Interaction Of Dislocations Research Articles

Exceptional cryogenic strength and sufficient ductility of a nanotwinned high-entropy alloy fabricated by cryogenic multi-directional compression

Nanotwinned structures have been reported to produce superior mechanical properties. In this work, nanotwinned Al0.1CoCrFeNi high-entropy alloy (HEA) was fabricated by cryogenic multi-directional compression (CMC) followed by stress-relief annealing. The nanotwinned HEA samples exhibited excellent cryogenic tensile properties with a yield strength of ∼1.2 GPa and uniform elongation of ∼27%, which is attributed to the introduction of high-density dislocations as well as hierarchical nanotwins (NTs) in the HEA samples. During the cryogenic tensile testing, more NTs were activated, increasing the possibility of dislocation and NTs interactions, further activating multiple deformation mechanisms, including the formation of stacking faults (SFs), microbands, and shear bands, thereby increasing the yield strength and maintaining a stable strain hardening ability. This strategy provides new insights into the development of high-performance HEAs applied in cryogenic environments.

Interaction of dislocations and cracks in grains based on discrete dislocations

Interaction of dislocations and cracks in grains based on discrete dislocations

Influence of Co content on the simultaneous enhancement of strength and ductility in severely drawn textured Ni-Co microwires

ABSTRACT This study examines the deformation behaviour of Ni-Co microwires exhibiting high strengths and ductility, where the addition of Co is intended to decrease the stacking fault energy. A reduction in stacking fault energy is likely to retard the recovery processes, refine grain size and enhance strain hardening by dislocation interactions at twin boundaries. Microwires were drawn to a strain of 5.88, from annealed Ni-Co alloys with Co content varying from 30 to 60 wt%. Subsequent tensile testing revealed a simultaneous increase in strength and ductility with an increase in Co content. The enhanced strength is a consequence of the finer grain size with an increase in Co, and the larger ductility is related to a combination of greater strain hardening and a higher strain rate sensitivity with an increase in Co. The textured drawn Ni-Co wires exhibited higher strengths than those obtained by severe plastic deformation with comparable grain sizes.

The effect of neighboring grain orientation on dislocation-grain interaction in Ti-5553 alloy

The accumulated dislocations are widely reported to be easily transmitted at low angle grain boundaries (GBs), while the associated mechanism at high angle GB needs further investigation. In this paper, the mechanisms associated with the interaction between GBs and 〈111〉 dislocations are investigated in Ti-5553 alloy with body-centered cubic structure. Slip traces transmitted into the neighboring grain or blocked by GBs are observed on the surface of the sample via secondary electron imaging. The corresponding slip system is determined with the combined use of EBSD-detected grain orientation and slip traces. The experimental results indicate that most slips can transfer at low angle GBs, while some slips can also transmit into the neighbor at high angle GBs. The misorientation angle of GBs is invalid to evaluate the mechanism induced by the interaction of dislocations with GBs. Crystallographic alignment-based analysis shows that slip transfer happens at GBs with favorable parameters of high geometric alignment ( m ′) and low residual Burgers vector magnitude (|∆ b |). The general relation of GB misorientation angle with m ′ and |∆ b | is established. The results indicate that low angle GBs always possess high m ′ and low ∆ b , while only some high angle GBs meet the requirement of large alignment parameters. This difference contributes to their preference of slip transfer. Finally, the orientations of best neighboring grains that are favorable for slip transfer are calculated, which is in agreement with the experimentally detected examples. • Dislocations can transmit at some high-angle grain boundaries with special orientations. • General effect of grain boundary misorientation angle on slip transmission is studied. • The best orientations of neighboring grains favorable for slip transmission are established for low- and high-angle GBs.

Dynamic strength, reinforcing mechanism and damage of ceramic metal composites

• We fully study the shock responses of ceramic metal composites via NEMD . • The interaction of shock-wave with the interface affects dislocation strengthening. • An abnormal softening effect is captured under shock loading . • The interface possesses negative effect on damage for composites. Shock tolerance is desirable for ceramic particles-reinforced metal matrix composites in many applications, where the dislocation dynamics evolution under the extreme load is the key but still elusive. Herein, we have investigated the dislocation motion and interaction under shock loading of SiC/Al nanocomposites using molecular dynamics simulations. We have demonstrated that the plastic deformation occurs at an impact velocity (0.5 km/s) lower than the Hugoniot elastic limit of aluminum due to the reflected shear wave effect. The Al/SiC interfaces act as a dislocation emitter to control dislocation multiplication density and slip direction, opening a new pathway to achieve ultrahigh-strength via shock loading. When the impact velocity (1.0 or 1.5 km/s) exceeds the Hugoniot elastic limit, the effect of nanoparticles on dislocation structure has changed from multiplying to retarding dislocations. The spall strength of composites improves due to dislocations pile-up at interface. Instead, the damage in the matrix is exacerbated, owing to the enhanced residual peak stress and interface reflection waves. In addition, the effect of abnormal shock softening determined by atomic velocity is revealed, which could be suppressed by increasing impact energy dissipation. Meanwhile, dynamic compressive strength depends on pressure and dislocation structures evolution. Our atomistic insights might be helpful in designing advanced shock-tolerant materials.

