Dislocation Jogs Research Articles

Modelling the spatial evolution of excess vacancies and its influence on age hardening behaviors in multicomponent aluminium alloys

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys.

The influence of processing methods on creep of wrought and additively manufactured CrCoNi multi-principal element alloys

The influence of vacuum arc melting and mechanical processing (wrought) versus additive manufacturing (AM) on the creep behavior of multi-principal element alloys (MPEA) is investigated in this study. Annealed wrought and hot isostatically pressed (HIP) AM CrCoNi were creep tested under constant tensile stress of 40 to 200 MPa at temperatures of 1023 to 1173 K. Stress exponents of 4.5 ± 0.2 and 5.9 ± 0.1 and activation energies ranging from 240 to 259 and 320 to 331 kJ/mol were found for wrought and AM CrCoNi, respectively. The results indicate that the AM material exhibits superior creep resistance and inferior creep ductility compared to the wrought alloy. This difference is attributed to the AM material having a higher percentage of Cr-rich oxides, a smaller total low angle grain boundary (LAGB) length on a percentage basis, and a greater total twin boundary (TB) length on a percentage basis. The AM and wrought materials have similar grain sizes; however, the smaller LAGB length and greater TB length in the AM material reduce slip transmission on the other side of the boundaries and contribute to strength. The dislocation structures of the AM and wrought materials consist of individual curved dislocations with dislocation multijunctions and jogs, which is similar to the arrangements previously observed in CrMnFeCoNi.

Breaking the high-temperature strength-ductility trade-off in TiAl alloys through microstructural optimization

Designing a novel microstructure with higher strength-ductility than lamellar microstructure (LM), which has been considered the most valuable engineering application in the service temperature range, is desired for lightweight TiAl alloys. Our prior publication reported a three-phase tri-modal (T-T) structure for Ti-43.5Al-4Nb-1Mo-0.1B(.at%) with superior mechanical properties than LM at room temperature (Ref: Acta Mater. 225 (2022) 117,585). However, its strength-ductility at working temperature has yet to be fully exploited. This work fills the much-needed gap. The qualitative and quantitative optimizations indicate that the T-T structure's yield strength increases first and then decreases with increasing pearlitic-like microstructure (PM, which consist of three-phase (α2, γ and βo phase) with clear orientation relationships that form in a lamellar/cellular manner) content while the ductility keeps rising. When the T-T structure contains ∼20 % PMs, it exhibits a yield strength about 130MPa higher than LM coupling with doubled ductility at 750 °C. PMs can induce a prominent twinning induced plasticity and dislocation jog dragging (TWIP&DJD) effect, resulting in better strength-ductility below 700°C; above this temperature, the TWIP&DJD effect significantly plasticizes the alloys. The higher strength of the T-T structure at 750 °C is caused by hetero-deformation-induced strengthening between LMs and PMs, which cannot be offset by the plasticizing caused by PM when the volume fraction of PMs is less than 50 %. This study demonstrates that introducing an appropriate amount of plasticizing structures (similar to PM) with excellent work hardening capacity is a promising strategy for enhancing the strength-ductility synergy of TiAl and possibly other brittle materials.

High strain rate deformation behavior of Al0.65CoCrFe2Ni dual-phase high entropy alloy

An industrially-cast high entropy alloy with a nominal composition of Al0.65CoCrFe2Ni, having FCC and BCC phases, was compressed at high strain rates using a split Hopkinson pressure bar under a range of test temperatures. The strain rate sensitivity and strain hardening behaviour of the alloy were analyzed. The alloy exhibited high strain hardening and excellent strain rate sensitivity owing to its large solid solution strengthening and low athermal strength contributors. The alloy also displayed outstanding dynamic strain rate sensitivity compared to similar dual-phase high entropy alloys of the AlCoCrFeNi alloy system. The alloy exhibited excellent resistance to thermal softening at high strain rate loading due to higher phonon drag associated with an increase in the operating temperature. The substructures formed in the alloy during high strain rate deformation at room temperature comprised of Lomer-Cottrell (L-C) locks, deformation twins, planar deformation bands, and high dislocation pile-ups at interphase boundaries and subgrain boundaries. The high strain rate deformation at high temperatures resulted in the formation of dislocation cells, kinks and jogs, enabling the alloy to retain strain hardening behavior. The Johnson-Cook (J-C) model accurately predicted the plasticity behavior of the alloy under high strain rate loading.

