High Voltage Electron Microscope Irradiation Research Articles

Formation of Vacancy-Type Dislocation Loops in Hydrogen-Ion-Implanted Fe–Cr Alloy

Fe–10 at.%Cr alloy was implanted with hydrogen ions at room temperature, followed by annealing at high temperatures. The annealing process made the defects develop into large dislocation loops. The nature of the dislocation loops formed after annealing was studied by the evolution of loops under in situ electron irradiation in high-voltage electron microscope. It indicated that only interstitial-type loops were observed when annealed at 550 °C and below, but vacancy-type loops started to form at the temperature higher than 600 °C. According to the previous study of our group, the presence of chromium element made the formation temperature of vacancy-type loops higher than that in pure iron. The effect of alloying elements on the formation temperature of the vacancy-type loops was discussed.

Evolution of dislocation loops in annealed iron pre-irradiated with hydrogen ion in high-voltage electron microscope

The nature of dislocation loops in the annealed pure iron pre-irradiated with hydrogen ion at room temperature was studied by the evolution of loops under electron irradiation in high-voltage electron microscope (HVEM). Only interstitial-type loops were observed when annealed and electron irradiated at 350 °C but only vacancy-type loops formed at temperature higher than 500 °C. When annealed at temperatures from 450 °C to 490 °C, both interstitial-type and vacancy-type loops formed simultaneously in the specimen and vacancy-type loops accounted for an increasing fraction with increasing annealing temperature, from 28.5% at 450 °C to 55% at 490 °C. The bias factor of interstitial-type and vacancy-type loops was compared based on the growth rate or shrinkage rate of the dislocation loops. The bias factor of interstitial-type loops was demonstrated to be higher than that of vacancy-type loops at all three annealing temperatures.

Point defect behavior in electron irradiated V–4Cr–4Ti alloy

To investigate the migration behavior of interstitials and vacancies in vanadium alloy V–4Cr–4Ti, high purity V alloys were irradiated with 2 MeV electrons using a high voltage electron microscope (HVEM) and 28 MeV electrons using a electron linear accelerator. Interstitial type dislocation loops formed quickly in all cases of HVEM irradiation. The saturation density of the dislocation loops was independent of irradiation temperature and defect production rate at temperatures in the range of 323–448 K. However, the logarithm of saturated dislocation loop density was inversely proportional to the irradiation temperature, and dislocation loop density was directly proportional to the square root of the defect production rate above 448 K. The dissociation energy of the interstitial-impurity complex in V–4Cr–4Ti alloy was estimated to be 1.1 eV from the HVEM experiment. It was found from positron lifetime measurements that the vacancies form vacancy clusters at 373 K.

A computational study of dislocation evolution and radiation-induced segregation at a grain boundary

Radiation-induced segregation (RIS) influences the chemical and mechanical properties of the structural materials of fusion and fission reactors. In this study, such RIS and dislocation sink strength evolution were investigated on the basis of computer simulation for solute segregation at and near a grain boundary. A new continuity equation model for RIS, based on the rate theory, which includes the simultaneous evolution of faulted dislocation loops and network dislocations, has been developed. A di-interstitial was assumed to be the nucleus of a dislocation loop in the present work, and we calculated dislocation evolution due to absorption of interstitials and vacancies. The dependencies of irradiation temperature, irradiation rate and dose on the RIS at a grain boundary under electron irradiation in high-voltage electron microscope (HVEM) were clarified. We also considered the effect of probe size used in TEM-EDS analysis in order to satisfy the relationship with measured compositional profiles. The present work sheds a light on the accomplishment of performing ‘combined calculation’ which deals with both RIS and various kinds of internal sink evolution.

Microstructural evolution of He-preinjected austenitic stainless steels under HVEM irradiation

Two kinds of austenitic stainless steels, 316R (standard type 316 stainless steel) and alloy K (low-C and 0.29 wt% Ti) were irradiated in a high voltage electron microscope (HVEM) after He injection to 2.5 and 10 appm. 45 keV He was injected into the prepared transmission electron microscope (TEM) foils and HVEM irradiation was performed at 773 K to 14 dpa. Without helium preinjection, alloy K shows very few void production and thus the void swelling of alloy K was lower than that of 316R. Injected helium of 2.5 and 10 appm led to the void number densities of 10 22/m 3 level or more in both steels. Void swelling was largest for 2.5 appm He specimens for both steels and the void growth rate was higher in alloy K than in 316R, and void swelling was higher in alloy K. The order of swelling resistance of these two steels as thus changed after 2.5 appm He injection.

Development of specimen holders for measuring electrical properties of oxides under electron irradiation in HVEM

Two types of specimen holders have been developed for in situ measurements of electrical properties during microstructural observation in a high voltage electron microscope (HVEM). Each specimen holder contains an electrode cell consisting of electrodes, a heater, a thermocouple and a base made from MACOR insulating ceramic. The temperature dependence of the electrical conductivity in ZrO2-3mol% Y2O3 (YSZ) and the dielectric loss in MgO have been investigated without irradiation for evaluating the reliability and the limitation of the specimen holders. YSZ specimens were irradiated with 1 MeV electrons at temperatures from 450 K to 870 K in the HVEM. The radiation induced electrical conductivity was detected by in situ measurements. The dielectric loss in MgO was also measured and found to increase under electron irradiation.

