Type Of Dislocation Structure Research Articles

Mapping Sliding Ferroelectricity in Bulk (PbS)1.11VS2 Crystals

The recent discovery of sliding ferroelectricity opens up the field of 2D ferroelectricity. However, the limitation to switching the polarization due to the topological protection of the domain walls limits the potential of these heterostructures. Here, the switching of sliding ferroelectric bulk misfit layer compound (PbS)1.11VS2 crystals is studied using switching spectroscopy piezoresponse force microscopy. A large number of measured hysteresis loops can be efficiently analyzed using unsupervised machine‐learning tools. Different switching behavior of the domains is observed on different types of domain structures, and it is shown that full domain switching can be observed for the right type of dislocation structure in the PbS layer.

Cascade overlap simulation of formation of dislocation loops in Ti-V-Ta multi-principal element alloy

Among the currently developed multi-principal element alloys (MPEAs), Ti-V-Ta MPEA stands out because of its good high-temperature strength, good room-temperature plasticity, stable organizational structure, and low neutron activation, and becomes a first option for cladding material of special power reactors. The radiation resistance of Ti-V-Ta MPEA is the focus of current research. Dislocation loops are the main irradiation defects in Ti-V-Ta MPEA, which can significantly affect the mechanical properties. Therefore, clarifying the formation mechanism of dislocation loops in Ti-V-Ta HEA can help to understand its radiation resistance. The formation behavior of dislocation loops in Ti-V-Ta MPEA is studied based on molecular dynamics method in this work. Cascade overlap simulations with vacancy clusters and interstitial clusters are carried out. The cascade overlap formation mechanism of dislocation loops is analyzed and discussed. In Ti-V-Ta MPEA, the cascade overlap with defect clusters can directly produce different types of dislocation structures. The defect configuration after cascade overlap is determined by the primary knock-on atom (PKA) energy and the type and size of the preset defect clusters. Cascade overlap can improve the formation probability of <inline-formula><tex-math id="M6">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math></inline-formula> dislocation loops in Ti-V-Ta MPEA. Cascade overlap with vacancy clusters is an important mechanism for the formation of <inline-formula><tex-math id="M7">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math></inline-formula> vacancy dislocation loops, and the size of vacancy clusters is the dominant factor for the formation of <inline-formula><tex-math id="M8">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math></inline-formula> vacancy dislocation loops. When the PKA energy is enough to dissolve the defect clusters, <inline-formula><tex-math id="M9">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math></inline-formula> vacancy dislocation loops are more likely to form. Furthermore, cascade overlap with interstitial clusters in Ti-V-Ta MPEA is a possible mechanism for the formation of <inline-formula><tex-math id="M10">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math></inline-formula> interstitial dislocation loops. This study can contribute to understanding the evolution behavior of irradiation defects in Ti-V-Ta MPEA, and provide theoretical support for designing the composition and optimizing the high-entropy alloys.

ВЛИЯНИЕ ХИМИЧЕСКОГО СОСТАВА НА ТВЕРДОРАСТВОРНОЕ И ДЕФОРМАЦИОННОЕ УПРОЧНЕНИЕ МОНОКРИСТАЛЛОВ ГЦК ВЫСОКОЭНТРОПИЙНЫХ СПЛАВОВ

A characteristic feature of high-entropy alloys is high strength at maintaining plasticity, wear and corrosion resistance, and fracture toughness at cryogenic temperatures. Currently, CoCrFeNiMn is the best-investigated high-entropy compound. However, its application is limited in the high-temperature region due to the low values of the deforming stress level at the plasticity breaking point at T>296 K. One of the common ways to improve the material durability is the addition of substitution atoms of larger atomic radius, and Al, Ti, and Mo are some of these atoms. The paper presents the analysis of the mechanical behavior of single crystals of CoCrFeNiMn and CoCrFeNiMо FCC high-entropy alloys (at. %) oriented along the [001] direction: the author studied the temperature dependence of critical shear stresses τcr(T) within the temperature range of T=77–973K, the type of dislocation structure, strain hardening coefficient θII, plasticity and fracture at Т=296 K under tension. The study shows that the alloying with Mo atoms 4 at. % of the CoCrFeNi system (at. %) causes the solid solution hardening, and critical shear stresses τcr increase within the entire studied temperature range. The onset of plastic deformation is associated with slip at all temperature tests. At T=296 K, the author identified a planar dislocation structure with flat dislocation pile-ups and dislocation networks in CoCrFeNiMo while in equiatomic CoCrFeNiMn, at such test temperature, a uniform distribution of dislocations was observed in several systems without flat pile-ups. Work hardening coefficient, plasticity, and the level of stresses before fracture turn out to be similar in [001]-crystals of CoCrFeNiMo and CoCrFeNiMn high-entropy alloys, which are determined by the development of slip deformation simultaneously in several systems. Crystals are destroyed viscously at 296 K at the same level of stress.

