Cross Slip Of Screw Dislocations Research Articles

Advanced Computational Analysis of Cobalt-Based Superalloys through Crystal Plasticity.

This study introduces an advanced computational method aimed at accelerating continuum-scale processes using crystal plasticity approaches to predict mechanical responses in cobalt-based superalloys. The framework integrates two levels, namely, sub-grain and homogenized, at the meso-scale through crystal plasticity finite element (CPFE) platforms. The model is applicable across a temperature range from room temperature up to 900 °C, accommodating various dislocation mechanisms in the microstructure. The sub-grain level explicitly incorporates precipitates and employs a dislocation density-based constitutive model that is size-dependent. In contrast, the homogenized level utilizes an activation energy-based constitutive model, implicitly representing the γ' phase for efficiency in computations. This level considers the effects of composition and morphology on mechanical properties, demonstrating the potential for cobalt-based superalloys to rival nickel-based superalloys. The study aims to investigate the impacts of elements including tungsten, tantalum, titanium, and chromium through the homogenized constitutive model. The model accounts for the locking mechanism to address the cross-slip of screw dislocations at lower temperatures as well as the glide and climb mechanism to simulate diffusions at higher temperatures. The model's validity is established across diverse compositions and morphologies, as well as various temperatures, through comparison with experimental data. This advanced computational framework not only enables accurate predictions of mechanical responses in cobalt-based superalloys across a wide temperature range, but also provides valuable insights into the design and optimization of these materials for high-temperature applications.

Optimizing Tensile Properties and Hardness of Inconel 718 by Cold Rolling

The as-received commercial Inconel 718 material was solid solution heat treated (ST), cold-rolled (CR), and precipitation-hardened (PH) to investigate the effects of deformation on the tensile properties, hardness, and texture. Three sets of specimens (0%, 20%, and 50% CR) were ST at 1100 °C/1 h, CR, and aged (720 °C/8 h + 650 °C/8 h) for the analysis. The ultimate tensile strength (UTS), 0.2% yield strength (YS), and elongation of 50% deformed condition were 1645 MPa, 1512 MPa, and 3.8%, respectively. The 20% deformation resulted in a balanced UTS (1348 MPa), YS (1202 MPa), and elongation (11%). The contribution of precipitation hardening to the strength decreased, while the contribution of CR increased with an increasing percentage of deformation. As the level of deformation increased, the size and quantity of γ″ decreased proportionally. The CR specimens produced a high density of nano/micro twins with twin planes oriented perpendicular to the RD-ND surface. The gradient of crystal orientation and internal features of large austenitic grains were generated by their preferred rotation. The cross-slip of screw dislocations induced a complete β-fiber consisting of {110}<112>, {112}<111>, and {123}<634>, with doubled intensity at the higher deformation. Additionally, the specimens were highly susceptible to the twinning-induced orientation emerged by a predominant <110>//ND. In the as-deformed condition, an incomplete but intense α-fiber, clustered between {110}<001> and {110}<112>, was characterized. Apart from achieving the highest strength, the current work demonstrates the effects of CR on the material strength without the complex influences of δ precipitates.

Effect of Si on the deformation behavior of retained austenite and annealed martensite in medium Mn steels

The present study systematically investigated the effect of Si additions from 0 to 4 wt % on the deformation mechanism of a Fe-0.2C-6.8Mn (wt. %) medium Mn steel. The duplex microstructure consisting of film-like retained austenite (RA) and lath-shaped annealed martensite (AM) was produced for different Si alloyed steels by annealing treatments. They have similar volume fraction of RA. The effect of Si content on the dynamic strain aging (DSA) of RA and the serration type of tensile curves, as well as the fine structure evolution within AM during deformation was studied in detail. For the different Si alloyed steels, their tensile curves all exhibit flow serrations, accompanied with the propagation of the Portevine-Le Châtelier (PLC) band caused by DSA in RA. The serration type was sensitive to Si content. For the high Si steel, only type B serrations appear. This is attributed to the high Si content in RA, promoting aged dislocations by C atoms due to the repulsive interaction between C and Si atoms. For the low Si and Si free steels, type A serrations occur after type B serrations disappear at a certain strain. The mechanical stability and Mn/Si contents of RA play an important role in the PLC band propagation. The Si content has also an important effect on the evolution of dislocation configuration within AM during deformation. In the low and Si free steels, the dislocation multiplication and tangles, as well as the formation of dislocation walls, dividing up the laths appear in the AM. While in the high Si steel, it is found that the dislocation cells appear within the AM due to Si addition promoting the formation of screw dislocations by neutron diffraction technique, and the cross slip of screw dislocations as a necessary mechanism for dislocation cell formation. The formation of dislocation configuration with low energy is believed to be another essential factor improving the mechanical properties, besides the transformation-induced plasticity effect. This work paves a new pathway towards tailoring dislocation configuration for designing a new generation advanced high-strength steel.

