Coplanar Slip Research Articles

Cell structure development during cyclic deformation of near-[001] and near-[011] copper single crystals

Dislocation cell structures significantly influence the fatigue failure process. Research on the cell structures in multiple-slip-oriented copper single crystals has focused primarily on the [1‾11] orientation, whereas formation mechanisms and development processes of cell structures in [001] and [011] copper single crystals remain unclear. Therefore, this study examines the dislocation structures of near-[001] and near-[011] copper single crystals under cyclic deformation at high strain amplitudes. The cyclic stress–strain curve of the [011] crystals showed a distinct plateau at plastic shear strain amplitudes ≤7.5 × 10−3. In the plateau region, the dislocation structures consisted of cell structures along the (111) primary slip plane and matrix wall structures. Furthermore, (111) cell structures were formed in the single-slip deformation bands (DBs). After the plateau region, several types of new DBs could be observed due to the activation of multiple-slip systems. Multiple-slip DBs have a higher degree of plastic strain concentration than single-slip DBs, leading to the preferential formation of cell structures. For the near-[001] crystals, (111) cell structures were uniformly formed throughout the matrix, without the formation of DBs. The formation mechanism of the (111) cell boundary was independent of the stress axis and resulted from the interaction between the primary and coplanar slip systems.

Deformation mechanism of dislocation slip and kinking behaviour in dynamic compression response of TiZrHfNb alloy fabricated by laser directed energy deposition

ABSTRACT In this study, the dynamic response and deformation mechanism of TiZrHfNb RHEA fabricated by laser directed energy deposition (L-DED) were researched under a dynamic compression test at a nominal strain rate of 3500 s−1. The L-DED TiZrHfNb alloy with columnar grains exhibits exceptional mechanical properties with a uniform dynamic flow stress of 1670 ± 10 MPa and a maximum plastic strain of 0.28 ± 0.03. The dislocation slipping and kinking behaviour dominate uniform plastic deformation. First, the dislocations are primarily confined within dislocation channels due to coplanar slip, and the evolution of them in dynamic compressive deformation process has undergone spacing refinement and crossing. Furthermore, the kinking behaviour induced by the lattice rotation with the T = <011> axis denotes the work hardening effect. Particularly, the second kinking behaviour effectively relieves local stress concentration and delays fracture before the formation of adiabatic shear band (ASB).

Microstructural origins of impact resistance of AlCoCrFeNi2.1 eutectic high-entropy alloy

In this work, the mechanical properties and related deformation mechanisms of AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) under dynamic loading were systematically investigated at a medium strain rate range from 500 to 1500 s−1. The EHEA presents significant strain rate hardening and continuous strain hardening without any adiabatic shear bands. The yield strength increases from 803.8 MPa at 500 s−1 to 1006.8 MPa at 1500 s−1. The strain rate sensitivity and strain hardening exponent were evaluated to be 0.198 and 0.567–0.683, respectively, responding for the superior capability to withstand dynamic loading. Post-deformation microstructural analysis reveals that increased strain rate induces higher dislocation density. In the FCC phase, dislocation planar slip at {111} planes dominates at 500 and 1000 s−1, while high dense dislocation walls (HDDWs) form at 1500 s−1 from coplanar slip along {111} planes. In the BCC phase, high dense nano-precipitates result in a significant precipitate-strengthening effect, featured by arrow-shaped dislocations, dislocation loops and dislocation tangles.

Four-dimensional microstructurally small fatigue crack growth in cyclically loaded nickel superalloy specimen

We nondestructively image a low-solvus high refractory (LSHR) nickel base specimen microstructure during cyclic loading to characterize crack propagation during the microstructurally small crack (MSC) growth regime. Micro-computed tomography (μCT) is used to characterize the material density and identify crack interfaces, while high-energy diffraction microscopy (HEDM) provides snapshots of the microstructural orientation field changes. From these measurements, data are computationally reconstructed and coregistered to describe the specimen’s configuration before load, before crack nucleation, and throughout the crack propagation/loading process. An MSC forms at a FIB-cut notch, which was previously placed to concentrate stress at the center of the specimen gauge, and propagates along a facet parallel to the local {111} crystallographic plane. Analysis of available crystallographic slip directions and measured point-to-point disorientation axes indicate probable single and coplanar slip activity along FCC octahedral planes within this region. Similar analyses of other crack propagation facets reveal generally slower growth along directions favoring zig-zag cross and codirectional cross slip, which are often disfavored at room temperature.

