Dislocation Kinks Research Articles

Anomalous hardening of two-component disordered crystals

The nature of increasing the strength of disordered two-component solid solutions in comparison with materials consisting of atoms of one component is studied. For this purpose, the contribution of extreme fluctuations in the distribution of solution atoms, which create obstacles for the movement of dislocation kinks, is calculated. It is shown that a slow - power - decrease in the probability of large delays on such obstacles leads to anomalous kinetics of kinks. It is accompanied by a slowdown in the movement of dislocations. This may be the reason for the hardening of the material.

Deformation mechanisms of additively manufactured Hf10Nb12Ti40V38 refractory high-entropy alloy: Dislocation channels and kink bands

To date, the reported refractory high-entropy alloys (RHEAs) with strength and ductility synergy all have up to 1 GPa strain hardening rate (SHR). Keeping the SHR of more than 1 GPa in the RHEAs can postpone the point of plastic instability, thus prolonging the uniform tensile ductility (UTD). In this paper, Hf10Nb12Ti40V38 RHEA with high strength and ductility is successfully manufactured by laser-directed energy deposition (L-DED). The yield strength of the alloy is about 1011 MPa with the tensile ductility of ∼12.6 %. Interestingly, the SHR of the alloy is very low, far below 1 GPa. The stress-strain curve of the alloy does not only show no premature dropout, but also exhibits a flat plateau-type curve. For this interesting phenomenon, the deformation mechanisms of Hf10Nb12Ti40V38 alloy are systematically investigated by investigating the deformation microstructures. The results indicate that the dislocation modes are diversifying, but the limited number of tangles generated by the intersection of dislocation channels are the main source of SHR. Various dislocation configurations such as single dislocation channel and cross-slip are stimulated sequentially, and although they have no positive effect on the SHR of the alloy, they allow alloy to accommodate plastic deformation. Combined with the kink bands as a strain softening mechanism, it improves the ductility while decreasing the SHR. Multiple dislocation channels interactions and kink bands are clearly identified as the two deformation mechanisms governing strain hardening and strain softening. The competition between two mechanisms generates the phenomenon mentioned above. This study does not only shed new insights on the unique deformation behavior of Hf10Nb12Ti40V38 RHEA but also complement the current general perception about the relationship between SHR and ductility of RHEAs.

Microstructural evolution, mechanical behavior, and deformation mechanisms of a lightweight Ti–Nb–Cr–V refractory high-entropy alloy at room and elevated temperatures

Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for high-temperature applications owing to their distinctive mechanical properties. However, their limited tensile ductility and high density pose hurdles for further engineering utilization. In this study, we introduce a novel RHEA composition, Ti45Nb30Cr15V10 (at.%), distinguished by its reduced density (∼6.2 g/cm3), achieved through the assistance of CALPHAD (CALculation of PHAse Diagrams) modeling. The as-cast RHEA exhibits a single body-centered cubic (BCC) structure, demonstrating a tensile yield strength of ∼904 MPa along with a total elongation of ∼9.7 %. By employing cold rolling followed by recrystallized annealing (designated as the CRRA sample), we achieve notable enhancements in both strength and ductility. Specifically, the CRRA sample manifests an impressive tensile yield strength of ∼1010 MPa and a total elongation of ∼20.6 % at room temperature. Remarkably, even at an elevated temperature of 700 °C, the CRRA sample maintains notable strength, displaying a yield strength of ∼695 MPa. The observed dislocation behavior, transitioning from dual-system slip to multi-system slip, along with the intricate interactions of dislocation substructures (e.g., loops, tangles, networks, bands, and cells) and kink bands, are identified as the principal deformation mechanisms contributing to the exceptional tensile properties of the CRRA sample at room temperature. At 700 °C, the presence of wavy-slipping dislocations, induced by the strong pinning effect of component fluctuations, predominantly contributes to the strength of the CRRA sample. However, the emergence of Cr2(Ti, Nb, V) Laves phase at grain boundaries results in premature fracture. The extraordinary tensile properties exhibited by the CRRA sample at both room and elevated temperatures underscore its potential for advanced engineering applications.

Mechanism of Solid-Solution Hardening: Quasi-Localization of Dislocation Kinks

Mechanism of Solid-Solution Hardening: Quasi-Localization of Dislocation Kinks

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.