A TEM Study on a Polycrystalline Olivine Sample Deformed in a D-DIA under Mantle Conditions

We carried out an electron microscopy study on a polycrystalline olivine sample that was deformed with multiple deformation cycles under controlled differential stresses and strain rates at high pressures and high temperatures. Low-angle backscattered electron images thereof showed randomly oriented grains. Most of the grains were about 10–20 μm wide. The grains were irregular with wavy grain boundaries, indicating high grain boundary mobility during deformation. Transmission electron microscopy (TEM) images showed complex dislocation microstructure characteristics of high temperature, high pressure, and high strain. Free dislocations were predominantly either short and straight screw dislocations or curved dislocations with mixed screw and edge characters. Many of them split into partial dislocations. The differential stress estimated with the free dislocations was ~780 MPa, which was close to the value of differential stress attained in the final deformation cycle. We also observed dense dislocation tangles, which formed dislocation cell substructures under high strain. The existence of dislocation loops and jogs indicated significant climbing activity, providing evidence for high-temperature creep as the dominant deformation mechanism. All of the dislocations observed in this study were exclusively with a [001] Burgers vector. Dislocations with a [100] Burgers vector were absent, suggesting that the activity of the a-slip (i.e., (010)[100] and (001)[100] slip systems) was completely suppressed. These observations support a conclusion that was reported based on an X-ray texture analysis, which considered that a high pressure promotes the activities of the c-slip (i.e., (010)[001] and (100)[001] slip systems). It appears that the transition from the a-slip to the c-slip was complete with multiple deformation cycles at a relatively lower pressure of 5.1 GPa than previously thought, corresponding to a depth of 165 km in the mantle.

In situ observation of the effect of the twin boundary orientation on the mechanical properties of single crystalline Ni

The construction of twin boundaries (TBs) in materials is a remarkable way of promoting their strength and ductility. However, the effects of TB orientation on the mechanical properties have not been reported experimentally so far. Using a state-of-the-art in situ tensile stage equipped in a transmission electron microscope, uniaxial tensile tests were performed on three single-crystalline Ni samples with TB parallel and perpendicular to the tensile direction and no TB. The results showed that the uniform tensile elongation strongly depends on TB, 120% for the perpendicular TB sample, 99% for the parallel TB sample, and only 55% for the no TB sample. In addition, dislocation interaction before reaching the perpendicular CTB contribute to cross-slip and dynamic formation of dislocation jogs, thereby improving strain hardening and resulting in a large uniform tensile elongation.

Deformation twinning in Ti48.9Zr32.0Nb12.6Ta6.5 medium entropy alloy

Abstract The {112} β deformation twinning has never been confirmed in refractory high or medium entropy alloys (HEAs or MEAs) at room temperature. Here a Ti48.9Zr32.0Nb12.6Ta6.5 MEA shows {112} β twins was designed based on the d-electron alloy design method learned from β Ti alloys. The solution-treated (ST) samples show equiaxed grains, single β phase, homogenous chemical composition and good tensile properties. Transmission electron microscopy (TEM) investigation shows that the ST samples is free of athermal ω phase. Following TEM results revealed abundant dislocations and their structures such as jogs, loops, cross-slips and slip bands in the deformed samples. The {112} β twins were observed in the fractured samples, with interfacial ω phase on the twin boundary. Elemental segregation has not been detected in the twinned region by scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) mapping. The interaction between dislocations and twins was revealed by low-angle annular dark field (LAADF), thus twins in addition to the dislocation jogs, loops, cross-slips and slip bands, contribute to the work hardening in the present MEA. Deformation twinning in HEAs and MEAs has been discussed and twins can be expected in these alloys when the effects of their constituent elements were carefully considered. In particular, the present strategy of starting from β Ti alloys can be applied to design refractory HEAs and MEAs with desirable deformation microstructures.

Gradient Microstructure Induced by Surface Mechanical Attrition Treatment (SMAT) in Magnesium Studied Using Positron Annihilation Spectroscopy and Complementary Methods.

