The dislocation structure of slip bands in deformed high entropy alloy nanopillars

Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sud-den crystal slips.While large crystal slips can be easily observed on the surface of deformed crystals,unraveling the underlying dislocation interaction mechanisms,however,has been a longstanding chal-lenge in the study of single-crystal plasticity.A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al0.1CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar.Here,we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip.Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations,secondary dislocations,and dislocation pileups.The results were compared with the previously proposed slip band models.The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.     

Plastic Deformation in Quench-and 650℃ Tempered Steel

The variations of the dislocation structuresin the quench and 650℃ tempered steel withincrease of elongations have been investigatedby using transmission electron microscopy. Inthe small elongation stage, the boundaries betweenferrite and carbide in this steel can releasedislocations. As the elongations increase, themoving dislocations in the ferrite slip ontothe carbides. Then, the interaction betweenmoving dislocations and dislocations releasedfrom this boundaries, and the interaction betweenthe dislocations moving to the carbides in everyslip plane occurs. Thereby, the dislocationtangles around the carbides can be formed.In the large elongation stage, the dislocationtangles with high dislocation density and thedeveloped dislocation cells are formed.     

The identification of faulted prismatic dislocation loops in single crystals of undoped InP

High-voltage transmission electron microscopy has shown that undoped single crystals of indium phosphide, grown by the liquid-encapsulated Czochralski technique, can contain linear arrays of faulted dislocation loops. The plane of the loops is (1 1 0), the fault vector is 1/n [1 1 0] and the Burgers vector of the dislocation loop is 1/n [1 1 0]. A direct correlation has been obtained between these loops and arrays of both ridge and prism features revealed by chemical etching. Sequential use of two etchants has also established a direct link between the faulted dislocation loops and the slip dislocations induced by thermal stresses during crystal growth. A study of a number of crystals grown from differently prepared starting materials suggests that the formation of the loops is associated with departures from stoichiometry and inhibited by the presence of dopant impurities.     

Dislocations and the plasticity of ionic crystals

Abstract

Some recent developments in the dislocation theory of plastic deformation in ionic crystals are reviewed. It has been found that the Peierls force operates on slip planes of type {110} and {00l}, that interactions between dislocations and point defects are different on these planes, that point defects are produced by jogged dislocations, and that cross-slip is responsible for polycrystalline ductility. There are still many interesting questions to be answered in the field of ionic crystals, which provides an excellent testing ground for dislocation theory.

MST/268     

Dislocation-mediated migration of interphase boundaries

Faceted interphase boundaries (IPBs) are commonly observed in lath-shaped precipitates in alloys consisting of simple face-centred cubic (fcc), body centred-cubic (bcc) or hexagonal closed packed (hcp) phases, which normally contain one or two sets of parallel dislocations. The influence of these dislocations on interface migration and possible accompanying long-range strain field remain unclear. To elucidate this, we carried out atomistic simulations to investigate the dislocation-mediated migration processes of IPBs in a pure-iron system. Our results show that the migration of these IPBs is accompanied with the slip of interfacial dislocations, even in high-index slip planes, with two migration modes were observed: the first mode is the uniform migration mode that occurs only when all of the dislocations slip in a common slip plane. A shear-coupled interface migration was observed for this mode. The other interfaces propagate in the stick-slip migration mode that occurs when the dislocations glide on different slip planes, involving dislocation reaction or tangling. A quantitative relationship was established to link the atomic displacements with the dislocation structure, slip plane, and interface normal. The macroscopic shear deformation due to the effect of overall atomic displacement shows a good agreement with the results obtained based on the phenomenological theory of martensite crystallography. Our findings have general implications for the understanding of phase transformations and the surface relief effect at the atomic scale.     

Contraintes induites dans un support de molybdénite par un dépôt épitaxique de magnésium

Misfit dislocations are often observed between epitaxial layers. The introduction of these dislocations at the interface depends upon the presence of suitable slip planes between the crystals. The aim of this paper is to describe a crystal system in which such slip planes do not occur.Epitaxial Mg films were grown by evaporation onto hot MoS₂ thin platelets in high vacuum. In such a combination there are no slip planes to allow the introduction of interfacial misfit dislocations. Another type of dislocation with curved lines appears in the substrate as a consequence of the stresses induced by the overgrowth.     

Climb of dislocation spirals and loops during room-temperature oxidation of zinc crystals

Dislocation spirals and loops are observed by Berg-Barrett X-ray topography in the basal cleaved zinc single crystals in the presence of large numbers of slip dislocations of all three basal Burgers vectors. Dislocation spirals and loops are often composed of bundles of these basal slip dislocations. The climb of dislocation spirals and loops, associated with vacancy injection into the lattice during oxidation, is also observed during the surface oxidation process at room temperature. The results imply that the oxidation process is closely related to and controlled by the lattice defect density and dislocation arrangements near the crystal surface.     