CALPHAD-aided design for superior thermal stability and mechanical behavior in a TiZrHfNb refractory high-entropy alloy

TiZrHfNbTa refractory high-entropy alloys (RHEAs) are now at the research frontier of advanced metallic materials due to their exceptional mechanical performance, particularly at high temperatures. However, the TiZrHfNbTa RHEAs exhibit poor phase stability at intermediate temperatures (600 – 1,000 °C). The present study aimed to tailor their phase stability and mechanical properties via the calculation of phase diagrams (CALPHAD) approach. We found that Ta and Hf were detrimental to the phase stability of the TiZrHfNbTa RHEAs. Accordingly, a Ta-free and Hf-depleted Ti30Zr30Hf16Nb24 RHEA with outstanding phase stability was designed, which could remain a single-phase body-centered cubic (BCC) structure after annealing at 600 °C for 200 h. Furthermore, numerous (Ti, Zr)-rich nano-precipitates were dispersedly formed in the cold-rolled plus recrystallization-annealed (CR+A) Ti30Zr30Hf16Nb24 RHEA. The nano-precipitates controlled by the spinodal decomposition mechanism had an identical BCC lattice. The lattice fringes with (11¯0) Miller indices bent from the matrix phase to the nano-precipitates, causing a strong local strain field near the phase boundaries. The CR+A alloy possessed a yield strength of ∼ 800 MPa and tensile fracture elongation of ∼ 34.0%, showing a superior strength-ductility combination. The strain measurement by a digital image correlation indicated that the CR+A alloy exhibited a more substantial plastic stability than the as-cast alloy. Detailed observations of deformation microstructures through a transmission electron microscope and electron back-scattered diffraction revealed the origin of strength and ductility. Dislocation cross-slip and kink bands tended to form in the CR+A alloy during deformation and were capable of accommodating dislocation slip against stress concentration. Labusch's model uncovered that solid-solution strengthening contributed the most to yield strength. The present study provides a paradigm for the superior thermostability and controllable nanophase-precipitation behavior in RHEAs.

High-temperature deformation behavior and microstructural evolution of NbZrTiTa refractory high entropy alloy

The hot deformation behaviors and microstructural evolution of an equiatomic NbZrTiTa refractory high entropy alloy (RHEA) were studied by using isothermal compression tests in a range of temperatures (900 °C∼1200 °C) and strain rates (10−3 s−1∼1 s−1). A category of Arrhenius power law relationship was constructed and the activation energy was calculated to be 336–403 kJ mol−1 throughout the strain. The dislocation piling-up and kink bands found in local microstructural areas at low temperatures and high strain rates contributed to intense strain hardening and subsequent strain softening. As the temperature increased and the strain rate decreased, a sharp drop in flow stress was observed after the peak stress. This may be explained by dislocations pinned by short-range ordering (SRO) structures being unlocked. The higher dislocation density and newly emerging fine grains near deformed grain boundaries suggested that the dynamic recrystallization (DRX) was responsible for the continuous flow softening. Discontinuous DRX (DDRX) was the main DRX mechanism in this work, however, the isolated equiaxed grains in initial grains and large cumulative misorientation along the interior of the deformed grains were also found at the high temperatures and low strain rates, which demonstrated that the continuous DRX (CDRX) occurred.

Temperature-dependent nanoindentation and activation volume in high-purity body-centered cubic chromium

The thermally activated dislocation plasticity of body-centered cubic (BCC) metals is controlled by the nucleation of dislocation kink pairs at low temperatures. We probed this deformation mechanism with high-temperature nanoindentation in chromium using very fine temperature increments. The established kink pair nucleation models are modified for the high-temperature nanoindentation method and applied to characterize the deformation behavior of high-purity BCC chromium from room temperature to 698 K. The measured hardness, strain rate sensitivity, and apparent activation volume are compared with a modified kink-pair nucleation model based on Seeger’s work. A change in the apparent slip plane from {110} to {112} with increasing temperature is inferred from self-consistent predictions and experimental measurements of the hardness-dependent activation volume from temperature-dependent hardness measurements using the line tension model for kink-pair nucleation.

