Twin Lamellae Research Articles

Study on the plasticity improvement mechanism and grain refinement of AZ80 Mg alloy under cryogenic multi-directional forging

The introduction of twin lamellae and dislocation interaction through Multi-directional forging (MDF) at room temperature (RT) has been reported to significantly improve the mechanical properties of Mg alloys. However, the Mg alloys typically exhibited poor plasticity at RT, making it highly susceptible to cracking. This study further enhanced the deformation capacity of AZ80 Mg alloy by adding a pre-cryogenic step before MDF. Compared to direct MDF at RT, MDF at cryogenic temperature (CT, −196 °C) further increased the alloy's deformation capacity from 0.72 (4P) to 1.08 (6P), with the tensile yield strength (TYS) and ultimate tensile strength (UTS) increasing to ∼420 MPa and ∼512 MPa. The alloy subjected to cryogenic MDF exhibited denser twin lamellae and a higher proportion of {10–12}-{10–12} twin interactions. The more frequent twin interactions under CT inhibited excessive twin growth and further increased the dislocation density of the matrix, facilitating the nucleation of new twins. Additionally, with increasing deformation passes, the texture of the RT specimens became more single, while the CT specimen showed a trend toward texture dispersion. The study elucidated that poor forgeability of the alloy under MDF at RT was hindered by unfavorable texture and limited twin activation. However, this issue was mitigated under CT, where the activation of more atypical twins with low Schmid factors (SFs) significantly enhanced the deformation coordination ability. Furthermore, the activation of atypical twins and extensive twin interactions promoted texture dispersion in CT specimens, which facilitated the activation of various slips/twins and suppressed excessive twinning-induced hardening, thereby improving forgeability.

The toughening effect of twins on fracture in nanotwinned Cu during cyclic loading

Extensive studies have been focused on improving the fatigue performance of materials, as fatigue fracture is a primary mode of failure in material applications for structural components. The introduction of high-density twins emerges as a novel strategy for enhancing fracture toughness; however, the mechanisms driving this enhancement during fatigue remain elusive. In this work, the impact of twins on fracture toughness in nanotwinned Cu was investigated with correlated transmission electron microscopy under cyclic loading. The results revealed three toughening mechanisms, i.e., bridging, bending of twin lamellae and ductile cracking along twin boundaries. The occurrence of the three toughening mechanisms strongly depends on twin thickness and cracking orientations, as different dislocation modes are activated in varying scenarios. Specifically, bridging and ductile cracking along twin boundaries are facilitated by the pile-up and continuous emission of hard mode I dislocations, respectively, while the bending of twin lamellae is governed by the accumulation of same-signed soft mode dislocations triggered by crack-tip stress. These findings unveil the underlying toughening mechanisms of twins in nanotwinned metals during cyclic loading and offer valuable guidance for the design of high-toughness nanolayered materials.

Modulation on the twinning microstructure of Cu nanowires and its effect on oxidation behaviour

In this study, nanotwinned Cu nanowires (nt-Cu NWs) with varying twin characteristics were fabricated via pulse electrodeposition using a novel H3BO3 based electrolyte, and their impact on the oxidation process was investigated. The density and direction of twin lamellas within nt-Cu NW were adjusted by modifying the concentration of H3BO3 and the current density. Specifically, the average twinning thickness of Cu NWs increases from 26.7 nm to 424.5 nm as the concentration of H3BO3 increases from 0 g/L to 6 g/L. Moreover, three types of nt-Cu NWs with distinct twinning directions, namely waterfall type, bevel type, and ladder type were fabricated by changing the current density from −0.16 ASD to -1.6 ASD (vs Ag/AgCl). A nanowire Winand (NWW) diagram was established to describe the change of twin density and direction based on the difference between the growth behaviour of thin films and nanowires. The thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) were further used to characterize the oxidation process of nt-Cu NWs. It revealed that the oxide thickness was closely related to the thickness of twin lamellas for the ladder type nt-Cu NWs. Also, the waterfall type Cu NWs exhibited a unique bead-chain like oxide morphology and a slower oxidation rate through the impact on the nanoscale Kirkendall effect (NKE). This study provides valuable insights into the controllable fabrication of nt-Cu NWs and Cu oxide NWs.