Surface mechanical attrition treatment (SMAT) was used to generate a gradient microstructure in commercial grade magnesium. Positron annihilation lifetime spectroscopy and variable energy positron beam measurements, as well as microhardness tests, electron backscatter diffraction, X-ray diffraction, and electrochemical corrosion tests, were used to investigate the created subsurface microstructure and its properties. It was found that SMAT causes an increase in dislocation density and grain refinement which results in increased hardness of the subsurface zone. The mean positron lifetime values indicate trapping of positrons in vacancies associated with dislocations and dislocation jogs. The increase of the SMAT duration and the vibration amplitude influences the depth profile of the mean positron lifetime, which reflects the defect concentration profile. Electrochemical measurements revealed that the structure induced by SMAT increases the susceptibility of magnesium to anodic oxidation, leading to the enhanced formation of hydroxide coverage at the surface and, as a consequence, to the decrease in corrosion current. No significant effect of the treatment on the residual stress was found.

A phase field model for dislocation climb under irradiation: Formalism and applications to pure bcc iron and ferritic alloys

A phase-field model allowing to describe dislocation climb under irradiation is presented. Whereas previous phase-field models of the literature devoted to this phenomenon only take into account vacancies, our approach includes the effect of self interstitial atoms (SIAs) as required in the context of irradiated metals. Beyond the fact that it rigorously ensures the balance between the quantity of point defects, vacancies and SIAs, absorbed or emitted by the loop and the loop evolution, the present model has several originalities. First, it is capable to quantify the climb rate for systems far from equilibrium, which is commonly the case under irradiation. This required supplemental methodological developments since we clearly show that a mere generalization of existing phase-field models is not satisfactory to tackle this specificity. Secondly, it alleviates the often adopted assumption of perfect sink through the introduction of a kinetic parameter related to the dislocation jog density. A preliminary generic study of dislocations considered as nonperfect sinks leads to nonintuitive results, since the climb rate decreases when the dislocation jog density increases. Thirdly, the possibility to consider different types of interacting microstructural defects in the model allows to show the significant role of the point defect thermal equilibrium fractions on the climb rate in pure bcc iron under irradiation conditions. Finally, the climb model is coupled to the chemical diffusion equations in the same phase-field formalism. For this purpose, a multi-time step algorithm is proposed in order to couple phenomena with different characteristic time scales by several orders of magnitude, namely climb, point defect and chemical diffusion. It allows to study the interaction between the motion of the dislocation and the well-known phenomenon of radiation induced segregation in a Fe–Cr alloy. It is shown that the shape and size of the solute atmosphere can strongly depend on the dislocation motion under irradiation.

Study of the extended defects in CdZnTe crystal

Etch pit real-time observation (EPRTO) technique was employed to study the extended defects in CdZnTe materials. By tracking the pyramidal etch pits on the (1 1 1) A surface etched by Nakagawa etchant, the microstructures inside the extended defects were revealed. The extending directions of the extended defects and corresponding crystallographic orientations were determined by measuring the displacement vectors of the etch pit tips. Then the orientation distributions of the extended defects in CdZnTe samples were obtained and displayed on the [1 1 1] pole figures. The results show that the extended defects in CdZnTe material consist of dislocation lines connected by micro precipitates or dislocation jogs. The extending directions of the defects situate on the connection lines of [0 1 1], [1 0 1] and [1 1 0] on the [1 1 1] pole figure. The defects with extending directions of 〈2 1 1〉 or 〈3 2 1〉 outside the connection lines of [0 1 1], [1 0 1] and [1 1 0] cannot be revealed by Nakagawa etchant. The orientation distribution of the extended defects shows that the pyramidal etch pit density (PEPD) is not sufficient to describe the property of the extended defects in CdZnTe materials. Some additional parameters are proposed for a more comprehensive evaluation.

Calculations of the free energy of dislocation defects in lamellae forming diblock copolymers using thermodynamic integration

State-of-the-art directed self-assembly (DSA) of block copolymer (BCP) methods still yield defect densities orders of magnitude higher than is necessary in semiconductor fabrication. The defect free energy of a dislocation pair or jog defect, one of the most common defects found in BCP-DSA, is calculated via thermodynamic integration using a coarse-grained molecular dynamics model as a function of χ and the degree of polymerization (N). It is found that χN is not the best predictor of defect free energy and that a single χN value can yield vastly different free energies when χ and N are different. Defect free energy was highly dependent on defect location relative to the underlayer, and free energy differences ∼100 kT were found among the three possible defect locations on a 1:3 guiding pattern. It was found that increasing molar mass dispersity (Ð) significantly reduced defect free energy. Extrapolating from Ð up to 1.5 suggests that the defect will occur in equal proportions to the defect free state at a Ð of around 1.6 for this system. It was found that long chains tended to concentrate near the defect and stabilize the defect.

