Dislocation Loops In Aluminium Research Articles

Implementation of the nudged elastic band method in a dislocation dynamics formalism: Application to dislocation nucleation

We propose a framework to study thermally-activated processes in dislocation glide. This approach is based on an implementation of the nudged elastic band method in a nodal mesoscale dislocation dynamics formalism. Special care is paid to develop a variational formulation to ensure convergence to well-defined minimum energy paths. We also propose a methodology to rigorously parametrize the model on atomistic data, including elastic, core and stacking fault contributions. To assess the validity of the model, we investigate the homogeneous nucleation of partial dislocation loops in aluminum, recovering the activation energies and loop shapes obtained with atomistic calculations and extending these calculations to lower applied stresses. The present method is also applied to heterogeneous nucleation on spherical inclusions.

Formation of stacking-fault tetrahedra in aluminum irradiated with high-energy particles at low-temperatures

Stacking-fault tetrahedra (SFT) are typical vacancy clusters in fcc metals. SFT have been, however, considered to be unstable in aluminum because of its high stacking-fault energy, until the recent finding of SFT in thin aluminum foils subjected to a tensile fracture. This study confirmed that SFT form in aluminum following irradiation with high-energy particles. In electron irradiation, SFT with an average size of 2 nm formed below 203 K, while a larger irradiation intensity at lower temperature induced SFT with a larger number density. Irradiation with 60-keV Al + ions induced SFT below room temperature, although the defect yield (the ratio of the number of defect clusters to the number of incident ions) was about 10 3 , considerably smaller than that in the other fcc pure metals. With neutron irradiation below 15 K to a fluence of 2 × 10 2 1 neutrons m - 2 , SFT were not observed at room temperature. Instead, dislocation loops were observed to form and to disappear during observation with 120-kV electrons that do not cause atomic displacements. Tensile fracture of aluminum thin foil induced SFT at up to 400 K, which is close to the temperature at which SFT become unstable in isochronal annealing experiments. These results suggest that a high concentration of vacancies at lower temperature tends to cause SFT to form rather than vacancy-type dislocation loops in aluminum.

Highlights in the history of X-ray topography 1

Abstract X-ray topography provides a non-destructive method of mapping point-by-point variations in orientation and reflecting power within crystals. The discovery, made by several workers independently, that in nearly perfect crystals it was possible to detect individual dislocations by X-ray diffraction contrast started an epoch of rapid exploitation of X-ray topography as a new, general method for assessing crystal perfection. Another discovery, that of X-ray Pendellösung, led to important theoretical developments in X-ray diffraction theory and to a new and precise method for measuring structure factors on an absolute scale. Other highlights picked out for mention are studies of Frank-Read dislocation sources, the discovery of long dislocation helices and lines of coaxial dislocation loops in aluminium, of internal magnetic domain structures in Fe-3 wt.% Si, and of stacking faults in silicon and natural diamonds.

Effect of hydrogen on the formation of interstitial loops in hydrogen-irradiated aluminum

The role of hydrogen atoms in the formation of interstitial type dislocation loops in aluminum irradiated with 15 keV hydrogen ions was investigated by electron microscopy. It was found that the loop density satisfied and Arrhenius relation with the pre-irradiation temperature, yielding an apparent activation energy of about 0.3 eV. Experimental results are interpreted in terms of the mechanism that small hydrogen-vacancy complexes formed by the pre-irradiation of hydrogen ions act as nucleation sites of dislocation loops in the subsequent electron irradiation. A basically identical mechanism seems to be acting in the irradiation with hydrogen ions at room temperature.

Pipe diffusion along the segment of faulted dislocation loops in aluminum

The motion of a perfect dislocation loop during annealing was examined by several workers. They proposed a mechanism of pipe diffusion and analyzed this motion. However, these studies on pipe diffusion may have some errors caused by the prismatic glide. In this paper we studied on the motion of faulted dislocation loops under the mutual interaction and estimated the activation energy for pipe diffusion along the dislocation segment of faulted loops.Figure 1 shows schematically a pair of regular hexagonal faulted dislocation loops lying on the (111) plane in quenched aluminum as well as their images observed with an electron microscope, where (a) is the one taken from [001] direction before annealing, (b) and (c) are those taken from [001] and [011] directions after annealing at 95°C for 2 hours, respectively. In order to determine the vector S which connects the center of loop 1 and that of loop 2 in Fig. 1, three different coordinate systems (X1, Y1, Z1), (X2, Y2, Z2), (X3, Y3, Z3) are set up on a specimen, an electron microscope and a micrograph, respectively,(Fig. 2).

Further Studies of Dislocation-Loop Annealing in Aluminium-Base Alloys

Abstract The detailed annealing behaviour of large faulted dislocation loops in aluminium and aluminium-base alloys has been studied by quantitative electron microscopy. Changes in loop shrinkage rates, loop character and dislocation pinning have been observed and used to derive information on solute clustering and vacancy/solute interactions. Although generally small, differences between the measured shrinkage rates of loops in pure Al and A1-base alloys were found to be dependent upon the state of solute dispersion. The results are consistent with a model in which extensive clustering occurs during quenching and ageing, and the differences in apparent activation energies are attributed to binding between vacancies and solute atom clusters, or zones. The apparent discrepancy between the large solute atom/vacancy binding energies measured in quenching studies and the much smaller ones measured in equilibrium studies is thus partially explained.