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

Role of slip in hydrogen-assisted crack initiation in Ni-based alloy 725.

We conduct in situ tensile straining experiments to investigate the role of hydrogen and slip in crack initiation in nickel-based alloy 725. Our experiments reveal no tendency for hydrogen to enhance localized slip and no necessity of slip for crack initiation. We use electrochemical charging to introduce hydrogen into samples, melt extraction to measure hydrogen content, and digital image correlation to analyze localized plastic strains during in situ tensile tests in a scanning electron microscope. Cracks initiate both in regions with and without nearby localized slip. Moreover, the fraction of cracks initiating with no nearby slip is greater at higher hydrogen content. Slip-assisted crack initiation generally occurs at locations where intergranular slip is arrested, especially at intersections of slipping coherent twin boundaries with thin twin lamellae. Cracks that initiate without nearby slip occur at a wider variety of microstructural features, including inclusions, triple junctions, and surface flaws.

Effect of Gd on preparation of nanocrystalline Mg alloys via cold rotary swaging

The effects of Gd element on the nanocrystallization of Mg alloys during swaging were studied by means of the microhardness test, tensile test, and transmission electron microscopy. The results indicate that it is impossible to achieve nanocrystallization in pure Mg via rotary swaging, which may be attributed to the difficulty of forming dislocation arrays. In contrast, adding Gd alone can achieve a remarkable grain refinement effect after swaging. In the early stage of swaging, twin lamellae are all formed in the pure Mg and Mg−Gd alloys. As the swaging process continues, more non-basal dislocations are formed in the Mg−Gd alloys than in pure Mg. This promotes the formation of dislocation arrays and the resulting nanocrystallization in the Mg−Gd alloys. As the Gd content increases, the refinement effect is obviously enhanced. The critical content for the refinement of the grain size below 100 nm is about 5.4 wt.%.

Ilmenite phase transformations in suevite from the Ries impact structure (Germany) record evolution in pressure, temperature, and oxygen fugacity conditions

Abstract Aggregates of ilmenite with varying amounts of rutile, ferropseudobrookite, and pseudorutile in suevites from the Ries impact structure have been analyzed by light microscopy, analytical scanning electron microscopy, electron microprobe analysis, and Raman spectroscopy to constrain their formation conditions. The tens to hundreds of micrometer aggregates comprise isometric ilmenite grains up to 15 µm in diameter that form a foam structure (i.e., smoothly curved grain boundaries and 120° angles at triple junctions). Grains with foam structure show no internal misorientations, indicating a post-impact formation. In contrast, ilmenite grains with internal misorientation occurring in the core of the aggregates are interpreted as shocked remnant ilmenite originating from the target gneisses. They can contain twin lamellae that share a common {1120} plane with the host, and the c-axis is oriented at an angle of 109° to that of the host. Similarly, the new grains with foam structure display up to three orientation domains, sharing one common {1120} plane for each pair of domains and c-axes at angles of 109° and 99°, respectively. This systematic orientation relationship likely reflects a cubic supersymmetry resulting from the transformation of the initial ilmenite upon shock (>16 GPa) to a transient perovskite-type high-pressure phase (liuite), subsequent retrograde transformation to the polymorph wangdaodeite, and then back-transformation to ilmenite. Whereas, the new grains with foam structure formed from complete transformation, the twin domains in the shocked ilmenite are interpreted to represent only partial transformation. Ferropseudobrookite occurs mostly near the rim of the aggregates. An intergrowth of ferropseudobrookite, ilmenite, and rutile, as well as magnetite or rarely armalcolite occurs at contact with the (devitrified) matrix. The presence of ferropseudobrookite indicates high temperature (>1140 °C) and reducing conditions. The surrounding matrix provided Mg2+ to form the ferropseudobrookite-armalcolite solid solution. Rutile can occur within the aggregates and/or along the ilmenite boundaries; it is interpreted to have formed together with iron during the decomposition of ilmenite at lower temperatures (850–1050 °C). We suggest magnetite in the rims formed by electrochemical gradients driven by the presence of a reducing agent, where Fe2+ within ilmenite diffused toward the rim. Subsequent cooling under oxidizing conditions led to the formation of magnetite from the iron-enriched rim as well as pseudorutile around ilmenite grains. Our study demonstrates that the specific crystallographic relationships of ilmenite grains with foam structure indicate a back-transformation from high (shock) pressures >16 GPa; moreover, the presence of associated Fe-Ti-oxides helps indicate local temperature and oxygen fugacity conditions.

