L.M. Howe

A study of the irradiation behaviour of Zr3Al

Abstract Zr 3 Al is an ordered f.c.c. alloy of the L1 2 type. In this investigation, the stability of the ordered phase when subjected to Ar + ion bombardment was explored using transmission electron microscopy. At low ion fluences (up to 10 12 ions/cm 2 ) individual damaged regions were observed. Imaging with fundamental reflections revealed those regions having a spherically symmetrical strain field whereas imaging with superlattice reflections also revealed disordered regions. At ion fluences above 10 12 ions/cm 2 overlap of the damage regions occurred thus resulting in a complex damage configuration. In the fluence regime 5 × 10 14 to 1 × 10 16 ions/cm 2 , complete disordering of Zr 3 Al initially occurred and this was followed by complete transformation to the amorphous state. The fluence required to reach a particular disordered state and degree of amorphicity was dependent upon the amount of annealing which occurred within the defect cascade at the temperature of the bombardment.

Features of collision cascades in silicon as determined by transmission electron microscopy

Abstract Studies have been performed on individual damaged regions in Si using high resolution transmission electron microscopy techniques. The damaged regions were produced during implantation at 40–50 K with monatomic and diatomic ions of As, Sb and Bi (10–120 keV energy) to fluences of 1 × 1011–6 × 1011 ions cm−2. All of the damaged regions investigated gave rise to contrast which was principally of the structure factor type and these regions are best described as being ‘amorphous’. The fraction of the theoretical collision cascade volume occupied by the damaged regions increased steadily as the average deposited energy density θ ν increased and at θ ν ⪆1 eV/atom the average diameter of the damaged regions was equal to or greater than twice the mean transverse straggling of the deposited energy distribution. At low energy densities θ ν ⪅0.1 eV/atom , multiple damaged regions were formed quite frequently within a single collision cascade i.e. sub-cascade structure had developed. The annealing behaviour of the induced damage was investigated using monatomic and diatomic Bi implants with θ ν ranging from 0.09 to 2.7 eV/atom. The damage produced by a diatomic ion was always more resistive to annealing than that produced by the monatomic ion of the same velocity. However, the temperature regimes in which the annealing occurred show no simple dependence upon θ ν .

Transmission electron microscopy investigations of ordered Zr3Al

Zr3Al is an ordered fcc alloy of the L12 type. The ordered phase appears to be very stable thermally as it has not been possible to disorder Zr3Al by quenching. It is possible, however, to disorder Zr3Al by irradiation. In deformed Zr3Al the superlattice dislocations were observed to be extended, i.e. they consisted of intrinsic stacking faults on {111} bounded by partial dislocations. _ The total Burgers vector is of the type and the superlattice dislocations have dissociated as follows: «110a> → 13«211a> + 13«121a>. During high-temperature annealing of the deformed material, dislocation networks were produced in which all of the dislocation nodes were extended; hence intrinsic and extrinsic stacking faults must alternate at the nodes. Weak beam images were obtained of long and relatively straight dislocation lines in annealed Zr3Al in order to obtain information on the equilibrium width of the extended dislocations.

A study of interstitial trapping configurations in fcc metals by ion channeling

Abstract Channeling measurements have shown that self-interstitial atoms in Al and Cu are trapped strongly by small solute atoms and weakly by large solute atoms. The strong trapping configuration is the 〈100〉 mixed dumbbell, in which the solute atom is displaced from its lattice site by 0.1–0.14 nm in a 〈100〉 direction. Mn, Cu, Fe, Zn and Ag solutes in Al, and Be solutes in Cu form strong traps which are stable up to stage III recovery. In the weak trapping configuration, the self-interstitial retains its identity and the solute atoms are displaced very little. Mg and Sn solutes in Al and Sb, Ag and Au solutes in Cu are weak traps, which release trapped interstitials during stage II recovery.

Heavy ion damage in silicon and germanium

High resolution transmission electron microscopy techniques have been used to obtain information on the contrast, spatial distribution, size and annealing behaviour of the damaged regions produced within individual collision cascades by heavy ion bombardment of Si and Ge. Of particular interest was the dependence of the above features on the average deposited energy density θ ν within the collision cascade. Implants were performed at 35–50 K using monatomic and diatomic ions of 75 As, 121 Sb, 128 Te and 209 Bi, having incident energies of 10–120 keV, which meant that θ ν varied from 0.02 to 5.5 eV/atom. The bombarding ion fluences used covered the range 1.0 × 10 11 to 1.0 × 10 13 ions cm −2 . At high energy densities (θ gn 1.0 eV/atom), the visible damage produced in the main cascade consisted of a single, isolated damaged region. With decreasing values of θ ν (i.e. increasing ion implant energies), there was an increasing tendency for multiple damaged regions to be produced within the main cascade. Also, the fraction of the theoretical cascade volume occupied by a heavily damaged region steadily increased as θ ν increased. For high energy implants, at ion fluences > 5 × 10 11 ions cm- 2 , additional regions appeared which had lighter contrast than the multiple damaged regions formed in the initial stages. It is proposed that as overlap of the individual cascades occurs, the damage level in the peripheral regions becomes great enough to produce diffraction contrast in more extensive regions of the cascades. The annealing behaviour of the various damaged regions was also investigated at temperatures ranging from 300 to 775 K.

