Dislocation Glide Plane Research Articles

Effect of minor elements Al and Ti on dynamic deformation and fracture of CoCrNi-based medium-entropy alloys

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, are investigated under plate impact loading. Free surface velocity histories are obtained along with microstructure characterizations of postmortem samples. The influences of the addition of minor elements Al and Ti on the microstructures, dynamic mechanical properties and underlying deformation and damage mechanisms of MEAs are elucidated. Compared to the single-phase CoCrNi MEA, spherical nanosized L12 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively. The addition of minor elements Al and Ti leads to an increase in the dynamic yield stress approximately by 70% (to 1.0 GPa) and 130% (to 1.4 GPa) for (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively, due to the solid solution strengthening and precipitate strengthening. Multiple deformation mechanisms are observed. Planar dislocation glide and stacking faults are the dominant deformation mechanisms for all the MEAs. However, deformation twinning is deactivated in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi)94Al3Ti3, but brittle for (CoCrNi)90Al5Ti5. Under similar shock stress, the spall strength of (CoCrNi)94Al3Ti3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi)90Al5Ti5 because intergranular brittle fracture activation does not require substantial plastic deformation. The minor addition of Al and Ti in medium-entropy alloys significantly affects their dynamic mechanical properties, deformation and damage mechanisms.

EBSD patterns simulation of dislocation structures based on electron diffraction dynamic theory

Electron backscatter diffraction (EBSD) technology is a powerful tool for materials characterization including crystal orientation mapping, phase identification, and strain analysis. However, it is still challenging for using EBSD to identify crystallographic defects due to the insufficient understanding of the diffraction patterns of different defect structures. In the present work, EBSD patterns of FCC-Fe with 1/2 < 110 > edge dislocation dipole and 1/6 < 11̅2 > screw dislocation quadrupole structures are simulated by the revised real space (RRS) method. Our results showed that the presence of dislocations deteriorates the overall quality of the diffraction pattern and have different effects on different Kikuchi bands and Kikuchi poles. The edges of the Kikuchi band corresponding to the edge dislocation glide plane are sharp and the diffraction details within the band are clear. The sharpness of the edges of the Kikuchi band corresponding to the crystal plane normal to the dislocation Burgers vector is reduced, but the intra-band diffraction details are clear. Other Kikuchi bands show obvious anisotropic blurring. The diffraction details of the Kikuchi pole corresponding to the screw dislocation Burgers vector are clear, the edges of the Kikuchi bands across this pole are sharp, and the diffraction details within the bands are clear in the segments close to this pole and blurred in the segments far away from it. Other Kikuchi bands and Kikuchi poles are blurred. Our results indicate that the EBSD pattern can be simulated based on the electron diffraction dynamic theory and the correlation between dislocation structure and EBSD pattern is revealed, which provides theoretical guidance for the resolution of dislocation structures by the EBSD technique.

Interaction of an edge dislocation with a 〈110〉 tilt boundary in nickel: molecular dynamics simulation

The interaction of a lattice edge dislocation with a 〈110〉 tilt boundary in nickel was studied by the molecular dynamics method in the case when the dislocation glide plane is parallel to the grain tilt axis. The dependence of the local threshold stress on the grain misorientation angle is obtained. It is found that with increasing misorientation angle, the local threshold stress increases, but the growth rate for low- and high-angle boundaries is different: for high-angle boundaries, this growth is slower. When studying the mechanism of overcoming the low-angle 〈110〉 tilt boundary by an edge dislocation in the considered orientation, it was found that the dislocation, in fact, does not pass through the boundary, but exchanges places with a grain-boundary dislocation, which are ordinary perfect edge dislocations in the low-angle 〈110〉 tilt boundaries. When studying the interaction of a dislocation with high-angle boundaries, a peculiarity was noticed, which consists in pushing two or three grain-boundary dislocations out of the boundary at once.