Interaction of hydrogen with dislocations and grain boundaries in Tungsten

A kinetic model of hydrogen capture and accumulation in tungsten under low-energy bombardment with hydrogen atoms in the “subthreshold” mode is proposed in this paper. Primary defects do not form in the bombarded material in this mode. This model takes into account the physical mechanisms for hydrogen bubble nucleation and growth at dislocations, i.e., the presence of “traps” in the dislocation core and dislocation jog formation due to interstitial tungsten-atom emission during the bubble growth process. Features of the dynamics of bubble growth as a function of the temperature and the hydrogen flux are studied. It is shown that there is a “threshold” temperature that is dependent on the flux of implanted particles and above which the process of hydrogen-bubble formation is suppressed significantly. The obtained estimates of the “threshold” temperature (600–700 K for a flux in the range of 1022–1023 m–2 s–1) agree with existing published experimental data.

Irradiation damage from low-dose high-energy protons on mechanical properties and positron annihilation lifetimes of Fe–9Cr alloy

Nuclear reactions in accelerator-driven systems (ADS) result in the generation of helium within the ADS materials. The amount of helium produced in this way is approximately one order of magnitude higher than that generated by nuclear fusion. As helium is well-known to induce degradation in the mechanical properties of metals, its effect on ADS materials is an important factor to assess. The results obtained in this study show that low-dose proton irradiation (11 MeV at 573 K to 9.0 × 10−4 dpa and 150 MeV at room temperature to 2.6 × 10−6 dpa) leads to a decrease in yield stress and ultimate tensile strength in a Fe–9Cr alloy. Moreover, interstitial helium and hydrogen atoms, as well as the annihilation of dislocation jogs, were identified as key factors that determine the observed softening of the alloy.

Interface structure and the inception of plasticity in Nb/NbC nanolayered composites

Molecular dynamics (MD) simulations were performed to explore the effect of interface structure on the inception of plastic deformation in Nb/NbC nanolayered composites. Using the atomistically informed Frank–Bilby method and disregistry analysis, we characterized the structure of the Nb/NbC interface, including misfit dislocations, dislocation nodes and three coherent interface structures. According to the crystallographic analysis of the interface, four possible coherent interface structures were identified. However, study of the interface energy showed that only three of these are energetically stable. After the relaxation of the interface, the unstable coherent region, which features Nb atoms in the Nb layer on the top of the Nb atoms in the NbC layer, evolves into a condensed interface dislocation node. Three stable coherent interface regions are retained in association with the formation, glide and reaction of interface misfit dislocation loops. Disregistry analysis of the Nb/NbC interface revealed that (i) all misfit dislocations are edge type, and (ii) misfit dislocations enclosing the coherent extended nodes have Burgers vectors along 〈110〉Nb. The role of the interface structure in the plastic deformation of Nb/NbC nanolayered composites was studied under two loading conditions, i.e. uniform compression and nanoindentation. Under uniform compression, lattice dislocations primarily nucleate from the condensed nodal regions where the local strains are the highest. Dislocations propagate in two {110} slip planes with the same Schmid factors. Under nanoindentation, by proper positioning of the indenter, lattice dislocations nucleate from the segments of the misfit dislocations and propagate in {112} slip planes. MD simulations also show cross-slip of lattice dislocations from {112} planes into {110} planes and the formation of vacancies as a result of climb of dislocation jogs.

Anisotropy of the EPR line shape of dislocation acceptor centers in semiconducting type Ic diamonds

The shape anisotropy of the EPR lines of broken carbon bonds in a dislocation core with an edge component in natural semiconducting type Ic diamonds has been investigated. It has been found that electrically active acceptor centers are formed at dislocation steps, jogs, or kinks. The distance between the paramagnetic centers is determined.

Investigations on the deformation mechanisms of single-crystalline Cu nanowires under bending and torsion

Molecular dynamics simulations are used to investigate the deformation behavior of 〈001〉/{100} single-crystalline Cu nanowires under bending and torsion. For 〈001〉/{100} Cu nanowires subjected to bending loadings, fivefold deformation twins are observed at large bending angles. The formation process can be understood by two dominate factors, i.e., the reciprocal phase transformation from atoms of other 12-coordinate lattice to hcp lattice and the annihilation of dislocations on adjacent slip planes to form a twin boundary. In addition, an orientation change of applied stress is essential for its formation. For 〈001〉/{100} Cu nanowires under torsion, the creation of full dislocations from the ends of nanowires is responsible for the plastic deformation mechanism. Moreover, damage is found to occur nearby the ends of nanowires, which is caused by the formation of dense network of dislocation jogs.