Temperature-dependent universal dislocation structures and transition of plasticity enhancing mechanisms of the Fe40Mn40Co10Cr10 high entropy alloy

The FCC Fe40Mn40Co10Cr10 (at.%) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in deformation at room temperature (298 K) and subzero temperature (223 K) of the cold-rolled and annealed Fe40Mn40Co10Cr10 alloy. The material deformed at 223 K shows a higher strain hardening rate than the material deformed at room temperature while both materials show large ductility, 48% for the 223 K deformation and 55% for the 298 K deformation. The main deformation mechanisms of the investigated HEA include the development of inhomogeneous dislocation structures and interaction between dislocations and deformation twin/mechanically induced HCP martensite. The stacking fault energy measured using TEM weak-beam dark-field imaging of dissociated dislocations is 20±9 mJ/m2 at 298 K. The Fe40Mn40Co10Cr10 alloy exhibiting a positive temperature dependence of SFE leads to a decrease of SFE as deformation temperature decreases from 298 K to 223 K. The decrease of SFE results in the transition from deformation twinning to deformation-induced HCP transformation. Further, at higher strains at 223 K, kink banding of HCP and reverse transformation from HCP to FCC were observed, which could account for strain accommodation and stress relaxation, and the large ductility. The 298 K deformation leads to various types of dislocation structures: Extended dislocations, Taylor lattice of perfect dislocations, dislocation loops, highly dense dislocation walls, cell blocks, and cell structures. The observed dislocation structures at 298 K and 223 K are similar suggesting the minor effect of SFE on dislocation structures.

Orientation and temperature dependence of a planar slip and twinning in single crystals of Al0.3CoCrFeNi high-entropy alloy

This paper repots the effect of crystal orientation and test temperature on the tensile deformation behaviour, the type of dislocation structure (planar or cellular) and the deformation mechanism (slip and twinning) in single crystals of the fcc Al0.3CoCrFeNi (at%) high-entropy alloy. It was shown that, in the temperature range 77–573 K, the tensile deformation behaviour at 0.1% < ε < 15% along [001], [1¯11] and [011] orientations is related to the slip, at which a planar structure with dislocation pile-ups develops in the wide temperature range of 77–423 K. The planarity of the dislocation structure is determined by the strong effect of solid-solution hardening via Al atoms, since the effective radius of Al atoms exceeds the effective radii of the other elements included in the composition of the Al0.3CoCrFeNi alloy, and existence of short-range order. In slip deformation, the strain hardening coefficient in the linear stage of hardening, i.e., ΘII(T)/G(T), depends on the crystal orientation and the test temperature. In the [011]-oriented single crystals where the slip deformation develops predominantly in one system, ΘII(T)/G(T) has small values and it increases with an increasing number of active slip systems in the [1¯11]- and [001]-oriented single crystals. Twinning was detected at 77 K in the [1¯11]- and [011]-oriented single crystals after 15% and 20%, respectively. Deformation twinning in the [1¯11]- and [011]-oriented single crystals leads to an increase in ΘII(T)/G(T) and determines more complex deformation stages in the σ–ε curve in the [011]-oriented single crystals compared with the development of slip deformation at high test temperatures.

Thermal and Lattice Misfit Stress Relaxation in Growing AlN Crystal with Simultaneous Evaporation of SiC Substrate

AlN single crystals were prevented from cracking by simultaneous growth and evaporation of SiC substrates. The freestanding crystals (&lt;1 mm thick) were proved continuous by synchrotron phase contrast imaging and used as a model system to investigate the type of dislocation structure near AlN/SiC interface by x-ray diffraction techniques. We have found that, unlike the situation in GaN films, where predominantly edge-type threading dislocations cross the layer along its normal, the dislocations configure to form mosaic structure. We suggest a theoretical model that describes the misfit stress relaxation in growing AlN crystal.

Structures and electronic properties of defects on the borders of silicon bonded wafers

The complex research of the structure and electronic properties of defects occurring on the bonding border of twist misfit Si(001) wafers of n-type conductivity was carried out by the methods of transmission electron microscopy, deep level transient spectroscopy (DLTS), and photoluminescence. The revealed defects main are two types of dislocation structure: the orthogonal dislocation network composed of two screw dislocation families and zigzag mixed dislocations. The dislocation structures observed are the sources of intense luminescence, whose spectrum differ considerably from the standard dislocation luminescence spectrum at all investigated misfit angles of bonded Si wafers. It is shown that an increase of the misfit angle results in a strong transformation of the dislocation luminescence spectra consisting in the changes of the spectra form and a decrease in the integral luminescence intensity. In the studied samples, by means of the deep level transient spectroscopy method, the presence of deep centers whose concentration increases with an increase of the misfit angle of the bonded wafers is revealed. It is established that the deep centers are related to the dislocation structures observed by means of transmission electron microscopy.