Microstructures and mechanical properties of novel MoTaVW refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) containing multiple principal alloying elements have created a growing interest in exploring superalloys applications due to their high melting points and excellent softening resistance. The room-temperature brittleness and oxygen sensitivity greatly restrict their further processing and application. Here, utilizing the unique high strength of Vanadium (V) among alloy elements and cocktail effect of HEAs, we designed a MoTaVW RHEAs with equal atomic ratio, that exhibit multiphase phases (Ta-rich, Mo-rich and V-rich phase) with different composition at different sintering temperatures and the alloy elements in BCC solid solution tend to be uniformly distributed as the increased sintering temperature. Exploring the room-temperature compressive properties and underlying deformation mechanisms, the MoTaVW RHEAs exhibit the yield strength (σy) of 1420 MPa and the plastic strain (εp) of 18 %, that achieved good combination of strength and plasticity compared with other most known BCC RHEAs, which are attributed to high solid solution strengthening caused by lattice distortion and the cross slip of screw dislocations on the close-packed plane that can enhance the synergistic effect of strength and plasticity. These results reveal the MoTaVW RHEAs is a very promising materials for compositional space in RHEAs.

Stress-dependent activation entropy in thermally activated cross-slip of dislocations

Cross-slip of screw dislocations in crystalline solids is a stress-driven thermally activated process essential to many phenomena during plastic deformation, including dislocation pattern formation, strain hardening, and dynamic recovery. Molecular dynamics (MD) simulation has played an important role in determining the microscopic mechanisms of cross-slip. However, due to its limited timescale, MD can only predict cross-slip rates in high-stress or high-temperature conditions. The transition state theory can predict the cross-slip rate over a broad range of stress and temperature conditions, but its predictions have been found to be several orders of magnitude too low in comparison to MD results. This discrepancy can be expressed as an anomalously large activation entropy whose physical origin remains unclear. Here, we resolve this discrepancy by showing that the large activation entropy results from anharmonic effects, including thermal softening, thermal expansion, and soft vibrational modes of the dislocation. We expect these anharmonic effects to be significant in a wide range of stress-driven thermally activated processes in solids.

Direct comparison between experiments and dislocation dynamics simulations of high rate deformation of single crystal copper

A long standing challenge in computational materials science is to establish a quantitative connection between the macroscopic properties of plastic deformation with the microscopic mechanisms of dislocations in crystalline materials. Although the discrete dislocation dynamics (DDD) simulation method has been developed for several decades with the goal of addressing this challenge, a one-to-one comparison between the DDD predictions on single crystal stress–strain curves and experimental measurements under identical conditions has not been possible to date. Such a comparison is an essential step towards establishing a dislocation-physics based theory of plasticity and a multiscale framework of the plastic behaviors of crystalline materials. Here we provide direct comparisons between the stress–strain curves of Cu single crystals under high strain rate loading in the [001] and [011] directions obtained from miniaturized desktop Kolsky bar experiments and those from DDD simulations under identical loading conditions. With an appropriate set of parameters, DDD simulations can produce stress–strain curves that are in reasonable agreement with the experimental results. However, the dislocation mobility values needed to achieve this agreement are an order of magnitude lower than expected based on previous measurements and atomistic simulations. We hypothesize that this discrepancy could be caused by drag forces from jogs and point defects produced during the plastic deformation. Cross-slip of screw dislocations is also found to be necessary to capture the experimental stress–strain behavior, especially for the [011] loading direction. This work provides an example of how direct comparisons between DDD simulations and experimental measurements can provide new insight into the fundamental mechanisms of plastic deformation.

