Influence of crystallographic orientation on cyclic strain-hardening behaviour of copper single crystals

The cyclic strain-hardening behaviour of copper single crystals with various slip orientations is considered systematically. It is shown that the crystallographic orientation has a strong effect on the cyclic hardening behaviours of double- and multiple-slip-oriented copper single crystals. The initial cyclic hardening of differently oriented copper single crystals is mainly dependent on the modes and intensities of dislocation interactions between slip systems operating in the crystal, as well as on the possibility of cross-slip. A distinctive strain burst phenomenon has been frequently observed in the very early stage of cyclic hardening for critical double-slip-oriented crystals. A secondary cyclic hardening stage occurs readily late in cyclic deformation of coplanar doubleslip-oriented crystals.     

Scanning electron microscopy-electron channelling contrast investigation of recrystallization during cyclic deformation of ultrafine grained copper processed by equal channel angular pressing

Cyclic deformation has been performed at ambient temperature on ultrafine-grained copper processed by equal channel angular pressing. Observations by electron channelling contrast in a scanning electron microscope revealed that pronounced recrystallization occurred and several distinct dislocation configurations such as dislocation walls and labyrinth structures were found within the recrystallized regions. A model is proposed to account for the evolution of recrystallization, emphasizing that its development results from an interaction between the cyclic deformation and the growth of recrystallized regions. The formation of recrystallized regions of different sizes is discussed.     

Stacking fault and dislocation glide on the basal plane of graphite

Generalized stacking-fault energies for the basal plane of graphite are calculated from first principles for slip along two high-symmetry directions. The rhombohedral fault energy compares well with experiment and the anisotropy in behaviour is consistent with observed dislocation network geometry. Utilizing these calculated fault energies within a modified Peierls-Nabarro model, we estimate the barrier for basal dislocation motion based on lattice friction. This is found to be extremely small, from which we conclude that dislocation network interaction and pinning, rather than the Peierls barrier, must determine the practical shear strength of graphite. However, at low dislocation densities or over small crystallite regions, the shear strength should tend to this lower limit. We discuss the relevance of this to the mechanism of lubrication.     

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.     

Effect of film thickness and grain size on fatigue-induced dislocation structures in Cu thin films

This letter presents systematic experimental observations of fatigue damage and corresponding dislocation structures in thin Cu films as a function of film thickness made using transmission electron microscopy and focused-ion-beam microscopy. It is found that, in thick films and grains of at least 3.0 μm diameter, coarse surface extrusions and dislocation wall and cell structures occur whereas, in thin films or in small-diameter grains, finer extrusions occur but no clearly defined dislocation structures are present. This minimum required dimension of 3.0 μm for fatigue damage formation may be caused by constrained dislocation motion in small dimensions.     

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.     

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.     

Gamma-surfaces in quasicrystals

The concept of generalized stacking faults and the associated gamma-surfaces provides a useful tool for the analysis of dislocation core structures in crystalline materials. Applied to quasicrystals, suitable Burgers vectors for a given slip plane can be identified by calculating the minima of the gamma-surface. Instead of using a specific interaction model, we motivate a relation between structural properties and a subset of the minima of the gamma-surface. The results agree with calculations based on pair potentials. As a consequence the continuum of possible Burgers vectors in physical space is reduced to a quasiperiodic set, described in terms of a strip-projection method. Further conclusions for the plasticity of quasicrystals are discussed.     

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.     

The fatigue limits of copper single crystals cyclically deformed at constant plastic strain amplitudes

Based on systematic surface observations, experimental measurements on the fatigue limits of differently oriented copper single crystals cyclically deformed at constant plastic strain amplitudes are summarized, and an orientation dependence of the fatigue limit is sketched. It was found that the fatigue limits of crystals depend mainly upon the low-strain-amplitude dislocation structures, which are characteristic of the particular crystal orientations. When multiple-slip deformation is the main mode of deformation and ultimately produces stable lowenergy dislocation structures such as labyrinth and cell structures, the fatigue limit can be improved notably.     

Application of electron channelling contrast to the investigation of strain localization effects in cyclically deformed fcc crystals

The dislocation substructure and the glide activity in cyclically deformed nickel single crystals have been studied using the channelling contrast of backscattered electrons in a scanning electron microscope. Dislocation arrangements which arise in the saturation region after cyclic loading at room temperature and at one elevated temperature are considered. The electron channelling contrast technique is shown to be a useful instrument for the qualitative and quantitative characterization of the dislocation pattern on a macroscopic and a mesoscopic scale. The correlation between the specific dislocation structure in persistent slip bands PSBs and the localized glide activity of PSBs and PSB macrobands are considered.     

