Torben Leffers

Nucleation of helium bubbles on dislocations, dislocation networks and dislocations in grain boundaries during 600 MeV proton irradiation of aluminium

Abstract High-purity aluminium (99.9999%) was irradiated with 600 MeV protons with a damage rate of 3.5 × 10 −6 dpa/s . Irradiation with 600 MeV protons produces helium and hydrogen at the rate of 140 and 615 appm per dpa, respectively. Specimens irradiated at temperatures in the range 116 to 318 °C to doses in the range 0.04 to 5 dpa were examined in a transmission electron microscope (TEM). The TEM investigation has shown that helium bubbles are formed on dislocations in the grains as well as dislocations in the grain boundaries. Dislocation nodal points whether present in dislocation walls or in grain boundaries are found to be the most favourable sites for bubble nucleation. The mean diameter of the bubbles on individual dislocation lines are found to be larger than those for the bubbles in the matrix. The bubble size and density on grain boundaries vary from boundary to boundary. The size of these bubbles on the boundaries is larger than or equal to the size of those in the matrix. It is suggested that helium atoms once arrived at a dislocation remain bound to the dislocation line but at the same time remain mobile within the dislocation core; the bubble nucleation behaviour in the core would thus be affected by the core structure of the different dislocations. An estimate of the effective helium diffusion in the dislocations relative to that in the lattice has been made on the basis of the measured bubble parameters and the width of the bubble-denuded zone along dislocation lines; the diffusion coefficient of helium in the dislocations is found to be about the same as that in the lattice.

Dislocation and void segregation in copper during neutron irradiation

Abstract High-purity (99·999%) and fully annealed copper specimens have been irradiated in the DR-3 reactor at Ris⊘ to doses of 1 × 1022 and 5 × 1022 neutrons (fast)m−2 (2 × 10−3 dpa and 1 × 10−2 dpa, respectively); the irradiation experiments were carried out at 250°C. The irradiated specimens were examined by transmission electron microscopy. At both doses, the irradiation-induced structure was found to be highly segregated; the dislocation loops and segments were present in the form of irregular walls and the voids were distributed between these walls. The dislocation walls were practically free of voids and generally had a void-denuded zone along them. The density of dislocations (loops and segments) was very low in the region containing voids (i.e. between the dislocation walls). Even with this low dislocation density, the void swelling rate was very high (∼2·5% per dpa). The implications of the segregated distribution of sinks for void formation and growth are briefly discussed. It is pointed out th...

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.

Simulation of structure and annihilation of screw dislocation dipoles

Abstract Large scale atomistic simulations are used to investigate the properties of screw dislocation dipoles in copper. Spontaneous annihilation is observed for dipole heights less than 1 nm. Equilibrated dipoles of heights larger than 1 nm adopt a skew configuration due to the elastic anisotropy of Cu. The equilibrium splitting width of the screw dislocations decreases with decreasing dipole height, as expected from elasticity theory. The energy barriers, and corresponding transition states for annihilation of stable dipoles are determined for straight and for flexible dislocations for dipole heights up to 5.2 nm. In both cases the annihilation is initiated by cross-slip of one of the dislocations. For straight dislocations the activation energy shows a linear dependence on the inverse dipole height, and for flexible dislocations the dependence is roughly linear for the dipoles investigated.

Formation of cavities at and away from grain boundaries during 600 MeV proton irradiation

Abstract High-purity aluminium (99.9999%) was irradiated with 600 MeV protons at the Swiss Institute for Nuclear Research (SIN) with a damage rate of 3.5 × 10 −6 dpa/s . Irradiation with 600 MeV protons produces helium, hydrogen, and other impurities through mutational reactions. The irradiation experiments were carried out at ~120°C (~0.42T m , where T m is the melting temperature in K). Transmission electron microscopy on specimens irradiated to 0.2 and 0.6 dpa has shown the presence of (a) cavity-denuded zones (CDZ) along grain boundaries, (b) cavity-containing zones (CCZ) adjacent to the CDZ and (c) regions beyond the CCZ containing a very low density of cavities. At the dose level of 2 dpa, a dense population of very small cavities is resolved on the grain boundaries and also in their immediate vicinity (in the CDZ). Furthermore, at the dose levels of 0.6 and 2 dpa, a well defined dual size distribution of cavities is observed in the CCZ. The results are discussed in terms of agglomeration of helium atoms which are considered, during irradiation, to diffuse mainly via vacancies.

Effects of heterogeneous sink distribution on void swelling

Abstract Swelling rates are calculated for two types of material with heterogeneous distributions of dislocations and voids, namely copper irradiated with neutrons to low dose at 250°C and heavily cold-worked copper irradiated with 1 MeV electrons in a HVEM at 250°C. Both materials are considered to consist of non-interacting spherical components with a wall and an inner cell with different dislocation and/or void densities. We subdivide the sphere (wall plus cell) in a number of concentric shells and find a quasi-static solution for the interstitial and vacancy concentrations in the different shells by a finite-difference method. From these concentrations the local and the average swelling rates are calculated. The effect of the heterogeneities in sink distribution on swelling rate and the dependence of this effect on various structural parameters are investigated. We find that a heterogeneous sink distribution may increase or decrease the swelling rate relative to the swelling rate for a homogeneous dis...

Calculated and experimental orientation distributions of twin lamellae in rolled brass

Abstract The orientation distribution of lamellae of mechanical twins in f.c.c. material is calculated with the Taylor model, the Sachs model and the “modified Sachs model” (which includes random stresses), and the calculated distributions are compared with the experimental distribution in brass rolled to 40% reduction. The twin orientation distributions calculated with the Taylor model are rather different from that observed experimentally, whereas some of the distributions calculated with the Sachs-type models are rather close to the experimental distribution. The modified Sachs model, which reproduces the experimental twin distribution quite well, at the same time produces a good simulation of the development of the brass texture.

The temperature dependence of void and bubble formation and growth in aluminium during 600 MeV proton irradiation

Abstract As a part of a continuing program, we report in the present paper results obtained from irradiating pure aluminium samples in the PIREX facility installed in the 600 MeV proton beam of the accelerator at the Swiss Institute for Nuclear Research (SIN). The aluminium foils have been irradiated at 8 different temperatures in the range from 130°–430°C, to displacement doses of up to 5 dpa and helium contents of over 1000 appm. The TEM examinations have shown that at all irradiation temperatures and displacement doses, helium bubbles are formed uniformly through the whole grain interior. No voids are observed at temperatures above 160° C. At all temperatures, irradiation induced dislocations have been observed, most of them linked to bubbles. At higher temperatures and doses, clear evidence of irradiation induced precipitation has been observed; the precipitates are normally decorated with helium bubbles.

The Relation Between Texture and Microstructure in Rolled FCC Materials

The relations between texture and microstrueture in rolled fcc metals and alloys are described. In materials with low stacking fault energy texture and microstructure together provide a consistent empirical description of the deformation process. In materials with intermediate and high stacking fault energy things are more complicated. There are a number of textural and microstructural features which are not understood and which probably require a combination of texture and microstructure to be understood. A substantial effort is needed in order to establish this connection.

Grain boundary related effects in aluminium during 600 MeV proton irradiation at different temperatures

Abstract Samples of high-purity aluminium were irradiated with 600 MeV protons at temperatures in the range 130 to 433°C; in these experiments 615 and 125 appm of hydrogen and helium, respectively, are produced per dpa. Bubble formation and growth at grain boundaries and in the zone adjacent to the bubble-denuded zone are described. Precipitation at grain boundaries and migration of grain boundaries during irradiation are also reported.