Singularity-free theory and adaptive finite element computations of arbitrarily-shaped dislocation loop dynamics in 3D heterogeneous material structures

The long-standing problem of arbitrarily-shaped discrete dislocation loops in three-dimensional heterogeneous material structures is addressed by introducing novel singularity-free elastic field solutions as well as developing adaptive finite element computations for dislocation dynamics simulations. The first framework uses the Stroh formalism in combination with the biperiodic Fourier-transform and dual variable and position techniques to determine the finite-valued Peach–Koehler force acting on curved dislocation loops. On the other hand, the second versatile mixed-element method proposes to capture the driving forces through dissipative energy considerations with domain integrals by means of the virtual extension principle of the surfacial discontinuities. Excellent agreement between theoretical and numerical analyses is illustrated from simple circular shear dislocation loops to prismatic dislocations with complicated simply-connected contours in linear homogeneous isotropic solids and anisotropic elastic multimaterials, which also serves as improved benchmarks for dealing with more realistic boundary-value problems with evolving dislocations. Examples of sophisticated dislocation applications include the short-range core reaction between intersecting dislocation loops in interaction with a spherical cavity, as well as the Orowan dislocation-precipitate bypass mechanism in a compressed micropillar of polycrystalline copper. The latter multiscale investigation spans three orders of magnitude in size scale, and is thus enabled by computationally efficient and robust adaptive mesh generation procedures for explicit dislocation propagation, interaction, and coalescence in three-dimensional materials and material structures.

Anomalous slip in body-centred cubic metals.

Crystal strength and plastic flow are controlled by the motion and interaction of dislocations, the line defects carrying atomic shear increments. Whereas, in most crystals, deformation develops in the crystallographic planes in which the glide force acting on dislocations is maximum, plasticity in body-centred cubic metals is more complex. Slip systems in which the resolved shear stress is not the highest can dominate at low temperature, leading to anomalous slip1,2. Using in situ tensile tests in a transmission electron microscope we show that anomalous slip arises from the high mobility of multi-junctions3, that is, junctions between more than two dislocations, which glide at a velocity several orders of magnitude larger than single dislocations. These multi-junctions result from the interaction of a simple binary junction with a gliding dislocation. Although elasticity theory predicts that these binary junctions should be unstable in crystals with a weak elastic anisotropy such as tungsten, both experiments and atomistic simulations reveal that such junctions can be created under dynamic conditions, in agreement with the existence of anomalous slip in almost all body-centred cubic metals, including tungsten4,5.

Thermal effect on dislocation interactions in magnesium alloy

In this work, dislocation interactions are directly observed and analyzed by using in-situ transmission electron microscope during heating processes in a pre-deformed Mg-3Al-1Zn magnesium alloy. Thermodynamic behaviors of two types of dislocations are investigated, i.e., individual dislocations and low-angle grain boundary arranged by dislocations. The results show that the low-angle grain boundary remains stable as the heating temperature increases to 673 K, while most of the individual dislocations disappear until the heating temperature reaches 473 K. Furthermore, the low-angle grain boundary tends to absorb individual dislocations nearby. The stability and the annihilation of both types of dislocations are discussed.

Deformation temperature impacts on the microstructure evolution and mechanical properties of a novel medium-heavy alloy (MHA)

Medium-heavy alloys (MHAs) were rolled under different temperatures to characterize the microstructural evolution and mechanical properties by using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, electron back scattered diffraction, tensile testing and microhardness testing. The results show that with an increase in the degree of deformation, the equiaxed grains are elongated along the rolling direction. A large number of slip bands are generated to coordinate intense plastic deformation, which leads to the formation of a fibrous texture. The sharp increase in dislocation density promotes significantly the dislocation interaction, which, in turn, refines the grain size of the MHAs to the nanometer level. Compared with the room-temperature rolling specimens, the cryorolling specimens underwent intense deformation. After 90% cryorolling deformation, the MHA grains were refined to 16.1 nm. The strength and hardness of the cryorolling specimens were significantly higher than those of the room-temperature rolling specimens, though the elongation was slightly lower. The fracture morphology of both specimens changed from ductile fracture (before deformation) to a mixed ductile–brittle fracture (after deformation).

