Glissile Dislocations Research Articles

Interaction of carbon with vacancy and self-interstitial atom clusters in α-iron studied using metallic–covalent interatomic potential

The presence of even small amount of carbon interstitial impurity affects properties of Fe and Fe-based ferritic alloys. From earlier experiments it follows that carbon exhibits considerably strong interaction with lattice defects and therefore influences their mobility, hence affecting the evolution of the microstructure under irradiation. This work is dedicated to understanding the interaction of carbon–vacancy complexes with glissile dislocation loops, which form in Fe, Fe-based alloys and ferritic steels under irradiation. We apply large scale atomistic simulations coupled with the so-called ‘metallic–covalent bonding’ interatomic model for the Fe–C system, known to be the most consistent interatomic model available today. With these techniques we have studied (i) the stability of vacancy–carbon clusters; (ii) the interaction of octahedral carbon with ½〈1 1 1〉 loops; (iii) possibility of the dynamic drag of carbon by ½〈1 1 1〉 loops and (iv) the interaction of ½〈1 1 1〉 loops with the most stable vacancy–carbon clusters expected to occur under irradiation. Finally, we have shown that carbon–vacancy complexes act as strong traps for ½〈1 1 1〉 loops.

Electron microscopic study of Ti 3Al microstructure after deformation at 1073–1273 K

An electron microscopic analysis of the dislocation structure of the Ti 3Al intermetallic after high-temperature deformation is performed. It is found that the microstructure of the samples deformed at T = 1073–1173 K contains mobile a and 2 c + a superdislocations. Dislocation configurations including a superstructural intrinsic stacking faults are revealed after deformation at T = 1273 K. A scheme of formation of these configurations by the interaction of a superdislocations in the basal and prism planes is proposed. Glissile dislocations with the Burgers vector [0 0 0 1] in prism planes and superdislocations with the Burgers vectors 1 / 3 〈 2 1 ¯ 1 ¯ 3 〉 and 1 / 3 〈 0 1 ¯ 1 3 〉 are observed at T = 1273 K. Possible models of the barrier destruction on 2 c + a superdislocations in pyramid planes are discussed considering results of the computer simulation of the superdislocation core structure in Ti 3Al.

A new look at attractive/repulsive junctions and cleavage crack formation in BCC materials

In this paper, the formation of sessile cleavage crack nuclei of the type a 〈 0 0 1 〉 from glissile dislocations of the type a 2 〈 1 1 1 〉 is considered. While an energy reduction is necessary, formation of a nucleus necessarily involves interaction forces. The propensity for cleavage crack formation in BCC crystals is explained in terms of the fact that glide dislocations inevitably interact to form microcrack nuclei. The calculated dislocation interactions are consistent with the observation that cleavage cracking in BCC alloys on {0 0 1}-type planes requires plastic deformation. Continued growth of a microcrack is examined in terms of distance between pre-existing sessile dislocations and the external force necessary to drive glissile dislocations to a common junction. Pre-existing sessile dislocations significantly alter the orientation dependence of the glide force on the glissile dislocations from that computed for glissile/glissile interactions. An important feature of this work was to provide a general framework for the computation of dislocation interactions using closed-form solutions in a computational environment.

Glissile Dislocations with Transient Cores in Silicon

We report an unexpected characteristic of dislocation cores in silicon. Using first-principles calculations, we show that all of the stable core configurations for a nondissociated 60 degrees dislocation are sessile. The only glissile configuration, previously obtained by nucleation from surfaces, surprisingly corresponds to an unstable core. As a result, the 60 degrees dislocation motion is solely driven by stress, with no thermal activation. We predict that this original feature could be relevant in situations for which large stresses occur, such as mechanical deformation at room temperature. Our work also suggests that postmortem observations of stable dislocations could be misleading and that mobile unstable dislocation cores should be taken into account in theoretical investigations.

