Individual Screw Dislocations Research Articles

Quantitative tests revealing hydrogen-enhanced dislocation motion in α-iron

Hydrogen embrittlement jeopardizes the use of high-strength steels in critical load-bearing applications. However, uncertainty regarding how hydrogen affects dislocation motion, owing to the lack of quantitative experimental evidence, hinders our understanding of hydrogen embrittlement. Here, by studying the well-controlled, cyclic, bow-out motions of individual screw dislocations in α-iron, we find that the critical stress for initiating dislocation motion in a 2 Pa electron-beam-excited H2 atmosphere is 27-43% lower than that in a vacuum environment, proving that hydrogen enhances screw dislocation motion. Moreover, we find that aside from vacuum degassing, cyclic loading and unloading facilitates the de-trapping of hydrogen, allowing the dislocation to regain its hydrogen-free behaviour. These findings at the individual dislocation level can inform hydrogen embrittlement modelling and guide the design of hydrogen-resistant steels.

Evolution of superdislocation structures during tertiary creep of a nickel-based single-crystal superalloy at high temperature and low stress

A second-generation nickel-based single crystal superalloys DD6 were creep tested in the [001] direction (within 15°) at 1100 °C/140 MPa. The specimen tested until rupture was investigated using scanning and transmission electron microscopy to determine the evolution of the dislocation behaviors during tertiary creep. It was found that the tertiary creep deformation was highly localized and inhomogeneous along the gauge length, and the types of superdislocations in γ′ rafts varied with the distances from the rupture surface, including individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. It can be concluded that γ′ rafts shearing events occur in the following sequence with the evolution of tertiary creep: individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. The origin of these transformations of superdislocation types and its influence on tertiary creep rate are discussed. It is proposed that at the microscopic level, a more reasonable explanation for the strain softening mechanism during the tertiary creep of nickel-based superalloys at high temperatures and low stresses is the emergence of new superdislocation types with higher mobility rather than the density rise of a single type of superdislocation produced during the later secondary creep stage.

On the stress discrepancy at low-temperatures in pure iron

We present a complete analysis of the velocity of individual screw dislocations in pure iron, as a function of stress and temperature, by means of in situ straining experiments in a transmission electron microscope. The results show a very pronounced deviation from the classical laws of thermodynamics, which is at the origin of the discrepancy between experimental and calculated deformation stresses at low temperature. This strongly supports the occurrence of large quantum effects proposed in a recent theoretical study.

Kinetics of dislocations in pure Fe. Part II. In situ straining experiments at low temperature

In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250 K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {1 1 0} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250 K. They have been interpreted in terms of a locking–unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250 K can account for the surprisingly low value of the flow stress extrapolated to 0 K (much lower than the theoretical Peierls stress).

Surface effects on the growth of solution processed pentacene thin films

13,6- N-Sulfinylacetamidopentacene (SAP) has been transferred on solid substrates both by water-rich environment and organic solvents without altering his chemical structure. Thermal conversion of SAP to pentacene leads to crystals showing similar features of vacuum-sublimated pentacene both in the bulk and thin-film phases. The thin-film phase is strongly affected by the substrate surface properties. Low energy Si–CH 3 rich surfaces allow for the formation of compact micrometric crystals following a Volmer–Weber like growth with orientation suitable for field-effect carrier mobility. Screw dislocations along with straight edges are found on these systems as an indication of a loose interlayer interaction with respect to the lateral π–π intermolecular forces. In contrast, higher energy SiOH rich surfaces result in non-compact systems (Stransky–Krastanov like growth) arising by in-space randomly oriented crystals as well as by individual screw dislocations.

Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centred cubic metals

We propose a mesoscopic model that explains the discrepancy (at a factor of 2–3) between experimentally measured yield stresses of bcc metals at low temperatures and typical Peierls stresses determined by atomistic simulations of isolated screw dislocations. The model involves a Frank–Read-type source emitting dislocations that become pure screws at a certain distance from the source and, owing to their high Peierls stress, control its operation. However, due to the mutual interaction between emitted dislocations, the group consisting of both non-screw and screw dislocations can move at an applied stress that is a factor of 2–3 lower than the stress needed for the glide of individual screw dislocations.

Asymmetry of the screw dislocation core structure in cadmium sulfide crystals

The local symmetry of dislocation centers in CdS crystals was studied by piezospectroscopy of space resolved photoluminescence of individual screw dislocations. The limited set of dislocation Cs centers (2 from a total of 6) was found to be characteristic for each screw dislocation and that set determines the asymmetry of the core structure. The space- and time-nonstationarity of the dislocation domains observed in the polarized emission of dislocations are explained by the reorientation of one-dimensionally ordered Cs centers in the core of the screw dislocation gliding by the Peierls mechanism.

