Initial Dislocation Structure Research Articles

Compression wave structure under plane impact deformation of a molybdenum single crystal [100] with different initial density of dislocations

The features of the evolution of the elastic-plastic compression wave in Mo single crystals along the crystallographic plane [100], having a different initial dislocation structure formed by small deformations of varying degrees of static compression at room temperature, have been studied. The analysis of wave profiles recorded using the VISAR laser interferometer in samples of different thicknesses showed a non-monotonic change in the Hugoniot elastic limit depending on the initial dislocation density pre-strain of a single crystal by compression reduces the Hugoniot elastic limit by 3 times. Keywords: Single crystal, small deformations, compression wave evolution, wave profile, VISAR interferometer.

Структура волны сжатия при плоском ударном деформировании монокристалла молибдена [100] с различной начальной плотностью дислокаций

The features of the evolution of the elastic-plastic compression wave in Mo single crystals along the crystallographic plane [100], having a different initial dislocation structure formed by small deformations of varying degrees of static compression at room temperature, have been studied. The analysis of wave profiles recorded using the VISAR laser interferometer in samples of different thicknesses showed a non-monotonic change in the hugoniot elastic limit depending on the initial dislocation density – pre-strain of a single crystal by compression reduces the hugoniot elastic limit by 3 times.

Restoration of plasticity of cold-deformed aluminum alloy by short-term irradiation with accelerated Ar+ ions

Based on results of structural analysis and mechanical tests of initially cold-rolled 1-mm-thick sheets made of 1441 aluminum alloy, the efficiency of radiation annealing was proved, which was used instead of standard furnace annealing. It has been established that exposure of cold-deformed metastable alloy 1441 of the Al-Li-Cu-Mg system to beams of accelerated Ar+ ions (E = 20–40 keV) initiates the process of explosive rebuilding of the initial cellular dislocation structure in them with the formation of subgrains (at fluences of ~1015–1016 cm−2; a corresponding irradiation time from ~1 to 10 s; T ≤ 130°С), as well as recrystallization and grain growth processes (at fluences of 1.75·1016–2·1017 cm−2). In the course of irradiation, a reduction in size and partial dissolution of Al8Fe2Si and Al3Zr coarse intermetallic particles formed during crystallization and the formation of θ′ (Al2Cu) and S1 (Al2LiMg) strengthening phases also take place.It was established that the ion irradiation-induced fast processes occur in the whole volume of 1-mm-thick samples far beyond (by factor ~104) an Ar+ projected range at temperatures significantly below (by 100–200°С) the temperature of intermediate recrystallization annealing used in cold-rolling practice.The non-thermal nature of the fast-flowing initiating processes under irradiation has been shown. It was shown that of fast flowing radiation annealing, which may be qualified as a giant long-range effect under ion irradiation, is comparable with standard furnace annealing.

Proton irradiation and characterization of additively manufactured 304L stainless steels

Irradiations were performed with 1.5 MeV protons to 0.6 dpa at 40–150 °C on additively manufactured (AM) 304L stainless steel and the changes in microstructure and mechanical behavior after irradiation were compared to wrought 304L stainless steel. All microstructural and hardness results after irradiation suggest the samples evolve toward a similar state, despite significant differences in the unirradiated microstructures and hardness values. A TEM and nanoindentation-based investigation of before and after proton irradiation at 40–150 °C is presented. Results are interpreted in terms of initial dislocation content, dislocation structures, and microstructural and chemical homogeneity.

The effects of initial void and dislocation on the onset of plasticity in copper single crystals

Based on molecular dynamics, the effects of void and initial dislocation structure on incipient plasticity are investigated with single crystal copper samples. By introducing a void of different sizes and shapes and dislocations of different initial structures around the void in copper single crystals, we explore their effects on yielding as well as the dominated deformation mechanism. The main findings from this study are as follows: (1) for the samples with a void of the same size, the yield stress approaches to a specific value as the initial dislocation density increases; (2) for samples with a void of varying size, the yield stress becomes independent of the void size as it reaches a certain level, due to the saturation of preexisting dislocations around the void; and (3) as the strain rate increases, the effects of voids and preexisting dislocations on the yield stress become limited due to the response inertia, for which the phase transition from face-centered cubic to an amorphous phase occurs at an extremely high strain rate.