Physics-Based Flow Stress Model for Alloy 718

A dislocation density-based model for alloy 718 in the annealed state is proposed in order to accurately describe the deformation behavior of this alloy for a wide range of thermo-mechanical loadings. The model accounts for numerous microstructural mechanisms, including strain hardening, grain size effect, dynamic strain aging (DSA), solid solution strengthening, as well as phonon and electron drag which affects dislocation movements at high strain rates. Two types of recovery mechanisms are also included: recovery due to dislocation glide and recovery associated with cross-slip of screw dislocations. The model is calibrated using experimentally determined stress–strain curves for both low and high strain rates in the order of 10–3 to 103 s−1, and for temperatures in the range 20 °C to 800 °C. The stress–strain data computed with the model are in good agreement with the experimental data. The inclusion of DSA is found to be effective in the combination of temperatures and strain rates corresponding to experimental observations. The solid solution strengthening contribution increases with decreasing temperature and increasing strain rate. The drag effect in the model proves to be significant only for deformation at high strain rate (~ 103 s−1).

Novel strategy of electroshock treatment for improving mechanical performances of Al–Zn–Mg–Cu alloy by edge dislocation increment

The application of high-strength Al–Zn–Mg–Cu alloy is seriously limited because of its poor ductility and fatigue property. In this work, an electroshock treatment (EST), which is an expeditious and inexpensive strategy, has been adopted to improve significantly the ductility and fatigue life of Al–Zn–Mg–Cu alloy without reducing its strength. After analyzing microstructure evolution by multi-scale characterization, we demonstrated that the reason for improving the properties could be attributed to the homogeneous distribution of dislocations and dislocation increment, especially edge dislocation. An underlying mechanism that the cross-slip of screw dislocations was induced by the electron-dislocation scattering has been proposed to illustrate the phenomenon of microstructure evolutions.

Grain orientation dependence of deformation microstructure evolution and mechanical properties in face-centered cubic high/medium entropy alloys

This study revealed characteristics of the deformation behavior in high/medium entropy alloys (HEAs/MEAs) with face-centered cubic (FCC) structure. A Co60Ni40 alloy and a Co20Cr40Ni40 MEA having low and high friction stresses (fundamental resistance to dislocation glide in solid solutions), respectively, but similar in other properties, including their stacking fault energy and grain sizes, were compared. The MEA exhibited a higher yield strength and work-hardening ability than those in the Co60Ni40 alloy at room temperature. Deformation microstructures of the Co60Ni40 alloy were composed of coarse dislocation cells (DCs) in most grains, and a few deformation twins (DTs) formed in grains with tensile axis (TA) nearly parallel to <111>. In the MEA, three microstructure types were found depending on the grain orientations: (1) fine DCs developed in TA~//<100>-oriented grains; (2) planar dislocation structures (PDSs) formed in grains with other orientations; and (3) dense DTs adding to the PDSs developed in TA~//<111>-oriented grains. The results imply difficulty in cross-slip of screw dislocations and dynamic recovery in the MEA, leading to an increase in the dislocation density and work-hardening rate. Our results suggest that FCC high-alloy systems with high friction stress inherently develop characteristic deformation microstructures advantageous for achieving high strength and large ductility.

Impact characteristic and crack propagation mechanisms of large-size molybdenum alloy single crystal and polycrystal

The impact characteristic and crack propagation mechanism for large-size Mo3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo3Nb single crystals is lower than that of Mo3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.