Formation of prismatic loop alignments in crystals deformed in single slip

In γ-TiAl deformed at room temperature in single slip, prismatic loops ar often organized in a staircase-like configuration, called strings whose generation by cross-slip is facilitated by the dislocation tendency to form screw locks as described recently by Grégori and Veyssière. The present letter is aimed at showing that strings may be encountered in virtually any crystal provided that the two impinging dislocations exhibit significantly different velocities. The crossslip annihilation mechanism of crossed dislocations considered by Tetelman is revisited and shown to evolve into a configuration significantly distinct from that originally predicted.     

Strain-induced submicrocrystalline grains developed in austenitic stainless steel under severe warm deformation

Strain-induced grain evolution in a 304 type austenitic stainless steel has been studied in multiple compression with the loading direction being changed in each pass. The tests were carried out to total strains above 6 at 873 K (0.5 T _m) at a strain rate of about 10^-3 s^-1. Multiple deformation promotes the rapid formation of many mutually crossing subboundaries because various slip systems operate from pass to pass. The gradual rise in misorientations across dislocation subboundaries with increasing strain finally leads to the evolution of very fine grains with large-angle boundaries. It is concluded that a new grained structure can result from a kind of continuous reaction during deformation, namely continuous dynamic recrystallization. Such deformation-induced grains are characterized by relatively low densities of dislocations, and considerable lattice curvatures developed in their interiors. The latter observations suggest that high elastic distortions are developed in the grain interiors and so such strain-induced grain structures are in a non-equilibrium state.     

Resolving individual Shockley partials of a dissociated dislocation by STEM

A practical method was developed to image detailed features of defects in a crystal using STEM. This method is essentially a STEM version of the conventional CTEM g/3g weak beam dark field (WBDF) method. The method was successfully applied to resolving individual Shockley partials of a dissociated dislocation in a Cu-6.44at.%Al alloy.     

Survey of plateau behaviour in the cyclic stress-strain curve of copper single crystals

Experimental results relating to the plateau behaviour in the cyclic stress-strain (CSS) curve of copper single crystals located on different sides of the stereographic triangle are summarized. Unlike the situation for single-sliporiented crystals, the crystallographic orientation has a strong effect on the plateau behaviour in the CSS curves of double- and multiple-slip-oriented crystals. The existence or non-existence of a plateau in the CSS curves, as well as the corresponding plateau stress amplitude, depend not only on the modes and intensities of dislocation interactions among slip systems operating in the crystals but also on the slip deformation characteristics associated with crystal orientations. The plateau region in CSS curve disappears only when multiple slip plays a determining role during cyclic deformation.     

Fracture mechanisms of symmetrical tilt grain boundaries

The mechanisms of failure of symmetrical tilt boundaries have been investigated using an atomistic technique. The fracture process advances in steps that are related to the structural unit of the symmetrical tilt grain boundary. These boundaries are shown to fail through a combined mechanism involving dislocation emission in certain regions of the structural unit, followed by microcleavage, and again dislocation emission. The overall toughness of the boundaries is given by the combination of the toughness of these two processes.     

Dissociation of dislocations in MgAl2O4 spinel deformed at low temperatures

Abstract

When spinel is deformed in compression at 400°C along 〈110〉, the primary slip plane is found to be {111} with cross-slip occurring on a {001} plane. A comparison of weak-beam images of dislocations from both systems indicates that all dislocations which belong to the primary slip plane are dissociated out of the {111} plane independent of the character of the dislocation. It is proposed that deformation occurs by motion of dislocations in their dissociated state and that the partial dislocations actually glide on parallel glide planes. Movement of these dissociated dislocations is then accompanied by a concurrent migration of the stacking fault which takes place by a local shuffling of the cations. A stacking fault energy for conservative dissociation at 400°C on {001} of 530±90mJ m^?2 has been determined from weak-beam images of screw dislocations.     

Investigation of jog motion in γ-TiAl via molecular dynamics

The elastic displacement field of a jogged screw dislocation is obtained analytically from Burgers equation. With this analytic solution, a pair of jogs in a screw dislocation is implemented into molecular-dynamics simulations. The dislocation line bows out between two jog pinning points and breaks away when the line tension of the dislocation exceeds a certain critical value. The creation of vacancies and interstitials is observed during the non-conservative motion of the jogged screw dislocation in γ-TiAl. The structures of vacancies and interstitials are discussed.     

Electropulsing-induced strengthening of steel at high temperature

Electropulsing usually promotes a metastable phase to evolve towards its equilibrium state. This work reports an alternative case, where electropulsing promotes the decomposition of the stable δ-phase in duplex stainless steel at high temperature. This decomposition enables both the γ-phase and the σ-phase in the steel to survive and hence to strengthen it at high temperature. The hardness of the quenched sample with electropulsing treatment is 49.4% higher than that without electropulsing treatment. A fundamental understanding of the observation is developed.     

On the shape of a dislocation shear loop in stable equilibrium

It is demonstrated that the shape of a dislocation shear loop held in stable equilibrium by an infinitesimal shear loop of the same sign at its centre is a circle. This unexpected result may help to explain some discrepancies between the shape of loops predicted by de Wit and Koehler and experimental observation.