Tuning strain-induced γ-to-ε martensitic transformation of biomedical Co–Cr–Mo alloys by introducing parent phase lattice defects

In this study, we examined the effect of pre-existing dislocation structures in a face-centered cubic γ-phase on strain-induced martensitic transformation (SIMT) to produce a hexagonal close-packed ε-phase in a hot-rolled biomedical Co–Cr–Mo alloy. The as-rolled microstructure was characterized by numerous dislocations as well as stacking faults and deformation twins. SIMT occurred just after macroscopic yielding in tensile deformation. Using synchrotron X-ray diffraction line-profile analysis, we successfully captured the nucleation of ε-martensite during tensile deformation in terms of structural evolution in the surrounding γ-matrix: many dislocations that were introduced into the γ-matrix during the hot-rolling process were consumed to produce ε-martensite, together with strong interactions between dislocations in the γ-matrix. As a result, the SIMT behavior during tensile deformation was accelerated through the consumption of these lattice defects, and the nucleation sites for the SIMT ε-phase transformed into intergranular regions upon hot rolling. Consequently, the hot-rolled Co–Cr–Mo alloy simultaneously exhibited an enhanced strain hardening and a high yield strength. The results of this study suggest the possibility of a novel approach for controlling the γ → ε SIMT behavior, and ultimately, the performance of the alloy in service by manipulating the initial dislocation structures.

Direct observation of dislocation structure evolution in SRF cavity niobium using electron channeling contrast imaging

Two major goals of superconducting radio frequency cavity research and development are achieving higher accelerating gradient and gaining the highest quality factor, a measure of efficiency. However, the consistent improvement of these performance metrics is restricted by many factors, one of which is microstructural defects, such as dislocation substructures within the material. In this work, dislocation evolution is compared in four samples extracted from a 2.8 mm thick large-grain niobium slice, with tensile axes chosen to generate specific dislocation structures during subsequent deformation. The four samples are (1) as-extracted, (2) extracted and annealed, (3) extracted and then deformed to 40% tensile strain, and (4) extracted, annealed at 800 °C 2 h, and then deformed to 40% strain. Electron channeling contrast imaging was performed on all samples to characterize initial dislocation density and dislocation structure evolution due to annealing and deformation, and related to the mechanical behavior observed in stress-strain curves. A fundamental understanding of dislocation evolution in niobium is necessary to develop computational models to simulate cavity forming, which could enable new processing methods for cavity fabrication to be identified that could lead to improved performance.

A physically based yield criterion I. Determination of the yield stress based on analysis of pre-yield dislocation behaviour

In this paper a physically based method to determine the yield stress of metals from tensile curves is proposed. The criterion is based on the identification of the transition in dislocation behaviour from limited reversible glide in the pre-yield stage, without essential changes in the dislocation structure, to dislocation multiplication in the post-yield stage. This transition point is a clearly distinguishable feature in a plot of the hardening rate vs. the flow stress (a Kocks-Mecking plot). Literature shows that this is a common feature shared by metallic alloys under different circumstances. The present paper shows, on the basis of stress-cycling experiments, that the pre-yield behaviour, i.e. at stresses below the observed transition point, is explained by reversible dislocation behaviour. Schoeck's classical theory of this anelastic deformation is extended to give a quantitative description of dislocation behaviour in the pre-yield stage. The extended theory provides a very accurate description of pre-yield deformation and can be used to determine structural parameters that characterise the initial dislocation structure.

Effect of powerful pulsed and continuous ion beams on the Al- Cu-Mg alloy structure

The paper considers the results of an electron microscopy study of the VD1 alloy of the Al-Cu-Mg system after cold working and subsequent irradiation with a powerful pulsed ion beam (70% C+ + 30% H+, E = 180 keV) in the pulsed-periodic mode (t = 80 ns, f = 0.1 Hz, j = 200 A/cm2, F = 1-1014 cm-2) and under the conditions of the generation of only one pulse (t = 180 ns, j = 100 A/cm2, F = 2-1015 cm-2). It is established that this irradiation noticeably affects the microstructure of the cold-worked 3 mm thick sheets of VD1 alloy. The initial cellular dislocation structure transforms into a subgrain one. The intensity of structural transformations in the alloy increases with ion current density of a pulse. A similar transformation of a dislocation structure over the entire thickness of the sample is observed under irradiation with continuous Ar+ ion beams (E = 20-40 keV) with not high fluences (1015-1016 cm-2).

