Role Of Dislocations Research Articles

Influence of dislocations on hydrogen diffusion and trapping in an Al-Zn-Mg aluminium alloy

Cu-lean Al-Zn-Mg alloys are known to be susceptible to hydrogen embrittlement (HE), which currently limits their use in automotive industry. Several works suggested that the resulting loss of mechanical properties was related to hydrogen trapping in different metallurgical sites. The present work attempts to provide a better understanding of the hydrogen-dislocations interactions to evaluate their influence on the loss of mechanical properties of hydrogen-embrittled Al-Zn-Mg alloys. Pre-strained samples of 7046 aluminium alloy (AA7046) were therefore prepared in order to increase the density of motionless dislocations. Tensile samples, pre-strained or not, were then corroded in 0.6 M NaCl and mechanically tested to evaluate their HE susceptibility and the role of dislocations on hydrogen diffusion. Results highlighted a significant improvement of the HE resistance of the alloy with the increase in the density of motionless dislocations induced by the pre-strain step. This was attributed to preferential hydrogen trapping on motionless dislocations leading to a decrease in the hydrogen amount in the grain boundaries. The measurements of hydrogen penetration depth by Scanning Kelvin Probe Force Microscopy (SKPFM) for cathodically charged samples provided further evidence to support these assumptions.

Experimental Characterization of Geometric Aspects of the Behavior of Magnetic Shape Memory Materials and Theoretical Interpretation

Small samples of a Ni-Mn-Ga single crystal of three different geometries were subjected to bending by applying a rotating magnetic field. The magneto-mechanical behavior of the sample in cantilever was analyzed and special attention was given to elongations and curvature along the deformation process. A sequence of 3000 images was made using a high-resolution camera and the data was analyzed using a code in Matlab. Furthermore, the geometric analysis showed that, when the magnetic field is equal to cero, the sample do not recover its original shape totally and the presence of a pseudo-elastic behavior was observed. Analysis and interpretation of the data allows the presentation of some hypotheses concerning to the crystalline structure and the role of dislocations, represented by a dislocation density, in the martensite phase of these materials. These hypotheses are discussed more formally in the second part of this paper. Some experiments are proposed that would give the opportunity to a wider theoretical knowledge of MSMM.

Experimental Characterization of Geometric Aspects of the Behavior of Magnetic Shape Memory Materials and Theoretical Interpretation

Small samples of a Ni-Mn-Ga single crystal of three different geometries were subjected to bending by applying a rotating magnetic field. The magneto-mechanical behavior of the sample in cantilever was analyzed and special attention was given to elongations and curvature along the deformation process. A sequence of 3000 images was made using a high-resolution camera and the data was analyzed using a code in Matlab. Furthermore, the geometric analysis showed that, when the magnetic field is equal to cero, the sample do not recover its original shape totally and the presence of a pseudo-elastic behavior was observed. Analysis and interpretation of the data allows the presentation of some hypotheses concerning to the crystalline structure and the role of dislocations, represented by a dislocation density, in the martensite phase of these materials. These hypotheses are discussed more formally in the second part of this paper. Some experiments are proposed that would give the opportunity to a wider theoretical knowledge of MSMM.

Chemical decomposition along dislocations during plastic deformation

Chemical decomposition of compound phases has been observed in many engineering alloys subjected to plastic deformation. However, little is known about the role of dislocations moving within compounds in their deformation-induced chemical decomposition. We report observations of chemical decomposition of compound crystals along dislocations with atomic resolution, which is inconsistent with the well-established dislocation theory. Transition-metal (TM) and rare-earth element (RE) atoms in the wake of moving partial dislocations are removed out of Mg-TM-RE compounds in magnesium alloys, producing magnesium nanoslices inside. Glide of partial dislocations forces local transformation from face-centered cubic to hexagonal structures with supersaturated TM and RE in Mg-TM-RE compounds. TM and RE atoms behind moving partial dislocations are then relocated into adjacent magnesium grains through pipe diffusion, under the driving force of supersaturated chemical potential. Our observations demonstrate atomically spatial coupling of individual moving dislocations and chemical decomposition within bulk crystals during plastic deformation, providing insights into dislocations and their roles in materials.

