Planar Dislocation Slip Research Articles

Dynamic strain aging studied at the atomic scale

Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloy. It initiates jerky dislocation motion and abrupt softening, causing negative strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress–strain response and deformation banding. These macroscopic features are referred to as the Portevin–Le Chatelier effect (PLC). Here we study the atomistic origin of dynamic strain aging in an Al-4.8at.% Mg alloy using atom probe tomography (APT) and transmission electron microscopy (TEM). Samples were prepared from as-cold rolled (90% thickness reduction), stabilized (120°C, 20h) and recrystallized sheets (400°C, 10min), respectively. In the stabilized state, Mg was found to decorate <110> aligned dislocations with up to ∼12.5at.%. Tensile tests in combination with thermographic and laser speckle observations were used to map the deformation bands for the site-specific extraction of APT samples from regions inside the PLC bands. We observed an asymmetrical Mg distribution along some of the dislocations, matching model predictions for high dislocation speeds at peak drag stress by Zhang and Curtin. In this case, the Mg distribution is characterized by depletion in the compressive regime above the dislocation slip plane and enrichment in the dilatation region below the slip plane. Mg also depletes in a tail-like form behind fast-moving dislocations, further promoting slip localization.

Powder Iron–Glass Material: Mechanism of Wear in Vacuum

The wear mechanism of powder iron–glass material is analyzed. The behavior of PS5GSh–PS5GSh and of PS5GSh–hardened steel 45 during friction in vacuum is studied. It is established that the glass inclusions and “white layer” formed on the friction surface influence the wear-resistance of the iron-glass material. It is noted that increased hardness leads the glass inclusions to form a near-surface framework, resisting the plastic deformation on the surface and inside the material. It is shown that the white inclusions have low friction adhesion and thus prevent seizure with steel. The “white layer” adheres strongly to the material. It acts as a solid lubricant and, along with the glass inclusions, increases the wear resistance. Extreme gross wear is accompanied by active growth and failure of the “white layer” as scales on dislocation slip planes located at an angle to the friction surface. The wear mechanism of the powder iron–glass materials under simultaneous contact of steel and glass is studied.

Hydrogen-induced Modification in the Deformation and Fracture of a Precipitation-hardened Fe–Ni Based Austenitic Alloy

Hydrogen-induced modification in the deformation and fracture of a precipitation-hardened Fe–Ni based austenitic alloy has been investigated in the present study by means of thermal hydrogen charging experiment, tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the γ′ particles are subjected to the multiple shearing by dislocations during plastic deformation, which promotes the occurrence of the dislocation planar slip. Moreover, the alloy will be enhanced by hydrogen resulting in the formation of strain localization at macroscale. So, the mechanisms of deformation and fracture in the alloy have been proposed in terms of serious hydrogen-induced planar slip at microscale which can lead to macroscopic strain localization.

Cross slip of dislocation loops in GaN under shear

AbstractThis work explores possible cross‐slip mechanisms of gliding type &lt;a &gt; = a /3[1 –2 1 0] dislocation loops in wurtzite gallium nitride (GaN) as a function of slip plane. A modified form of the dislocation dynamics code ParaDiS was employed using isotropic linear elasticity and dislocation mobilities estimated in part from molecular dynamics (MD) simulations. Under an externally applied uniform stress, the occurrence of cross slip events is highly dependent on the initial dislocation slip plane. The basal plane is the preferred active plane, owing to the greater mobility of &lt;a &gt; type segments on that plane, over the other planes considered including the prismatic (–1 0 1 0) and two equivalent pyramidal planes (–1 0 1 1) and (1 0 –1 1). For an applied stress state, cross slip processes are more readily seen from the prismatic‐to‐basal planes or the pyramidal‐to‐basal planes, and neither is found to occur in reverse. Cross slip by climb is not presently considered. In all cases, cross‐slip events occur after the loop expands until a greater number of screw‐oriented segments are able to access the cross slip plane and the resolved stresses on the plane become sufficiently large. In comparison to dislocations found in GaN previously, the calculations suggest that some threading dislocations along the [0001] direction that have edge character may have been formed from loops whose screw segments slip and escape on basal planes leaving only the edge segments.