Asymmetric tension-compression mechanical behaviors of extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy with double-peak fiber texture

To clarify asymmetric mechanical behaviors of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy during room temperature tension and compression, the evolution of microstructure, texture and strain hardening rate were systematically investigated when loading along the extrusion direction (ED). The results found that the as-extruded alloy exhibited <101̅0>-<112̅0> double-peak fiber texture, leading to obvious tension compression yield asymmetry and compressive yield plateau. Fiber texture kept stable and only texture intensity gradually decreased with the increasing tensile strain, while fiber texture was exhausted due to the extension twinning and <0001> texture component was aggravated with the increasing compressive strain. Moreover, twinning parts within grains with <101̅0> fiber texture was re-oriented to the c-axis paralleling to the ED, and that in <112̅0> fiber texture deviated about 30° away from the ED. This soft orientation would be finally rotated to the <0001> paralleling to the ED under the action of basal <a> slip, explaining why the <0001> pole dispersed at first and then concentrated with the increasing compressive strain. Three stages of strain hardening rate were both observed for tensile and compressive tests, while increment of Stage II was only observed under compression. Stage II in compression could be further divided into two parts with different slopes. Such phenomenon was seldom reported in Mg alloys, which was caused by different propagation and re-orientation of extension twinning in double-peak fiber texture, and weakened the effect of twin lamella segmentation with the increasing compressive strain. Findings in this work given us a better understanding of asymmetric tensile-compressive mechanical behaviors of extruded Mg alloys with double-peak fiber texture.

New insights into lamellar and twinning transformations in TiAl/Nb composites with core-shell structure during high temperature annealing

Lamellar and twinning transformations play a crucial role in stabilizing the ordered phases in TiAl based composites for property improvement. In this work, we have systematically investigated the lamellar and twinning transformations in TiAl/Nb composites with core-shell structure and proposed novel insights on γ → α2 and γ → γT processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α2 and γT twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γT can nucleate in B2 regime by accidental misalignment of atoms on a 110B2 plane adhering to Kurdjumov-Sachs relationship, and then grow even into α2 lath driven by the release of internal stress via Shockley partial slips on the 111γ plane. The formation of γT mediated by 9R involves γ → 9R and γ ← 9R → γT transformations. Among them, a full period of atomic stacking …ABC|BCA|CAB… (LPSO) can be achieved through three Shockley partials being sequentially activated on the C, A and B atomic layers in the stacking of …ABC|ABC|ABC… (L10) with the total shift of 3δA. Then 9R will change synchronously to γ and γT once the migrating Shockley partials react with Frank partials to yield a full dislocation, i.e., δA + Dδ→DA and δA + δC + δD→BD, which can also be considered as detwinning and twinning behaviors, respectively. Additionally, the transition of bulk γ to α2 or γT lamellae is closely associated with the stacking faults, i.e. whether the activated atomic layer is located at the boundary or periphery of the updated stacking fault region.

Threshold and structure of HCP/FCC nucleation in BCC iron under arbitrary triaxial compression: Atomistic simulations

Shear deformation is one of the primary factors determining the threshold of structural transition (ST). It is important to quantitatively express the relationship between shear deformation and the ST threshold for the in-depth development of ST dynamics models. This work used classical molecular dynamics methods to study the effect of shear deformation on the ST in iron by controlling the triaxial ratio of compression. Based on the simulation results, there are significant differences in the microstructure for different loading paths. HCP lamellar twins, HCP-FCC thin twins, and crossed HCP twins embedded with parallel hexahedral FCC grains were all observed, as uniaxial loading transitions to equiaxial loading. The changes in pressure and the softening range of shear stress under different triaxial compressions were revealed. By introducing the strain triaxiality (i.e., the ratio of shear strain to volumetric strain), a general expression of the pressure threshold of ST under high strain rate compression-shear was proposed. In addition, whether the close-packed plane is easily activated under different strain environments was explained based on the results of stacking fault energy.