Effect of Rolling Temperature on Annealing of aa1050 Aluminium Alloy

Commercially pure, dilute aluminium alloys, such as AA1050, dynamically generate sub-grains during cold rolling. If AA1050 is rolled at cryogenic temperatures (liquid nitrogen), this large decrease in temperature minimises the occurrence of dynamic recovery during rolling. The result is that the material has a large amount of stored energy and a high dislocation density, thereby giving it a high strength. This research looks at the recovery and recrystallization processes during annealing after cryo-rolling, and compares the formation of sub-grains and recrystallized grains to those where rolling was performed at room temperature. The type of dislocation structure that forms during the rolling process directly affects the evolution of the microstructure post-deformation. Owing to the extreme temperatures of cryo-rolling, the dislocation structure cannot undergo dynamic recovery. Instead, a distinct cell-like structure forms, with dense dislocation walls that are high energy. During subsequent annealing, the driving force for recrystallization is increased with a decrease in the rolling temperature, with the cryo-rolled material having a greater number of nucleation seed and consequently, a fine grained recrystallized microstructure.

Structures and Electronic Properties of Defects on the Borders of Silicon Bonded Wafers

Comprehensive studies of the structure and electronic properties of defects occurring on the connection boundary of disarranged n − type Si(001) wafers have been made by the methods of transmission electron microscopy, deep level transient spectroscopy (DLTS) and photoluminescence. The main revealed defects are two types of dislocation structure: orthogonal dislocation network composed of two screw dislocation families and zigzag mixed dislocations. The dislocation structures observed are sources of intense luminescence whose spectra are appreciably different from the standard dislocation luminescence spectra at all the investigated misfit angles of the Si bonded wafers. We show that an increase of the misfit angle results in a strong transformation of the dislocation luminescence spectra consisting in changes of the form of the spectra and a decrease in the integral luminescence intensity. In the samples in question the DLTS method revealed the presence of deep centers the concentration of which increased with increasing of twist misorientation of bonded wafers. It has been established that the deep centers are related to the dislocation structures observed by means of transmission electron microscopy.

Modification of the microstructure of TiN-based columnar coatings in indentation zones

The columnar structure of titanium nitride-based coatings deposited by magnetron sputtering is studied. The structure-phase state of the coatings is analyzed after deposition and in the indentation zone. The type of dislocation structure, the grain size, the subgrain size, the misorientations at boundaries, and their change during coating growth are determined. The detected decrease (several tens of percent) in the coating thickness under an indenter indicates plastic deformation of a coating. On a microscopic level, this deformation manifests itself in an increase in the density, the misorientation, and the nonequilibrium of boundaries.

High cycle fatigue behavior of the second generation single crystal superalloy DD6

The second generation single crystal superalloy DD6 with 0.10%Hf and 0.34%Hf (in mass fraction) was subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C in ambient atmosphere. SEM was used to determine the initiation site and the failure mechanism. Evolution of the microstructure was investigated by TEM observation. The results show that fatigue limit of DD6 alloy with 0.34%Hf is a little smaller than that of the alloy with 0.10%Hf. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along {111} octahedral slip planes, rather than perpendicular to the loading axis of specimen. Typical fatigue striation formed in steady propagation of fatigue crack. The fracture mechanisms of the high cycle fatigue of DD6 alloys with 0.10%Hf and 0.34%Hf are quasi-cleavage fracture. Different types of dislocation structures were developed during high cycle fatigue deformation.

Application of orientation imaging to the study of substructural development in cold deformed Al-0.3%Mn single crystal of {110} orientation

A systematic study of the deformation microstructures over 3 perpendicular surfaces was carried out in the present work in order to correlate better the substructure with slip patterns, initial and final crystal orientation, and the macroscopic coordinate system. The microstructure and texture evolution of high purity Al-0.3 wt% Mn alloy of initial near brass {110}<112> (or Bs) orientation, channel-die compressed to the strain level of 1.5, were studied by TEM (including TEM orientation mapping) and high-resolution FEG-SEM/EBSD techniques to observe crystal subdivision deformation patterns at the microscopic scale. The grain orientation dependent structures were analyzed in terms of active slip systems focussing on the crystallography of extended planar boundaries. It was concluded that the type of dislocation structure (one or two sets of microbands) in {110}<112> oriented crystallites, at moderate strains (< 1.5) depended strongly on the crystallographic grain orientation. In this non-ideal initial crystal orientation the applied deformation mode activates a double slip, of which one system predominates and leads to further rotation away from Bs. A general rotation combined with a wide orientation spread is observed after a strain up to 1.5. The microband boundary alignment corresponds very well to the traces of crystallographic {111} planes, on which most of the slip occurs.

