Experimental observation of the dislocation walls in heterostructures with two interfaces: Ge/Ge0.5Si0.5 10 nm/Si(001) as an example

High-resolution electron microscopy (HREM) at the atomic scale has been applied to study the edge dislocation redistribution between interfaces in Ge/Ge_0.5Si_0.5/Si(0?0?1) heterostructures. Our results provide a direct explanation that plastic relaxation of the GeSi buffer layer proceeds owing to motion of Lomer-type dislocation complexes consisting of a pair of complementary 60° dislocations with the ends of the {1?1?1} extra planes being located at a distance of ~2–12 interplanar spacings from each other. It is demonstrated that edge dislocations belonging to the upper and lower interfaces become arranged one under the other and dislocation walls are formed. The distributions of tension and compression in the [0?0?1] direction between two edge dislocations, obtained by processing the HREM image, testify to superposition of strain fields.     

Interactions of persistent slip bands with a grain boundary on the common primary slip plane in a copper bicrystal

In this letter, the dislocation patterns on the common primary slip plane in a fatigued \[1-34]-\[182-7] copper bicrystal with a Sigma=19b grain boundary (GB) have been investigated using the electron channelling contrast technique in a scanning electron microscope. The results show that the two-phase dislocation structure, such as veins and persistent slip band (PSB) walls, embedded within veins, can be clearly seen on the common primary slip plane. In particular, the interactions of PSBs with the GB are clearly revealed. It is found that there are three kinds of interaction mode between the GB and the dislocations during cyclic deformation, and those are discussed. It is suggested that the dislocations carried by PSBs cannot transfer through the GB continuously even though the bicrystal has a common primary slip plane and its surface slip bands are continuous across the GB.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Dislocation arrangements and crystallographic characterization of deformation band in fatigued copper single crystal

This letter reveals the dislocation arrangements and crystallographic characterization of deformation bands (denoted DBII) in a copper single crystal fatigued at a high strain amplitude gamma_pl = 8 x 10^-3. The results show that the surface deformation morphology of the crystal displays the following features. (1) Primary slip bands (SBs) were formed after 2 x 10⁴ cycles and these carried a relatively homogeneous and small plastic strain. (2) Secondary slip bands did not operate during cyclic deformation. (3) Deformation bands (DBs) with a width of 50 mum were homogeneously distributed over the whole surface of the crystal and were perpendicular to the SBs. (4) Dislocation patterns within the SBs often consisted of irregular structures, which did not show a persistent feature. The results indicate that these SBs are not typical persistent slip bands (PSBs). (5) Within the DBII, the microstructure can be classified into two types. One type consists of regular 100% ladder-like parallel PSBs. The other type is full of dislocation walls parallel to DB direction, which have not been reported previously. By crystallographic analysis of the DBII, it is shown that the habit plane of the DBII should correspond to the (101) plane. Based on the observations above, it is suggested that the formation of DBII should be attributed to the local regularization of dislocation walls within primary slip bands.     

Dislocation nucleation and vacancy formation during high-speed deformation of fcc metals

Recently, a dislocation-free deformation mechanism was proposed by Kiritani et al. on the basis of a series of experiments where thin foils of fcc metals were deformed at very high strain rates. In the experimental study, they observed a large density of stacking fault tetrahedra but very low dislocation densities in the foils after deformation. This was interpreted as evidence for a new dislocation-free deformation mechanism, resulting in a very high vacancy production rate. In this paper we investigate this proposition using large-scale computer simulations of bulk and thin films of copper. To favour such a dislocation-free deformation mechanism, we have made dislocation nucleation very difficult by not introducing any potential dislocation sources in the initial configuration. Nevertheless, we observe the nucleation of dislocation loops, and the deformation is carried by dislocations. The dislocations are nucleated as single Shockley partials. The large stresses required before dislocations are nucleated result in a very high dislocation density, and therefore in many inelastic interactions between the dislocations. These interactions create vacancies and a very large vacancy concentration is quickly reached.     