Effect of high current pulsed electron beam on surface microstructure and properties of cold-rolled austenitic stainless steel

High current pulsed electron beam (HCPEB) was used to modify the surface microstructure and performance of AISI 304 stainless steel with different cold-rolling deformation. The phase, microstructure, microhardness, and corrosion performance were characterized and analyzed after cold-rolling and HCPEB treatment. The results show that after cold-rolling, deformation-induced martensites (DMs) are generated in the deformation twin lamellae or at the intersections of multiple twin lamellae, resulting in a grid-like morphology, smaller size, and a certain orientation relationship with the parent austenite. The content of DMs increases with increasing cold-rolling deformation. After HCPEB treatment, many craters with different sizes and similar densities are produced on the sample surface, and the DMs are transformed into austenite again in the surface layer. The difference is that the grain size of the regenerated austenite after HCPEB is much smaller than that of the initial austenite (before cold-rolling). Such high-density equiaxed ultrafine grains (UFGs) of austenite formed after HCPEB present a twin feature, which is the inheritance of the orientation relationship between DMs and austenite formed during rolling. HCPEB treatment can effectively reshape the phase and microstructure of the surface layer of the cold-rolled deformed austenitic steel but reduce the surface hardness and increase pitting corrosion tendency.

The Microstructures and Deformation Mechanism of Hetero-Structured Pure Ti under High Strain Rates.

This study investigates the microstructures and deformation mechanism of hetero-structured pure Ti under different high strain rates (500 s-1, 1000 s-1, 2000 s-1). It has been observed that, in samples subjected to deformation, the changes in texture are minimal and the rise in temperature is relatively low. Therefore, the influence of these two factors on the deformation mechanism can be disregarded. As the strain rate increases, the dominance of dislocation slip decreases while deformation twinning becomes more prominent. Notably, at a strain rate of 2000 s-1, nanoscale twin lamellae are activated within the grain with a size of 500 nm, which is a rarely observed phenomenon in pure Ti. Additionally, martensitic phase transformation has also been identified. In order to establish a correlation between the stress required for twinning and the grain size, a modified Hall-Petch model is proposed, with the obtained value of Ktwin serving as an effective metric for this relationship. These findings greatly enhance our understanding of the mechanical responses of Ti and broaden the potential applications of Ti in dynamic deformation scenarios.

Dynamic recrystallization of commercially pure titanium during cryogenic compression

In this study, a systematic investigation on the twin-induced dynamic recrystallization (DRX) of commercially pure titanium (CP-Ti) was carried out under uniaxial compression at room (RT) and cryogenic temperature (CT, −150 °C). The compression tests were intentionally interrupted at 0.02, 0.05 and 0.1 strain levels at both deformation temperatures and a detailed post-mortem analysis was performed via electron backscattered diffraction (EBSD) to examine the progressive evolution of microstructure. The results revealed that two major types of twins i.e. {10−12} extension twin (ETW) and {11−22} compression twin (CTW) were effectively activated at both deformation temperatures. During RT deformation, increased strain levels resulted in the higher evolution of ETWs and CTWs, where numerous twin lamellas, lateral twin thickening and twin-twin interactions (ETW-ETW, ETW-CTW) were observed. On the other hand, CT deformation led to significant grain refinement with the average grain size reduced by 94% (3.7 μm for CT-10 sample as compared to 63 μm of initial material). The recrystallization kinetics were understood with respect to three region of interests (ROIs). The DRX mechanism was identified as tDRX, where intersectional {10–12} ETWs and {11–22} CTWs network provided the preferential sites for the nucleation of DRXed grains due to high strain energy from the dislocation pile-ups at the TBs.