Molecular dynamics simulations of grain boundary migration during recrystallization employing tilt and twist dislocation boundaries to provide the driving pressure

Molecular dynamics simulations of grain boundary migration, where the driving pressure P is the excess stored energy due to dislocation structures, have been performed. This represents recrystallization in metals. Two types of dislocation structures have been simulated: (a) tilt dislocation boundaries, where edge dislocations are arranged as parallel arrays, (b) twist dislocation boundaries, where screw dislocations are arranged in interconnected dislocation networks. The velocity v and mobility M of the migrating grain boundaries have been calculated from the simulations. v and M are higher in twist-type simulations than in tilt-type simulations, although the activation energies are similar in the two cases. v ∝ P is observed for tilt simulations where the driving pressure is changed by varying the density of dislocation boundaries and for twist simulations where the driving pressure is changed by varying the misorientation across dislocation boundaries. When the misorientations across edge dislocation boundaries are varied, however, the simulations show v ∝ P2. It is suggested that this deviation from the usual v ∝ P-relationship is due to local interactions between the grain boundary and nearby individual dislocations. Misorientation variations across grain boundaries have also been simulated, but the mobilities show little dependence on this. The present simulations result in mobilities and activation energies that are, respectively, significantly higher and somewhat lower than experimental values. A direct mimic of experimental observations is, however not the purpose of this study. Rather the present simulations are based on idealized dislocation structures and suggest that variations in the dislocation structures may play a dominant role in recrystallization dynamics and that local effects are very important phenomena, essential for the interpretation of recrystallization mechanisms.

High cycle fatigue behavior of a single crystal superalloy at elevated temperatures

Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [0 0 1] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C, 760 °C, 850 °C and 900 °C in ambient atmosphere. The results demonstrate that the fatigue strength of smooth specimens reached the maximum at 760 °C and decreased with increasing temperature. The alloy became more notching sensitive with increase of temperature while the notch sensitivity declined at 900 °C. Analysis on fracture surfaces of both smooth and notched specimen shows that a transition from ductile fracture at lower temperatures to cleavage mode at higher temperatures were observed. Evolution of the microstructure was investigated by SEM and TEM observation. With the process of cyclic plastic deformation at elevated temperatures, the primary cuboidal γ′ precipitates tended to dissolve into the matrix channels, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix. In addition, different types of dislocation structures were developed during the cyclic deformation, which would have a significant impact on the fatigue life of the material.

Cyclic stress–strain behavior of polycrystalline nickel

The influence of strain amplitude and fatigue cycles on cyclic deformation of polycrystalline specimens of commercially 99.53% purity nickel was investigated using fully reversed strain-controlled fatigue tests. The fatigue tests were performed under constant strain amplitude, in air and at room temperature, at a constant strain rate of 0.0001 s−1. The objective was to study cyclic deformation behavior and characterize cyclic hardening response by microstructural observations using transmission electron microscopy. The cyclic stress–strain (CSS) curve of cyclically deformed polycrystalline nickel exhibited three distinct regions with a short quasi-plateau region in the intermediate amplitude range and a slight increase of saturation stress with plastic strain amplitude. The plastic strain amplitude at which the test was conducted influences the cyclic hardening rate. Dislocation structures in fatigued polycrystalline nickel are amplitude dependent and are classified into three types of dislocation structures corresponding to the three regions in the CSS curve. Vein structures consisting of loop patches were observed at low strain amplitudes. These structures become mixed with labyrinth structures at intermediate amplitudes. In addition, persistent slip bands (PSBs) were observed in the quasi-plateau regions of CSS curve. Cellular structures were observed at higher amplitudes which become increasingly equiaxed and smaller with fatigue cycles. A correlation between stress–strain response and development of dislocation structures in deformed polycrystalline nickel revealed that the saturation stress is linearly related to the inverse wall spacing. This relationship is equivalent to the mesh-length theory of work hardening over the strain range used in this study.

