent lack of precise data on point defect formation, migration and solute binding energies highlighted. Analytical electron microscopy results are presented on solute segregation to lath boundaries in fast reactor irradiated FV448 martensitic steel, and compared with calculated behaviour. Predictions of enhanced interfacial Si segregation after irradiation at 400°C are in accord with experimental observations.

Detection of the role of free point defects from the variation of defect structures near permanent sinks in neutron irradiated metals

The role of freely migrating point defects during neutron irradiation in the defect structure development with the presence of permanent sinks is studied using rate equations. Three specific cases are examined as a function of the distance from the surface: (1) only interstitial cluster existing system, (2) interstitial cluster and vacancy cluster coexisting system, and (3) only vacancy cluster existing system. The obtained results are as follows. In case (1): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen; no clusters in the deeper region of thick specimen. In case (2): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen just the same as those in case (1); the growth of both types of clusters in the deeper region of thick specimen. In case (3): easy formation of vacancy clusters near the surface of the specimen. These results are compared with the experimental results.

Electrical and structural properties of silicon layers heavily damaged by ion implantation

The effects of high-dose silicon and arsenic ion implantation on the electrical and structural properties of silicon layers are investigated. Combining electrical, transmission electron microscopy, and triple-crystal x-ray-diffraction measurements made it possible to characterize the effects of thermal annealing both on defect annihilation mechanisms and on electrical doping activation. It is clearly shown that a low-temperature (≤450 °C) electrical activation process is taking place in the amorphous surface layer induced by high-dose ion implantation. This phenomenon is found to be completely independent of the recrystallization regrowth by solid phase epitaxy which occurs at higher temperature. This electrical activation process is found to be well described by a local relaxation model involving point defect migration.

Influence of different heat treatment methods on the fatigue behaviour of sintered steels: Sonsino, C.M. Materialwissen. Werkstofftech. Aug 1991 22, (8), 308–315 (in German)

The role of point defects in the formation of surface relief and in the initiation of a fatigue crack in crystalline materials is analyzed. The dislocation interactions in the bands of intensive cyclic slip (persistent slip bands – PSBs) are specified and relations describing the formation and annihilation of interstitial and vacancy type defects in the channels of the ladder-like PSB are derived.The continuous formation, annihilation and primarily the migration of point defects are proposed to be responsible for the mass redistribution within PSB and between PSB and the PSB/matrix boundary. The redistribution of the matter results in local tensile and compressive stresses that are the sources of the principal irreversibility of slip within PSB. Local tensile and compressive stresses are relaxed by dislocation movement within PSB in the direction of the active Burgers vector and lead to the formation of characteristic surface relief in the form of extrusions and intrusions. The intrusions represent crack-like defects and fatigue cracks initiate in the tip of intrusions.

Life prediction for a nickel-based alloy Nimonic PE 16 under low-cycle-fatigue loading at 923 K: Mathew, M.D., Singh, V., Chen, W. and Wahi, R.P. Acta Metall. Mater. July 1991 39, (7), 1507–1513

The role of point defects in the formation of surface relief and in the initiation of a fatigue crack in crystalline materials is analyzed. The dislocation interactions in the bands of intensive cyclic slip (persistent slip bands – PSBs) are specified and relations describing the formation and annihilation of interstitial and vacancy type defects in the channels of the ladder-like PSB are derived.The continuous formation, annihilation and primarily the migration of point defects are proposed to be responsible for the mass redistribution within PSB and between PSB and the PSB/matrix boundary. The redistribution of the matter results in local tensile and compressive stresses that are the sources of the principal irreversibility of slip within PSB. Local tensile and compressive stresses are relaxed by dislocation movement within PSB in the direction of the active Burgers vector and lead to the formation of characteristic surface relief in the form of extrusions and intrusions. The intrusions represent crack-like defects and fatigue cracks initiate in the tip of intrusions.

Synchrotron-radiation topographic studies on dynamic behavior of lattice defects: Dislocation climb and point-defect diffusivity

Abstract An X-ray topographic method for investigating migration of point defects is described as a summary of previous work carried out on self-interstitials in ice. Since the method presented is based on in situ observations of dislocation climb in a thick specimen, the advantages of synchrotron-radiation topography for this method is emphasized. It is shown that the diffusion coefficients of the point defects can be determined as functions of both temperature and pressure.

Point-Defect Migration to Cracks during Irradiation

Point-Defect Migration to Cracks during Irradiation

X-ray powder diffraction study of annealed uraninite

Natural UO 2+ x (uraninite) samples were annealed in a reducing atmosphere up to 1200°C. The initial unit cell parameters of 0.5385, 0.5439 and 0.5458 nm increased after annealing to 0.5469, 0.5447 and 0.5462 nm, respectively. In one sample the unit cell parameter decreased from 0.5463 nm to 0.5456 nm. Migration of point defects in the oxygen sub-lattice occurs at 300°C. In the interval of 500 to 700°C, uranium vacancies migrate and cluster. The formation of uranium vacancies is enhanced by the presence of radiogenic Pb. Above 750°C, the unit cell parameter increases as a result of reduction U 6+ to U 4+. Uraninite incongruently decomposes at 1100°C. Density increased after annealing except for one sample in which a large number of bubbles (1–2 μm in size) formed. The complex annealing behavior of uraninite is due to the presence of impurities (Pb, Ca, Th, Zr, REE), mixed oxidation states of uranium, and α-decay event radiation damage.

The effect of ion-ion correlations on the ionic hall effect in crystals

Abstract Because an external magnetic field does no work on moving charges, it is argued that independently migrating point defects in an ionic crystal cannot exhibit any Hall effect, to first order. This is in sharp contrast to the cooperative motions displayed by superionic conductors. Experimental results on interstitial migration in AgBr and α—AgI agree with these expectations.