Alterable tension-compression asymmetry in work hardening of an additively manufactured dual-phase high-entropy alloy

High-entropy alloys (HEAs) as potential structural materials appear to exhibit tension-compression asymmetry (TCA) in work hardening. However, the intricate origin of the TCA, particularly in multi-domain HEAs, remains unclear. Herein, an additive manufactured AlCoCrFeNi2.1 dual-phase (FCC + BCC) HEA was used as a prototypical model system to gain a fundamental understanding of the TCA in multi-domain HEAs. Microscopic characterizations demonstrate that the TCA in work hardening is primarily attributed to the dominant stacking fault (SF)-dislocation interaction, SF-SF interaction, and subordinate twin-dislocation interaction, all of which are observed only in the FCC phase in compression. The former two mechanisms not only offer strong strengthening by impeding dislocation motion, but also bring considerable plasticity. The latter mechanism occurs in the dense SF regions to accommodate the plastic deformation, and contribute to additional work hardening. However, in tension, there are only dislocations generated inside the FCC phase. The marked difference in the deformation of the FCC phase is closely related to the FCC/BCC alternating lamellar heterostructure. Dislocations will preferentially nucleate and accumulate at the phase boundary, resulting in localized stress concentration. In tension, the stress concentration will lead to rapid crack propagation parallel to the phase boundary, resulting in premature failure of the sample. In compression, various types of cracks are formed and develop slowly, postponing the failure of the sample. Also, this TCA has the potential to be tuned by tailoring the FCC/BCC lamellar heterostructure; specifically, the width ratio of the FCC phase to the BCC phase. Our findings deliver cutting-edge insights relative to the TCA in new forms of alloys and offer a promising strategy to achieve the tunable possibility of the TCA in the work hardening by designing hierarchical heterostructures.

Nanoindentation creep behavior of diverse microstructures in a pre-strained interstitial high-entropy alloy by high-throughput mapping

Time-dependent plastic deformation, also known as creep, can occur in high-melting-point materials at room temperature when subjected to submicron plastic contact (e.g., components in micro-electronic mechanical systems) due to the extremely high stress conditions. Previous investigations of creep behavior at submicron scale mainly focus on uniform microstructures, whereas recently developed high-performance structural materials usually show complex microstructural heterogeneities. Here, we demonstrate a high-throughput nanoindentation approach to unveil the creep behavior of locally diverse microstructures, via a case study on an interstitial high-entropy alloy (iHEA). The prototype iHEA specimen was firstly pre-strained by severe cold-rolling to achieve diverse microstructure features including dense dislocations, nanotwins and nanograins. Nanoindentation mapping with 144 indents is conducted to collect creep data from all types of diverse microstructures, followed by systematical calculations based on power-law analysis. Then intuitive contour maps of nanohardness (H), strain rate (ε˙) and stress exponent (n) are attained. The individual creep mechanism of each microstructure variant is evaluated by correlating local microstructure features with corresponding creep responses from these contour maps. With the increase of n value for different sample regions, the ε˙ value inversely reduces and the corresponding dominant creep mechanism gradually converts from diffusion-mediated mode to dislocation-controlled one. The presence of nanograins and nanotwins significantly hinders dislocation interactions and promotes stress-induced diffusion. This work verifies that nanoindentation creep behavior is highly correlated with local microstructure features, and the mapping approach is feasible for evaluating creep-resistance of locally diverse microstructures under extremely high-stress conditions.

In-situ neutron diffraction study on a dislocation density in a correlation with strain hardening in Al–Mg alloys

It is well known that variations of alloy and microstructural parameters i.e., alloy composition and grain size are favorable ways to improve the mechanical properties in non-heat-treatable aluminum alloys including Al–Mg alloys. It leads to a change in stress-strain curve behaviors. The flow stress is mainly correlated with dislocation interactions. In this study, in-situ neutron diffraction experiment was conducted to study evolution of dislocation density and characteristics during tensile deformation of Al–Mg. The dislocation density and related characteristic i.e., dislocation rearrangement were quantitatively analyzed by using neutron diffraction line profile analysis with using Convolution Multiple Whole Profile (CMWP) method. Variations of solute (Mg) contents and grain sizes in Al–Mg were investigated in a correlation with the dislocation densities and dislocation interactions. The higher Mg content and smaller grain size resulted to higher dislocation density and, consequently, higher flow stress and strain hardening rate.

