Faulted Dislocation Loops Research Articles

Defect structure and tetrahedral precipitates in sulphur-doped gallium phosphide

Transmission electron microscopy has been used to study the defect structure in liquid encapsulated Czochralski GaP doped with 2×1018 S atoms cm−3. Dislocation clusters, stacking faults and precipitates were observed and analysed in as-grown, annealed and zinc diffused materials. In as-grown and annealed GaP both the perfect and faulted dislocation loops were shown to be interstitial in nature while the precipitates were found to be incoherent and tetrahedral in shape. Prolonged zinc diffusion was shown to increase the dislocation density and cause the precipitates to become coherent.

The Climbing Motion of Faulted Dislocation Loops Induced by Elastic Interaction

The movement of faulted dislocation loops during annealing has been studied with transmission electron microscopy using the usual stereotechnique. It is revealed that the distance between two nearby faulted dislocation loops in a thin foil varies during annealing at 95°C for 2 hours without their sizes changing. This variation in distance is considered to occur by pipe diffusion due to the elastic interaction between the loops. The vacancy concentration and the vacancy flow along the perimeter of the loops are calculated by considering the interaction force between their segments. It is shown that the vacancy flow that moves loops exists when the distance between loops is less than 200 Å. The activation energy for pipe diffusion is estimated to be about 0.90 eV.

Preferential precipitation on dislocation loops in Te-doped gallium arsenide

Abstract Heat-treated tellurium-doped gallium arsenide contains prismatic and faulted dislocation loops with precipitate particles lying on the dislocation core. The position of the precipitates on the loops has been studied revealing preferential precipitation on 〈110〉 line segments for both faulted and prismatic loops. From a consideration of the atomic arrangements at the dislocation cores it is concluded that the precipitates preferentially decorate the prismatic loops where the dislocation core is identical to either a βss or αgs Lomer dislocation. Precipitates on the {111} faulted loops, however, can appear on all possible 〈110〉 dislocation segments. The faulted loops are thought to consist of a layer of gallium telluride, and, since this is a defect lattice containing gallium vacancies, β type dislocation core structures can occur along all six 〈110〉 directions. By analogy with the faulted loops, preferential precipitation on prismatic loops is believed to occur on βss Lomer dislocations.

A quantitative study of anomalous annealing phenomena in quenched aluminum

It is demonstrated how a quantitative study of some anomalous thin-foil annealing phenomena, if interpreted correctly, leads to the determination of important physical constants. The necessary theory is developed and applied to measure the activation energy for dislocation pipe diffusion in thin aluminum foils from the annealing rate of quenched-in and subsequently truncated faulted dislocation loops. The results are compatible with those obtained from void annealing by another group. Triangular faulted defects present in bulk-aged and thinned samples of aluminum are shown to consist of a mixture of extrinsic and intrinsic loops. They can readily be distinguished by their annealing rates. From the relative values of these rates, the relation between the intrinsic and extrinsic stacking-fault energies has been determined.

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).

Microdefects and striations in dislocation-free LEC-GaP crystals

Dislocation-free GaP crystals grown by the liquid encapsulated Czochralski technique were characterized chiefly by the use of X-ray transmission topography and EBIC-mode scanning electron microscopy. Preliminary results obtained with transmission electron microscopy are also presented. X-ray topographs reveal a crystal inhomogeneity which is mainly attributed to the presence of non-stoichiometric striations. High densities (10 11-10 12 cm -3 of microdefects were detected, distributed both homogeneously and in a striated pattern. Various types of microdefects (perfect and faulted dislocation loops and (semi) coherent precipitates) were revealed. Although most microdefects are less than 1 μm across, significantly larger defects can form. Evidence for macroscopic dislocation generation due to point defect condensation is presented.

Solute Segregation in Stainless Steel under Irradiation

Solute element additions can significantly influence the behavior of an alloy under irradiation. The aggregation of irradiation-induced vacancies into three-dimensional clusters, voids, and the volume expansion, swelling, associated with void formation is of prime concern in the design of nuclear reactors. A modified type 316 stainless steel, LS1A, has been developed which exhibits high resistance to swelling. This alloy, containing ˜1.0 wt % silicon and 0. 15 wt % titanium, swells 30 times less under nickel ion irradiation than a nominal type 316 stainless steel.In addition to the observation of swelling resistance in LS1A, it was observed that the evolution of dislocation portions of the damage structure was modified by the silicon and titanium additions. Specifically, the stability of faulted dislocation loops in LS1A is quite high and the growth of large loops is severely curtailed. At higher doses, radiation-induced precipitates of the same size and shape as the dislocation loops were observed.