Early-Stage Dislocation Structures inside the Dislocation Channels of Face-Centered-Cubic Metals with Point Defect Clusters

Early-stage dislocation structures inside dislocation channels of rapid-cooled and tensile-deformed aluminum single crystals were investigated by using the bright field imaging mode in a scanning transmission electron microscope (STEM-BF). Inside dislocation channels, arrays of the prismatic dislocation loops originated from dislocations of the primary slip system, i.e., (111)[101], were mainly formed. Dislocations of the primary coplanar slip systems such as (111)[011] and (111)[110] were activated owing to internal stresses caused by the primary dislocations pile-up inside the cleared channels. The activated primary coplanar dislocations left the dislocation loops elongating along the edge dislocation directions behind them. Inter-dislocation-loop interactions occur especially at the arrays of the prismatic dislocation loops originated from dislocations of the primary slip systems and produce “butterfly shape” dislocation loops. As the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. When the interactions proceed, “tangled structures” would be formed around the arrays of the prismatic dislocation loops originated from dislocations of the primary slip system.

On the low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing

The low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing (NIA) was examined at different strain amplitudes. We showed that NIA improved the low cycle fatigue life (more than 7000 cycles) by optimising the precipitate configuration within 5.5 h while maintaining comparable mechanical properties (570 MPa for tensile strength) and conductivity (nearly 39% IACS) to conventional isothermal ageing, simultaneously. Experimental observation combined with molecular dynamic simulation revealed that precipitation configuration manipulated by NIA had a crucial effect on fatigue resistance. A great number of repeatedly sheared and locally destructed GP zones enhanced co-planar slip and slip localisation in the under-aged alloy during the early stage of NIA, responsible for the dramatic displacement steps on the surface and resultant poor fatigue performance. As the NIA further proceeded, moderately coarsened precipitates with an average dimension of 6.0 nm and elevated number density effectively impeded the dislocation movement and weaken the slip localisation to a great extent, improving the fatigue performance within a few hours.

Hydrogen-induced hardening of a high-manganese twinning induced plasticity steel

High-manganese twinning-induced plasticity (TWIP) steels exhibit high strain hardening, high tensile strength, and high ductility, which make them attractive for structural applications. At low tensile strain rates, TWIP steels are prone to hydrogen embrittlement (HE). Here though, we study the hardening and strengthening resulting from electrochemical hydrogen-charging of a surface layer of a Fe-26.9Mn-0.28C (wt.%) TWIP steel. We observed a 20% increase in yield strength following the electrochemical hydrogen-charging, accompanied by a reduction in ductility from 75% to 10% at a tensile strain rate of 10−3s−1. The microstructural evolution during tensile deformation was examined at strain levels of 3%, 5% and 7% by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to study the dislocation structure of the hardened region. As expected, the microstructure of the hydrogen-hardened and the uncharged regions of the material evolve differently. The uncharged areas show entangled dislocation structures, indicating slip from a limited number of potentially coplanar slip systems. In contrast, hydrogen segregated to the grain boundaries, revealed by the deuterium-labelled atom probe tomography, delays the dislocation nucleation by blocking dislocation sources at the grain boundaries. The charged areas hence first show the formation of cells, indicating dislocation entanglement from more non-coplanar slip systems. With increasing strain, these cells dissolve, and stacking faults and strain-induced ε-martensite are formed, promoted by the presence of hydrogen. The influence of hydrogen on dislocation structures and the overall deformation mechanism is discussed in details.

Microstructural evolution and twins formation under tensile test of Ni-free Mn-N austenitic stainless steel using laser powder bed fusion method

At the initial uniaxial tensile stage, predominant planar dislocation slip occurs to generate dislocation pile-ups and dipole bundles. Low-energy Taylor lattice structure is then formed by co-planar slip bands along the primary slip system along [001]γ and [111]γ directions to form primary twins. Increasing the straining and generation of numerous micro-bands and crystallographic defects led to the strong restriction for dislocation motion, enhancing the formation of secondary twins with stacking fault near the deformation twin's energy. Decreasing the mean slip band and micoband spacing and configuration of complex dislocation arrangement would cause the dislocation slip, generating an extensive number of dislocation walls and cell blocks, resulting in forming a noncrystalline structure with multi-orientation. Molecular dynamics (MD) analyses revealed that deformation began by nucleation of Shockley partial dislocations with a burger vector of 1/6 〈112〉 from none-periodic boundaries. Afterwards, dislocations junctions created Hirth dislocations with a Burger vector of 1/3 〈100〉, which causes strain hardening in the system. At this point, Shockley dislocations nucleated from Hirth dislocations and strain hardening increases.