Excellent room-temperature tensile ductility in as-cast Ti37V15Nb22Hf23W3 refractory high entropy alloys

An as-cast Ti37V15Nb22Hf23W3 refractory high entropy alloy (RHEA) with outstanding tensile properties designed by the natural mixing method and solid solution strengthening theoretical model is reported. The current as-cast RHEA presents a random solid solution with a body-centered cubic (BCC) crystal structure and inherits the natural-mixing characteristics. Interestingly, When compared to the W-free Ti38V15Nb23Hf24 RHEA, minor addition of 3 at. % W atoms increases the yield strength by 26.6% to 980−19+17 MPa while preserving 19.8−0.5+0.5 % tensile ductility at room temperature. Toward a better understanding of high yield strength, a solid solution model is proposed to explain strong strengthening, and the experimental value is in good agreement with the theoretical one, indicating the significant solid solution strengthening effect in strengthening the RHEAs. The widespread planar dislocation glide favors the production of complicated dislocation structures (e.g., jogs, dipoles, and loops) as well as dense dislocation networks, which can further obstruct dislocation motion meanwhile effectively relieving stress concentration. The dislocation tangles, loops, dipoles, and jogs together with their interactions postpone the local deformation, results in the excellent ductility.

A feasible route to produce 1.1 GPa ferritic-based low-Mn lightweight steels with ductility of 47%

High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility. However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facile strategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embedded in a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain, which act through dislocation engineering. We ascribe the simultaneous improvement in strength and total elongation to both the size-dependent dislocation movement in austenite grains and the controlled deformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation and high dislocation density-induced hardening and by interface strengthening. Additionally, high-carbon-partitioned small austenite grains enhance the strength and ductility by planar dislocation glide (in the low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strain regime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP may be desirable for broad industrial applications at an economic cost.

Ultrahigh strength-high ductility 1 GPa low density austenitic steel with ordered precipitation strengthening phase and dynamic slip band refinement

Low density steels are of significant interest because of light weight combined with high strength-high ductility combination. We describe here a comprehensive understanding of the mechanical behavior of Fe–27Mn–9Al–1C austenitic steel microalloyed with Nb in terms of work hardening behavior and plasticity mechanisms. The evolution of deformed microstructure with strain was studied via post-mortem electron microscopy and electron back scattered diffraction (EBSD). The experimental steel was characterized by ultimate tensile strength and elongation to fracture of 1125 MPa and 30.8%, respectively. Plastic deformation was accommodated by pronounced planar dislocation slip and characterized by a single planar dislocation glide at low strain and multiple planar slip at high strain. Electron microscopy studies indicated that some of the ordered kappa carbide precipitates of size ∼5–7 nm, which were uniformly distributed in the austenite matrix were sheared by planar glide dislocations during deformation. The slip band spacing decreased gradually during straining and cross-slip of dislocations occurred at high strain. The planar dislocation glide phenomenon is attributed to the superposition of sheared ordered phase and short range ordering (SRO), which can lead to glide plane softening. Dynamic slip band refinement and shearing of precipitates led to constant strain hardening of austenitic steel. Annealing twin boundaries acted as dislocation source to generate dislocations during deformation, which were helpful in further sub-division of slip bands and suppressing strain localization.

Strain hardening recovery mediated by coherent precipitates in lightweight steel

We investigated the effect of κ-carbide precipitates on the strain hardening behavior of aged Fe–Mn-Al-C alloys by microstructure analysis. The κ-carbides-strengthened Fe–Mn-Al-C alloys exhibited a superior strength-ductility balance enabled by the recovery of the strain hardening rate. To understand the relation between the κ-carbides and strain hardening recovery, dislocation gliding in the aged alloys during plastic deformation was analyzed through in situ tensile transmission electron microscopy (TEM). The in situ TEM results confirmed the particle shearing mechanism leads to planar dislocation gliding. During deformation of the 100 h-aged alloy, some gliding dislocations were strongly pinned by the large κ-carbide blocks and were prone to cross-slip, leading to the activation of multiple slip systems. The abrupt decline in the dislocation mean free path was attributed to the activation of multiple slip systems, resulting in the rapid saturation of the strain hardening recovery. It is concluded that the planar dislocation glide and sequential activation of slip systems are key to induce strain hardening recovery in polycrystalline metals. Thus, if a microstructure is designed such that dislocations glide in a planar manner, the strain hardening recovery could be utilized to obtain enhanced mechanical properties of the material.