Formation of gradient microstructure and elimination of tension-compression yield asymmetry of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy via free-end torsion

Evident tension-compression yield asymmetry limited the potential use of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy as a structural material. Pre-torsion deformation (free-end torsion) was conducted on the extruded alloy and its effects on microstructure and tensile-compressive mechanical properties were investigated in this study. It was found that the as-extruded alloy exhibited homogenous microstructure and <100>−<110 > double-peak fiber texture. Gradient structures along the radial direction were formed after pre-torsion, which contained gradient distribution of dislocations, grain size, {102} extension twin lamellas and texture components. <100>−<110 > double-peak fiber texture was gradually weakened with the torsional angle increasing from 90° to 135° and 180°, and it was related with the re-orientation of extension twinning and dislocation movement. Moreover, extension twinning was more easily to be activated in grains with <110> fiber texture component, and this phenomenon was revealed by calculating the global Schmid factor of extension twinning under pure shear stress state. Tensile and compressive yield strength increased monotonically with the increasing torsional angle. Meanwhile, ductility almost remained unchanged. Tension-compression yield asymmetry was gradually reduced, and it was eliminated after 180° pre-torsion, which was attributed to the texture weakening. The studying provided a promising and cost-effective method to enhance yield strength and reduce tension-compression yield asymmetry while also keep ductility for extruded Mg alloys.

Enhanced strength of AlCoCrCu0.5FeNi high entropy alloy thin films reinforced by multi-phase hardening and nanotwins

High entropy alloy (HEA) thin films have become increasingly popular because they exhibit favorable properties but with lower material consumption. Here, this research demonstrates that annealing of AlCoCrCu0.5FeNi thin films results in multiphase hardening and the formation of nanotwins. HEA thin films were atmospherically annealed at different temperatures (300 – 700 °C) for 5 h. A thin dense oxide layer consisting of Al2O3 and Cr2O3 appeared at the top surface of the 700 °C annealed film. This oxide layer endows the HEA thin film with excellent antioxidant properties. As such, vacuum and inert gas might not be required for HEAs annealing process at up to 700 °C. A phase transformation was observed in the film annealed at 700 °C for 5 h from X-ray diffraction patterns. Results from transmission Kikuchi diffraction and energy dispersive X-ray spectroscopy mapping indicated the formation of three new phases, including the FeCo phase, Cr-rich phase and Cu nano-clusters. Variations also happened in HEA FCC matrix where two types of texture existed in the HEA FCC grains i.e., {012} <110> and {112} <110>. Of particular interest was that high-resolution transmission electron microscopy revealed a high density of annealing nanotwins among the HEA FCC grains, including lamellar and co-axial twins. The influence of these structural phenomena on mechanical properties was evaluated. The results showed that the hardness and modulus of 700 °C annealed HEA thin film increased by 25% and 24% to 8.4 ± 0.2 GPa and 149.4 ± 6.1 GPa when compared to the properties of as-deposited films.

Effect of cerium alloying on microstructure, texture and mechanical properties of magnesium during cold-rolling process

Hot-extrusion and cold-rolling were conducted on Mg–Ce binary alloys to explore the effect of cerium (Ce) on microstructure, texture and mechanical properties of Mg alloys. The addition of Ce results in significant grain refinement both in as-cast and hot-extruded samples. The basal texture is also weakened after extrusion and an inclined basal texture is formed with Ce addition. The strength and elongation of the Mg–Ce alloys are improved simultaneously due to such grain refinement and texture weakening effect. After cold-rolling, plenty of twins are found in the pure Mg and AZ31 plates while grains in the Mg-0.3Ce plate deform more uniformly without lamellar twin structure. Furthermore, Mg-0.3Ce alloys own strong and continuous strain hardening because Ce atoms and Mg–Ce precipitated phases serve as obstacles for dislocation slip.

Influence of additives induced microstructural parameters on mechanical behavior of (111)-oriented nanotwinned microcrystalline copper

Depth sensing nanoindentation experiments were performed to investigate the strain rate dependent-independent behaviors of nanotwinned microcrystalline Cu. The specimens were electro-deposited using three different additives, which resulted in controlled variations in microstructural parameters, viz., grain size, and twin spacing. One of the three samples with the lowest twin lamella spacing showed a lower stress exponent, which might result in enhanced ductility. The stress exponent and activation volume increased with increasing twin spacing. In contrast, the hardness revealed a dependence on the effective length scale considering the weighted average of the twin spacing and grain size. The mechanical properties obtained by introducing nanotwins in coarser initial grain size are comparable to nanotwinned nanocrystalline materials. Thus, the possibility of using a coarser initial grain size for nanotwinned materials paves the way for a new paradigm with a broader range of nanotwinned materials.