A physically based yield criterion II. Incorporation of Hall Petch effect and resistance due to thermally activated dislocation glide

The anelastic model developed in part I of this paper [1] is extended to incorporate the Hall–Petch effect and the thermally activated pre yield phenomena that were revealed in the stress cycling experiments The yield criterion developed in [1] is tested by application to Hall–Petch experiments and measurements of the yield stress as a function of strain rate and temperature. The results of the strain rate/temperature experiments support the conclusion that reversible dislocation glide occurs before yield and massive dislocation multiplication. The model is substantiated by analysis of parameters pertaining to the initial dislocation structure viz. the average initial dislocation segment length and density computed from experimental data by application of the model equations.

Atomistic view of thin Ni/Ni3Al (0 0 1) under uniaxial tension of twist grain boundaries

Atomic motion and the structure response of grain boundaries (GBs) are essential to the plastic deformation of small-volume polycrystal systems, especially for thin materials that exhibit some dramatic characteristics. Here, the microstructure and properties of thin Ni/Ni3Al (0 0 1) with the uniaxial tension of twist GBs are investigated using molecular dynamics (MD) simulations with an embedded atom (EAM) potential. We find that low angle twist GB dislocations propagate mainly from the corners to the center of the interface, and for high angle twist GBs they originate from the edge of the interface but do not extend to the center. In the process of plastic deformation, both low angle and high angle twist GB fractures occur in the center of the interface, which is completely different from the situation of thick materials reported by other references. More interestingly, the fracture patterns between low angle and high angle twist GBs exhibit extremely different performance due to the different characteristics of the initial dislocation structures in the interfaces.

X-ray diffraction study of microstructural changes during fatigue damage initiation in pipe steels: Role of the initial dislocation structure

The present work is the second part of an ongoing study of microstructural changes during fatigue damage initiation in pipe steels [B. Pinheiro et al., Mat. Sci. Eng., A 532 (2012) 158–166]. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction during alternating bending fatigue tests on samples taken from an API 5L X60 grade steel pipe. Microdeformations are evaluated from the full width at half maximum (FWHM) of the diffraction peak and residual stresses are estimated from the peak shift. Here, to understand the role of the initial dislocation structure, annealed samples are considered. As previously found for as-machined samples, the evolution of microdeformations shows three regular successive stages, but now with an increase during the first stage. The amplitude of each stage is accentuated with increasing stress amplitude, while its duration is reduced. Residual stresses show a similar trend, with stages of the same durations than those observed for FWHM, respectively. Additionally, changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. The results are very encouraging for the development of a future indicator of fatigue damage initiation for pipe steels based on microstructural changes measured by X-ray diffraction.

Calculation of Interfacial Dislocation Structures: Revisit to the O-lattice Theory

This article clarifies a geometric method for calculating interfacial dislocation structures, to provide virtually unique initial dislocation structures for further relaxation. Ambiguities in specifying the misfit deformation and Burgers vectors are effectively eliminated. The physical basis for the method is a hypothesized periodicity correspondence between the structure within a good matching site (GMS) cluster (GMSC) and the conserved structure between dislocations. It is proposed to attribute each interfacial dislocation with a couple of correlated Burgers vectors. A set of Burgers vectors from each real lattice is identified according to the translation symmetry in a GMSC. A GMSC principle is suggested to guide the formulation of the deformation matrix A. The O-lattice theory is applied to quantify the distribution of GMSCs at the O-elements and poor matching regions at the O-cell walls. Configuration of dislocations in a general semicoherent interface is determined according to the effective O-cell wall traces in the interface. The relationship between GMS/GMSC and coincidence-site-lattice/O-lattice is elucidated for describing secondary dislocations. Several controversial issues on dislocation descriptions are discussed, including the decomposition of a net Burgers vector content, the influence of reference lattice, and the relationship between secondary dislocations and atomic steps.