Mechanical spectroscopy study of as-cast and additive manufactured AlSi10Mg

The AlSi10Mg alloy produced by casting (AC) and additive manufacturing (AM) technology of laser powder bed fusion (L-PBF) has been investigated through mechanical spectroscopy (MS). In addition to the grain boundary peak PGB the Q−1 curves of both materials exhibit two other relaxation peaks, P1 (H = 0.8 ± 0.05 eV; τ0 = 10−11±1 s) and P2 (H = 1.0 ± 0.05 eV; τ0 = 10−13±1 s), depending on the interaction of dislocations with solute elements (Si and Mg). Relaxation strengths of P1, P2 and PGB of AM alloy are greater than those of the AC one owing to the finer structure of Al cells and the higher amount of Si and Mg in supersaturated solid solution induced by the rapid solidification typical of the L-PBF process. After successive MS test runs relaxation strengths of P1 and P2 peaks in both the examined materials decrease due to the precipitation of Si atoms and dislocation density recovery. Such decrease is more pronounced in AM alloy where change of cell shape and increase of cell size is observed. Dynamic modulus of AM alloy exhibits an anomalous trend in the first test run that is no more present in successive runs. The irreversible process giving rise to such anomalous behavior is the closure of pores of nanometric size.

Coupled diffusion-mechanics framework for simulating hydrogen assisted deformation and failure behavior of metals

Understanding of years-old multifaceted hydrogen-assisted damage evolution necessitates efforts to model the coupled diffusion-mechanics response in metallic materials. Informed by the dislocation-hydrogen interactions, understood earlier via experiments and/or multi-scale modeling techniques, this work presents a dislocation density-based crystal plasticity model coupled with a hydrogen diffusion/trapping model to simulate the hydrogen-assisted deformation and failure under the HELP mechanism of hydrogen embrittlement. The important role of hydrogen on dislocation multiplication, annihilation and dislocation interaction weakening are included in the presented framework. Two possible scenarios under HELP mechanism, leading to H-induced macroscopic softening or hardening as a result of trade-off between the hydrogen-induced weakening of dislocation interactions and hydrogen-induced increased dislocation density emerges from the simulation studies. These finding points towards the inevitable role of HELP mechanism to cause early failure in metals either working independently or in support with additional mechanisms.

Nanoindentation investigation of incoherent twin boundary migration in Au nanocrystalline films

Incoherent twin boundary (ITB) evolution and migration induced by nanoindentation in Au nanocrystalline films were systematically investigated by Cs-corrected transmission electron microscopy (TEM). With a gradient stress distribution at various indentation depths, 9R/3R phase nucleation is greatly facilitated at the ITBs. The 9R/3R phase formation can be achieved by collective glide of triple partial dislocations with different Burgers vectors at two phase boundaries. With 9R/3R phase at ITBs, atoms on consecutive {111} planes shear in a helical manner, generating zero macroscopic strain during ITB migration. Dislocation interactions with ITBs were further analyzed by combining atomic-scale high-angle annular dark-field scanning TEM (HAADF-STEM) with geometric phase analysis method. ITBs can both absorb partial dislocations and emit partial dislocations from the junctions between coherent twin boundaries and ITBs, providing complementary mechanisms for ITB migration in Au nanocrystalline films under nanoindentation. These results suggest the critical role of 9R/3R phase nucleation in ITB migration and provide a deeper understanding of nanoindentation-induced deformation twinning in nanocrystalline films.

Correlating in-plane strength anisotropy with its microstructural counterpart for a hot rolled line pipe steel

The present study focuses on the microstructural entities, emphasizing the role of precipitates on the in-plane directional tensile responses, including the strain hardening behaviour of a hot rolled X-65 Line pipe graded steel. Tensile samples have been prepared from four different directions lying on the RD-TD (rolling direction-transverse direction) plane at the varying angle of 0°, 30°, 60°, and 90° with respect to the RD of the hot-rolled plate. Superior mechanical properties have been evidenced in the 90° sample compared to other orientations in terms of maintaining yield continuity and better strength-ductility combination. The yielding behaviour and tensile response have been explained from the viewpoint of grain/grain boundary characteristics, dislocation-precipitation interaction, and elasto-plastic incompatibility across grain boundaries. The role of interaction of dislocations with fine precipitates along with the Elastic Modulus (EM) and Schmid Factor (SF) differences across high angle grain boundaries (HAGBs) have been found to be significant in determining the superiority of the mechanical properties of 90° sample.