Room temperature tensile behaviour of K640S Co-based superalloy

The room-temperature tensile properties of K640S cobalt-based superalloy were investigated by tensile tests and microstructure observation. The experimental results showed that the deformation mechanism of K640S alloy is dislocation slip; that the dislocation could be decomposed into continuous stacking faults with different orientations; and that with the increase of the number of dislocations, the dislocation tangle interacts with the decomposed stacking fault to increase the tensile strength of the alloy. As the stretching proceeded further, a plurality of slip systems were activated between different grains to coordinate the deformation, and the grains were gradually plastically deformed. The stress concentration occurred at the carbide interfaces, and microcracks were formed, causing mixed crystal fracture to the alloy.     

Geometrical effect on dislocation nucleation at crystal surface nanostructures

Crystal surface nanostructures such as steps and trenches in microelectronic layer structures have been considered as stress concentrators that may facilitate dislocation nucleation. Quantitative characterization of the critical condition of this atomic scale process is of considerable interest for the development of high performance electronic devices. This paper addresses this issue using a multiscale approach based on the variational boundary integral formulation of the Peierls–Nabarro dislocation model. By representing the profiles of embryonic dislocations as the relative displacements between the two adjacent atomic layers along the slip planes, the critical conditions for dislocation nucleation are obtained by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. The geometrical effect of surface nanostructures such as steps and trenches on dislocation nucleation is ascertained quantitatively. Our results show that the atomic scale surface nanostructures can reduce the critical stress for dislocation nucleation by nearly an order of magnitude and the trench configurations are more prone to dislocation nucleation than the step configurations. Nucleation of versatile dislocations in multiple slip systems at crystal surfaces may be attributed surface nanostructures of a variety of geometries.     

Dislocation structures in deformed Cu-Ni-Zn alloy single crystals

It is known that Cu-Ni-Zn alloy has an ordered structure Cu₂NiZn (Ll₂) by annealing between 573 and 623 K. In the present experiments, the effects of annealing on the dislocation structure were studied on Cu-Ni-Zn single crystals with several compositions. Thin foils cut parallel to the {111} planes were observed in a transmission electron microscope. The results obtained are as follows. (i) In Cu-5Ni-5Zn, Cu-10Ni-10Zn and Cu-15Ni-15Zn (at%), the stress-strain behaviour, slip mode and dislocation structure did not change by annealing at 573 K. However, the slip mode became more concentrated and localized, and dislocations emitted from a source tended to stay on the same slip plane, as the nickel and zinc concentrations increased. (ii) However, those properties in Cu-20Ni-20Zn and Cu-25Ni-25Zn changed drastically by annealing. As the ordering proceeded, uniform distributions of superlattice dislocations were observed. A typical dislocation configuration, with long screw and wavy-edged superlattice dislocations, took the place of piled-up unit dislocations. (iii) The facts that edge-type superlattice dislocations formed dipoles and their clusters, and that the secondary dislocation density was much lower than the primary one, implied that the elastic interaction of the primary edge-type superlattice dislocations on the nearby parallel slip planes would control the work-hardening of ordered Cu₂NiZn alloy single crystals.     

Transmission electron microscopic observation of dislocations in x-phthalocyanine crystals

Transmission electron microscopy has been utilized to directly reveal the defects that are present in thin single crystals of x-phthalocyanine polymorph. The bright field images are characteristic of dislocation arrays while the associated diffraction patterns indicate that the parent monoclinic structure of x-phthalocyanine may undergo a stress-induced phase transformation into a daughter orthorhombic structure. The transformation is akin to a martensitic process as a result of the operation of an invariant plane strain. The dislocation arrays observed have been interpreted in terms of slip dislocations.     

Influences of Re on low-cycle fatigue behaviors of single crystal superalloys at intermediate temperature

Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760 °C. During heat treatment, Re retards γ′ phases coarsening and equalizes the distribution of γ′ phases. As Re content increases, fatigue life increases and slip bands distribute more inhomogeneously. Moreover, adding Re not only reduces stacking fault energy of the matrix, but also promotes the element segregation to increase the lattice misfit. However, the larger lattice misfit does not lead to the formation of dislocation networks, but which activates dislocation movement and promotes dislocations cross-slip and climbing movement under high temperature and applied stress. On the other hand, with the addition of Re, cyclic deformation behaviors change from cyclic hardening to cyclic stability, mainly depending on a transformation of deformation mechanisms from slip bands cutting through γ and γ′ phases to stacking faults shearing.     