Size distribution of dislocation loops

The annealing behavior of dislocation loops in aluminium obtained by bulk quenching is discussed. If the mean distance between loops is small compared with the grain-size of the specimen, the total number of vacancies condensed in loops remains constant, whilst the difference of chemical potential at loops of different sizes provides a driving force for coarsening. Thus loops with radii larger than the average radius grow and loops with smaller radii shrink. After a transient the mean loop radius increases with the square root of time and the loop radii are described by a steady state distribution function which shows a cut off at twice the average radius. Specimens containing only prismatic or only faulted loops show this behaviour, but coarsening is much faster for faulted loops. In specimens where both faulted and prismatic loops are present, the latter grow at the expense of the former. Similar coarsening reactions should take place in h.c.p. and b.c.c. metals.

The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation

The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation

Multi-faulted dislocation loops in electron bombarded aluminium

The high voltage electron microscope has been used to introduce and observe multi-faulted dislocation loops in aluminium. A wide variety of different loop configurations, including double and triple-layer defects, was encountered in helium preinjected aluminium and compared with that from quenching investigations by other authors. The observation that faulted dislocation loops formed under a wider range of irradiation temperatures and with a wider variety of configurations in aluminium previously doped with a few ppm helium than they did in uninjected material, suggests that helium can influence the process of fault formation. Le microscope électronique à haute tension a été utilisé dans le but d'introduire et d'observer des boucles de dislocations à fautes multiples. Une grande variété de configurations de boucles, y compris des défauts d'empilement doubles et triples, fut observée dans de l'aluminium préinjecté d'hélium et comparées à celles observées dans d'autres travaux publiés par différents auteurs. Le fait que les bouches de dislocations furent obtenues dans un intervalle de températures plus étendu et assumèrent un plus grand nombre de formes dans les échantillons dopés de quelques ppm d'hékium que dans ceux qui ne l'étaient suggère que l'hélium peut influencer le procédé de formation des défauts. Mit einem Hochspannungselektronenmikroskop wurden mehrfach defekete Veschiebungsschleifen in Aluminiumproben erzeugt und beobachtet. Eine Vielfalt verschiedener Schleifenformen, unter anderen doppelte und dreifache Schichtdefekte, wurden in Aluminium aufgefunden, das mit Helium präinjiziert wurde, und diese Befunde mit Untersuchungen anderer Autoren verglichen, die an gelöschtem Material arbeiteten. Wenn das Aluminium zuerst mit einigen ppm Helium dotiert wurde, traten defekte Verschiebungsschleifen in einem größeren Bestrahlungstemperaturbereich auf und zeigten eine größere Vielfalt der Konfigurationen als im nicht injizierten Material. Daraus läßt sich scließen, daß Helium die Defektbildung beeinflulsBt.

The effects of different microstructures on the formation of dislocation loops in aluminium during electron irradiation

Abstract Exposure to the beam in a 200kV electron microscope caused defect clusters to form in high-purity aluminium foils. The damage was due to the direct displacement of atoms by electrons. The point defect clusters nucleated randomly and grew to form dislocation loops. The microstructure of the aluminium was varied by quenching or alloying. In high-purity foils containing previously quenched-in Frank sessile vacancy loops the subsequent electron irradiation-induced defects were seen to interact with the pre-existing defects. In 99% aluminium and in 2024 aluminium (4.5%Cu, 1.5%Mg, 0.6%Mn, balance Al) no electron-induced damage was seen. However, when the 99% aluminium specimens were first neutron irradiated, subsequent bombardment by electrons caused the development of visible dislocation loops. The important conclusion of this electron microscope study is that the presence and distribution of electron irradiation damage in aluminium is extremely sensitive to microstructure.

Nuclei of strain in a cubic material

Abstract Small imperfections in a crystal cause diverse patterns of contrast on an electron micrograph. The shape of these patterns is calculated by a new method, the result being a simple algebraic function of x and y. Detailed results are given for the symmetrical centre of pressure, for double forces and for prismatic dislocation loops in aluminium, copper and beta brass.

Formation of Dislocation Loops at Low Temperatures in Quenched Aluminum

Formation of dislocation loops in aluminum below room temperature down to -90°C is studied by the two step aging method after quenching. The beginning stage of the vacancy clustering is similar to that around room temperature, that is the small vacancy clusters growing to dislocation loops have a fairly wide range in their size of weak bindings and a well defined nucleus of dislocation loop does not exist. It is concluded that the main part of the beginning stage of the clustering is played by quenched-in di-vacancies, especially at low temperatures. Two kinds of influences of impurity atoms are found. The major influence is to enhance the formation of the loops, but the increase in the stability of vacancy clusters by impurity is not enough to make well defined stable nuclei of dislocation loops.