Pulsed Eddy Current Testing of Thermally Aged and Cold-Rolled Fe–Cu Alloys

In this work, Fe-1wt%Cu alloy samples exposed to thermal aging and cold rolling (CR) were studied as an attempt to emulate material degradation in neutron irradiated nuclear reactor pressure vessels. CR was tested by pulsed eddy currents (PECs) using a directional probe which was rotated in the sample plane. Normalized PEC signals, represented by typical “figure of eight” on the polar plot, showed very strong anisotropy, which has a monotonic dependence on the amount of cold work. Since the normalized PEC signals represent the response due to electrical conductivity and reduce the ferromagnetic effect, it is concluded that the change is due to anisotropic magnetoresistivity of the rolled steel. The normalized difference PEC responses have been found to be described by the quadratic dependence of the cosine of in-plane angle shifted by an angle, which is believed to correspond to slip planes of dislocations induced by rolling. The technique advantageously combines effects of rolling on electrical conductivity and magnetic permeability and does not require referencing to an untreated standard, which is important for future material and device health management.

Origin of Significant Grain Refinement in Co-Cr-Mo Alloys Without Severe Plastic Deformation

We have previously reported that ultrafine-grained (UFG) microstructures can be obtained in a Co-29Cr-6Mo (wt pct) alloy by utilizing dynamic recrystallization (DRX) that occurs during conventional hot deformation (Yamanaka et al.: Metall. Mater. Trans. A, 2009, vol. 40A, pp. 1980−94). The present study investigates the novel DRX mechanism of this alloy in detail. The microstructure evolution during hot deformation under relatively high Zener–Hollomon (Z) parameter conditions for which ultrafine grains can develop was systematically investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy. This alloy exhibited a different flow stress behavior and microstructural development from conventional DRX mechanisms. The deformation microstructure contained a large number of stacking faults, which implies that planar dislocation slip is the primary deformation mechanism in the hot deformation of the Co-29Cr-6Mo alloy due to its abnormally low stacking fault energy (SFE) at elevated temperatures. Inhomogeneities in local strain distributions induced by planar slip will enhance grain subdivision by geometrically necessary (GN) dislocation boundaries. Deformation twinning may also contribute to grain refinement. The DRX mechanism operating in the Co-29Cr-6Mo alloy is discussed by considering the relationships between anomalous dislocation structures, flow stress behavior, texture development, and nucleation behavior.

Mechanism of hydrogen embrittlement in a gamma-prime phase strengthened Fe–Ni based austenitic alloy

The mechanism of hydrogen embrittlement (HE) in a γ′-Ni3(Al,Ti) phase strengthened Fe–Ni based austenitic alloy has been investigated in detail. Hot hydrogen charging experiment and tensile test reveal that the alloy with coherent γ′ phase exhibits a much higher decrease in reduction of area (RA) than that of the alloy in the solution-treated state. However, three-dimensional atom probe (3DAP) experiment shows that segregation of hydrogen atoms is not found at the coherent interface between the γ′ phase and the matrix, which indicates that the interface is not a strong hydrogen trap. Furthermore, high-resolution transmission electron microscopy (TEM) observation indicates that the interface coherency is maintained during the deformation, even tensile to fracture. It is found that macroscale slip band rupture and intergranular fracture are promoted by serious dislocation planar slip, which become the predominant features in the tensile-to-fracture sample after hydrogen charging. This phenomenon has been interpreted as a result of combined effects of the γ′ phase and hydrogen in the precipitation-strengthened Fe–Ni based austenitic alloy.