Early-Stage Dislocation Structures inside the Dislocation Channels of Face-Centered-Cubic Metals with Point Defect Clusters

Early-stage dislocation structures inside dislocation channels of rapid-cooled and tensile-deformed aluminum single crystals were investigated by using the bright field imaging mode in a scanning transmission electron microscope (STEM-BF). Inside dislocation channels, arrays of the prismatic dislocation loops originated from dislocations of the primary slip system, i.e., , were mainly formed. Dislocations of the primary coplanar slip systems such as and were activated owing to internal stresses caused by the primary dislocations pile-up inside the cleared channels. The activated primary coplanar dislocations left the dislocation loops elongating along the edge dislocation directions behind them. Inter-dislocation-loop interactions occur especially at the arrays of the prismatic dislocation loops originated from dislocations of the primary slip systems and produce “butterfly shape” dislocation loops. As the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. When the interactions proceed, “tangled structures” would be formed around the arrays of the prismatic dislocation loops originated from dislocations of the primary slip system.

Phase dissolution strengthens and ductilizes a high-entropy alloy

Annealing is widely used in the thermomechanical processing of metallic alloys. Usually, ductility enhances while strength decreases because defects annihilate with the increase in annealing temperature. Here, we report a simultaneous improvement in strength and ductility of a TiZrNbTa high-entropy alloy by promoting phase dissolution with increasing annealing temperature. When annealing is performed from 800 to 1250°C, the tensile yield strength increases by 40% to 1003 ± 16 MPa, and the elongation is nearly doubled to 16.79% ± 1.03%. The annealing-induced phase dissolution promotes lattice distortion, contributing to enhanced lattice friction stress and yield strength. The correspondingly decreased interface mismatch alleviates stress concentration, and the stimulated formation of local chemical orders encourages coplanar slip and dislocation multiplication. Both occurrences boost ductility. These results not only expand our understanding of the phase dissolution and annealing effect in metallic materials, but also provide an insight into designing strong and ductile alloys.

Remarkable strength-impact toughness conflict in high-strength Al-Mg-Sc-Zr alloy fabricated via laser powder bed fusion additive manufacturing

Laser powder bed fusion (LPBF) of AlMgScZr alloys have a combination of high strength and good ductility, which may apply to damage-tolerant structures. However, there have been only limited studies on the toughness performance of this promising alloy system. In the present work, the impact toughness of an LPBF-processed AlMgScZr alloy, which is critical for the application that involves high-strain-rate loading, was systematically investigated using instrumented Charpy impact testing. The results demonstrated that the AB(as-built)-LPBF-processed AlMgScZr alloy had a good combination of yield strength and impact toughness (338 ± 3 MPa, 27.47 ± 1.04 J), which were about 2 and 4 times those of the AC(as-cast)-processed AlMgScZr alloy (160 ± 1 MPa, 6.21 ± 0.66 J). However, post-aging treatment greatly deteriorated the impact toughness. On one hand, the strain-hardening capability of the LPBF-processed AlMgScZr alloy was reduced after post-aging; on the other hand, the coplanar slip associated with the L12 superlattice structure of the secondary Al3(Sc,Zr) phase promoted the intergranular brittle fracture. This resulted in the remarkable strength-impact toughness conflict in the AA(as-aged)-LPBF-processed AlMgScZr alloy (512 ± 2 MPa, 4.27 ± 0.09 J). This study not only provides valuable insights into the dynamic fracture resistance of the LPBF-processed AlMgScZr alloys but also sheds light on the importance of choosing whether to post-age the LPBF-processed AlMgScZr alloy based on service loading conditions.

Crystal Plasticity Model Analysis of the Effect of Short-Range Order on Strength-Plasticity of Medium Entropy Alloys

Numerous studies have demonstrated the widespread presence of chemical short-range order (SRO) in medium and high entropy alloys (M/HEAs). However, the mechanism of their influence on macroscopic mechanical behavior remains to be understood. In this paper, we propose a novel dislocation-based model of crystal plasticity, by considering both the dislocation blocking and coplanar slip induced by SRO. The effect of SRO on the plastic deformation of CoCrNi MEAs was investigated. We found that the yield strength increases monotonically with increasing SRO-induced slip resistance, but the elongation first appeared to increase and then decreased. Further analysis suggested that the plastic elongation is a result of the competition between grain rotation-induced deformation coordination and stress concentration, which depends on the slip resistance of the SRO.