Climb kinetics of dislocation loops in aluminium

Abstract The annealing kinetics of dislocation loops in quenched aluminium has been studied using transmission electron microscopy techniques. An influence of the vicinity of the foil surface on the rate of climb was confirmed. The apparent activation energy for climb of faulted loops was found to be 1·18 ± 0·02 ev and the pre-exponential factor, (7 ± 3)1013 Å/S. The stacking-fault energy was estimated as 110 ergs/cm2 by comparing shrinkage rates of ⅓⟨111⟩ and ½⟨110⟩ loops. A simple model, taking into account rapid diffusion of vacancies along the dislocation, is suggested to account for the observed maintenance of circular shape during loop shrinkage.

The influence of a dilute magnesium addition on the growth ant shrinkage of dislocation loops in aluminium

Abstract The influence of a dilute magnesium addition to pure aluminium on the climb of dislocation loops has been studied by isothermal annealing of thin foils of Al-0·65% Mg. Faulted loops were observed to shrink at temperatures above 130°C, whereas prismatic loops above a critical size grew at temperature? ≳200°C; in pure aluminium they generally shrink. The various mechanisms previously proposed to account for excess vacancies have been critically examined and shown to be inconsistent with the experimental observations, and an alternative mechanism based on the growth of a surface oxide is proposed. Experimental observations consistent with the oxide growth mechanism are described. The variation of shrinkage rate with temperature for both prismatic and faulted loops over a wide temperature range has been determined. The results, which do not conform to emission controlled kinetics, have been analysed in terms of diffusion controlled behaviour to deduce an accurate value for the intrinsic stacking-fault energy of Al-0·65% Mg. Comparison of the annealing rate of faulted loops in the alloy and pure aluminium indicates that the Mg atom-vacancy binding energy is extremely small.

Climb kinetics of dislocation loops in aluminium

Abstract The climb kinetics of dislocation loops in thin foils in terms of either the emission of vacancies from the loop or the diffusion of vacancies to the surface has been critically examined. Although both mechanisms give similar rate equations, predictions can be made which allow the rate controlling process to be determined experimentally. The annealing behaviour of faulted and prismatic loops in quenched aluminium has been studied using quantitative electron microscopy techniques and shown to support a theory based on vacancy diffusion. Hence by comparing the annealing rates of faulted loops with those of prismatic loops as a function of temperature the stacking-fault energy of aluminium has been determined to be 135±20 ergs/cm2. This value is shown to be consistent with the physical properties of aluminium in relation to other metals.

The effect of pressure on the annealing of dislocation loops in aluminium

Thin foils of quenched aluminium have been annealed at hydrostatic pressures up to 60 kb and temperatures up to 289°C. Observation by transmission electron microscopy before and after treatment has enabled a determination of temperature ranges within which dislocation loops anneal out at different pressures. It is found that the temperature required for annealing increases with pressure by between 1.26 and 2.62°C/kb, corresponding to an activation volume lying between 0.44 and 0.87 atomic volumes. This range accords with values for other f.c.c. metals and with values calculated firstly from the pressure variation of the melting temperature and secondly from expressions relating activation volume and entropy to activation energy. The work shows that the decrease in point defect mobility with pressure is a much larger effect than the increase in driving force. Two alternative general expressions have been derived for lines of equal annealing to predict the effect of pressure on diffusion controlled processes in any solid.

The Effect of Solute Concentration on the Formation of Loops in Aluminium‐Magnesium Alloys during Fission Fragment Irradiation

AbstractA study has been made of the effect of magnesium concentration (up to 0.5 at%) on the formation of dislocation loops in aluminium during fission fragment bombardment at 60 °C. Increasing magnesium content reduces the initial rate of increase of the total loop area with dose, but the loop area saturates at a higher level in more concentrated alloys. A concentration as low as 0.02% reduces the initial rate of loop growth by a factor of 4 to 5. Considerable attention has been given to the problems of obtaining accurate measurements of the loop area per unit volume in foils irradiated in the pre‐thinned state, and a number of corrections have been applied. An unusual surface denudation effect is observed. The results are explained in terms of the effect of trapping on the mobility of both the interstitial atoms and the vacancies.

On interstitial dislocation loops in aluminium bombarded with alpha-particles

Abstract Electron transmission microscopy and selected area diffraction have shown that the dislocation loops in aluminium bombarded with 38 Mev alpha-particles lie generally upon the {110} planes with a [110] Burgers vector normal to the loop. These loops are only seen where the alpha-particles come to rest. The theory of diffraction contrast at dislocations (Hirsch et al. 1960) has been used to determine the sense of the Burgers vectors and shows that the loops are formed from interstitial atoms as previously inferred from their annealing behaviour (Barnes and Mazey 1960).

On the stability of quenched dislocation loops in aluminum

On the stability of quenched dislocation loops in aluminum