Creep behaviour of ice single crystals loaded in torsion explained by dislocation cross-slip

Torsion creep experiments are carried out in order to understand the physics of ice plasticity. A dislocation spreading mechanism based on double cross-slip of basal dislocations is proposed to explain the strong plastic anisotropy and the power law relationship between stress and strain rates. The scenario is tested using three-dimensional dislocation dynamics simulations. Numerical investigations give a stress exponent n?=?2.3 in agreement with experimental measurements. This dislocation spreading mechanism sheds a new light on the interpretation of former experimental observations.     

The annihilation by glide of prismatic loops in walls

Two mechanisms are conjectured for the non-diffusional refinement of prismatic dislocation loops. These mechanisms offer an alternative to the widely accepted concept of spontaneous disintegration below some critical dipole height. In one, a loop array is refined by reaction with a mobile dislocation having the same Burgers vector. Loop shrinkage then depends on the geometry of the array and on the position of the plane of incidence. In principle, any loop array can be refined via this mechanism provided that all the loops have the same sign. An alternative mechanism may take place in dense walls formed of randomly arranged dipolar loops of both signs. As the wall is densified by impacting dislocations that push pre-existing loops on their glide prism against each other, refinement occurs conservatively when two loops of opposite sign come into contact.     

Characterization of room-temperature plastic deformation of ß-Si3N4 by atomic force microscopy and transmission electron microscopy

Dislocation microstructures induced by room-temperature microhardness tests have been investigated in silicon nitride. Surface analysis of the residual indent by atomic force microscopy reveals intragranular slip bands and demonstrates that room-temperature plastic deformation involves dislocation motion as well as cross-slip events. Cross-slip events have been found to occur between {1010} prismatic planes. Transmission electron microscopy shows that dislocations have a Burgers vector b = [0001] and are located along the screw direction. Based on these observations, specific dislocation core configurations are discussed.     

Microstructure of icosahedral Al-Pd-Mn quasicrystals deformed at room temperature in an anisotropic confining medium

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.     

Pop-in phenomenon in MgO and LiF: Observation of dislocation structures

This letter is based on recent progress in the observation of dislocation distributions around nanoindentations by chemical etching. This so-called nanoetching technique is used to determine the dislocation mechanisms associated with the pop-in phenomenon in MgO and LiF. Successive stages of highly controlled chemomechanical polishing have revealed that these dislocations are half-loops lying in the classical slip systems of MgO or LiF. However, they do not extend on the surface in the classical rosette-arms pattern but stay concentrated around the imprint. A mechanism of dislocation interactions, enhanced by the fact that the dislocations are suddenly nucleated in a small volume, is proposed to explain this specific distribution.     

Abnormally high residual dislocation density in pure aluminum after Al/Ti/Al laminate annealing for seven days

We report an abnormally high residual dislocation density in aluminium in an Al/Ti/Al laminate annealed at 873 K for seven days. The residual dislocation density reaches 7.5 × 10¹⁴ m^?2, higher than that in aluminum after severe plastic deformation processes such as accumulative roll bonding and high-pressure torsion. It is proposed that the high residual dislocation density may result from obstruction of the movement of TiAl₃ nanoparticles by the grain boundary and Ti atoms conglomerating at vacancies distributed in the aluminium matrix at a high temperature for a sufficient time to allow a relatively stable crystal.     

A theory of work softening

Observed features of the yield drop in work softening support the view that the structure in a slip band is heterogeneous in a metal such as aluminium or copper. The centre of the band is almost empty of dislocations. These are concentrated round its boundary, particularly as walls of edge dipoles at its ends. It is argued that work hardening is due to the prevention of the passage of these dislocations by a forest of obstacles with small activation volumes and energies. Thermal energy enables them to cut through their obstacles and so to penetrate further into their walls, thereby reducing their back stress and enabling the Frank-Read sources, within the bands, to become active at lower applied stresses. The yield drop is explained as due to an overshooting, resulting from this thermally activated cutting, of the stress to operate the sources.     