Cyclic plasticity and internal dislocation structure in two-phase alloy

Austenitic-ferritic stainless steel has been cycled in symmetrical push-pull regime with constant strain rate and different strain amplitudes. Hysteresis loops were recorded and analysed. The plot of the second derivative of the half-loop vs. relative strain shows one peak for low amplitude straining corresponding to the cyclic deformation of softer phase, austenite. With increasing strain amplitude second peak appears and corresponds to the cyclic deformation of the harder phase, ferrite. Transmission electron microscopy study revealed parallel bands in austenite and initial virgin dislocation structure in ferrite at low amplitude while cell structure in austenite and secondary wall structure in ferrite at high amplitude.

Internal stress evolution in Fe laths deformed at low temperature analysed by dislocation dynamics simulations

Stress evolution in Fe laths undergoing plastic deformation is investigated using three-dimensional dislocation dynamics simulations adapted to body centred cubic crystals, in the ductile to brittle transition temperature range. The selected boundary conditions, applied stress tensor and initial dislocation structures account for the realistic microstructure observed in bainitic steels. The effective stress field projected in the three different {1 0 0} cleavage planes is calculated for two different temperatures (50 and 200 K) and presented quantitatively, in the form of stress/frequency diagrams. It is shown that plastic activity tends to relax the stress acting in certain cleavage planes (the (0 1 0) and (0 0 1) planes) while, at the same time, amplifying the stress acting in other cleavage planes (the (1 0 0) planes). The selective stress amplification in the latter planes depends on the applied load direction, in combination with the limited set of available slip systems and the lath geometry. In the examined configuration, this selection effect is more pronounced with decreasing temperature, emphasizing the role of thermally activated plasticity on deformation-induced stress concentrations.

Discrete dislocation dynamics simulations of plasticity at small scales

Discrete dislocation dynamics simulations in three dimensions have been used to examine the role of dislocation multiplication and mobility on the plasticity in small samples under uniaxial compression. To account for the effects of the free surfaces a boundary-element method, with a superposition technique, was employed. Cross-slip motion of the dislocation was also included, and found to be critical to the modeling of the dislocation behavior. To compare directly with recent experiments on micropillars, simulation samples at small volumes were created by cutting them from bulk three-dimensional simulations, leading to a range of initial dislocation structures. Application was made to single-crystal nickel samples. Comparison of the simulation results and the experiments are excellent, finding essentially identical behavior. Examination of details of the dislocation mechanism illuminates many features unique to small samples and points directly to the importance of both the surface forces and cross-slip in understanding small-scale plasticity.

Initial dislocation structures in 3-D discrete dislocation dynamics and their influence on microscale plasticity

Realistic dislocation network topologies were generated by relaxing an initially pinning point free dislocation loop structure using three-dimensional discrete dislocation dynamics simulations. Traction-free finite-sized samples were used. Subsequently, these equilibrated structures were subjected to tensile loading and their mechanical behavior was investigated with respect to the initial configuration. A strong mechanical size effect was found. The flow stress at 0.2% plastic deformation scales with specimen size with an exponent between −0.6 and −0.9, depending on the initial structure and size regime. During relaxation, a mechanism, also favored by cross-slip, is identified which leads to rather stable pinning points. These pinning points are comparable to those of the isolated Frank–Read sources often used as a starting configuration in previous discrete dislocation dynamics simulations. These nodes act as quite stable dislocation sources, which can be activated multiple times. The influence of this source mechanism on the mechanical properties of small-scale specimens is discussed.