Path Dependency of Plastic Deformation in Crystals: Work Hardening, Crystallographic Rotation and Dislocation Structure Evolution

This paper reviewed the research progress of studies on the crystal rotation of single crystals that were deformed by tension and shear and the influences of crystal rotation and dislocation evolution on strain hardening behavior in crystals that were deformed with different initial orientations. The crystal rotation is entirely different depending on whether the single crystal was deformed by tension or shear. A three-stage work hardening behavior, which is not one of the intrinsic properties of materials, is generated when FCC metallic single crystals are deformed by tension along unstable oriFigurFigurentations, but single crystals do not exhibit this three-stage hardening behavior when they are deformed by simple shear at room temperature. Under tension, crystal rotation causes the transition from work hardening stage I to stage II, while the transition from work hardening stage II to III is caused by dislocation evolution. The evolution of the dislocation structure is related to deformation loading and can be classified into three types when a crystal is deformed by tension. Different from tension, shear stress can directly act on one of the 12 slip systems when a crystal is deformed by simple shear. When FCC single crystals are deformed by shear along the (11¯1)[110], (111)[112¯] and (001)[110] orientations, the single slip system, co-planar slip systems and co-directional slip systems are activated, respectively, and the crystals hardly rotate under the shear conditions. The slip direction of [110] forces the crystal to rotate toward the shear direction under simple shear. The dislocation tangles tend to form the dislocation cells and wall structures when multiple slip systems are activated under simple shear.

Lattice rotations during channel-die compression at high strain rates in copper single crystals of {110}, {112} and {123} orientations

The microstructure and texture evolutions of high purity copper single crystals of Br(1-10)[112], C(112)[11-1] and S(346)[63-5] initial orientations were investigated to evaluate the influence of high strain rate on the intensity of twinning and shear banding. The samples were deformed up to 60% in a channel-die at a strain rate of 7 × 105 s-1 using explosive energy to propel the punch. The microstructure was characterized by optical microscopy as well as scanning and transmission electron microscopy. High strain rates of deformation of C and S-oriented single crystals lead to massive deformation twinning. Despite the identification of three families of compact clusters of twins of two generations, the microstructure and texture of both single crystals are practically dominated by the twinning on only one co-planar slip plane. The common rotation of twin-matrix layers, within the narrow area, combined with deformation twinning in the re-oriented matrix are responsible for final SB texture formation. This mechanism contributes to the formation of orientations from the neighborhood of the {110}<100> orientation. The {110}<112> orientation, stable during deformation in channel-die in the range of ‘conventional’ strain rates, lost its stability giving rise to intense twinning and deformation bands across the deformed specimen.

Slip-induced mechanism of the orientation-dependent deformation banding in cold rolled Al-Mg alloy

The mechanism of deformation banding in cold rolling AlMg alloy is investigated in detail with regards to local lattice rotation and related dislocation slip. The formation of deformation bands (DBs) has a strong orientation dependency where they populate in grains with 〈111〉 and 〈001〉 directions close to the normal direction and 〈101〉 directions close to the rolling direction. On the contrary, DBs are hardly observed in grains close to the {101}〈111〉 orientations. Experimental and numerical analysis indicates that activated secondary slips and their related lattice rotation account for this orientation dependency. The activation of secondary single slip or secondary coplanar slip plays a vital role in the formation of DBs. DBs are formed when the primary and secondary slips lead to sustainable opposite lattice rotations, which gradually split the grain and develop into DBs. Meanwhile, the tendency of the development of Goss texture during the early rolling stage is observed to arise from the generation of DBs.

Notch strength and notch fracture mechanisms of a cast Mg-Gd-Y alloy

This study aims to elucidate the mechanical properties and fracture mechanisms of cast Mg-6Gd-3Y-0.4Zr (wt.%) (GW63) alloy in the presence of geometrical discontinuities induced by notches. Notch strength is evaluated by room-temperature tensile tests using circumferentially notched bars of different notch acuity. Finite element calculations suggest that notch fracture is controlled by the von Mises stress and effective plastic strain at the notch tip. Through fractographic observations and EBSD analysis on samples subjected to interrupted tests, a cleavage micro-cracking behavior uncommon for conventional Mg alloys is found to dominate notch fracture. Grain-sized cleavage microcracks tend to occur along the basal plane of grains favorably oriented for basal slip, which is explained by crack nucleation at co-planar slip bands blocked by the grain boundary according to Smith and Barnby's theory. Failure is initiated by the accumulation and coalescence of such microcracks in the notch vicinity. The unexpected cleavage micro-cracking behavior is mainly ascribed to the RE solutes that suppress {10–12} twinning, and is responsible for the limited deformation capacity and notch strength of the cast-T6 GW63 alloy with coarse grains and significant strengthening due to precipitation.