Investigation on the deformation mechanisms and size-dependent hardening effect of He bubbles in 84 dpa neutron irradiated Inconel X-750

Microstructural characterization and calculation of how several intrinsic features contribute to the mechanical properties of highly strained Inconel X-750 alloy specimens irradiated to 84 dpa at two distinct temperature ranges (300 °C–330 °C and 120 °C–280 °C) were performed. The individual contributions of different microstructural features to the critical resolved shear stress (CRSS) of the material, including the γ matrix, γ′ precipitates, irradiation induced defects, and helium bubbles, were calculated. The obstacle strength of He bubbles was found to be size-dependent and a critical size was determined. For bubbles < 2.4 nm, the obstacle strength is only approximately 0.03 while it can be as high as 0.55 for bubbles > 6.6 nm. In the high temperature range (300–330 °C) irradiated specimen, localised dislocation slip was found responsible for the failure of the specimen during micro-tensile testing, associated with significant helium bubble elongation. Elongation of helium bubbles on both the primary and adjacent secondary {1 1 1} planes was likely induced by the cross slip of screw dislocations. Nanotwins were also found adjacent to the shear failure surface in the high temperature specimen, but no elongated bubbles were found within the nano-twinned region. In the low temperature range (120–280 °C) irradiated highly strained specimen, a dislocation network structure was revealed.

On the cross-slip of screw dislocations in zirconium

We develop here a cross-slip model for zirconium based on discrete dislocation dynamics and explicit representation of partial dislocations. It is found that the basal-to-prismatic cross-slip is always energetically favorable. The corresponding stress-free activation enthalpy is ΔHa*∼0.5eV, and the critical nucleation length is close to zero. The Escaig stress on the primary plane is found to be the most effective component in influencing the activation enthalpy. By contrast, prismatic-to-basal cross-slip is activated only if the Schmid stress on the conjugate plane is higher than ∼1GPa, with an activation enthalpy larger than 5 eV. We propose that basal slip of 〈a〉 screw dislocations in Zr is mediated by the formation and subsequent lateral migration of kink pairs formed by double cross-slip. The proposed mechanism is consistent with experimental observations on the temperature-dependence of the critical resolved shear stress (CRSS) and the wavy motion of 〈a〉-basal slip.

Effect of plastic bending on high temperature creep resistance of molybdenum single crystals

Using molybdenum single crystals, it is shown how plastic bending multiplies the high-temperature strength of products for pipelines and tube shells of small diameter (10–30 mm), which are subjected to high internal pressure. It is very convenient to do this when the plastic bending process is included in the manufacturing technology of such products, for example, in the form of spiral winding a strip into a pipe. It has been established that during 1000 h, the creep rate of a pre-bent Mo single crystals at 1633 K and an applied stress of 10 MPa permanently decreases by 4–5 orders of magnitude as compared to an unstrengthened single crystal. Using light microscopy, X-ray diffractometry, and transmission electron microscopy, changes in the dislocation structure and the mechanism leading to such strengthening have been found. During the creep test, the edge components of dislocations are depleted and disappear, and the cross slip of screw dislocations is blocked by numerous loops and helicoids that were formed under bending and transformed during the creep process. As a result, dislocation creep gradually turns into diffusion creep during one test. It is safe to assume that similar effects will be observed in polycrystals when the dominant texture direction will correspond to the required bending axis.

Rheological behavior and dynamic softening mechanism of AA7075 sheet under isothermal tensile deformation

Rheological behavior and dynamic softening mechanism of high-strength AA7075 sheet were investigated by performing isothermal uniaxial tensile tests at deformation temperatures of 573−723 K and strain rates of 0.01–10 s−1. On the basis of the obtained flow stress, response surface models for the ultimate tensile stress (UTS) and fracture strain were developed as a function of temperature and strain rate, respectively. Error analysis certified the reliability of the models. Besides, a constitutive model coupling with dislocation evolution was built to predict the deformation behavior of the alloy, and the dislocation evolution law was revealed based on the established constitutive model. In addition, the dynamic softening mechanism of the alloy during hot deformation was exposed according to the Zener-Hollomon parameter map, microstructural observation and apparent activation energy. Owing to precipitates pinning effect on the motion of dislocation and migration of subgrain boundaries, the dominant softening mechanism is concluded to be dynamic recovery (DRV), relating to cross-slip of screw dislocation.