Local phase transformations in alloys during charged-particle irradiation

Among the various mechanisms and processes by which energetic irradiation can alter the phase stability of alloys, radiation-induced segregation is one of the most important phenomena. Radiation-induced segregation in alloys occurs as a consequence of preferential coupling between persistent fluxes of excess defects and solute atoms, leading to local enrichment or depletion of alloying elements. Thus, this phenomenon tends to drive alloy systems away from thermodynamic equilibrium, on a local scale. During charged-particle irradiations, the spatial nonuniformity in the defect production gives rise to a combination of persistent defect fluxes, near the irradiated surface and in the peak-damage region. This defect-flux combination can modify the alloy composition in a complex fashion, i.e. it can destabilize pre-existing phases, causing spatially- and temporally-dependent precipitation of new metastable phases. The effects of radiation-induced segregation on local phase transformations in Ni-based alloys during proton bombardment and high-voltage electron-microscope irradiation at elevated temperatures are discussed.

High voltage electron microscope irradiation and observations in metallic glasses

High voltage electron microscope (hvem) has been extensively used to produce radiation damage and to study the characteristics of defects so produced in crystalline solids. To understand the defect production in metallic glasses and to evaluate the influence of such defects on physical properties like crystallisation temperature etc., high voltage microscopy and subsequentin situ heating and observation has been extremely useful technique. This paper gives a qualitative overview of such work performed in metallic glasses. In particular results obtained on a nickel based metallic glass using ahvem and an electron accelerator are presented. The advantages and limitations ofhvem irradiation are highlighted.

Compositional redistribution in alloy films under high-voltage electron microscope irradiation

The problem of nonequilibrium segregation in alloy films under high-voltage electron microscope (HVEM) irradiation at elevated temperatures is re-examined in the present work, taking into account the damage-rate gradients caused by radial variation in the electron flux. Axial and radial compositional redistributions in model solid solutions, representative of concentrated Ni-Cu, Ni-Al and Ni-Si alloys, were calculated as a function of time, temperature, and film thickness, using a kinetic theory of segregation in binary alloys. The numerical results were achieved by means of a new software package (DISPL2) for solving convection-diffusion-kinetics problems with general orthogonal geometries. It was found that HVEM irradiation-induced segregation in thin films consists of two stages. Initially, due to the proximity of the film surfaces as sinks for point defects, the usual axial segregation (to surfaces) occurs at relatively short irradiation times, and rapidly attains quasi-steady state. Then, radial segregation becomes more and more competitive, gradually affecting the kinetics of axial segregation. At a given temperature, the buildup time to steady state is much longer in the present situation than in the simple case of one-dimensional segregation with uniform defect production. Changes in the alloy composition occur in a much larger zone than the irradiated volume. As a result, the average alloy composition within the irradiated region can differ greatly from that of the unirradiated alloy. The present calculations may be useful in the interpretation of the kinetics of certain HVEM irradiation-induced processes in alloys.

The voids formation of Cu-1.5%Fe alloy during irradiation in HVEM

IntroductionThe high voltage electron microscope (HVEM) is a method useful for simulating the effects of fusion and fission reactor and has the advantage that direct observation can be made of the void and other defects of the specimen during irradiation. Also, the determination of the conditions under which voids can be nucleated and grown by electron irradiation ought to be helpful in separating the effects of the various parameters involved. This paper presents the effects of aging on void formation in Cu-Fe alloy during irradiation in the HVEM.Experimental ProcedureAlloy was quenched into water from 1263 K and aged at 973 K for 1 hr, 4hrs and 4 days. Electron irradiation was performed at 523 K by using a 650 kV HVEM in a flux of 3xl023e/m2 (9xl0-4 displacement/atom/s). The foil plane was a (110). For comparison with the alloy, pure Cu also irradiated under the same dcondition.

Irradiation of U 3Si-based compounds in the high-voltage electron microscope

U 3Si and U-3.5 wt% Si-1.5 wt% Al have been irradiated in a high-voltage electron microscope (HVEM) at 500–1180 keV and 300–660 K. The creation of ‘black spot’ damage and removal of deformation twins are observed. Defects about 10 nm diameter averaging 4 × 10 21 m −3 are seen only in samples pre-injected with 10 −5 atomic fraction argon and are tentatively identified as voids. The atomic displacement rate during HVEM irradiation of U 3Si-based compounds is about two orders of magnitude higher than that for fuel at power reactor ratings. It is inferred that displacement of silicon, and possible uranium, atoms in U 3Si-based compounds occurred in the HVEM at accelerating voltages in the range 700–1180keV.

Loop and Void Formation in Copper During High Voltage Electron Microscope Irradiation

The formation of dislocation loops and voids under neutron, ion, and electron irradiation is well documented. Since irradiation produces Frenkel pairs, the presence of biased interstitial sinks is required for void formation to procede. It is generally accepted that dislocations, generated either by cold-work or irradiation, act as the preferential sinks.During irradiation in the high voltage electron microscope, both interstitials and vacancies are lost to foil surfaces and corresponding denuded zones are set up. As a result of image forces, dislocation loops formed upon irradiation tend to unfault and glide out of the foil. Thus, below a given thickness, the foil is unable to maintain a sufficiently high dislocation density to support void nucleation and growth.