Swelling and microstructure of austenitic stainless steel ChS-68 CW after high dose neutron irradiation

Austenitic stainless steel ChS-68 serving as fuel pin cladding was irradiated in the 20% cold-worked condition in the BN-600 fast reactor in the range 56–84 dpa. This steel was developed to replace EI-847 which was limited by its insufficient resistance to void swelling. Comparison of swelling between EI-847 and ChS-68 under similar irradiation conditions showed improvement of the latter steel by an extended transient regime of an additional ∼10 dpa. Concurrent with swelling was the development of a variety of phases. In the temperature range 430–460 °С where the temperature peak of swelling was located, the principal type of phase generated during irradiation was G-phase, with volume fraction increasing linearly with dose to ∼0.5% at 84 dpa. While the onset of swelling is concurrent with formation of G-phase, the action of G-phase cannot be confidently ascribed to significant removal from solution of swelling-suppressive elements such as silicon. A plausible mechanism for the higher resistance to void swelling of ChS-68 as compared with EI-847 may be related to an observed higher stability of faulted dislocation loops in ChS-68 that impedes the formation of a glissile dislocation network. The higher level of boron in ChS-68 is thought to be one contributor that might play this role.

Two different types of shear-deformation behaviour in Au–Cu multilayers

Localised shear deformation of a material is usually identified as a particular feature of deformation inhomogeneity. Here, we show two different types of shear deformation-behaviour that occurred in Au–Cu multilayers subjected to microindentation load, namely, a cooperative-layer-buckling-induced shear banding in a nanoscale multilayer and a direct localised shearing across a layer interface along a shear plane in a submicron-scale multilayer. Theoretical analysis indicates that the formation of the two different types of shear deformation in the multilayers depends on a competition between the dislocation-pile-up-induced stress concentration at the layer interface and the barrier strength of the layer interface for glissile dislocation transmission.

Formation of stable sessile interstitial complexes in reactions between glissile dislocation loops in bcc Fe

Clusters of self-interstitial atoms (loops) are commonly observed in the microstructure of irradiated metals. These clusters can be formed directly in high-energy displacement cascades or by growth as a result of interaction between individual self interstitials. The majority of these clusters have features of glissile dislocation loops and migrate by fast one-dimensional glide. In this paper, we present results of a systematic molecular dynamics (MD) study of reactions involving glissile interstitial loops. By the example of bcc iron we demonstrate that the reactions can produce a number of specific, stable microstructural features, with different properties compared to the reactants. Namely, the reactions between the most common glissile clusters of 〈1 1 1〉 crowdions can result in coarsening or formation of immobile self interstitial complexes. The coarsening leads to a decrease of the total dislocation line length and therefore is favourable. The structure and stability of the junction formed in the reactions has been studied using many-body potentials and density functional theory (DFT) techniques. No evidence of the formation of a 〈1 0 0〉 loop from two glissile 〈1 1 1〉 clusters was found among the studied reactions. The immobile self interstitial complexes that form as a result of these reaction have, however, high binding energies, of the order of tens of eV, implying that a relatively long life time should be assigned to the resulting configurations and therefore that such objects are expected to contribute to the evolution of the microstructure under irradiation.

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: IV. Thermoactivated straightening of dislocations along a preferred direction in TiAl

As a result of experiments which include plastic deformation and subsequent heating without stress, self-blocking of dislocations in TiAl has been revealed. The transformations of glissile dislocations into dislocation barriers has been observed both for superdislocations with Burgers vectors <101] and 1/2<112] and for single dislocations. The preliminary deformation was performed at room temperature; the heating without stress, at temperatures both below and above the peak temperature T max in the temperature dependence of the yield stress σy(T). With a proper choice of the temperature of heating and its duration, it proved to be possible to fix the initial stages of the straightening of single dislocations along a preferred direction parallel to their Burgers vector and the subsequent formation of long blocked dislocations. As a result of TEM analysis, it has been shown that the barriers were formed during heating without stress but they were not destroyed. It has been revealed that the barriers with a total Burgers vector <101] in TiAl, in contrast to Ni3Al, remain indestructible even when the experiments included a repeated deformation.