Elucidating the transition of cryogenic deformation mechanism of CrMnFeCoNi high entropy alloy

Here, the evolution of the deformation mechanism of the well-known CrMnFeCoNi high entropy alloy (HEA) under a cryogenic temperature was investigated via using transmission electron microscopy characterization after compressive experiments. It is found that, in contrast to the dominate mechanism of dislocation slip at 298 K, the deformation mode of the studied HEA at 123 K can be divided into two stages: (i) at the initial plastic stage, the plasticity is mainly induced by the planar dislocation glide; and (ii) at the late plastic stage, the combination of dislocation hardening and mechanical twinning facilitates high strength for the studied alloy. These findings are helpful to understand the temperature dependent deformation mechanism of high entropy alloys.

Elastoplastic transition in a metastable β-Titanium alloy, Timetal-18 – An in-situ synchrotron X-ray diffraction study

The elastoplastic transition of a metastable β-Ti alloy, Timetal-18, is studied using in-situ high energy synchrotron X-ray diffraction microscopy (HEDM). The measured evolutions of the complete elastic strain (and stress) tensor(s), resolved shear stress, lattice rotation and rotation of the stress state of the grains are compared with the predictions of the elasto-viscoplastic Micromechanical Analysis of Stress-Strain Inhomogeneities with fast Fourier transform (MASSIF) code instantiated with an experimentally measured microstructure which matched that of the sample. The preferred glide plane of dislocations with ½<111> Burgers vectors of the BCC alloy was explored. It was found that the polycrystalline stress-strain response could be equally well described by any of the candidate glide planes or combinations thereof (i.e., pencil glide). However, simulations involving slip on {112} planes yielded a marginally better description of the individual grain-level responses, as compared to the simulation involving only the {110} planes. The small (typically <1°) crystallographic reorientations that the grains undergo during the elastoplastic transition, are insufficient to permit discrimination between candidate slip modes. The resolved shear stress (RSS) distributions showed a sharp increase in skewness around macroscopic yield and it was found that the hardening during the elastoplastic transition is primarily due to intergranular interactions. Analysis of “hard” and “soft” grains suggests non-Schmid effects may be present, even in these low strain rate, room temperature experiments. Finally, examination of the individual responses revealed “strain softening” in some of the grains. Intragranular heterogeneity in the orientation and stress state are highlighted as important areas for future investigations, which may reveal answers to unresolved questions in this research.

Aspects on numerical integration of dislocation surface traction fields for discrete dislocation dynamics FEM coupling: the case of emerging dislocations

In discrete dislocation dynamics frameworks for finite samples, the evaluation of surface tractions caused by dislocations requires special attention to capture the contributions of dislocations piercing through the surface. Here we focus on this aspect only, and the convergence of an empirically improved Gauss integration and the Duffy and Duffy distance integration schemes commonly used for 1/r singularities, is investigated for a finite element surface area. It is found, that traction from a dislocation, using singular segment stress fields, can be integrated with an error smaller than 1% with the distance Duffy integration using a total of 48 integration points only. This scheme was tested for a wide range of dislocation Burgers vectors and glide plane orientations with respect to the surface. Furthermore the FEM nodal forces behave smoothly during the motion of exit point of the dislocation (the singular point) to a neighbouring surface element in the mesh.

Interaction mechanism of an edge dislocation with a void in Fe–Ni–Cr concentrated solid-solution alloy

The interactions of a 1/2<110>{111} edge dislocation with a void in Fe10Ni20Cr and Fe33Ni33Cr concentrated solid-solution alloys are investigated by using molecular dynamics simulation method. The edge dislocation dissociates into two shockley partial dislocations with stacking fault in both alloys. Compared to the Fe10Ni20Cr alloy, the dislocation motion becomes difficult in Fe33Ni33Cr alloy due to the big fluctuation of stacking fault energy. The obstacle strength for a void with diameter of 2 nm in Fe33Ni33Cr alloy is slightly weaker than that in Fe10Ni20Cr alloy at temperature range from 300K to 800K. It is attributed to easy vacancies migration in Fe33Ni33Cr alloy. Interestingly, a significant increase of obstacle strength for void at 900K is noted only in Fe10Ni20Cr alloy because of the transformation from void to stacking fault tetrahedra (SFT). It is found that the compress strain upon the edge dislocation glide plane promotes the transformation from void to SFT at 900K in Fe10Ni20Cr alloy. While in Fe33Ni33Cr alloy, sluggish diffusion induced by atomic-level heterogeneity suppresses the transition from void to SFT. Consequently, Fe33Ni33Cr concentrated solid-solution alloy exhibits better irradiation hardening resistance than Fe10Ni20Cr alloy, especially at high temperature.