Phase transition in medium entropy alloy CoCrNi under quasi-isentropic compression

Under high strain-rate loading, prominent increases in pressure usually triggers phase transition (PT), but the concomitant temperature rise may also cause melting. Quasi-isentropic (QI) compression provides a strategy to explore solid-state phase transition by reducing the temperature rise while retaining high pressure. Using large-scale molecular dynamics simulations, we investigate PTs in single crystal CoCrNi medium entropy alloys (MEAs) under QI compression. With the applied strain rates ranging from 108 s−1 to 1011 s−1, the strain-rate dependence and anisotropy of yield stress and solid-state PT path are revealed by comparing the mechanical responses along three compressed crystallographic orientations ([100], [11¯0], and [111]). Positive strain-rate sensitiveness is found in the yield stress along the [11¯0] and [111] directions, while insensitiveness along the [100] direction. Various PTs occur alongside massive plastic deformation in the post-yield regime. As the strain rate rises, face-centered-cubic (FCC) to body-centered-cubic (BCC) PT overrides the stacking fault-induced hexagonal-close-packed (HCP) phase formation and dominates the plasticity for the [100] loading. By contrast, crystalline PTs give way to amorphization for [11¯0] and [111] loading at high strain rates. Chemical short-range order hinders dislocation slip and promotes dislocation interactions, which further facilitate early formation of the BCC phase, suggesting a potential strategy to tailor polymorphism in MEAs.

Phase-field, dislocation based plasticity and damage coupled model: Modelling and application to single crystal superalloys

In the present work, we propose a novel model coupling phase-field, dislocation density based plasticity and damage. The dislocation density governing equations are constructed based on evolutions of mobile and immobile dislocations. Mechanisms including dislocation multiplication, annihilation, mobile–immobile transfer due to dislocation interactions and block of interfaces are incorporated in the model. Especially, the “swallow-gap” problem surrounding the coarsened second phase, which often appears in dislocation and phase-field coupled simulations, is solved in the present model. Moreover, the phenomenon of dislocation cutting into the second phase during tertiary creep, which has rarely been considered in previous phase-field simulations of single crystal superalloys, is successfully captured in the present model with the coupling of damage. The long range stresses induced by external loading, coherent interface misfit, plastic activity and damage, as well as the short range stresses induced by antiphase boundary, dislocation line tension and forest dislocation trapping are considered in the dynamics of the model. High temperature 〈001〉 creep simulations of single crystal superalloys under 200MPa and 350MPa are conducted using the coupled model and compared with experiments. The results show that simulated phase microstructures, dislocations and creep properties principally agree with experiments during the whole creep stage, in terms of both microscopic and macroscopic features.

Synergetic strengthening of heterogeneous interface far beyond rule of mixture in Cu/1010 steel bimetal laminar composite

This short communication presents a mechanical performance of the Cu/1010 steel bimetal laminated composites with typical heterogeneous interface. Interfacial mismatch allows for superior strength that can exceed the predicted value of the rule of mixture. This special behavior is afforded by pile-up and interaction of geometrical necessary dislocations caused by interfacial mismatch.

Effect of γ' particles morphology on dynamic strain aging behavior of Udimet 500 superalloy

The Udimet 500 superalloy is a promising candidate material for power plant gas turbine blades due to its excellent mechanical properties at high temperatures. However, dynamic strain aging (DSA) behavior of the Udimet 500 superalloy during high-temperature tensile test has not been investigated so far. In the present study, the effect of γ' particles size and volume fraction on dynamic strain aging behavior of the Udimet 500 superalloy were investigated in the terms of serration frequency and magnitude. The serrated flow was observed in the stress-strain curves during plastic straining at 800 °C. The results illustrated that the γ' particles morphology has significant effect on serration frequency, serration magnitude, and fracture mechanism of the Udimet 500 superalloy. Although, the serration frequency decreased with reducing γ' particles size and increasing γ' particles volume fraction, the serration magnitude increased. The governing mechanism for dynamic strain aging of the Udimet 500 superalloy have been proposed by dislocation interactions with solute atoms and γ' particles. Furthermore, a non-zero work hardening rate is observed during Lüders elongation which was attributed to high volume fraction of γ' particles in precipitation strengthened superalloys. The effect of dynamic strain aging on fracture mechanism was also investigated. When dynamic strain aging was absent, the dimples initiated at the γ/γ' interfaces during room temperature tensile test. Dynamic strain aging inhibits dimples formation at the γ/γ' interfaces.