Study of Defects Structure in Fe-Al Alloys

In the literature it is proven that thermal vacancies have a great influence on the mechanism of hardening of Fe-Al alloys. Moreover, in these alloys, we observed a long-range ordering, which can significantly affect the mechanical and physical properties and their stability. In this paper, influence of low-temperature annealing on elimination of excess vacancies was investigated. TEM observation of annealed specimens for the alloys with 28 and 38 at.% aluminum have helped elucidate the phenomena responsible for vacancies elimination due to the occurrence of particular interactions between point and linear structure defects. It was shown that the aluminum content influences significantly changes in defects structure. The alloy with 28 at.% aluminum has mainly superdislocations in the structure, while in alloy with 38 at.% aluminum, mainly unit dislocations and high-energy dislocation configurations, like dislocation loops, dislocation dipoles, and dislocation jogs, were observed. The results suggest that different defect types may control the diffusion process during low-temperature annealing and that it is affected by alloy composition.

Precipitation in Al-Alloy 6016 – The Role of Excess Vacancies

6xxx Al alloys owe their superior mechanical properties to the precipitation of finely dispersed metastable β´´ precipitates. These particles are formed in the course of optimized heat treatments, where the desired microstructure is generated in a sequence of precipitation processes going from MgSi co-clusters and GP zones to β´´ and β´ precipitates and finally to the stable β and Si diamond phases. The entire precipitation sequence occurs at relatively low temperatures (RT to approx. 200 °C) and is mainly controlled by the excess amount of quenched-in vacancies, which drive the diffusional processes at these low temperatures. Very recently a novel model for the prediction of the excess vacancy evolution controlled by the annihilation and generation of vacancies at dislocation jogs, grain boundaries and Frank loops was developed and implemented in the thermo-kinetic software MatCalc. In the present work, we explore the basic features of this model in the simulation of the excess vacancy evolution during technological heat treatments. The focus of this article lies on the effect of vacancy supersaturation during different heat treatment steps, such as quenching, heating, natural and artificial aging.

Effects of Silicon Doping and Threading Dislocation Density on Stress Evolution in AlGaN Films

ABSTRACTIn-situ wafer curvature measurements were used to study the effect of Si doping on intrinsic growth stress during the metalorganic chemical vapor deposition (MOCVD) growth of AlxGa1-xN (x=0-0.62) layers on SiC substrates. Post-growth transmission electron microscopy (TEM) characterization was used to correlate measured changes in stress with changes in film microstructure. Si doping was found to result in the inclination of edge-type threading dislocations (TDs) in AlxGa1-xN which resulted in a relaxation of compressive stress and generation of tensile stress. The experimentally measured stress gradient was similar to that predicted by an effective climb model. Dislocation inclination resulted in a reduction in the TD density for Si-doped layers compared to undoped AlxGa1-xN likely due to increased opportunities for dislocation interaction and annihilation. The TD density, which increased with increasing Al-fraction, was found to significantly alter the stress gradients in the films. Film stress was also observed to play a role in TD inclination. In undoped AlxGa1-xN, TD inclination was observed only when the film grew under a compressive stress while in Si-doped AlxGa1-xN, TD inclination was observed independent of the sign or magnitude of the film stress. Si dopants are believed to alter the concentration of surface vacancies which gives rise to dislocation jog via a surface-mediated climb mechanism.

Dislocation junctions and jogs in a free-standing FCC thin film

Dislocation junctions and jogs in a free-standing FCC thin film have been studied using three-dimensional dislocation dynamics simulations. Due to the unconstrained motion of surface nodes and dislocation annihilation at the free surface, junctions and jogs are unstable except for some uncommon conditions. If the film is thin enough for a significant portion of the dislocation network to be terminated at the free surface, junctions and jogs can exist for only a finite time during deformation. Thus, the creation of junction/jog-related dislocation sources and their performance are more limited as the film thickness decreases. This effect could lead to insufficient dislocation multiplication to balance dislocation annihilation at the free surface.