The thermal stability of epitaxial GeSn layers

We report on the direct observation of lattice relaxation and Sn segregation of GeSn/Ge/Si heterostructures under annealing. We investigated strained and partially relaxed epi-layers with Sn content in the 5 at. %-12 at. % range. In relaxed samples, we observe a further strain relaxation followed by a sudden Sn segregation, resulting in the separation of a β-Sn phase. In pseudomorphic samples, a slower segregation process progressively leads to the accumulation of Sn at the surface only. The different behaviors are explained by the role of dislocations in the Sn diffusion process. The positive impact of annealing on optical emission is also discussed.

Role of fluttering dislocations in the thermal interface resistance between a silicon crystal and plastic solid He4

A quantum solid (solid 4 He) in contact with a classical solid defines a new class of interfaces. In addition to its quantum nature, solid 4 He is indeed a very plastic medium. We examine the thermal interface resistance upon solidification of superfluid 4 He in contact with a silicon crystal surface (111) and show that dislocations play a crucial role in the thermal interface transport. The growth of solid 4 He and the measurements are conducted at the minimum of the melting curve of helium (0.778 K and ∼25 bar). The results display a first-order transition in the Kapitza resistance from a value of R K,L = (80 ± 8) cm 2 K/W at a pressure of 24.5 bar to a value of R K,S = (41.7 ± 8) cm 2 K/W after the formation of solid helium at ∼25.2 bar. The drop in R K,S is only of a factor of ∼2, although transverse phonon modes in solid 4 He now participate in heat transmission at the interface. We provide an explanation for the measured R K,S by considering the interaction of thermal phonons with vibrating dislocations in solid 4 He. We demonstrate that this mechanism, also called fluttering, induces a thermal resistance R F l ∝ N d T −6 , where T is the temperature and N d is the density of dislocations. We estimate that for dislocation densities on the order of ∼10 7 cm −2 , R F l predominates over the boundary resistance R K,S. These fundamental findings shed light on the role of dislocations and provide a quantitative explanation for previous experiments which showed no measurable change in the Kapitza resistance between Cu and superfluid 4 He upon solidification of the latter. This demonstrates the possibility of using dislocations as an additional means to tailor thermal resistances at interfaces, formed especially with a plastic material.

Role of dislocations and carrier concentration in limiting the electron mobility of InN films grown by plasma assisted molecular beam epitaxy

We report the molecular beam epitaxy growth of device quality InN films on GaN epilayer and nano-wall network (NWN) templates deposited on c-sapphire by varying the film thickness up to 1 μm. The careful experiments are directed towards obtaining high mobility InN layers having a low band gap with improved crystal quality. The dislocation density is quantified by using high resolution X-ray diffraction rocking curve broadening values of symmetric and asymmetric reflections, respectively. We observe that the dislocation density of the InN films grown on GaN NWN is less than that of the films grown on the GaN epilayer. This is attributed to the nanoepitaxial lateral overlayer growth (ELOG) process, where the presence of voids at the interface of InN/GaN NWN prevents the propagation of dislocation lines into the InN epilayers, thereby causing less defects in the overgrown InN films. Thus, this new adaptation of the nano-ELOG growth process enables us to prepare InN layers with high electron mobility. The obtained electron mobility of 2121 cm2/Vs for 1 μm thick InN/GaN NWN is comparable with the literature values of similar thickness InN films. Furthermore, in order to understand the reasons that limit electron mobility, the charge neutrality condition is employed to study the variation of electron mobility as a function of dislocation density and carrier concentration. Overall, this study provides a route to attaining improved crystal quality and electronic properties of InN films.

Electrical properties of planar AlGaN/GaN Schottky diodes: Role of 2DEG and analysis of non-idealities

A detailed study of the electrical properties of planar AlGaN/GaN Schottky diodes is presented, the focus being on the role of the two dimensional electron gas (2DEG) depletion and the diodes non-idealities in different voltage regimes. The 2DEG depletion behavior is inferred from the analysis of capacitance and current measurements with transition from vertical to lateral diode operation occurring at Vpinch-off = 4 V. In particular, the sub-micrometer depletion width, laterally extending from the edge of the Schottky contact under high reverse voltages, is evaluated on the basis of a simple fringe capacitance model. Current transport mechanisms are discussed, investigating the interrelation between 2DEG, Poole-Frenkel effect, and defects. With regard to defects, the role of dislocations in the AlGaN/GaN diode non-idealities, usually interpreted in terms of Schottky barrier inhomogeneities, is critically addressed. Photocurrent spatial mapping under high reverse voltage points out the not uniform electric field distribution around the Schottky contact and highlights the presence of local photo-conductive paths, likely associated with the dislocations near the edge of the Schottky contact.