Surface integrity and material removal mechanisms in high-speed grinding of Al/SiCp metal matrix composites

SiC particle reinforced Al metal matrix composites (Al/SiCp MMCs) are typical difficult-to-machine materials due to the heterogeneous constituent. Poor surface integrity is commonly caused in conventional machining methods. To explore material removal mechanisms in high-speed grinding, this study carries out high-speed grinding (HSG) on an Al/SiCp MMC at a grinding speed from 30.4 m/s to 307.0 m/s, and assesses surface integrity including surface damage and subsurface damage (SSD) to explore how different grinding speeds take effect therein. The results reveal that improved surface quality is attained in HSG in which continuous and discontinuous dynamic recrystallization mechanisms govern Al grain refinement, and the latter is inclined to occur in the upper part of the ground surface. The distribution of the O-rich zone is closely associated with subsurface cracks. The workpiece ground at a higher grinding speed is with less damage than at a lower grinding speed due to the reduced O-rich zone. Three different layers in the subsurface below ground workpiece are identified based on various features, which are relatively narrower compared to that in low-speed grinding. The range of plastic deformation of the Al alloy matrix is suppressed in HSG because of larger Al grains and a reduced depth of lateral cracks in Al alloy matrix. Distinctly denser dislocation kinks formed at the boundary of SiC particles in HSG indicate the increased ductility of SiC particles. In HSG of Al/SiCp MMCs, strain-rate effect prevails for Al alloy matrix as a result of reduced ductility, and size effect plays the dominant role for SiC particles due to increased ductility, which facilitate reducing the property discrepancies between these two very different components. Therefore, an improved surface integrity of the Al/SiCp MMCs is realized through HSG. This study enhances the understanding of the surface and subsurface formation and material removal mechanisms in HSG of Al/SiCp MMCs, which can provide a theoretical basis and practical reference for achieving better surface quality for Al/SiCp MMCs and other composites machined by HSG.

High strength and plasticity in Cr-Al-C composite

The microstructure and mechanical behavior of a Cr-Al-C composite rapidly consolidated by spark plasma sintering at a relatively low temperature of 900 °C were investigated. The sintered composite showed a multi-phase microstructure consisting of Cr2AlC and Al4C3 in Cr5Al8 matrix, as confirmed by high intensity X-ray diffraction and transmission electron microscopy (TEM). The multi-phase composite showed high hardness of about 11 GPa, high compressive strength of up to 5.81 GPa and strain-to-failure of 1.76%–10.82% at room temperature. High resolution TEM revealed that the dislocation slip and kink bands within the Cr2AlC MAX phase grains contribute significantly to the plasticity, whereas, the Cr5Al8 and Al4C3 improves the strength and hardness of the overall composite. These results highlight the importance of combining ceramic phases with intermetallic matrix for designing high-performance engineering materials.

Transition from solid-solution softening to hardening in the plasticity of crystalline materials as a manifestation of quasi-localization of dislocation kinks

The mechanical properties of materials are highly sensitive to the content of impurities and point defects due to their interaction with plastic deformation carriers-dislocations. This opens up the possibility to some extent control the mechanical properties of materials through alloying and creating solid solutions. In a number of crystalline materials, in addition to the well-known solid-solution strengthening in the region of a low concentration of solution atoms, the opposite behavior is also observed --- an increase of plasticity, or softening. In the present work, the mechanism of competition of these effects is revealed and it is shown how the boundary of behavior change can be used to estimate the microscopic parameters of materials. The theory predicts trends in the dependence of plasticity on the concentration of alloying elements, stress, temperature, strain rate, and a number of material parameters. Keywords: competition of solid-solution hardening and softening, dislocation kinks, random processes.

Microstructures, tensile properties and serrated flow of Al CrMnFeCoNi high entropy alloys

Abstract Microstructures and mechanical properties of dual-phase AlxCrMnFeCoNi (x=0.4, 0.5, 0.6, at.%) alloys were investigated. Thermomechanical processing leads to a microstructural evolution from cast dendritic structures to equiaxed ones, consisting of face-centered cubic (fcc) and body-centered cubic (bcc) phases in the two states. The volume fraction of bcc phase increases and the size of fcc grain decreases with increasing Al content, resulting in remarkably improved tensile strength. Specifically, the serrated flow occurring at the medium temperatures varies from type A+B to B+C or C as the testing temperature increases. The average serration amplitude of these Al-containing alloys is larger than that of CoCrFeNiMn alloy due to the enhanced pinning effect. The early small strain produces low-density of dislocation arrays and bowed dislocations in fcc grains while the dislocation climb and shearing mechanism dominate inside bcc grains. The cross-slip and kinks of dislocations are frequently observed and high-density-tangled dislocations lead to dislocation cells after plastic deformation with a high strain.