Multiple damage mechanisms facilitated by planar dislocation glide in a commercial-grade precipitation-strengthened Fe–Ni–Cr-based steel

The present study addresses the damage evolution mechanism in precipitation-strengthened Fe–Ni–Cr-based steel (SUH 660) at ambient temperature in air. Specifically, damage quantification and associated microstructure characterization were performed. The damage initiation sites were inclusion and grain boundaries impinging dislocation slip. The early stage of damage evolution was observed before necking of the specimen. Subsequent plastic straining caused damage growth along slip planes and coalescence of damage via localized shear in ligaments between damages, for which the behavior was associated with the planar character of the slip. The damage evolution behavior observed in the post-mortem specimen was consistent with fractographic characteristics.

Gaseous hydrogen embrittlement of a Ni-free austenitic stainless steel containing 1 mass% nitrogen: Effects of nitrogen-enhanced dislocation planarity

We investigated the effect of hydrogen on degradation of tensile properties in a Fe–25Cr–1N austenitic stainless steel. Hydrogen was introduced by exposure to a hydrogen gas atmosphere at 100 MPa and 270 °C. Hydrogen charging caused significant ductility loss associated with nitrogen-enhanced dislocation planarity. Specifically, even without hydrogen, the nitrogen-enhanced planar dislocation glide induced micro-stress concentration, which assisted the occurrence of hydrogen-induced intergranular and quasi-cleavage fractures. The hydrogen-assisted intergranular cracking occurred along boundaries of grains where primary slip was predominantly activated. On the other hand, the hydrogen-assisted quasi-cleavage fracture took place when multiple slip systems were activated. The hydrogen-related cracks emerged, but their growth was arrested via crack blunting associated with a significant plastic deformation. Instead, new cracks formed near the crack tips. Therefore, hydrogen-assisted crack propagation occurred through repetition of crack blunting, initiation, and coalescence.

First-Principles Calculations of Oxygen-Dislocation Interaction in Magnesium.

The interaction between interstitial oxygen atoms and <a>-type screw dislocations was investigated via first-principles calculations to elucidate the effect of oxygen solutes on the deformation behaviors of Mg. The results show that repulsive interactions exist between basal screw dislocation cores and oxygen atoms, which would enable the full basal dislocation to bypass the oxygen atoms in the dislocation glide plane through the cross-slip process. This repulsion also increases the resistance to the motion of dissociated basal dislocations. Moreover, the energy of prismatic <a>-type screw dislocation cores is reduced by the presence of oxygen, which would stabilize the screw dislocation core on the prismatic plane, accordingly facilitating the prismatic slip. This information can complement the fundamental knowledge of alloying Mg using interstitial solutes.

Correlation between open-circuit voltages and preferential glide planes of misfit dislocations of metamorphic InGaAs single junction solar cells grown on GaAs

Abstract Metamorphic growth of light absorber layers of solar cells introduces misfit dislocations due to strain relaxation, and their influences on the solar cell performance need to be characterized. In this work, we correlate the asymmetry of relaxation probabilities via different glide planes (which results in a tilt of the epilayer) with the open-circuit voltages (Voc) of several metamorphic InGaAs single junction solar cells. The devices contain graded InGaP buffer layers and are grown on a GaAs 001 wafer with a small miscut towards the 100 direction. We observe an inhomogeneous distribution of the solar cells’ Voc within the wafer and are able to correlate the Voc fluctuation to a difference in the asymmetry of the glide plane distributions of α and β dislocations. In the case of solar cells exhibiting a high Voc, the glide planes of both α and β dislocations are controlled by the surface orientation. In the case of low-Voc samples, the glide planes of the β dislocations are controlled by the ordering of InGaP, while those of the α dislocations are still controlled by the surface orientation.