Studying hydrogen effect on the core structure and mobility of dislocation in nickel by atomistically-informed generalized Peierls–Nabarro model

Hydrogen (H) atoms in the metallic crystalline lattice interact with the pre-existing dislocations and then remarkably affect the plastic deformation of metals. Thereby quantitatively characterizing the H-dislocation interaction is of great importance for understanding H-induced plasticity and failure. Most of the previous studies have focused on the long-range interaction between hydrogen and dislocation, but rarely considered the short-range interaction, especially the hydrogen effect on the dislocation core structure. Here, with the aid of the H-affected γ-surface calculated from atomistic modeling, an atomistically-informed generalized Peierls–Nabarro model is employed to study the hydrogen effect on the core structure of dislocation, the recombination energy of the extended screw dislocation, and the Peierls stress in nickel. Our results show that, on the one hand, hydrogen can decrease the stable stacking fault energy, leading to the increase of the stacking fault width for both extended edge and screw dislocations. Consequently, the recombination energy of extended screw dislocation is increased, indicating that hydrogen can suppress the cross-slip of screw dislocation and facilitate the slip planarity observed frequently in experiments. On the other hand, hydrogen can increase the unstable stacking fault energy, implying that hydrogen can inhibit the nucleation of partial dislocations. Moreover, hydrogen in the dislocation core increases the Peierls stress and thus increases dislocation slip resistance. Finally, quantitative relations between the Peierls stress and hydrogen concentration are given. These results are of great significance for understanding the H-affected dislocation plasticity mechanisms, and can be used for quantifying the hydrogen effect on dislocation dynamics.

A Crystal-Plasticity Cyclic Constitutive Model for the Ratchetting of Polycrystalline Material Considering Dislocation Substructures

A crystal-plasticity cyclic constitutive model of polycrystalline material considering intra-granular heterogeneous dislocation substructures, in terms of three dislocation categories: mobile dislocations, immobile dislocations in the cell interiors and in the cell walls, is proposed based on the existing microscopic and macroscopic experimental results. The multiplication, annihilation, rearrangement and immobilization of dislocations on each slip system are taken as the basic evolutionary mechanism of the three dislocation categories, and the cross-slip of screw dislocations is viewed as the dynamic recovery mechanism at room temperature. The slip resistance associated with the isotropic hardening rule results from the interactions of dislocations on the slip systems. Meanwhile, a modified nonlinear kinematic hardening rule and a rate-dependent flow rule at the slip system level are employed to improve the predictive capability of the model for ratchetting deformation. The predictive ability of the developed model to uniaxial and multiaxial ratchetting in macroscopic scale is verified by comparing with the experimental results of polycrystalline 316L stainless steel. The ratchetting in intra-granular scale which is obviously dependent on the crystallographic orientation and stress levels can be reasonably predicted by the proposed model.

Cross-slip in face centred cubic metals: a general full stress-field dependent activation energy line-tension model

ABSTRACT Cross-slip is a thermally activated process by which a screw dislocation changes its slip plane. Understanding and modelling the activation barrier of the cross-slip process as a free-energy barrier that depends on the stress conditions at the vicinity of the dislocation is crucial. In this work, we employ the line-tension model for the cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier when both Escaig stresses are applied on the primary and cross-slip planes and Schmid stress is applied on the cross-slip plane. We propose a closed-form expression for the activation energy for cross-slip in a large range of stresses, without any fitting parameters. The results of the proposed model are in good agreement with previous numerical results and atomistic simulations. We also show that, when Schmid stress is applied on the cross-slip plane, the energy barrier is decreased, and in particular, cross-slip can occur even when the Escaig stress in the primary plane is smaller than that on the cross-slip plane. The proposed closed-form expression for the activation energy can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality. This will allow cross-slip to be more accurately related to macroscopic plasticity.