Irradiation-Induced Growth and Microstructure of Recrystallized, Cold Worked and Quenched Zircaloy-2, NSF, and E635 Alloys

Abstract This paper is devoted to the study of the effect of the texture, phase composition, and microstructure on the irradiation-induced growth strain (GS) of zirconium-based alloys. GS measurements and TEM microstructural examinations were performed on Zry-2, NSF, and E635 samples in the recrystallized, beta quenched and cold-worked (CW) conditions. The samples were irradiated in the BOR-60 reactor in the temperature range of 315–325°C up to a neutron fluence level of 1.1 ×1026 n/m2 (E>1 MeV), i.e., up to a damage dose of 23 dpa. Growth strains of NSF and E635 alloys in all states and in the longitudinal and transverse directions are lower as compared to those of Zry-2, and do not exceed 0.2 % even at the maximum fluence level. As for recrystallized Zry-2, the GS kinetics are characterized by the appearance of the accelerated growth stage. A combination of a certain amount of Nb, Fe, and Sn in the matrix content plays a key role in GS kinetics. The higher the degree of CW, the higher the irradiation growth but its rate of increase with increasing fluence is different for alloys of different compositions. The maximum GS, reaching 0.72 %, is observed in the 20 % CW Zry-2 samples. Texture, along with the alloy composition, is one of the main GS-determining factors. Irradiation growth of the transversal samples is lower as compared to the longitudinal ones because of texture. As for quenched alloys, the texture is practically isotropic and GS values are low, independent of the alloy composition. In CW materials, the density of <c>- dislocations greatly affects the irradiation growth strain. Particles of Zr(Fe,Cr)2 and Zr2(Fe,Ni) phases in Zry-2 as well as Zr(Nb,Fe)2 in NSF and E635 are depleted in iron under irradiation. The Fe goes into the matrix and modifies its properties. The HCP lattice structure in the Laves phases in NSF and E635 changes into BCC (β-Nb-type). FCC (Zr,Nb)2Fe precipitates preserve on the whole their composition and structure; no amorphization of the Nb-containing precipitates is observed. The Zr2(Fe,Ni) precipitates with a BCT lattice remain crystalline, and HCP Zr(Cr,Fe)2 precipitates undergo amorphization. The average particle size in the irradiated alloys is larger and the concentration is a little lower as compared to the unirradiated ones. Irradiation-induced fine dispersed precipitates about 3 nm in size, probably enriched in niobium, appear in NSF and E635. The observed changes of microhardness are discussed from the viewpoint of generation of radiation defects (clusters, dislocation loops), evolution of the initial dislocation structure, and matrix composition (enrichment in Fe, Cr, and, probably, Nb).

Deformation Behavior of Intermetallics: Models and Experiments

AbstractA sufficiently general thermally activated mechanism for extension of dislocations in some preferential direction was proposed. This mechanism comprises a necessary step of dislocation transformations that lead to blocking. The reasons for blocking of different types of dislocations in different materials are diverse. A new concept was developed concerning the possibility of thermally activated blocking of superdislocations in the absence of external stresses. Some experiments with single crystals of Ni3(Al, Nb) were performed. They included no‐load heating after preliminary low‐ or high‐temperature deformation. It was found that the initial dislocation structure, which included curvilinear dislocations, transformed to a set of long rectilinear blocked superdislocations after no‐load heating. The experimental results confirmed theoretical assumptions on the possibility of thermally activated transformations of superdislocations to indestructible barriers in the absence of external stresses.

Relation between the deformation sub-structure after rolling or tension and the recrystallization mechanisms of an IF steel

The characteristics of the deformation microstructure of an IF steel sample have been studied using TEM after 40% cold rolling and 35% strain in tension. Different microstructures can be distinguished: lamellar bands for the {1 1 1} 〈1 1 2〉 grains belonging to the γ fibre and equiaxed cells for the {1 1 1} 〈1 1 0〉 grains that belong to both γ and α fibres as well as the {0 0 1} 〈1 1 0〉 grains that belong exclusively to the α fibre. Whatever the initial dislocation structure, the first steps of nucleus formation are characterized by coalescence and growth of deformation cells. The first steps of recrystallization mainly take place via continuous growth of subgrains for both orientations and are followed by bulging of grain boundaries in the case of the {1 1 1} 〈1 1 0〉 subgrains. The role of the morphology and size of the subgrains as well as that of the deformation energy stored by the sub-boundaries are discussed within the framework of the nucleation processes of low carbon steels.