Quantifying the dynamics of dislocation kinks in iron and tungsten through atomistic simulations

When high-Peierls-barrier materials such as iron (Fe) and tungsten (W) are deformed, dislocation kinks can be easily activated. The subsequent kink dynamics may dictate the dislocation mobility and the material's overall performance under certain conditions. In this work, taking the thermal-induced kink diffusion along ½<111> screw dislocation lines as an example, the kink dynamics in b.c.c. iron and tungsten are quantified through atomistic simulations. Results show that in both Fe and W, the kink dynamics, including its diffusion coefficient (Dkink) and dissipation parameter (γkink), are sensitive to the dimension (noted as L) of a simulation cell size along the dislocation line direction: the larger L, the higher Dkink, and the smaller γkink. A scaling law for describing the three-stage L-dependent kink dynamics is extracted from a series of computational analysis of the kink diffusion along dislocations with L ranging from tens to hundreds of nanometers. It is found that, if a converged Dkink is desired from atomistic simulations, the minimum L needs to be at least hundreds of nanometers. This is beyond the reach of an atomic-level model using a modest computational resource. To explain the L-dependent kink dynamics, we calculate the kink-induced local stress fields using two different atomistic stress formula, i.e., a widely-used Virial and a recently developed mechanical stress formula. Results suggest: (i) the L-dependent kink dynamics is caused by the long-range elastic interaction between the kink and its periodic images; and (ii) the Virial stress formula underestimates such interactions. This work lays the continuum description of kink-controlled dislocation dynamics on an atomistic foundation. It will also support the development of multiscale methods for addressing the coupled dynamics between the motion of a μm-long dislocation line and the atomic-level kink diffusion along the line itself in b.c.c. metals or other high-Peierls-barrier materials under deformation.

Effect of dislocation channeling and kink band formation on enhanced tensile properties of a new beta Ti alloy

Tensile properties and deformation mechanisms of a newly developed β Ti alloy, Ti–3Al–8Mo–7V–3Cr (wt.%), were investigated at room temperature. The alloy was found to exhibit a yield point at high strength level of 920 MPa and excellent total elongation of 31%. The yield point was presumably caused by the oxygen in the alloy, although the nano-sized omega precipitates were present, too. The examinations of deformed microstructures at different strains using transmission electron microscope and electron backscatter diffraction showed that during straining dislocation glide occurred in narrow channels and kink bands were formed leading to a high ductility. The competing effect of strain hardening by intersecting of channels and twins and softening by kink band formation results in a flat plateau-type engineering stress-strain curve.

Numerical Simulation of the Movement of Frenkel–Kontorova Dislocations in Aluminum Single Crystals at Low Temperatures

The motion of Frenkel–Kontorova dislocations in the single crystals of aluminum at low temperatures has been studied, by means of the computer simulation. It is shown that the dislocation movement is realized by the quantum tunneling of the kinks of dislocations through the Peierls barriers. It is shown that the action of the Peierls high barrier is analogous to the action of low temperatures, and if the Peierls barrier overcome, the dislocation moves unevenly, accelerating under the action of the Peierls barrier and slowing down after overcoming the Peierls barrier. Based on the numerical experiment, the mean free path of dislocation, the distance between the Peierls potential barriers and the width of the Peierls barrier are calculated. The computed values correspond to the real values.