High Temperature Deformation Behavior and Processing Maps of AZ31 Alloy Deformed in Tension versus Compression

The effect of the applied state-of-stress on the processing maps depicting the mechanisms for hot working of hot extruded Mg-3Al-1Zn alloy has been evaluated. Flow stresses at various temperatures in the range 300 – 500 °C and strain rates in the range 0.0003 – 1 s-1 have been measured by deforming in compression and in tension. Processing maps have been developed from the respective flow stress data at a strain of 0.1. The maps are essentially similar irrespective of the mode of deformation – compression or tension, and exhibited two domains in the temperature and strain rate ranges: (1) 375 – 500 °C and 0.0003 – 0.01 s-1, and (2) 450 – 500 °C and 0.1 – 1 s-1. On the basis of slower strain rates, high tensile ductility, and the apparent activation energy (152 kJ/mole closer to that for self-diffusion), Domain #1 is interpreted in terms of the occurrence of climb controlled dynamic recrystallization. In Domain #2, which occurs at higher strain rates and has an apparent activation energy near to 165 kJ/mole, dynamic recrystallization occurs that involves second order pyramidal slip {11-22} &lt;11-2-3&gt; and recovery by cross-slip of screw dislocations. The state-of-stress imposed on the specimen (compression or tension) does not have any significant effect on the processing maps or the kinetics of hot deformation.

Atomistic modeling of dislocation cross-slip in nickel using free-end nudged elastic band method

Cross-slip of screw dislocations plays an important role in the plastic deformation of face-centered cubic (FCC) metals and alloys. Here we use the free-end nudged elastic band (FENEB) method to determine the atomistic reaction pathways and energy barriers of cross-slip in an FCC single crystal of Ni. We focus on the cross-slip process mediated by an array of pinning vacancy clusters in the form of stacking fault tetrahedra. We also study a competing process of screw glide by direct cutting of those pinning obstacles on the original slip plane. The activation energies of both cross-slip and obstacle-cutting are determined for different stresses, obstacle spacings and sizes. Using FENEB-calculated energy barriers, we construct dislocation mechanism maps to reveal the effects of resolved shear stress, obstacle spacing and size on the rate-controlling dislocation process for plastic deformation. We further evaluate the activation volumes of cross-slip and obstacle-cutting. The latter result emphasizes the notion of finite strength of the atomically sized pinning obstacles to dislocation motion and also validates the Nabarro scaling law of the linear dependence of activation volume on obstacle spacing.

Fatigue behavior, microstructural evolution, and fatigue life model based on dislocation annihilation of an Fe-Ni-Cr alloy at 700 °C

Dislocation annihilation behavior of Sanicro 25 alloy was investigated based on various total strain amplitudes (Δεt/2). The slopes of cyclic stress response curves during stage ΙΙ were compared and showed that obvious stable cyclic behavior occurred at Δεt/2=0.3%. Microstructure analysis reveals that the number of slip plane increased with increasing Δεt/2. When Δεt/2=0.3%, dislocations can slip on the slip plane (111¯). When Δεt/2 increased to 0.4%, the slip planes included (1¯11) and (111¯). The fraction of substructured grains under Δεt/2=0.3% was obviously larger than that under Δεt/2=0.4%, which was attributed to the obvious dislocation annihilation. The deformed grains were dominant at Δεt/2=0.4%. The annihilation mechanisms of dislocations were analyzed during tensile and compressive loading based on the cyclic stress-time curves. The results showed that when Δεt/2=0.3%, during stage ΙΙ, dislocation annihilation mainly occurred during compressive loading. Dislocation annihilation included annihilation caused by cross-slip of screw dislocations and annihilation of adjacent edge dislocation dipoles. Furthermore, considering the critical annihilation distance between two adjacent edge dislocation dipoles, a modified dislocation annihilation model was presented. The dislocation annihilation model was verified based on the actual annihilation rate of dislocations. Further, the relationship of microstructure and fatigue properties was discussed. Based on the dislocation annihilation mechanisms, the fatigue life model considering dislocation annihilation was established. When the ratio of ρ¯− to ρ¯ increased, the fatigue life increased. Therefore, for cyclic hardening materials, obvious dislocation annihilation can increase fatigue life.