The effect of neutron irradiation on the tensile properties of zircaloy-2

Abstract Tensile specimens of annealed, 13.1 per cent coldworked, and tempered 25.5 per cent cold-worked Zircaloy-2 were irradiated at 220° C and 280° C with integrated fast-neutron fluxes of 3.6 × 10 19 n/cm 2 and 2.7 × 10 20 n/cm 2 respectively. Post-irradiation tensile tests performed at room temperature and 280° C showed that considerable irradiation hardening occurred in all the irradiated material. This change was characterized by an increase in the proportional limit, yield stress, and ultimate tensile strength, and a decrease in the total and uniform per cent elongations. The per cent changes in mechanical properties as a result of the irradiation were greater in the annealed material than in the cold-worked material. The tensile properties of irradiated 13.1 per cent cold-worked material were almost identical to those for the irradiated tempered 25.5 per cent cold-worked material for both levels of irradiation. A yield point was developed in irradiated annealed material tested at 280° C whereas no yield point was present in the same material tested at room temperature. Results are also given for post-irradiation annealing studies performed on the annealed and cold-worked material irradiated at 280° C.

Ion bombardment of ordered Zr3Al

Abstract The behaviour of the ordered alloy Zr3Al was investigated by transmission electron microscopy following ion bombardments at temperatures ranging from 30 to 850 K. Utilizing high energy(0.5–2.0 MeV) C+, N+ and Ar+ ion bombardments, observations were made of the irradiation-induced transformation from the ordered → disordered → amorphous states. Information about individual damage cascades was also obtained using both high energy (C+. N+ and Ar+) and low energy (15–120 keV Cu+) ion bombardments.

A diffusion calculation of axial dechanneling in Si and Ge

Abstract Analytical expressions have been derived for the diffusion functions which describe the increase in the transverse energy of channeled ions due to electronic and nuclear scattering, and also for various other functions which are required for a diffusion model of dechanneling. The electron density around the center of a channel was also taken into account in the theoretical treatment. Using these functions, the diffusion equation was solved numerically for H+ incident along the and axial channels of Si and for H+ and D+ incident along the same channels in Ge. The calculations were performed for various incident ion energies and target temperatures. Quite good agreement was obtained between the theoretical results and experimental dechanneling data.

Ion beam analysis of defect trapping

Abstract Channeling measurements using medium energy ions (e.g. 1 MeV He+) have been used to determine the positions of solute atoms which are displaced from lattice sites by the trapping of vacancies and self-interstitial atoms. In this way, some simple defect trapping configurations have been identified in fcc metals. One of these is the mixed dumbbell (created when a self-interstitial is trapped by a small solute atom), consisting of a host atom and solute atom stradding a normal lattice site. Another is the tetravacancy-solute atom complex, consisting of four nearest neighbour vacancies surrounding a solute atom displaced into the tetrahedral interstitial site. In addition, from detailed analyses of displacements into different crystallographic channels as a function of irradiation fluence and annealing temperature, the evolution of a variety of defect complexes containing self-interstitials or vacancies has been studied in Al, Cu, Ni, Fe, and Mg crystals. Information from channeling analyses will be compared with data obtained from measurements of electrical resistivity, Mossbauer effect, perturbed angular correlation, extended X-ray absorption fine structure, muon precession, positron annihilation and internal friction. The advantages of the different methods will be discussed.

Irradiation swelling of Zr3Al

Heavy ion (0.5–2 MeV Ar+) and 1 MeV electron irradiation of Zr3Al have established that the lattice parameter is increased by ~0.5% upon irradiation disordering and that the specific volume is increased by (5 ± 1) % following an irradiation-induced crystalline to amorphous transformation. These effects have been incorporated into a quantitative model of irradiation swelling which is shown to be in reasonable qualitative and quantitative agreement with fast-neutron induced dimensional changes in Zr3Al. The model predicts that swelling will saturate and that the largest changes will occur at low temperatures.