Dislocation generation associated with crack growth of silicon crystals containing precipitate defect structure

A precipitate defect structure was introduced into silicon crystals by diffusion of gold at elevated temperatures. The nature of the defect structure and the response of the crystals to deformation were studied by X-ray topography and transmission electron microscopy. The following results were obtained: 1. 1. The shape, size and the distribution of the lattice defects depended on the quenching temperature and cooling rate. High temperature and rapid quenching gave rise to a homogeneous distribution of small faulted dislocation loops throughout the matrix, while slow cooling resulted in plate-like defects precipitated at impurity striations. 2. 2. TEM analysis showed the faulted loops to be of the interstitial type. 3. 3. Deformation of notched crystals below the ductile - brittle temperature resulted in a cleavage type of fracture preceded by a microplastic zone at the notch tip. The precipitate structure removed from the (111) fracture plane remained unaffected by the fracture process and the lattice regions of silicon between the precipitates exhibited after fracture the characteristics of a perfect lattice (undisturbed PF pattern). 4. 4. Deformation of unnotched crystals above the ductile - brittle temperature induced microplastic zones which were formed by dislocation sources situated at surface sites and not by dislocation sources associated with precipitates.

Diffraction Fine Structure and Contrast from Octahedral Voids in Nickel

It is a well-recognized fact that planar interfaces, such as stacking faults and domain boundaries, can produce fine structure in an electron diffraction pattern consisting of satellites surrounding the main transmitted and diffracted beams. In some recent work, fine-structure darkfield images have been used to selectively study faulted dislocation loops lying on a specific set of lattice planes. It has been shown both theoretically and experimentally, however, that similar diffraction fine structure can be produced by the inclined faces of wedge-shaped crystals. The original work on these effects was done on wedges formed by the faces of MgO crystals and those formed by inclined twin boundaries in metals. The theory can be equally applied to internal "vacuum wedges" in metals formed by the faces of polyhedral cavities, such as the truncated octahedral voids observed in irradiated nickel.

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.

Annihilation of Faulted Dislocation Loops in Quenched Aluminum

The probability of the diffusing away of vacancies from the faulted loop has been calculated using the diffusion equation of vacancies under the effects of the permanent sinks and it has been clarified that the probability increases with decreasing the density of jogs on faulted loops and the distance from the faulted loop to the grain boundary, and it decreases with decreasing the void density. The annihilation processes of faulted loops in quenched aluminum have been experimentally studied, and it has been shown that the experimental results can be easily explained from the idea that the shrinkage speed of the faulted loop is proportional to the product of the emission rate and the diffusing away probability of vacancies from the faulted loop. The value of the stacking fault energy is estimated to be 130±30 erg/cm 2 from the shrinkage speed of the faulted loop in a thin foil.

A high voltage electron microscope study of the effects of preinjected helium on electron damage in aluminium

Defect clusters were formed and observed simultaneously in electron irradiated foils of aluminum, both uninjected and preinjected with about 8 atomic ppm helium, in a high voltage electron microscope operated at 650V. The nominal irradiation temperatures of the specimens were room temperature, 150°C and 200°C. The defect clusters in the form of dislocation loops formed in greater concentrations and persisted to longer times in the preinjected specimens than they did in the uninjected metal. At 200°C, no beam-induced loops were observed in the uninjected aluminum foil of usual thickness, but they were seen in the helium injected material. Faulted dislocation loops were more prevalent in the preinjected specimens. Under certain conditions the irradiated microstructure evolved to one resembling an unirradiated structure. It is concluded that the preinjected helium itself (rather than the associated alpha particle damage) causes the enhanced dislocation loop formation in the electron bombarded foils.