Computer simulation of primary damage creation in displacement cascades in copper. I. Defect creation and cluster statistics

Atomic-scale computer simulation has been used to investigate the primary damage created by displacement cascades in copper over a wide range of temperature (100K⩽T⩽900K) and primary knock-on atom energy (5keV⩽EPKA⩽25keV). A technique was introduced to improve computational efficiency and at least 20 cascades for each (EPKA,T) pair were simulated in order to provide statistical reliability of the results. The total of almost 450 simulated cascades is the largest yet reported for this metal. The mean number of surviving point defects per cascade is only 15–20% of the NRT model value. It decreases with increasing T at fixed EPKA and is proportional to (EPKA)1.1 at fixed T. A high proportion (60–80%) of self-interstitial atoms (SIAs) form clusters during the cascade process. The proportion of clustered vacancies is smaller and sensitive to T, falling from 30% to 60% for T⩽600K to less than 20% when T=900K. The structure of clusters has been examined in detail. Vacancies cluster predominantly in stacking-fault-tetrahedron-type configurations. SIAs tend to form either glissile dislocation loops with Burgers vector b=1/2<110> or sessile faulted Frank loops with b=1/3<111>. Despite the fact that cascades at a given EPKA and T exhibit a wide range of defect numbers and clustered fractions, there appears to be a correlation in the formation of vacancy clusters and SIA clusters in the same cascade. The size and spatial aspects of this are analysed in detail in part II [unpublished], where the stability of clusters when another cascade overlaps them is also investigated.

Influence of carbon addition on neutron-induced void swelling of Fe–15Cr–16Ni–0.25Ti model alloy

Addition of 0.05 wt% C to a model Fe–15Cr–16Ni–0.25Ti quaternary model alloy leads to a reduction in neutron-induced swelling at 430 °C. The transient regime of swelling is prolonged by carbon addition, most strongly at lower dpa rates. Contrary to the swelling behavior observed in carbon-free Fe–15Cr–16Ni and Fe–15Cr–16Ni–0.25Ti model alloys irradiated in the same experiment, Fe–15Cr–16Ti–0.25Ti–0.05C does not exhibit a strong dependence of swelling on dpa rate. It appears that carbon’s role, while not yet well-defined, operates via a solute-based or TiC complex mechanism rather than by a precipitate-based mechanism. A model is proposed whereby carbon stabilizes loop microstructures against unfaulting, where unfaulting is known to be a prerequisite to formation of the glissile dislocation network needed to establish a high swelling rate. This stabilization is proposed to counteract the tendency of loop unfaulting to occur more strongly at low dpa rates.

HRTEM study of defects in twin boundaries of ultra-fine grained copper

Twin boundaries (TBs) in ultra-fine grained (UFG) copper prepared by powder metallurgy were investigated using high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA). Specimens were analyzed both before and after mechanical deformation (compression of 40%) and emphasis placed on the study of TB defects. Twin boundaries in the as-processed specimens are mainly disoriented from the perfect Σ3 coincidence. They present a faceted structure with coherent {111} and incoherent {112} facets. The latter have a 9R structure and the {111}/{112} junctions are associated with sessile dislocations of Frank type . Shockley glissile dislocations with Burgers vector of type are also present. This microstructure is interpreted in terms of the absorption and decomposition at room temperature of lattice dislocations (60° type). After mechanical deformation, an enrichment of twins at dislocations and a decrease of step density and height is observed and quantified by statistical analysis. Deformation mechanisms of UFG copper are discussed in light of these observations.

Strain hardening due to deformation twinning in α-titanium: Mechanisms

Novel experiments were conducted to elucidate the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature. Orientation-imaging microscopy (OIM), microhardness, and nanohardness evaluations were employed in conjunction with optical microscopy and quasi-static compression testing to obtain insight into the deformation mechanisms. Hardness measurements revealed that the newly formed deformation twins were harder than the matrix. This observation is perhaps the first experimental evidence for the Basinski mechanism for hardening associated with twinning, arising from the transition of glissile dislocations to a sessile configuration upon the lattice reorientation by twinning shear. This work also provided direct evidence for two competing effects of deformation twinning on the overall stress-strain response: (1) hardening via both a reduction of the effective slip length (Hall-Petch effect) and an increase in the hardness of twinned regions (Basinski mechanism) and (2) softening due to the lattice reorientation of the twinned regions.