Rolling Texture of Cu–30%Zn Alloy Using Taylor Model Based on Twinning and Coplanar Slip

A modified Taylor model, hereafter referred to as the MTCS (Mechanical-Twinning-with-Coplanar-Slip)-model, is proposed in the present work to predict weak texture components in the shear bands of brass-type fcc metals with a twin–matrix lamellar (TML) structure. The MTCS-model considers two boundary conditions (i.e., twinning does not occur in previously twinned areas and coplanar slip occurs in the TML region) to simulate the rolling texture of Cu–30%Zn. In the first approximation, texture simulation using the MTCS-model revealed brass-type textures, including Y{1 1 1} &lt;1 1 2&gt; and Z{1 1 1} &lt;1 1 0&gt; components, which correspond to the observed experimental textures. Single orientations of C(1 1 2)[1¯ 1¯ 1] and S’(1 2 3)[4¯ 1¯ 2] were applied to the MTCS-model to understand the evolution of Y and Z components. For the Y orientation, the C orientation rotates toward T(5 5 2)[1 1 5] by twinning after 30% reduction and then toward Y(1 1 1)[1 1 2] by coplanar slip after over 30% reduction. For the Z orientation, the S’ orientation rotates toward T’(3 2 1)[2 1¯ 4¯] by twinning after 30% reduction and then toward Z(1 1 1)[1 0 1¯] by coplanar slip after over 30% reduction.

Analysis of deformation bands and cell structure in cyclically deformed near-[formula omitted]-oriented copper single crystals

In this study, we investigate the deformation bands (DBs), viz. DBI and DBII, and cell bands (CBs) in the near-1¯11-multiple-slip-oriented single crystals during cyclic deformation. Three different types each of DBIs [(111), (1¯1¯1), and (1¯11¯)] and DBIIs [(2¯11), (1¯21), and (1¯12)] can be summarized as the primarily activated slip plane and the combination of activated primary and coplanar slip systems, respectively. The formation planes of CBs [(121), (1¯1¯2), and (2¯11¯)] are perpendicular to the cross-slip plane and parallel to the primary slip direction. The highly symmetrical characteristics of multiple-slip-oriented single crystals can produce inconsistent correspondences between DBs and CBs.

Evolution of texture and precipitates in Al–Cu–Li alloy during deformation and heat treatment and its effect on anisotropy

Al–Li alloy offersa great advantage over traditional alloys and it is the ideal material for today’s aerospace industry. Therefore, here we examined the Al–Cu–Li alloy as the third generation of Al–Li alloy. Deformation and heat treatment on the texture and precipitate type and distribution of Al–Cu–Li alloy were studied by mechanical properties test, XRD, and TEM analysis. The effect of texture and precipitate on the plane anisotropy of yield strength was discussed. The results showed thatBrass texture was mainly formed in the deformation process. On the one hand, increasethe pass deformation or cross rolling,the total volume fraction of the texture (S{123}〈634〉, G{011}〈100〉, {011}〈011〉) was increased, which lowered thesheet anisotropy. While on the other hand, the δ′ phase precipitated rapidly during solution treatment and easily caused a coplanar slip, increasing plane anisotropy. However, owing to the difference distribution of the aged precipitated T1 on the two group of {111} habit plane, the strengthening effect of the Al–Cu–Li alloy sheet was weak at 45° direction, but stronger atthe rolling direction (0°) and transverse direction (90°).

Dislocation density-based plasticity model from massive discrete dislocation dynamics database

We present a dislocation density-based strain hardening model for single crystal copper through a systematic coarse-graining analysis of more than 200 discrete dislocation dynamics (DDD) simulations of plastic deformation under uniaxial tension. The proposed constitutive model has two components: a generalized Taylor relation connecting resolved shear stresses to dislocation densities on individual slip systems, and a generalized Kocks-Mecking model for dislocation multiplication. The DDD data strongly suggests a logarithmic dependence of flow stress on the plastic shear strain rate on each slip system, and, equivalently, an exponential dependence of the plastic shear strain rate on the resolved shear stress. Hence the proposed generalized Taylor relation subsumes the Orowan relation for plastic flow. The DDD data also calls for a correction to the Kocks-Mecking model of dislocation multiplication to account for the increase of dislocation density on slip systems with negligible plastic shear strain rate. This is accomplished by allowing the multiplication rate on each slip system to include contributions from the plastic strain rates of the two coplanar slip systems. The resulting constitutive model successfully captures the strain hardening rate dependence on the loading orientation as predicted by the DDD simulations, which is also consistent with existing experiments.