Set Of Parallel Dislocations Research Articles

The microstructures and mechanical properties of martensite Ti and TiN phases in a Ti6Al4V laser-assisted nitriding layer

A fiber laser assisted the preparation of a nitride layer on the Ti6Al4V surface in a pure nitrogen atmosphere. Through SEM, XRD, and XPS analyses, the nitride layer is found to be mainly composed of dendritic TiN phase and an interdendritic α′-Ti phase. Not only the true crystal structure of the two main phases (TiN and α′-Ti) were directly characterized using TEM, but also the mechanical properties were tested by micro-indentation and nano-indentation methods for the first time.It is found that the nano-hardness of the TiN phase is 2609 HV, and the elastic modulus is 208 GPa. During the TiN dendritic fracture process, the primary dendrites undergo near ductile fracture along the growth direction of the secondary dendrites, and microcracks, bifurcations, deflection, etc. occur to form a crack-toughening mechanism. The nano-hardness of α′-Ti is 632 HV, and the elastic modulus is 142.8 GPa. The entanglement of two sets of parallel dislocations in α′-Ti crystals forms subgrain boundaries, which exert a positive influence on work hardening and fine-grain strengthening. • A large number of parallel dislocation groups in the generated TiN. • TiN dendrites not only have near ductile fracture, but also have crack toughening; • A large number of parallel dislocation groups and subgrain boundaries in the α′-Ti.

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.

Revisiting the Al/Al2O3 Interface: Coherent Interfaces and Misfit Accommodation

We study the coherent and semi-coherent Al/α-Al2O3 interfaces using molecular dynamics simulations with a mixed, metallic-ionic atomistic model. For the coherent interfaces, both Al-terminated and O-terminated nonstoichiometric interfaces have been studied and their relative stability has been established. To understand the misfit accommodation at the semi-coherent interface, a 1-dimensional (1D) misfit dislocation model and a 2-dimensional (2D) dislocation network model have been studied. For the latter case, our analysis reveals an interface dislocation structure with a network of three sets of parallel dislocations, each with pure-edge character, giving rise to a pattern of coherent and stacking-fault-like regions at the interface. Structural relaxation at elevated temperatures leads to a further change of the dislocation pattern, which can be understood in terms of a competition between the stacking fault energy and the dislocation interaction energy at the interface. Our results are expected to serve as an input for the subsequent dislocation dynamics models to understand and predict the macroscopic mechanical behavior of Al/α-Al2O3 composite heterostructures.

Refined Prediction and Observation of Dislocation Structures in Low Σ Symmetric Grain Boundaries

The grain boundary dislocation structures of three Fe–4at.% Si bicrystals with an additional misorientation of the order of 2° were studied by transmission electron microscopy (TEM), one Σ5 [001] (310) and two Σ9 [110] (1\(\overline 1\)4). It was found that in the Σ5 case two Burgers vectors of the structure are not simple basic complete displacement shift lattice (DSC) vectors and are larger than any one of them. This enabled formation of the preferred grain boundary dislocation (GBD) structure with a hexagonal net composed of three screw dislocations and an independent set of parallel edge dislocations. In the Σ9 case the GBD structure comprises three apparently independent sets of parallel dislocations. The Burgers vectors of two sets could be established as basic DSC vectors in screw and edge orientation, respectively, with respect to the GB plane. For the third set formed by separated groups of four dislocations no corresponding basic DSC vector exists. This case needs further investigation. The previously published table of predicted GBD structures for symmetrical coincidence boundaries up to Σ11 was refined according to the new results.

A new epitaxial orientation of CoSi 2 on (111)Si

For cobalt films deposited and annealed under the non-ultrahigh vacuum conditions of 5 × 10 -5 and 5 × 10 -6 Torr respectively, a new epitaxy, type C CoSi 2, locally grown on (111)Si at 600–850 °C was found. Meanwhile both type A and type B epitaxy were also present and type B orientation dominated over type A and type C orientations. The orientation relationships between type C CoSi 2 and the silicon substrate are (001)CoSi 2(111)Si and [1 1 0]CoSi 2 [ 1 10]Si. Corresponding to the three equivalent 〈110〉Si directions, three type C variants, having structurally equivalent domains, were grown epitaxially on (111)Si. Only one set of parallel dislocations was observed at the interface C-CoSi 2/(111)Si. The parallel dislocations were identified as of mixed type with Burger's vectors of 1/2〈110〉 a. The epitaxial growth of type C CoSi 2 on (111)Si may be the result of contamination by carbon and oxygen in the cobalt layer.

Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue

An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760 °C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the present study.

Identification of interfacial structures in BiCaSrCuO thin film

BiCaSrCuO thin films with zero resistance temperature ( T 0) of 85 K were grown on yttrium-stabilized zirconia (YSZ) substrates using d.c. sputtering. Bi 2CaSr 2Cu 2O 8 grains were epitaxially grown one after another to form a multilayer twin with respect to the modulation by rotating 90° along the [001] direction. The orientation relationships were (001) A∥(001) B∥(001) YSZ and [100] A∥[010] B. Two sets of parallel dislocations along the [100] and [010] directions formed a square network at the twin boundary. The Burgers vectors of interfacial dislocations were 1 2 〈110〉a of mixed type. CaZrO 3 was formed at the interface between the BiCaSrCuO thin film and the YSZ substrate during heat treatment.

Conductivity of layered GeS at low temperatures connected with the presence of planar defects

The electric behaviour of orthorhombic layered GeS is examined in connection with the extended planar defects present in the crystals. Such defects are growth twins and typical dislocation networks. For this reason DC and AC measurements have been performed in the temperature range 340-140 K. The results have shown that the conductivity parallel to the c axis is affected by the sets of parallel dislocations which lie on the (001) layer planes. On the other hand, the (110) twins create potential barriers which influence considerably the conductivity on the layer planes and in the direction normal to the twinning walls.

The substructure of (252)f martensite formed in an Fe- 8Cr- 1 C alloy

This work deals with a transmission electron microscopy investigation of plate martensite in an Fe-8Cr-lC alloy, which exhibits a typical (252)f martensite transformation. It was found that macroscopic martensite plates, such as those used for optical microscopy determination of the habit plane and the shape strain, actually consist of many individual small plates having the same crystallographic orientation and habit plane. Individual plates within a macroscopic plate appear to form by auto-catalytic nucleation. The coalescence plane between individual plates is usually parallel to (011)b = (lll)f because the growth of the plates is restricted by stacking faults and twins on (lll)f, formed as a result of accommodation of the shape strain. Deformation of the martensite at the coalescence sites, mainly on the twin system (112)b \([\overline {11} 1]\) b, leads to a complex substructure of the macroscopic plates. Stacking faults and twins in the austenite may be inherited in the martensite, which leads to further complexity. Prior to the coalescence, the substructure of individual martensite plates is simple, and consists of a low density of irregularly distributed twins on (112)b \([\overline {11} 1]\) b and dislocations having Burgers vector ab/2\([\overline {11} 1]\). These defects are probably caused by accommodation deformation. The martensite/austenite interface contains a set of parallel dislocations having a spacing of about 13 A. These dislocations are likely to be screw dislocations with Burgers vector ab/2\([\overline {11} 1]\) = af/2\([\bar 101]\), which accomplish the complementary shear in the phenomenological crystallographic theory.

Resolution Of M23C6/Austenite Phase Boundary Defect Structures By Weak Beam Microscopy

SUMMARYThe weak beam dark field imaging technique has been successfully applied to the problem of imaging misfit dislocations at the interface between M23C6 precipitates and an austenite matrix. The technique eliminates Moiré fringe contrast which otherwise obliterates the contrast from interfacial dislocations. Three sets of parallel dislocations were observed with spacings and line vectors consistent with them being edge dislocations with Burgers vectors of a/3<111>.

Epitaxial dislocations

Abstract Bullough and Bilby's (1956) theory of a continuous distribution of surface dislocations has been applied to epitaxial interfaces formed with Burgers vectors limited to a discrete set. The types of misfit which can be relieved by one, two and three sets of parallel dislocations are examined. It is shown that the existence of epitaxial dislocations of 60° character in face-centred cubic materials can be rationalized by this approach.