To the Explanation of the Magnetoplastic Effect in Diamagnetic and Paramagnetic Solids

This work is devoted to the theoretical study of the magnetoplastic effect that manifests itself in the motion of dislocations in solids in the presence of a magnetic field without mechanical loading. An expression is obtained for the effective magnetic charge per unit length of dislocation, which occurs due to the action of the magnetic field on the dislocation. The effects associated with the induced magnetic charges at dislocations are studied. It is shown that the effective magnetic charge occurs due to the interaction between the magnetic field and only the edge components of dislocations. An expression for the force that acts on a dislocation in the magnetic field is derived and it is shown that in experiments on studying the motion of dislocations in the presence of magnetic fields the projection of this force onto the dislocation glide plane must play a dominant role.

Influence of Annealing Temperature on Microstructure and Three-Stage Strain Hardening Behavior in Cold-Rolled Fe-Mn-Al-C Steel

Microstructural evolution and tensile deformation behavior of cold-rolled Fe-27Mn-12Al-0.9C steel were investigated as a function of annealing temperature in a range from 900°C to 1050°C. The austenitic duplex steels exhibited an excellent combination of ultimate tensile strength of about 1000 MPa and elongation of about 40%. The influence of annealing temperature on deformation behavior was analyzed using a Hollomon equation, and the cold-rolled steel presented a three-stage strain hardening behavior during plastic deformation at room temperature. The superior mechanical properties substantially came from stage II and stage III. The annealing temperature mainly affected the deformation behavior by controlling the steady plastic deformation capacity and strain hardening capacity in stage II. The dislocation substructure of austenite showed planar dislocation glide characteristics. The deformation characteristics formed at a relatively lower strain than those in hot-rolled steel due to the grain refinement and internal stress caused by the cold rolling process.

The effects of interaction geometry on pinning strength induced by interstitial dislocation loop in BCC-Fe

Irradiation induced dislocation loops in Fe-based alloys can cause material hardening by impeding the mobility of dislocation. Using molecular dynamics (MD), the interaction between 1/2<111>{11¯0} edge dislocation (ED) and 1/2<111> interstitial dislocation loops with Burgers vector (BV) along different directions in BCC (body centered cubic)-Fe were investigated, and the influence of interaction geometry (means the relative position between dislocation glide plane (GP) and loop equator plane) on the CRSS (critical resolved shear stress) for dislocation to move again induced by loops was analyzed. It was found that the pinning strength of 1/2<111> loop was estimated to be 0.2–0.8, varying with the BVs direction of loops and the interaction geometries. The present study clearly showed that when parameterizing the dislocation dynamics, more interaction geometries should be considered in order to get a more quantitative pinning strength.

Comparison of the Dislocation Structure of a CrMnN and a CrNi Austenite after Cyclic Deformation

In the literature, the effects of nitrogen on the strength of austenitic stainless steels as well as on cold deformation are well documented. However, the effect of N on fatigue behaviour is still an open issue, especially when comparing the two alloying concepts for austenitic stainless steels—CrNi and CrMnN—where the microstructures show a different evolution during cyclic deformation. In the present investigation, a representative sample of each alloying concept has been tested in a resonant testing machine at ambient temperature and under stress control single step tests with a stress ratio of 0.05. The following comparative analysis of the microstructures showed a preferred formation of cellular dislocation substructures in the case of the CrNi alloy and distinct planar dislocation glide in the CrMnN steel, also called high nitrogen steel (HNS). The discussion of these findings deals with potential explanations for the dislocation glide mechanism, the role of N on this phenomenon, and the consequences on fatigue behaviour.

Oxygen-dislocation interaction in titanium from first principles

The interaction between screw dislocations and oxygen interstitial atoms is studied with ab initio calculations in hexagonal close-packed titanium. Our calculations evidence a strong repulsion when the solute atoms are located in the dislocation glide plane, leading to spontaneous cross-slip, which allows the dislocation to bypass the atomic obstacle. This avoidance process explains several experimental observations in titanium in presence of oxygen: (1) a larger lattice friction against screw dislocation motion, (2) a reduction of the dislocation glide distance in prismatic planes and (3) an enhancement of cross-slip in pyramidal planes.