Electrical activity of extended defects and gettering of metallic impurities in silicon

Abstract

Electrical activities of slip dislocations and Frank type partial dislocations in Si were investigated using the temperature dependent electron beam induced current technique. It was found that both types of dislocation are recombination active in the temperature range below about 200 K only and inactive in the higher temperature range if they are free from decoration by metallic impurities. From the temperature dependence of the electron beam induced current contrasts, the energy levels of recombination centres on these dislocations were determined to be less than 0.1 ev. They are not accompanied by any deep levels. Debris of defects generated from moving dislocations has deep levels about 0.3-0.4 eV from the band edge even if there is no decoration with metals. Both slip dislocations and Frank partials become recombination active at high temperatures when they getter metallic impurities. The characteristics of impurity gettering by dislocations depend on both the species of impurities and the cooling rate of a Si crystal after contamination. As a result, the recombination activities of dislocations are influenced by a variety of external parameters.     

A review of slip transfer: applications of mesoscale techniques

In this review article, we present and discuss recent mesoscale modeling studies of slip transmission of dislocations through biphase interfaces. Specific focus is given to fcc/fcc material systems. We first briefly review experimental, atomistic, and continuum-scale work that has helped to shape our understanding of these systems. Then several mesoscale methods are discussed, including Peierls–Nabarro models, discrete dislocation dynamics models, and phase field-based techniques. Recent extensions to the mesoscale mechanics technique called phase field dislocation dynamics are reviewed in detail. Results are compiled and discussed in terms of the proposed guidelines that relate composite properties to the critical stress required for a slip transmission event.     

An edge dislocation of constant velocity near a static internal crack

An edge dislocation of constant velocity near a static internal crack was investigated. The dislocation slip and climb and dislocation source were considered. The crack surface was simulated with static continuous dislocations. After obtaining the distribution of static dislocations in the crack, we calculated the stress field in the entire space. Using the stress distribution, we then computed the stress intensity factors at both crack tips and the image force on the edge dislocation. Numerical results are provided to describe in detail the effect of velocity and crack length on toughness and image force.     

Influence from geometrical features on the growth rate of a micro-structurally short fatigue crack

The influence on the crack growth rate on a micro-structurally short edge crack subjected to fatigue loading from changes in crack length, distance to grain boundaries and applied load has been investigated. The crack is assumed to grow in a single shear mechanism due to nucleation, glide and annihilation of dislocations along preferred slip planes in the material. The external geometry is modelled by distributed dislocation dipole elements in a boundary element approach under quasi-static and plane strain conditions. The evolving plasticity is described by individual discrete dislocations along a slip plane emanating from the crack in the crack direction. The crack growth rate is shown to be controlled by the plasticity, which in turn is controlled by geometrical parameters in combination with the external load.     

Room temperature strengthening mechanisms in a Co-Cr-Mo-C alloy

The various contributions to the flow stress at room temperature of a cast Co-Cr-Mo-C alloy were identified using transmission electron microscopy. These alloys are used as surgical implant materials. It was concluded that stacking fault intersections and twins make the largest contribution to the work hardening behaviour of the as-cast Co-Cr-Mo-C alloy. The stacking fault intersection was modelled as a form of dislocation dipole, from which the stress interaction with slip dislocations was estimated. In the case of twin-slip interactions, it is suggested that the incorporation of the slip dislocation into the twin is governed by the reaction 1/2[110](_¯11¯1)1/6[141](_¯1¯15)+1/6[2¯1¯1]_{coherent Twin Boundary (111)} A nucleation model for twinning in alloys with very low stacking fault energy is also proposed.     

Reactions of slip dislocations with twin boundary in Fe-Si bicrystals

Specimens for in situ TEM straining were prepared from Fe-5.5 at.%Si Σ 3 bicrystals with {112} grain boundary plane. They were strained under three different directions of the stress at the boundary with respect to the orientation of the grains. Transfer of slip across the boundary was analysed. In one case, the transfer of slip was realized by a transformation of the slip dislocation in one grain into the slip dislocation in the other grain. Low energy dislocation was created in the GB in accordance with general transfer criteria. In the second case, the incoming and outgoing slip systems were in direct contraction to the general transfer criteria. In the third case, oriented for common slip system in both grains, the trapped incoming slip dislocations dissociated into twinning dislocations which created twins on the other side of the boundary.     

A PROPOSED CRITERION FOR FATIGUE THRESHOLD: DISLOCATION SUBSTRUCTURE APPROACH

Abstract— A model for fatigue threshold has been proposed based on the dislocation subgrain cell structure that evolves at the crack tip in steels during the fatigue deformation process. The stabilized subgrain cells that develop in the material act as impenetrable barriers to dislocations in slip band pile-ups that emanate from the fatigue crack tip. The blocking of these dislocations tends to limit crack growth that occurs by crack tip emission of dislocations, thereby leading ultimately to the fatigue threshold condition. The grain size effect on threshold is deduced to be an indirect effect as it is proposed that the subgrain cell size is the controlling substructural parameter at the threshold stress intensity level. The subgrain cell size is shown to be proportional to the one-third power of the initial grain size.     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.