Phonon-kink scattering effect on the low-temperature thermal transport in solids

We consider contribution to the phonon scattering, in the temperature range of 1K, by the dislocation kinks pinned in the random stress fields in a crystal. The effect of electron-kink scattering on the thermal transport in the normal metals was considered much earlier \cite{Muk86}. The phonon thermal transport anomaly at low temperature was demonstrated by experiments in the deformed (bent) superconducting lead samples \cite{Mez79} and in helium-4 crystals \cite{Mez82, Mez84} and was ascribed to the dislocation dynamics. Previously, we had discussed semi-qualitatively the phonon-kink scattering effects on the thermal conductivity of insulating crystals in a series of papers \cite{mezmuk, ostmukmez}. In this work it is demonstrated explicitly that exponent of the power low in the temperature dependence of the phonon thermal conductivity depends, due to kinks, on the distribution of the random elastic stresses in the crystal, that pin the kinks motion along the dislocation lines. We found that one of the random matrix distributions of the well known Wigner-Dyson theory is most suitable to fit the lead samples experimental data \cite{Mez79}. We also demonstrate that depending on the distribution function of the oscillation frequencies of the kinks, the power low temperature dependences of the phonon thermal conductivity, in principle, may possess exponents in the range of $2\div 5$.

Kink deformation in a beta titanium alloy at high strain rate

Kink deformation is uncommonly observed in Ti-35V-15Cr-0.3Si-0.1C beta titanium alloy deformed at 5 × 103s−1. Band structures have formed on the deformed samples. Electron back scattered diffraction analyses prove those band structures are kink bands rather than commonly reported twins in beta alloys with high niobium. The kink bands are classified into three categories since the intragranular misorientation axis analyses reveal that three lattice rotation axes (Taylor axes) exist among the kink bands. The three Taylor axes are [11̅0], [54̅1], [121̅] and the corresponding three slip mode of the dislocation kink model are (112)[111̅], (123)[111̅], (101)[11̅1̅] respectively. It is demonstrated that the selection of the slip mode in a kink bands is dominated by the loading axis. A slip system would have the priority to be selected as the slip mode of the kink deformation if the loading axis is close to the normal direction of the slip plane and the perpendicular direction of the slip direction.

The tensile properties and serrated flow behavior of a thermomechanically treated CoCrFeNiMn high-entropy alloy

Serrated flow behavior of CoCrFeNiMn high entropy alloy with a single face-centered cubic phase was systematically examined over a wide range of temperature and strain rate. The alloy exhibited excellent fracture-resistance with RT elongation up to 56%. Prominent serrations occurred at the medium temperature range and evolved in the sequence of A→A+B→B→B+C→C with increasing temperature and decreasing strain rate. The initial small plasticity produced low density of dislocations piled up at grain boundaries and high density of dislocations were tangled to form cell substructures during subsequent severe deformation stage. Bowing and kinks of dislocations were frequently observed at 400 and 600°C. The critical strain for the onset of serrated flow decreased with increasing temperature and decreasing strain rate, indicating a normal and not inverse Portevin–Le Chatelier (PLC) effect due to the thermodynamic stability of the alloy. Two temperature-dependent values of activation energy for serrated flow (Q) were obtained. The estimated low value (116kJmol−1) implied that pipe diffusion is responsible for the pinning process of dislocations from 300 to 500°C. In contrast, a higher Q (295kJmol−1) between 500 and 600°C suggested that the pinning or unpinning process of dislocations is controlled by the most sluggish diffusion species (i.e. Ni) in the high-entropy alloy.

Dislocation dynamics in solid solutions of covalent crystals

The dislocation mechanism of solid solution strengthening of covalent semiconductor crystals has been studied. The change in the regularities of dislocation dynamics in solid solutions from those in the components of the solution is connected with the manifestation of the nonlinear drift of dislocation kinks. The theory developed suggests an explanation of specificities of the dislocation mobility in a Ge1–cSic solid solution.

Room-temperature solid solution softening in Fe-V binary system

Molecular dynamics simulations of the glide of an edge dislocation in the bcc matrix of Fe-V alloys were performed to investigate the room-temperature solid solution softening by V atoms. For this purpose, the glide velocity of an edge dislocation was calculated as a function of V concentration under the shear stresses of 100-300MPa using the Fe-V cross-potential constructed newly in the present study. Whereas the solid solution hardening occurred as the V concentration is less than 0.13 at% or more than 0.5 at%, the room-temperature solid solution softening was observed in Fe-(0.13-0.5) at% V alloys. The solid solution softening occurring in Fe-(0.13-0.5) at% V alloys was caused by the accelerated growth velocity of kinks by solute V atoms. The increase in kink velocity happened when the interatomic distance between solute V atoms was similar to the length of dislocation kinks.