An electron microscope study of the origin of latent hardening in NaCl

Abstract Latent hardening of NaCl single crystals, which is associated with the intersection of oblique {110} slip systems, has been studied by an electron microscope replica technique. The method images both the forest dislocations and the slip lines of individual screw dislocations moving through the forest dislocations so that the dislocation mechanisms could be studied in some detail. Accordingly the latent hardening is essentially determined by the long-range elastic interaction of the intersecting dislocations and the formation of junctions. Both processes generate obstacle structures with long-range stress fields, which limit the slip path of the gliding dislocations. The overcoming of the obstacle structures results in an athermal contribution to latent hardening.

Luminescence studies of individual dislocations in II-VI (ZnSe) and III-V (InP) semiconductors

Results are presented of high-resolution luminescence studies from individual dislocations and related defects in ZnSe and InP performed in a transmission electron microscope. In the case of ZnSe unusual luminescence bands (Y at 2.60 eV and S at 2.52 eV) originally observed in photoluminescence studies are attributed to dislocations. In some instances, complete quenching of the excitonic transitions was observed to correlate with the presence of Y emission from complex dislocation tangles. In the case of individual screw dislocations this quenching of the exciton luminescence was found to be variable; for example reduction of the exciton signal was not always observed. For InP, donor-exciton-related transitions were quenched at individual screw dislocations. Donor-acceptor pair/free-to-bound and deep level (band C) transitions were unaffected. For the case of InP, unlike ZnSe, no dislocation-related luminescence was observed within the system detection limit (0.7-4.0 eV).

Dislocation behaviour in annealed and aged LiF crystals

AbstractParallel investigation of the magnitude of the critical strear stress τ0 and dynamic characteristics of individual screw dislocations at temperatures 4.2, 77, and 300 K has been carried out in LiF crystals after annealing and after long aging at room temperature. It has been established that in impure LiF crystals, even at such low temperatures as room temperature (0.25Tmelt), diffusion processes take place which lead to quantitative and qualitative changes of the structure of obstacles determining macroscopic and microscopic mechanical properties.

Influence of temperature on the initial stage of plastic deformation of LiF crystals

The temperature dependence of the critical resolved shear stress, the stress relaxation parameters, and effects of the strain rate on the flow stress at the initial stage of plastic deformation in the temperature range 1.7 to 330 K are studied on LiF crystals containing below 10−3 wt% of divalent metal impurities. The velocity of individual screw dislocations versus stress in the range of 10−8 to 1 cm/s at temperatures 4.2, 77, and 300 K is measured. The experimental results allow to conclude that the plasticity of LiF crystals can be described by at least two basic thermally activated mechanisms in the temperature range studied. One, the overcoming of the Peierls barriers characterized by a Peierls stress τ*p = 35 MPa and an ultimate activation energy of ΔG0 = 0.08 eV prevails at lower temperatures. The other is dominant in the temperature range from about 100 K to room temperature (the overcoming by mobile dislocations of randomly arrayed impurities, as well as strain induced defects). [Russian Text Ignored].

γ′-Ni<SUB>3</SUB>(Al, Ti) 単結晶中の転位運動のHVEM観察

Dislocation movements during plastic deformation of L12-ordered Ni3(Al, Ti) single crystals at temperatures between 90 and 873 K have been investigated in-situ by high voltage electron microscopy. Either or both of the two slip systems, {111} octahedral slip and {100} cube slip, operated depending upon the testing temperature and the orientation of tensile axis of the specimen. Movements of {111} slip dislocations were quick and jerly. Once screw dislocations ceased to move, they were immobilized instantaneously. Movements of {100} slip dislocations were smooth and viscous showing high friction stress for the cube slip. The present observations have shown that the positive temperature dependence of the yield stress of γ′-Ni3(Al, Ti) can not be attributable to the intersections between {100} and {111} slip dislocations but to the increasing difficulty of the movements of individual screw dislocations in the {111} slip system with increasing temperature.

The introduction of dislocations and slip bands in molybdenum single crystals

The characteristics of selectively introduced slip bands and individual dislocations were investigated. It was found that slip bands produced by indenting the (112) surface had three types of Burgers vectors concentrated at the two edges of the slip band while the middle of the slip band consisted of primarily the ± ½ [111] Burger's vector. Scratching the specimen with a needle almost parallel to the (112) surface in the [111] direction produced individual screw dislocations near the scratch mark. Similar scratches normal to the [111] direction produced fine slip bands with a ± 1/2 [111] Burgers vector.

The stress field of a pile-up of screw dislocations at a cylindrical inclusion

The positions of the individual screw dislocations piled-up against a non-coherent cylindrical inclusion that has elastic constants different from those of the matrix have been calculated. The non-constant stress field close to the particle has been taken into account as well as the image stresses of the dislocations. For particles of higher hardness, this leads to an increase in dislocation spacing and an accompanying decrease in stress concentration at the particle-matrix interface.