Plastic deformation of nitrogen-containing austenitic stainless steel single crystals with low stacking fault energy

On single crystals of austenitic stainless steel with nitrogen C N =0-0.5 wt.% the stages of stress-strain curves, work-hardening coefficient, deformation mechanism - slip and twinning, the developing of dislocation structure in dependence on crystal tensile axis orientation, test temperature, nitrogen concentration have been investigated. The type of dislocation structure (cellular or planar) and deformation mechanism (slip or twinning) have been shown to determine by the value of matrix stacking fault energy, by the level of friction forces due to solid solution hardening with nitrogen atoms and by the crystal orientation. It has been established that the contribution of mechanical twinning to the plastic deformation of steel crystal increases with nitrogen content, and in high-strength states at concentration of interstitial atoms C>0.5 wt.% mechanical twinning develops from early stages of deformation and determines work-hardening coefficient. The achievement of high level of deformation stresses due to the solid solution hardening with nitrogen atoms in combination with low values of γ 0 results in twinning in [001] orientations.

Internal Strains in Single Grains of Fatigued Polycrystalline Nickel

Bragg diffraction profiles were measured on single grains of different axial orientation in cyclically deformed Ni polycrystals. Diffraction profile parameters were determined and compared with results from profile measurements on fatigued single crystals, showing the same type of dislocation structure as the grains with corresponding axial orientation. Conclusions are drawn concerning the deformation processes in the grains taking into account the activated slip systems, the dislocation density and long-range residual strains.

Strain hardening and fracture of austenitic steel single crystals with high concentration of interstitial atoms

Stages in the flow curves, mechanisms of deformation (slip or twinning), evolution of the dislocation structure and fracture are studied in austenitic stainless steel single crystals alloyed with nitrogen (CN = 0–0.7 wt. %) and Hadfield steel in relation to the orientation of the crystal axis of tension, test temperature, and atomic concentrations of nitrogen and carbon. The dislocation-structure pattern (cellular or planar) and deformation mechanisms (slip or twinning) are shown to depend on the matrix stacking-fault energy γsf, friction forces due to solid-solution hardening by interstitial atoms, and crystal orientation. An interrelation between the stages in the flow curves and the type of dislocation structure is found. The contribution of mechanical twinning to the plastic flow of steel crystals is shown to increase with increase in nitrogen and carbon concentrations. The mechanical twinning develops in the early stages of deformation and determines the strain-hardening coefficient and fracture of crystals in high-strength states for interstitial atomic concentration C ≥ 0.5–0.7 wt. %. High deforming stresses due to solid-solution strain hardening by interstitial atoms of nitrogen and carbon in combination with low γsf are found to result in twinning in the orientations. The values of γsf in Hadfield steel single crystals and in austenitic stainless steel single crystals are found experimentally depending on the concentration of nitrogen atoms and test temperature.

Dislocation structures in cyclically deformed nickel polycrystals

The effect of the grain orientation and the plastic strain amplitude ε pa on the saturated dislocation structure was studied on individual grains of cyclically deformed nickel polycrystals by means of scanning electron microscopy using the electron back scattering pattern technique and the channelling contrast of back scattered electrons. The main features of the dislocation configuration in a grain were found to be essentially determined by the crystallographic axial orientation of the grain. A labyrinth-like dislocation pattern is typical for grains with axial orientations near [001], a patch pattern exists in grains with a loading axis (LA) near [011] and fragmented dislocation walls are dominant in grains with LA near [1̄11]. Grains with axial orientations in the central part of the stereographic standard triangle contain a bundle arrangement of dislocation structures. All four types of dislocation structures, but mostly the bundle type, can occur together with the ladder structure of persistent slip bands. Cell patterns were found to be a result of a modification of the bundle and patch configuration at high deformation amplitudes. The mesoscopic dimensions of the dislocation patterns turned out to depend on ε pa in the same way for all grain orientations: while the thickness of regions with high dislocation density is reduced with increasing ε pa, the width of regions with low dislocation density remains roughly constant.

True Stress vs Temperature Diagram of Structural States for Polycrystalline bcc Metals

For the first time, a new type of a diagram of structural states is constructed for polycrystalline bcc metals. This true-stress-vs-temperature diagram is a regular collection of temperature dependences of critical stresses corresponding to successive changes in the type of dislocation structure from the onset of plastic deformation to fracture within the temperature range from completely brittle fracture to the formation of a new grain structure. On the basis of analysis of true-stress-vs-temperature diagrams for metals and alloys (Mo, Fe, and Fe–3%Si) with the help of the thermal activation analysis of the temperature dependences of critical stresses, we establish the regularities and distinctive features of the formation and behavior of the temperature–force boundaries and domains of existence of certain structural states and identify the mechanisms of plastic deformation within the limits of each of these domains.