Self-diffusion in aluminum at low temperatures

Values of the self-diffusivity of pure aluminum in the temperature range 130∮ to 200° have been determined by measuring the rate of annealing of prismatic and faulted dislocation loops in thin foils of quenched 99.999 pct Al using a diffusion-controlled climb model due to Seidman and Balluffi modified to take into account elastic interaction between vacancies and dislocations. Changes in line energy of prismatic loops with orientation were evaluated and found to produce a maximum error in D of 17 pct. Other possible sources of error were evaluated. The results giveD = 0.19 ± 0.06 exp—(1.28 ± 0.04ev/KT sq cm per sec. A direct method of determining the activation energy for self-diffusion from prismatic loop annealing rates is presented which minimizes the effects of many sources of variation in individual climb rates.

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 Nature of Defects in Quenched α-Brass

The defects such as faulted dislocation loops, perfect dislocation loops and stacking-fault tetrahedra are observed in quenched α-brass. The faulted loops lie on the {111} planes and have various polygonal shapes with edges along the <110> directions. The perfect loops have hexagonal, rhombic and irregular shapes, and the former two are on the {111} planes with their edges along the <110> directions. Analysis of the image contrast reveals that the faulted loops are of the vacancy type having a Burgers vector a/3<111> and the perfect loops are of the vacancy type having a Burgers vector a/2<110>. The perfect loops are favoured when the quenching temperature is high, while the faulted loops are favoured when the temperature is low. The stacking-fault tetrahedra are occasionally observed by high temperature quenching. The formation energy of a vacancy is estimated to be about (1.00±0.05) eV from the density and size of observed loops. The formation of three different defects is discussed by the simple energy comparison.

Quench-Induced Defect Structure in Al-6.5-at.% Zn

The defect structure induced in Al-6.5-at.% Zn upon quenching consisted of homogeneously nucleated dislocation loops, helical dislocations developed from screw dislocations, the latter being introduced by two different processes. The defect structure also contained perfect loops converted from helical dislocalocations and faulted dislocation loops whose nucleation was enhanced by vacancies generated by quench deformation. Based on experimental evidence, a model is presented explaining the formation of a row of perfect dislocation loops from a single helical dislocation. The interrelation of the various types of induced defects is shown by correlating their formation with the aid of vacancy mechanisms.

A rationalization of secondary defect structures in aluminium-based alloys

The structural relationship between the different planar secondary defects which are observed in Al and Al-based binary alloys has been considered and additional observations made to show that the changes in structure which occur in the alloy systems studied are both systematic and predictable. The structural variations have been explained in terms of a simple model relating the stability of faulted dislocation loops to the development of internal stress and changes in stacking-fault energy which occur with alloying.

500&amp;thinsp;kV電子顕微鏡中でのNiおよびNi-Cu合金の電子線照射損傷

The formation of point defects, their subsequent aggregation and effects on the recovery process in thin foils of nickel and a nickel-copper alloy have been investigated in a 500 kV electron microscope at a maximum beam current of 0.75 A/cm2 using a specimen heating stage. Dislocation loops and small black dots appeared only after heating above room temperature. Faulted dislocation loops were formed in an extra-pure nickel while prismatic loops in the nickel-copper alloy. The large loops were interpreted as an interstitial type in character from contrast experiments. Higher densities of dislocation loops were formed in a commercially pure nickel and the nickel-copper alloy. Deformation also assisted the formation of dislocation loops. Electron radiation damage accelerated the rate of annihilation of dislocations in deformed foils. Dislocations climbed into helices in the deformed nickel-copper alloy during irradiation and heating.

Dislocation loop growth in an electron irradiated thin foil

Abstract Large faulted dislocation loops are grown in thin foils of nickel at 450°c by electron irradiation in the beam of a high voltage electron microscope. Loop size is directly proportional to time, implying that surfaces are the dominant sinks for interstitials. A model of loop growth under these conditions is developed and predicts a growth rate in agreement with that observed. This paper is published by permission of the Central Electricity Generating Board.

Vacancy clustering in low order Cu3Au alloy

Abstract In electron-microscope micrographs of quenched and aged specimens of Cu3Au (a f.c.c. alloy with an order-disorder phase transition at ∼390°c) we have observed the presence of stacking-fault tetrahedra, faulted dislocation loops and prismatic dislocation loops. The most interesting experimental result seems to be some correlation between the type of observed defects and the degree of order in the same area of specimen. This feature is discussed and the results compared with those of other authors.