Screw misfit dislocations perpendicular to one or two free surfaces

ABSTRACT

When a screw dislocation pierces one or two free surface(s), noticeable elastic relaxation takes place nearby these surfaces. To refine the theoretical interpretation of an electron microscopy image this relaxation should be included in a precise imaging model. In the present work, the relaxation field is evaluated for a periodic set of misfit screw dislocations normal to the free surface of a semi-infinite or plate-like heterogeneous bicrystal. The proposed approach uses an appropriate combination of known biperiodic elastic fields. For a homogeneous medium and when the period increases, the results around a misfit screw dislocation converge to those of Eshelby and Stroh (1951) who considered an isolated translation screw dislocation.     

Homogeneous nucleation of dislocation loops under stress in perfect crystals

We present an analysis and results on the homogeneous nucleation of a dislocation loop under stress in a perfect crystal. By using a variational boundary integral method in the Peierls-Nabarro framework, we have determined the saddle-point configurations of embryonic dislocation loops and their associated activation energies under stress levels up to the ideal shear strength. The high-energy barriers under the usual levels of applied shear stresses, differing markedly from the ideal shear strength, confirm the widely held view that thermal motion should play no role in such nucleation. The result provides means for more definitive solutions of fundamental problems involving homogeneous nucleation of dislocation loops and has significant implications for models based on the mechanism of nucleation of dislocations from a perfect crystal.     

The atomic structure of the dislocation cores in a small-angle grain boundary in BaTiO3 thin films

The atomic structure of the cores of the dislocations forming a [110] tilt boundary of 10^o misorientation in a BaTiO₃ film is characterized by means of focal-series reconstruction. Along the small-angle grain boundary, facets with different inclinations are accompanied by different types of dislocation. The dislocation with a Burgers vector of a [001] appears as a perfect edge type. Those with a Burgers vector of a [111] are dissociated into three partials with a Burgers vector of (a/3)[111], leading to two segments of stacking faults. Structural models of these dislocation cores are suggested on the basis of the phase of the reconstructed wavefunction and tested by simulation of the wavefunction.     

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.     

HREM examination of [101] screw dislocations in Ni3Al

Abstract

The dissociation of [101] screw dislocations in Ni³Al has been examined using high-resolution electron microscopy. [101] superdislocations are found to be dissociated into (a/2)[101] superpartial dislocations on the (010) cube cross-slip plane. These superpartials in turn dissociate into complex stacking faults on the (111) or (111) which are bounded by Shockley partials in agreement with theoretical predictions. The degree of antiphase boundary spreading on (010) was found to increase with deformation temperature while the superpartial core dissociations remain unchanged.     

Dislocations in Bi-Pb-Sr-Ca-Cu-O superconductors

Abstract

Dislocations and dislocation networks in superconducting Bi-Pb-Sr-Ca-Cu-O have been observed by means of TEM, confirming that this compound is a layer structure with low stacking fault energy. On the basal plane, dislocations and dislocation networks dissociate into partial dislocations and partial dislocation networks. However, the dissociation of [010] dislocations is hardly observed, which is related to the structural characteristics of the compound.     

Onset of plasticity in a Σ = 5 GaAs compliant structure

Compliant structures have been fabricated in which a thin GaAs layer (thickness between 10 and 20nm) was bonded on top of a GaAs substrate with a large twist angle (about 37). This twist angle value was chosen so that the energy of the boundary (coincident boundary of type =5 (001)) was minimized. The structure of the interface was characterized and the onset of plasticity in such a compliant substructure was investigated using nanoindentation that allowed the low-load deformation regime to be observed. The results are compared with those obtained under the same conditions on a GaAs bulk substrate alone. No plastic zone was observed by transmission electron microscopy in the compliant structure under loads below 0.25mN while, under the same loads, plastic deformation was observed in the bulk substrate. For higher loads (2mN), plastic-flow enhancement was observed in the compliant structure. The results are discussed in the light of the arrangement of dislocations observed in the plastic zones.