Dislocation–obstacle interactions: Dynamic experiments to continuum modeling

Incorporating the interaction of dislocations with obstacles remains a challenge in the development of predictive large-scale plasticity models. The need is particularly important in the elastic–plastic transition region where these interactions can dominate the behavior. By combining post-mortem analysis with dynamic straining in the transmission electron microscope, the atomic processes governing glissile dislocation reactions and interactions with obstacles has been determined. This information has been incorporated at least phenomenologically in models to assess the macroscopic stress–strain response. Two examples will be presented to demonstrate the methodology. The first example considers the interaction of dislocations with small vacancy Frank loops and the formation of defect-free channels in copper, and the second with the influence of imperfect annealing twin boundaries on the macroscopic stress–strain response in silver. In both examples, the importance of grain and twin boundaries as dislocation sources will be demonstrated.

Microstructural changes taking place during the thermo-mechanical processing and cold working of steel 18Cr18Mn0.5N

Microstructural processes taking place during cold deformation were investigated using optical and transmission electron microscopy. Different interactions which contribute to the strengthening, i.e. slip/slip, slip/twin and twin/twin interactions, were observed with the following results: the partial dislocations were produced at twin boundaries, packets of stacking faults and consequently twins were formed by slip of partials. The high density of glissile dislocations and twins and the large number of their possible interactions resulted in high strength, ductility and toughness. The mechanical properties were improved further by the refinement of austenite grains using high temperature thermo-mechanical processing. Strain ageing of deformed steel before high temperature annealing did not influence recrystallization processes and did not decrease the grain size.

Orientation dependent elastic interaction between a truncated stacking fault tetrahedron and a glissile dislocation

The orientation dependence of elastic interaction between a stacking fault tetrahedron (SFT) and mobile dislocations is investigated for the possibility of unfaulting and subsequent absorption of the SFT. The obtained result indicates that 60° dislocations have stronger interaction with the spontaneously truncated SFT than pure screws or edges. Due to the high activation energy, the collapse and absorption of the SFTs seems to be limited to the cases where the approaching dislocations along 〈1 1 0〉 directly cut the SFTs symmetrically. The anisotropic energetics can contribute to the spatially limited growth of defect-cleared channels observed in the irradiated materials.

Calculations of Lengths of Dislocation Combinations in FCC Crystals

In the present work, lengths of dislocation combinations formed as a result of interaction of reacting dislocations in noncoplanar glide systems are calculated. A new model is suggested in which the geometry of dislocation combination is considered for an arbitrary asymmetrical intersection of dislocation segments. The lengths of dislocation combinations are determined on the example of one dislocation reaction. The statistics of an arbitrary intersection of the reacting dislocations are considered for an arbitrary (from screw to edge) orientation of a glissile dislocation.

Motion and rotation of small glissile dislocation loops in stress fields.

We derive here for the first time the equations that describe the combined motion and rotation of small prismatic dislocation loops in stress fields. When the applied torque is balanced by the self-torque of the loop, we show how the solution can be obtained for the loop orientation, and how this orientation affects the glide force on the loop.

Superdislocation core structure in type I and type II pyramidal planes of Ti3Al intermetallic: Glissile dislocations and dislocation barriers

The energies of surface defects in the basal and prismatic planes, as well as in the planes of type I and type II pyramids, are calculated by using N-particle interaction potentials for the Ti3Al intermetallic with the D019 superlattice. The core structure of 2c + a edge and screw glissile and sessile (barrier-forming) dislocations in pyramidal planes of type I, \(\{ 20\overline 2 1\} \), and type II, \(\{ \overline 1 \overline 1 21\} \), in Ti3Al is analyzed.

Coherent motion of interstitial defects in a crystalline material

Thermally activated Brownian motion of interstitial defects is one of the factors driving the evolution of microstructure of crystalline metals under irradiation. In the limit of relatively small system size the motion of defects can be followed on the atomistic scale by using molecular dynamics. However, understanding the kinetics of evolution of microstructure requires investigating how defects migrate and interact on a scale which is substantially greater than that accessible to molecular dynamics. This paper shows how mobile interstitial defects can be described by quasiparticle solutions of the multistring Frenkel–Kontorova (MSFK) model, which prove the equivalence between the crowdion and the glissile dislocation loop representations of small interstitial clusters. An exact solution of the MSFK model is found for the case of an infinite straight edge dislocation. This solution illustrates the fundamental link between the concepts of a crowdion and a dislocation in a crystalline material.

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10 −7 s −1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 〈110〉(001) system are about equally abundant, whereas 〈110〉{111} and [101] in (1̄31) to (2̄42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.