Adaptive atomistic‐to‐continuum modeling of propagating defects

SUMMARYAn adaptive atomistic‐to‐continuum method is presented for modeling the propagation of material defects. This method extends the bridging domain method to allow the atomic domain to dynamically conform to the evolving defect regions during a simulation, without introducing spurious oscillations and without requiring mesh refinement. The atomic domain expands as defects approach the bridging domain method coupling domain by fine graining nearby finite elements into equivalent atomistic subdomains. Additional algorithms coarse grain portions of the atomic domain to the continuum scale, reducing the degrees of freedom, when the atomic displacements in a subdomain can be approximated by FEM or extended FEM elements to within a certain homogeneity tolerance. The extended FEM approximations are created by fitting the broken inter‐atomic bonds of fractured surfaces and dislocation slip planes. Because atomic degrees of freedom are maintained only where needed for each timestep, the solution retains the advantages of multiscale modeling, with a reduced computational cost compared with other multiscale methods. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Dislocation-core symmetry and slip planes in tungsten alloys: Ab initio calculations and microcantilever bending experiments

Dislocation structure and slip plane are two key elements to understanding the basic mechanism of plastic deformation in metal alloys. Using density-functional theory (DFT), we investigate the influence of Ta and Re on the non-planar 1 2 〈 1 1 1 〉 screw dislocation in body-centered cubic W. The dislocation-core structure is modeled by the dislocation–dipole approach, and the effect of alloying is determined from the virtual-crystal approximation as well as the supercell method. Both reveal symmetric core structures for W–Ta alloys but asymmetric ones for W–Re. Our ab initio results allow for an evaluation of the inter-row model, which is found to produce dislocation-core structures consistent with DFT results, but fails to describe the dislocation movement in these alloys. DFT finds the slip plane to be altered by Re alloying. This result is confirmed by microcantilever bending tests.

The correlation between spatial alignment of dislocations, grain orientation, and grain boundaries in multicrystalline silicon

AbstractAn experimental study of the correlation between dislocations, grain orientation, and grain boundaries in multicrystalline silicon (mc‐Si) bulk crystals is presented. Selected mc‐Si samples revealing typical structural patterns like clustering or line‐up of dislocations in the vicinity of grain boundaries are measured by electron backscatter diffraction (EBSD). The orientation of dislocation slip planes and twin boundary planes as well as the relationship between the misorientation of neighbouring grains and the spatial alignment of dislocations are analyzed on the basis of the relevant pole figures calculated from EBSD scans. The effect of grain boundaries on the behaviour of dislocations seems to be related to the presence or absence of a common slip system on both sides of the boundary and hence, to the type of lattice misorientation between involved grains. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Internal Friction in Commercial Aluminium Alloy AW-2007

The aim of this work is to state internal friction on stress amplitude dependence in an experimental aluminium alloy AW - 2007. Dependence was examined on sample made of extruded material after annealing at the temperatures of 200°C and 250°C. Annealing at the increased temperature has significant effect on dislocation structure. All slip systems are activated in material and thermally activated dislocation transport also occurs. At the same time the annihilation of dislocations with opposite direction occurs. The Frank-Read sources are removed. After quenching at the room temperature the relatively immobile dislocation forest is present in the material that does not contribute to the internal friction and does not generate new dislocations. The amplitude dependence of internal friction can be described by Granato and Lücke model, which presumes the existence of weak attached barriers in slip plane of dislocation. This model involves average dislocation length, Young's elasticity modulus, interaction energy between dislocations and pinning points, magnitude of Burgers vector.The method of quality of resonant system was used to measure the internal damping. All experiments were carried out at the frequency range of 20kHz at the room temperature.

Lattice weakening by edge dislocation core under tension

Molecular dynamics simulations are carried out to investigate the influence of edge dislocations on the lattice strength in iron single crystals under tension perpendicular to the dislocation slip plane. Simulations are performed at different temperatures and with different densities of edge dislocations on the [1 1 1] slip system. It is found that cracks nucleate at dislocation cores in dislocation-containing systems at lower critical stresses compared with a perfect lattice, indicating a weakening caused by edge dislocations. Lattice thermal vibration plays an important role during the tensile deformation, resulting in a monotonic decrease in the yield strength as the temperature increases. There exists a critical temperature above which the lattice transforms from brittle to ductile, where dislocation nucleation and glide are favored instead of crack nucleation, and the critical temperature for the dislocated system is lower than that for the perfect lattice. The present simulation shows that higher dislocation density results in lower fracture strength, suggesting that the accumulation of dislocations during cyclic deformation may be the cause of lower strength under fatigue.

Dislocation subgrain structures and modeling the plastic hardening of metallic single crystals

A single crystal plasticity theory for insertion into finite element simulation is formulated using sequential laminates to model subgrain dislocation structures. It is known that local models do not adequately account for latent hardening, as latent hardening is not only a material property, but a nonlocal property (e.g. grain size and shape). The addition of the nonlocal energy from the formation of subgrain structure dislocation walls and the boundary layer misfits provide both latent and self-hardening of a crystal slip. Latent hardening occurs as the formation of new dislocation walls limits motion of new mobile dislocations, thus hardening future slip systems. Self-hardening is accomplished by an evolution of the subgrain structure length scale. The substructure length scale is computed by minimizing the nonlocal energy. The minimization of the nonlocal energy is a competition between the dislocation wall energy and the boundary layer energies. The nonlocal terms are also directly minimized within the subgrain model as they affect deformation response. The geometrical relationship between the dislocation walls and slip planes affecting the dislocation mean free path is taken into account, giving a first-order approximation to shape effects. A coplanar slip model is developed due to requirements while modeling the subgrain structure. This subgrain structure plasticity model is noteworthy as all material parameters are experimentally determined rather than fit. The model also has an inherit path dependence due to the formation of the subgrain structures. Validation is accomplished by comparison with single crystal tension test results.

Combined influences of micro-pillar geometry and substrate constraint on microplastic behavior of compressed single-crystal micro-pillar: Two-dimensional discrete dislocation dynamics modeling

2D discrete dislocation dynamic modeling of compressed micro-pillars attached on a huge base is executed to study the size-dependent microplastic behavior of micro-pillars and the corresponding size effect. In addition to the conventional dimensional parameters of the micro-pillar such as the micro-pillar size and the height-to-width ratio, the micro-pillar taper angle and the dislocation slip plane orientation angle in the micro-pillar are also considered to address the size effect and its rich underlying mechanism. Computational results show that there are at least two operating mechanisms responsible for the plastic behavior of micro-pillars. One is associated with the dislocation free slip-out from the micro-pillar sidewall; the other is related to the dislocation pile-up at the base and the top end of the pillar. The overall mechanism governing the size effect of the micro-pillar rests with multi-factors, including the micro-pillar size, the height-to-width ratio, the micro-pillar taper and the slip plane orientation angle; however, whether the “free slip band” exists or not is the most important denotation. The well-known Schmid law still validates in the slender micro-pillars due to existence of the free slip band, whereas it may fail in the podgier micro-pillars due to absence of the free slip band; as a result, a complicated even “reverse” size effect appears.

Structure and mobility of the [formula omitted] edge dislocation in BCC iron studied by molecular dynamics

In this paper, we carried out atomistic calculations to investigate in detail the core structure and motion mechanism of the 1 2 < 1 1 1 ¯ > { 1 1 2 } edge dislocation in α-iron. First, molecular statics simulations are used to characterise the dislocation-core structure in the framework of the Peierls–Nabarro model. It is shown that the accommodation of the distortion due to the insertion of the extra half-planes is not equivalent in the planes above and below the dislocation slip plane and that the relative atomic-displacement profile in the dislocation-core region is asymmetrical. Then, molecular dynamics simulations are used to study the mechanism of the dislocation motion at different temperatures. At low temperature, the dislocation is found to move by nucleation and propagation of kink-pairs along its line. Independently of temperature, when loading is performed in the twinning direction, the critical stress is found to be lower than the one corresponding to the antitwinning loading direction.

Effect of impact loading duration on yield strength

Plastic deformation of crystalline materials is mostly mediated by the motion of dislocations. It is known that the dislocation motion encounters drag related to the crystal lattice and various defects such as vacancies, impurity atoms, and grain boundaries. Under the conditions of quasi-static straining, the dislocations overcome this resistance due to the joint action of the applied stress and thermal fluctuations of atoms. The action of the thermal fluctuations of atoms on any region of a dislocation has a discrete character, since the time intervals during which the thermal fluctuations of atoms favor the motion alternate with time intervals where this action is absent. In accordance with the laws of statistical physics, this alternation takes place at a very high frequency. For this reason, the discrete character is not manifested under the usual experimental conditions. However, in connection with the investigations of material properties under impact loading of ultrashort duration, which have been conducted extensively in recent years [1], a question naturally arises: how short should the external loading duration be in order for thermal fluctuations of atoms to have, with a large probability, no time to set dislocations in motion during this loading? In this study, it is shown on the basis of the Einstein‐ Debye fluctuation theory and the results of our previous investigations [2] that, during the impact loading of a crystal with a duration of 10 –5 to 10 –6 s and a stress level on the order of the dynamic yield strength, the thermal fluctuations of atoms required for the dislocation motion do not occur (with a probability of nearly unity). As a result, the crystal remains plastically undeformed, though at the same stress level but for more prolonged loading it would experience plastic deformation. This effect can be compensated, that is, the plastic deformation of a sample subjected to a short loading pulse can be achieved, by increasing the stress level during the pulse. This is indicative of an increase in the yield strength of a material with decreasing duration of impact loading. Let us consider a crystal as an elastic isotropic body containing dislocations of one slip system. It is known that the following condition must be fulfilled in order for a dislocation at rest to be set in motion: in the vicinity of the dislocation, the tangential component of the stress tensor acting in the dislocation slip plane in the direction of the Burgers vector must exceed in absolute value a certain threshold σ 0 which characterizes the resistance impeding the dislocation motion. We will analyze a situation in which the external stress component σ B does not reach the threshold level; for certainty, let σ B be positive, so that we have 0 < σ B < σ 0 . In this case, favorable action of the thermal motion of atoms is needed for the onset of dislocation motion.

Structure and recombination properties of extended defects in the dislocation slip plane in silicon

AbstractIt is shown, that moving 60° dislocations in Si as well as in SiGe generate high density of extended defects in their slip plane. As a result, the plastic deformation introduces in the crystal volume in addition to the dislocations a very dense system of extended defects, spatially separated from dislocations. The EBIC imaging revealed strong recombination contrast associated with these extended defects. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Fatigue limits and SEM/TEM observations of fracture characteristics for three Pd—Ag dental casting alloys

The fatigue limits and fracture characteristics for three Pd-Ag dental casting alloys (Super Star, Heraeus Kulzer; Rx 91, Pentron; W-1, Ivoclar Vivadent) were studied. Specimens meeting the dimensions for ADA Specifications No. 5 and 38, and having the as-cast surface condition, were subjected to heat treatment simulating dental porcelain firing cycles and fatigued in air at room temperature under uniaxial tension-compression at 10 Hz. A ratio of compressive stress amplitude to tensile stress amplitude (R-ratio) of -1 was used. Alloy microstructures and fracture surfaces were examined with a scanning electron microscope and a transmission electron microscope. Fatigue limits for the three alloys had low values of approximately 15% of the yield strength for 0.2% permanent tensile strain. Complex fracture surfaces with characteristic striations were observed for all three fatigued alloys. Planar slip of dislocations occurred in the Pd solid solution matrix, along with dislocation-precipitate interactions and dislocation networks in the interfaces between the precipitates and surrounding matrix. Twinning occurred in the Pd solid solution matrix of Rx 91, and within discontinuous precipitates in Super Star and Rx 91. The low fatigue limits for these alloys are attributed to their complex microstructures and perhaps to casting defects.

Computer simulation of reactions between an edge dislocation and glissile self-interstitial clusters in iron

Clusters of self-interstitial atoms (SIAs) are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector, b. Atomic-scale computer simulation is used here to investigate their reaction with an edge dislocation gliding in α-iron under stress for the situation where b is inclined to the dislocation slip plane. The b of small loops (37 SIAs here) changes spontaneously and the interstitials are absorbed as a pair of superjogs. The line glides forward at critical stress τc when one or more vacancies are created and the jogs adopt a glissile form. A large loop (331 SIAs here) reacts spontaneously with the dislocation to form a segment with b = ⟨100 ⟩, which is sessile on the dislocation slip plane, and as applied stress increases the dislocation side arms are pulled into screw orientation. At low temperature (100 K), the ⟨100⟩ segment remains sessile and the dislocation eventually breaks free when the screw dipole arms cross-slip and annihilate. At 300 K and above, the segment can glide across the loop and transform it into a pair of superjogs, which become glissile at τc. Small loops are weaker obstacles than voids with a similar number of vacancies, large loops are stronger. Irrespective of size, the interaction processes leading to superjogs are efficient for absorption of SIA clusters from slip bands, an effect observed in flow localization.

Slip planes and kink properties of screw dislocations in high-purity niobium‡

Temperature and strain-rate dependence of the flow stress of cyclically pre-deformed high-purity niobium single crystals have been measured in the temperature range 120,K ≤T≤350 ,K with high accuracy and reproducibility for five or more resolved shear-strain rates between 6.5×10-5 ,s-1 and 3.5×10-3 ,s-1. The data are quantitatively interpreted in terms of kink-pair generation and kink diffusion in a 0⟨111⟩/ 2 screw dislocations (a 0= cubic lattice parameter). The prediction of a discontinuity in the stress dependence of the activation volume (occasionally dubbed ‘the hump’) at a strain-rate-independent effective flow stress has been verified. From the stress dependence of the activation volume and from the magnitude of the discontinuity the spatial period of the Peierls barriers of the screw dislocations could be derived without having to assume a special shape of the Peierls potential. In the temperature range investigated, the measured periodicity is in quantitative agreement with {112} as elementary slip planes (i.e. the slip planes of the screw dislocations between cross-slip events) but incompatible with predominant slip on {110} planes. Examples of further quantitative results are for the effective stress at the ‘upper bend’ of the flow-stress–temperature relationship, the enthalpy of formation of a pair of isolated kinks, 2H k=(0.68 ±0.02),eV, and the activation energy of kink diffusion, . In agreement with the above-mentioned prediction, the same H k values are obtained above and below . The Nb data are compared with those for Ta, Mo, W, and α-Fe, which all exhibit a similar pattern. The comparison with the internal-friction measurements of D'Anna and Benoit shows very clearly that the classical γ-relaxation of Nb – called irreversible by D'Anna and Benoit – is caused by the thermally activated generation of kink pairs in a 0⟨111⟩/ 2 screw dislocations on {112} planes. For the reversible γ-relaxation two alternative mechanisms are discussed. The one based on kink-pair formation in screw dislocations on {110} planes appears to be the more likely one. This interpretation implies that the reversible γ-relaxation is identical with the β-relaxation and thus substantiates Chambers' claim of the intrinsic nature of the β-relaxation. ‡Dedicated to Professor Frank Reginald Nunes Nabarro on the occasion of his 90th birthday on March 7th, 2006.