Complete dislocation of the ulnar nerve at the elbow: a protective effect against neuropathy?

Recurrent complete ulnar nerve dislocation has been perceived as a risk factor for development of ulnar neuropathy at the elbow (UNE). However, the role of dislocation in the pathogenesis of UNE remains uncertain. We studied 133 patients with complete ulnar nerve dislocation to determine whether this condition is a risk factor for UNE. In all, the nerve was palpated as it rolled over the medial epicondyle during elbow flexion. Of 56 elbows with unilateral dislocation, UNE localized contralaterally in 17 elbows (30.4%) and ipsilaterally in 10 elbows (17.9%). Of 154 elbows with bilateral dislocation, 26 had UNE (16.9%). Complete dislocation decreased the odds of having UNE by 44% (odds ratio = 0.475; P = 0.028), and was associated with less severe UNE (P = 0.045). UNE occurs less frequently and is less severe on the side of complete dislocation. Complete dislocation may have a protective effect on the ulnar nerve. Muscle Nerve 56: 242-246, 2017.

The effect of plastic pre-strain on low-temperature surface carburization of AISI 304 austenitic stainless steel

Abstract In this paper, the influence of plastic pre-strain on low-temperature surface carburization of AISI 304 austenitic stainless steel was investigated. The materials were strained to different degrees of engineering strain including 5%, 10%, 15%, 20% and 25%. Low-temperature surface carburization of the pre-strained specimens was performed at 470 °C for 30 h. In order to elucidate the effect of plastic pre-strain on low-temperature surface carburization, optical microscopy (OM), X-ray diffractometer (XRD), scanning electron probe micro-analyzer (EPMA), microhardness tester and residual stress analyzer were used. Meanwhile, dislocation density and the martensite volume fraction of the pre-strained specimens were quantitatively measured by means of XRD analysis. Based on the experimental results, the role of dislocations and martensite on carbon diffusion and related phase transformation during low-temperature surface carburization was discussed. The results show that the thicknesses of the carburized layers are independent of the pre-strain degree. Dislocations and strain-induced martensite almost have no effect on the growth of the carburized layers at the given carburizing temperature. Increasing dislocation density does not promote the development of carbides formation, while strain-induced martensite does. Due to the supersaturated carbon atoms introduced into the materials during carburization, strain-induced martensite transforms into austenite. As a result of carburization, an outstanding strengthening layer of the material develops, and plastic pre-strain has no effect on strengthening of carburization.

Cold deformation behavior of the Ti-15Mo-3Al-2.7Nb-0.2Si alloy and its effect on α precipitation and tensile properties in aging treatment

The microstructure and texture were investigated in cold-rolled Ti-15Mo-3Al-2.7Nb-0.2Si alloy with differing degrees of reduction. The texture of the alloy was prone to rotate to <110> orientation and more sub-boundaries were produced with the increase of the cold-deformation. Precipitation during aging was influenced by the dislocations, sub-boundaries and other defects formed in cold rolling, for they would provide assisted-nucleation sites and decrease the duration of β→α transformation. The α precipitates with the scale of 100–400 nm in the cold-rolled 20% plus aging sample, brought about the high-strength of 1612 MPa and the elongation of 6.5%. The highest strength of 1770 MPa was obtained in the cold-rolled 50% plus aging sample, which has little plastic deformation, due to the coarse α phase precipitated in sub-boundaries. The role of dislocations and sub-boundaries in influencing α precipitation and the tensile properties of Ti-15Mo-3Al-2.7Nb-0.2Si alloy are described in this paper.

Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

Here we show that cold-rolling of tungsten (W) decreases the stable crack growth onset temperature. Furthermore, we show that stable crack growth is accompanied by crack bridging, which in turn is triggered by dislocation activity. The entire stable crack growth regime shows ductile intergranular fracture.Our ductilisation approach is the modification of microstructure through cold-rolling. In this work, we assess two different microstructures obtained from (i) cold-rolled and (ii) severely cold-rolled tungsten plates. From these plates, single-edge cracked-plate tension (SECT) specimens were cut and tested in the L-T direction. Crack growth resistance (R) curves were obtained using the direct-current-potential-drop method (DCPM). The experiments show the following results: cold-rolled plates are brittle at room temperature (RT), but show stable crack growth at 250°C (523K) and a fracture toughness, KIQ, of about 100MPa(m)1/2 at a crack extension, Δa, of 0.6mm. Severely cold-rolled tungsten plates show stable crack growth at RT and a fracture toughness, KIQ, of 100MPa(m)1/2 at a crack extension, Δa, of 0.3mm. Scanning electron microscopy (SEM) analyses of the stable crack growth region show intergranular fracture with microductile character.The question of why cold-rolling causes the stable crack growth onset temperature to decrease (or in other words, why cold-rolling causes the brittle-to-ductile transition (BDT) temperature to decrease) is discussed against the background of (i) intrinsic and extrinsic size effects, (ii) crystallographic texture, (iii) impurities and (iv) the role of dislocations. Our results suggest that the spacing between the dislocation nucleation sites (high angle grain boundaries (HAGBs) act as dislocation source) is the most important parameter responsible for the decrease of the stable crack growth onset temperature.

On the reliability of powder diffraction Line Profile Analysis of plastically deformed nanocrystalline systems.

An iron-molybdenum alloy powder was extensively deformed by high energy milling, so to refine the bcc iron domain size to nanometer scale (~10 nm) and introduce a strong inhomogeneous strain. Both features contribute to comparable degree to the diffraction peak profile broadening, so that size and strain contributions can be easily separated by exploiting their different dependence on the diffraction angle. To assess the reliability of Line Profile Analysis, results were compared with evidence from other techniques, including scanning and transmission electron microscopy and X-ray small angle scattering. Results confirm the extent of the size broadening effect, whereas molecular dynamics simulations provide insight into the origin of the local atomic, inhomogeneous strain, pointing out the role of dislocations, domain boundaries and interactions among crystalline domains.

Hydrogen diffusion in ultrafine-grained palladium: Roles of dislocations and grain boundaries

Diffusion behavior of hydrogen in ultrafine-grained palladium (Pd) is investigated by electrochemical permeation tests. The ultrafine-grained structure is produced by severe plastic deformation through high-pressure torsion (HPT). The diffusion behavior is compared with an annealed state with a coarse-grained structure and a cold-rolled state with dislocations and subgrain structures. Hydrogen permeation is analyzed in absorption step and desorption step at five different temperatures in the range of 15–35 °C. Hydrogen diffusion is retarded due to hydrogen trapping by dislocations. Grain boundaries act as rapid diffusion paths for hydrogen so that hydrogen diffusion is enhanced in the HPT-processed Pd samples with the ultrafine-grained structures.

The role of defects in lowering the effective polariton temperature in electric and optically pumped polariton lasers

The role of dislocations and defects in the material constituting the active region of an exciton-polariton laser has been examined and elucidated in the context of dynamic condensation and the temperature of the lower polariton condensate, TLP. For a GaN microcavity diode polariton laser operated at room temperature and characterized in this study, the value of TLP obtained from analysis of measured occupation in momentum space is 270 K, which is lower than the lattice temperature. Similar results from other room temperature GaN devices and GaAs-based polariton lasers operated at cryogenic temperatures are presented and discussed.

Electrical spin injection and detection of spin precession in room temperature bulk GaN lateral spin valves

We report the measurement of diffusive electronic spin transport characteristics in an epitaxial wurtzite GaN lateral spin valve at room temperature. Hanle spin precession and non-local spin accumulation measurements have been performed with the spin valves fabricated with FeCo/MgO spin contacts. Electron spin relaxation length and spin-flip lifetime of 176 nm and 37 ps, respectively, are derived from analysis of results obtained from four-terminal Hanle spin precession measurements at 300 K. The role of dislocations and defects in bulk GaN has also been examined in the context of electronic spin relaxation dynamics.

Ductilisation of tungsten (W): On the shift of the brittle-to-ductile transition (BDT) to lower temperatures through cold rolling

Here we show that cold rolling decreased the brittle-to-ductile transitions (BDT) temperature of tungsten (W). Furthermore, we show that the BDT temperature correlates with the grain size (the smaller the grain size, the lower the BDT temperature) following a Hall–Petch-like equation. This relation between the grain size and the BDT temperature is well known from ferrous materials and is generally accepted in the steel community.Our ductilisation approach is the modification of the microstructure through cold rolling. In this work, we assess three different microstructures obtained from (i) hot-rolled, (ii) cold-rolled, and (iii) hot-rolled and annealed (1h/2000°C, annealed in H2) tungsten plates. From these plates, Charpy impact test samples with dimensions of 1×3×27 mm3, without notch, were cut and tested in the L-S and T-S directions. The results show the following BDT temperatures: 675°C/948K (L-S, “annealed”), 375°C/648K (L-S, “hot-rolled”) and 125°C/398K (L-S, “cold-rolled”). The microstructure of the plates is analysed by means of SEM (EBSD: grain size, subgrains, texture, KAM), FIB (channelling contrast) and TEM analyses (bright field imaging).The question of how cold rolling decreases the BDT temperature is discussed against the background of (i) microcracking, crack branching, and crack bridging effects; (ii) texture effects; (iii) the role of dislocations; and (iv) the impact of impurities, micropores, and sinter pores. Our results suggest that the availability of dislocation sources (dislocation boundaries, grain boundaries; in particular, IDBs and HAGBs) is the most important parameter responsible for the increase of the cleavage resistance stress, σF, or the decrease of the BDT temperature, respectively.

Investigation of dislocations in Nb-doped SrTiO3 by electron-beam-induced current and transmission electron microscopy

This paper aims to clarify the electrical activities of dislocations in Nb-doped SrTiO3 substrates and the role of dislocations in the resistance switching phenomenon in Pt/SrTiO3 Schottky contacts. The electrical activities of dislocations have been studied by electron-beam-induced current (EBIC) technique. EBIC has found that dislocations can exhibit dark or bright contrast depending on their character and band bending condition. The character of dislocations has been analysed based on chemical etching and transmission electron microscopy. These data suggested that not all the dislocations contribute to the switching phenomenon. The active dislocations for resistance switching were discussed.

Identification of ERAD components essential for dislocation of the null Hong Kong variant of α-1-antitrypsin (NHK)

Misfolded proteins or orphan subunits of protein complexes are removed from the endoplasmic reticulum (ER) by ER-associated degradation (ERAD). ERAD requires dislocation, also known as retrotranslocation, of those unwanted proteins from the ER lumen to the cytosol for destruction by the proteasomes. Over one hundred ERAD component proteins have been identified but their role in dislocation remain poorly understood. Here we assessed the requirement of ERAD components for dislocation of NHK in live cells using our recently developed dislocation-induced reconstituted GFP (drGFP) assay. RNAi revealed that 12 out of 21 ERAD components examined are required for efficient dislocation of NHK among which Hrd1, Sel1L, GRP94 and p97/VCP are critically required. In addition, knockdown of 7 of the 21 components enhanced NHK dislocation. This study uncovers a complex functional network of proteins required for NHK dislocation.

Local dislocation creep accommodation of a zirconium diboride silicon carbide composite

A grain boundary sliding creep mechanism, accommodated by “mantle” dislocation activities, is shown to allow for large strain (ε>0.08) during the creep of a ZrB2–20% SiC composite at 1800°C. We characterized the local grain deformation behavior using high-resolution electron backscatter diffraction microscopy and an indentation deformation mapping technique. Deformation gradients near grain boundaries (“mantle”) produced geometrically necessary dislocation (GND) densities of 1×1011–1×1012cm−2, about two orders of magnitude above that of the grain interiors (“core”). A deviation from single-crystal grain core deformation defines the mantle where excess GNDs accommodate the grain deformation gradient. Evidence supporting grain boundary sliding as the primary contribution to the creep strain appears in our earlier publication, but we show here the role of dislocations in the deformation of the grain mantle as the rate-controlling accommodation step.