Formation of nanocrystalline AZ31B Mg alloys via cryogenic rotary swaging

A bulk nanocrystalline AZ31B Mg alloy with extraordinarily high strength was prepared via cryogenic rotary swaging in this study. The obtained alloy shows finer grains, higher strength, and a negligible tension-compression yield asymmetry, compared with that prepared via room-temperature rotary swaging. Transmission electron microscopy investigations showed that at the initial stage, multiple twins, mostly tension twins, were activated and intersected with each other, thereby refining the coarse grains into a fine lamellar structure. Then, two types of nanoscale subgrains were generated with increasing swaging strain. The first type of nanoscale subgrain contained twin boundaries and low-angle grain boundaries. This type of subgrain appeared at the twin-twin intersections and was mainly driven by high local stress. The second type of nanoscale subgrain was formed within the twin lamellae. The boundaries of this type of subgrain did not contain twin boundaries and were transformed from massive dislocation arrays. Finally, randomly oriented nanograins were obtained via dynamic recrystallization, under the combined function of deformation heat and increased stored energy. Compared with room-temperature rotary swaging, cryogenic rotary swaging exhibits a slower grain refinement process but a remarkably enhanced grain refinement effect after the same five-pass swaging.

Self-Consistent Crystal Plasticity Modeling of Slip-Twin Interactions in Mg Alloys

Parsing the effect of slip-twin interactions on the strain rate and thermal sensitivities of Magnesium (Mg) alloys has been a challenging endeavor for scientists preoccupied with the mechanical behavior of hexagonal close-packed alloys, especially those with great latent economic potential such as Mg. One of the main barriers is the travail entailed in fitting the various stress−strain behaviors at different temperatures, strain rates, loading directions applied to different starting textures. Taking on this task for two different Mg alloys presenting different textures and as such various levels of slip-twin interactions were modeled using visco-plastic self-consistent (VPSC) code. A recently developed routine that captures dislocation transmutation by twinning interfaces on strain hardening within the twin lamellae was employed. While the strong texture was exemplified by traditional rolled AZ31 Mg alloys, the weak texture was represented by ZEK100 Mg alloy sheets. The transmutation model incorporated within a dislocation density based hardening model showed enhanced flexibility in predicting the complex strain rate and thermal sensitive behavior of Mg textures’ response to various mechanical loading schemes.

Grain and twin boundaries dependent mechanical behavior of FeCoCrNiCu high-entropy alloy

The nanoindentation response of FeCoCrNiCu high-entropy alloy is revealed through molecular dynamics simulation. The influences of various grain sizes and twin lamellae thicknesses on the mechanical characteristics and plastic deformation are carefully studied. The result indicates that the indentation force and substrate hardness increase as the grain size increases from 55.16 Å to 114.46 Å and the twin layer thickness rises from 6.74 Å to 30.04 Å, which reveals the inverse Hall-Petch effect in the relationship between the material strength versus grain size and the twin lamellae thickness. The shear strain and residual stress are converged around the indentation regions and then enhanced deep into the substrate along the grain boundary and twin boundary. The presence of grain boundaries significantly affects the movement of the atoms, resulting in an asymmetric dispersion of the atomic displacement vector during the deformation. Moreover, the surface morphology reveals that the pile-up height generally increases with increasing the grain size and decreasing the twin lamellae spacing. The microstructural evolution suggests that the rotation of the grain related to grain boundary sliding and the migration of the grain boundary are the dominant mechanism in the deformation with grain size reduction. Moreover, the dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic, which significantly affects the plastic deformation of the material. The result also shows that the dislocation density tends to increase with increasing grain size and twin lamellae thickness. The figure shows the simulation model of FeCoCrNiCu high-entropy alloy (a), atomic displacement vectors of samples with different grain sizes (b), atomic configuration of the specimens with various twin lamellar thicknesses (c). ● The inverse Hall-Petch effect is recognized with the variation of grain size and twin lamellae thickness. ● The deformation mechanism of FeCoCrNiCu HEA is significantly dependent on the grain/twin boundaries. ● The rotation of grain related to grain boundary sliding and the grain boundary migration are dominant mechanism. ● The dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic.