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The twin boundaries and inherent lattice strain of five-fold twin (5-FT) structures offer a promising and innovative approach to tune nanocrystal configurations and properties, enriching nanomaterial performance. However, a comprehensive understanding of the nonclassical growth models governing 5-FT nanocrystals remains elusive, largely due to the constraints of their small thermodynamically stable size and complex twin configurations. Here, we conducted in situ investigations to elucidate the atomic-scale mechanisms driving size-dependent and twin configuration-related aggregation phenomena between 5-FT and other nanoparticles at the atomic scale. Our results reveal that surface diffusion significantly shapes the morphology of aggregated nanoparticles, promoting the symmetrical formation of 5-FT, especially in smaller nanoparticles. Moreover, the inherent structural characteristics of 5-FT mitigate the dominance of surface diffusion in its morphological evolution, retarding the aggregation evolution process and fostering intricate twin structures. These findings contribute to advancing our capacity to manipulate the configuration of twinned particles, enabling more predictable synthesis of functional nanomaterials for advanced engineering applications. The mechanisms underlying the nonclassical growth of penta-twinned gold nanocrystals remain unclear. Here, the authors elucidate the atomic-level processes that drive size-dependent and twin configuration-related aggregation phenomena. [ABSTRACT FROM AUTHOR]

Ti48Al2Cr2Nb high-temperature alloy consisting of γ and α 2 phases, whose room-temperature brittleness limits its use in other lower temperature ranges, has been laser shocked to improve its surface properties. The study of the shock response and plastic deformation behaviour of material then provides further insight into the laser shock peening mechanism. In this paper, the molecular dynamics method and piston impact method are used to simulate laser shock peening of lamellar two-phase TiAl alloys to study the shock response and plastic deformation of material at different shock velocities, as well as the effects of shock velocity and two-phase interface on them. The results show that, in terms of shock response, the location of elastic–plastic wave separation as well as the atomic velocity and stress magnitude are affected by the shock velocity, elastic strain energy and interfacial energy, and some of stresses change significantly at the semi-coherent interface. At lower shock velocities, it is mainly stacking faults and twin boundaries in γ phase that initiate phase transitions across the interface to the α 2 phase, whereas at higher velocities, in addition to earlier α 2 phase transitions, clustered amorphous atoms and intermediate phases are also observed. The dislocations at the semi-coherent interfaces are affected by both interface type and shock velocity. The mechanical properties of lamellar two-phase TiAl are improved by the effect of laser shock at different shock velocities. These results provide experimental and theoretical guidance for improving the room-temperature properties of two-phase TiAl alloys. [ABSTRACT FROM AUTHOR]

The formation mechanism of FCC phase on prismatic-basal (PB) interfaces in { 10 1 ¯ 2 } < 1 ¯ 011 > twin in Zircaloy-4 under cyclic deformation at the strain level of 1.0% was systematically investigated by using transmission electron microscopy. Results showed that { 10 1 ¯ 2 } < 1 ¯ 011 > twin was presented in α-Zr grain and PB interfaces can be introduced into the twin boundaries. Under cyclic stresses, the emission of 1/3[ 0 1 ¯ 10 ] and 1/3 [ 01 1 ¯ 0 ] Shockley partial dislocations would be initiated on the PB interfaces of the twin and slipped symmetrically on every other ( 0001 ) planes of the twin, resulting in the HCP → FCC phase transformation. Moreover, when the shear stresses were large enough, 1/6 < 1 ¯ 1 ¯ 2 > Shockley partial dislocations were activated during cyclic deformation and the formation of { 111 } SFs in FCC phase was presented. [ABSTRACT FROM AUTHOR]

Fractional calculus is an important mathematical tool that is widely used in control systems. It is established in the literature that fractional order models are more accurate and more effective in system modelling. In this study, an alternative and novel technique is proposed to identify the fractional order time-delayed model of an unknown system. The method is based on obtaining the approximate stability boundary locus (SBL) curve of the unknown system by applying three different experimental tests. Three points on the SBL curve are determined by the experimental tests and then the parameters of the fractional order time-delayed model are computed by solving the nonlinear systems of equation. The system model with double fractional order element plus a time delay is obtained using the proposed method. The proposed method is explained through simulations on a twin rotor system. The proposed method is also used in model order reduction calculation of the higher order transfer functions. [ABSTRACT FROM AUTHOR]

In this study, the microstructural evolution of Ag films with different amounts of (111)-oriented grains and coincident site lattice Σ3 (CSL-Σ3) twin boundaries after low-temperature direct bonding was demonstrated. The highly (111)-oriented surface grains provided the rapid surface diffusion paths and the special twin boundaries contributed to structural stability of the Ag films during bonding. Combining both, perfect bonding can be achieved at a low temperature of 150 °C for 60 min in the bonding of Ag films with 99.4% (111) grains. Owing to the low recrystallization temperature, bonding interfaces with only a few small voids were observed after bonding at 250 °C for 30 min. Annealing twins grew across the bonding interface and extended to the other side of the Ag films, implying a strong bonding strength. Though increasing the bonding temperature and period could help to achieve a bonding interface with the least voids formed, it was suggested that increasing the amount of (111) grains on the Ag film bonding surfaces is the key point to achieved perfect bonding at temperatures lower than 250 °C. This would provide great potential for Ag as a suitable material for applications in three-dimensional integrated circuit (3D-IC) interconnections. [ABSTRACT FROM AUTHOR]

Semiconducting transition metal dichalcogenides (MX2) occur in 2H and rhombohedral (3R) polytypes, respectively distinguished by anti-parallel and parallel orientation of consecutive monolayer lattices. In its bulk form, 3R-MX2 is ferroelectric, hosting an out-of-plane electric polarisation, the direction of which is dictated by stacking. Here, we predict that twin boundaries, separating adjacent polarisation domains with reversed built-in electric fields, are able to host two-dimensional electrons and holes with an areal density reaching ~ 1013cm−2. Our modelling suggests that n-doped twin boundaries have a more promising binding energy than p-doped ones, whereas hole accumulation is stable at external surfaces of a twinned film. We also propose that assembling pairs of mono-twin films with a 'magic' twist angle θ* that provides commensurability between the moiré pattern at the interface and the accumulated carrier density, should promote a regime of strongly correlated states of electrons, such as Wigner crystals, and we specify the values of θ* for homo- and heterostructures of various TMDs. The authors investigate the twin boundaries in ferroelectric transition metal dichalcogenides, predicting their trap electrons with low densities, enhancing electron-electron interactions and potentially promoting Wigner crystallisation. [ABSTRACT FROM AUTHOR]

This study examined the effect of manganese (Mn) contents (3.12 to 8.97 pct) and three-pass deformation distributions on dynamic recrystallization (DRX) and dislocation evolution during hot compression of low-nickel duplex stainless steel (DSS). With a total deformation of 70 pct and a strain rate of 0.1 s−1, the study revealed that elevated Mn content enhances the ferrite phase's DRX, whereas austenite phase's dynamic recovery (DRV) varied with the deformation pass distribution. Changing the pass sequence (30 pct-10 pct-30 pct to 10 pct-30 pct-30 pct, and subsequently to 25 pct-35 pct-10 pct) in the 8.97 pct Mn sample, a reduction in the percentage of deformed grains in the austenite phase was observed. Furthermore, performing two consecutive passes with significant deformations was not favorable for ferrite DRX. Instead, it facilitated the development of subgrains within the austenite phase. The 25 pct-35 pct-10 pct deformation pattern in the 8.97 pct Mn sample led to twin boundaries formation, refining the coarse grains but impeding DRX, resulting in numerous austenite subgrains controlled by DRV. Conversely, a 30 pct-10 pct-30 pct deformation in the 8.97 pct Mn sample encouraged ferrite grains mainly displayed {001} and {111} recrystallization textures, thereby promoting DRX nucleation. The ferrite DRX process was primarily governed by continuous dynamic recrystallization (CDRX) mechanism, leading to the development of sizable, equiaxed grains with fewer dislocations. Kurdjumov-Sachs (K-S) relationship between the two phases enables dislocation slip from austenite to ferrite during compression deformation. With the increased addition of Mn, the ferrite underwent a gradual transition from a deformation texture to a recrystallization texture, whereas the austenite continued to be predominantly characterized by a deformation texture. The dynamic softening effect exhibited a more pronounced behavior in the 30 pct-10 pct-30 pct deformation condition for the sample with 8.97 pct Mn in comparison to the other deformation conditions. [ABSTRACT FROM AUTHOR]

Planar to cellular transition in the EH36 weld metal subjected to CaF2–TiO2 flux has been observed in situ under confocal scanning laser microscope. It is found that planars experience bifurcations, which preferentially evolve into cellular twins at the boundary, extending cellular arrays to interface. It is demonstrated that critical instability rate at the planar boundary is lower than that at the interface. Wavelengths of adjacent cellulars at both boundary and interface follow reverse relations with time. [ABSTRACT FROM AUTHOR]

In this work, the microstructure, and mechanical properties of cold-rolled low-density multi-principal-element Fe-30Mn-10Al-1.57C-2.3Cr-0.3Si-0.6Ti (wt.%) specimens were systematically investigated by annealing under different conditions. The microstructural evolution and strengthening mechanism were also examined. Results from x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) analyses confirmed that carbides were composed of TiC and κ-carbides. As the annealing temperature increased, both the volume fraction of κ-carbides and yield strength (YS) of the alloys decreased. TEM images indicated a pile-up of dislocations around carbides and boundary of twins. The increase in annealing temperature to 450 °C led to best mechanical properties of specimens with σ0.2% = 1270.28 MPa, Rm = 1318.67 MPa, and ε = 19.47%. Moreover, YS decreased by 9.28% and TE increased by 192.78%. Notably, the density of the as-obtained alloy reached 6.58 g/cm3, a value 15.6% lower than that of conventional steel. In sum, these findings are promising for future applications of low-density alloys. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) has long been known for its potential to replace Si and has rapidly emerged as a new power device material that provides opportunities to extend the power, temperature, or frequency capabilities of power modules. In this study, an innovative method for the die bonding of SiC chips with DBC alumina substrates has been performed at 250 °C using (111) preferentially oriented Ag nanotwinned films. For this purpose, the optimized conditions for the deposition of such Ag nanotwins using magnetron sputtering and electron beam evaporation techniques were investigated. The results indicated amounts of 36.4 and 3.5% for the proportions of Σ3 and Σ9 coincident twin boundaries to the total grain boundaries, respectively. AFM measurements revealed that the Ag nanotwinned films were quite uniform, with an average surface roughness of 10.5 nm, which is beneficial for the direct bonding of SiC power modulus. The hardness increased from 0.7 ± 0.2 to 1.57 ± 0.3 GPa when the substrate bias was increased from 0 to − 150 V, while the electrical resistivity was reduced from 2.71 to 2.02 μΩ·cm. It is obvious that applying an appropriate substrate bias can improve both the mechanical and electrical properties of Ag nanotwinned films on SiC chips. Finally, a satisfactory bonding strength of 39.4 ± 3.1 MPa without obvious voids at the interface has been achieved. [ABSTRACT FROM AUTHOR]

In this study, 6201 Al–Mg–Si cast ingots were investigated to determine the influence of total iron and silicon contents and homogenization on their microstructure and performance, including electrical and creep resistance. The tests included scanning electron microscopy, electron back scatter diffraction, and X-ray diffraction. The results showed that an increase in the total Fe and Si contents refined the microstructure, increased the weight of the ∑3 twin boundaries, and decreased the texture index FCGB in the initial Al–Mg–Si cast ingots. With increasing homogenization treatment time, the electrical conductivity (EC) in all three groups of homogenized ingots first decreased significantly (within 4 h) and then remained relatively stable. The increase in the total Fe and Si contents resulted in a decrease in the EC and an increase in the hardness of the as-homogenized cast ingot samples. The improvement in the EC of the initial and as-homogenized cast ingots was due to the high content of low-angle grain boundaries and strong Cube texture. The room-temperature creep resistance of the as-homogenized cast ingots was improved by high Fe and Si contents. The texture index FCGB and orientation streamline approach were determined to be powerful indicators for distinguishing and describing texture evolution during the homogenization of cast ingots with various chemical compositions. [ABSTRACT FROM AUTHOR]

Twin structures are commonly found in many metals and play a significant role in determining the mechanical properties and corrosion resistance of alloys. However, it is very challenging to observe the formation of twins in melt using traditional methods. In this paper, Fe90Al10 alloy was obtained by the vacuum non-consumable arc-melting method, and the results showed that the grain misorientation in the rapidly solidified Fe90Al10 alloys do not always follow a Mackenzie distribution, there is a peak at 60°, and it is well-known that Σ3 twin boundaries have an misorientation difference of 60°, Therefore, another nucleation method exists at the Σ3 grain boundary. Subsequently, Molecular dynamics simulations of Fe90Al10 alloys have shown that twin formation is sporadic and occurs through heterogeneous nucleation on the surface of previously formed grains, and only Σ3 twin boundaries can be formed in this way, which becomes a way for BCC metals to form twins during solidification, this is also consistent with experimental results, in addition to random nucleation, heterogeneous nucleation also exists during the rapid solidification of Fe-Al ally. Additionally, the aggregation of icosahedral-like (ICO-like) structures present in the under cooled melt leads to the heterogeneous nucleation of twinning and the difference in volume between the twin crystals will affect the subsequent evolutionary process. Therefore, the experimental and simulation results in this study strongly explain a formation mechanism of twins in BCC metals during rapid solidification. [ABSTRACT FROM AUTHOR]

This study employed molecular dynamics (MD) simulation to investigate the mechanical behavior of copper–silver (Cu–Ag) nanocrystalline alloys with twinning structures under solute gradient polarization conditions. The equidistant scaling method was used to identify different regions within the material, such as the solute concentration distribution from the grain interior to the GBs (GBs). MD simulations revealed that adjusting the twinning boundary spacing (TBS) can effectively improve the mechanical properties of Cu–Ag alloys. Tensile tests showed that stretching the twinning boundaries leads to a reduction in stress levels within the alloy. Furthermore, analysis based on the atomic structure volume fraction revealed that larger TBS values result in more pronounced structural changes within the alloy, particularly a significant transition from the face-centered cubic structure to the hexagonal close-packed structure. The latter typically possesses a higher density and closer packing, thus often exhibiting a higher strength and hardness. These findings reveal that adjusting the grain size, segregation gradients, TBS, and grain number can effectively improve the mechanical properties of Cu–Ag alloys. [ABSTRACT FROM AUTHOR]

We present a comprehensive investigation into the potential of n-type indium-doped cadmium selenide telluride (CST:In) as a high-performance candidate for solar cell applications, without the need for resource-intensive post-growth treatments that are required for CdTe:In. We compared undoped CST and CST:In crystals under different growth conditions, analyzing their structural and electronic properties using x-ray diffraction (XRD), electron probe microanalysis (EPMA), current–voltage (IV) and Hall effect measurements, time-resolved photoluminescence (TRPL), optical transmission, and photoluminescence (PL) mapping. The results reveal that as-grown CST:In crystals achieve nearly 100% carrier activation, yielding an electron concentration of 9.5 × 1018 cm−3, mobility of 653 cm2/V·s and a 5 ns lifetime which approaches the radiative limit. Furthermore, comparison of PL maps from crystal growths having different cooling profiles suggests a strong effect of cooling rate on selenium segregation and cubic/hexagonal/polytype phase distribution. Slower cooling leads to a more homogeneous cubic structure with lower Se segregation, while a faster cooling rate results in increased Se segregation, and twin boundaries and stacking faults with polytypic and hexagonal character. [ABSTRACT FROM AUTHOR]

The application of shape memory alloys is very suitable for developing pipe joints, and Fe-Mn-Si memory alloy pipe joints show great promise. This paper studied the variation rule and microscopic mechanism of the strain recovery rates of Fe-24Mn-6Si-7Cr-3Ni alloy pipe joints made by three different hot working technologies: casting, forging, and rolling. The strain recovery rate of as-cast pipe joints rose monotonously with the increasing thermomechanical training cycle. As-cast pipe joints' superior shape memory effect mainly came from the large grain size of 662.70 μm and the low density of annealing twin boundaries. On the other hand, the shape memory effects of forging and rolling pipe joints were restricted by the high densities of annealing twin boundaries and grain boundaries, respectively. Although the shape memory effects had a lower upper limit, their strain recovery rates climbed quickly. Following the third annealing, the remaining second-phase particles precipitated according to the ε -martensitic distribution orientation, and this contributed to the shape memory effects of as-cast and forging pipe joints. During adjusting the solution treatment parameters for as-cast pipe joints, it was found that as-cast pipe joints without solution treatment exhibited a notable superiority over the solution-treated ones when less thermomechanical training was performed. [ABSTRACT FROM AUTHOR]

Although the influence of grain boundaries (GB) or twin boundaries (TB) on corrosion resistance has been studied, there is no comparative study addressing the corrosion behavior of the two types of grain refinement. In this work, a comparative study of the corrosion behavior of samples with coarse grains (GB-coarse), fine grains (GB-refined), twin boundaries (TB-refined), and twin boundaries with solute atoms (TBS-refined) has been made. Results indicate that grain boundary refinement is more effective than twin boundaries in improving the corrosion resistance when grains are refined to similar grain sizes. The GB-refined sample show a typical corrosion band along rolling direction, while the corrosion in TB-refined counterparts preferentially occurs in twins. Twin boundaries improve corrosion resistance exclusively at an early stage, while an increase in corrosion rate after immersion occurs at longer times. In addition, segregation of solute atoms at twin boundaries further contributes to improving corrosion resistance. [ABSTRACT FROM AUTHOR]

The coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of 001 β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement. [ABSTRACT FROM AUTHOR]

Fe–Mn–Cr–N austenitic stainless steel is widely used in engineering fields because of its good ductility, but its relatively low yield strength limits its application range. In this study, Fe–Mn–Cr–N steel was fabricated with different parameters by friction stir processing (FSP). To explain the effect of the rotation rate and feed speed on the microstructure evolution and mechanical properties of Fe–Mn–Cr–N steel, four sets of parameters were selected. The numerical simulation results reveal that the maximum temperature and strain at the tool-workpiece interface on the advancing side (AS) are 948.8 °C and 39.3 mm/mm, respectively. Under the action of severe strain and frictional heat production during processing, the original coarse columnar grains were refined into uniform equiaxed grains with an average grain size of approximately 10 μm in the stir zone (SZ). Moreover, under the experimental conditions, the δ-ferrites gradually transformed into austenite, and all the δ-ferrites disappeared in the SZ with the greatest strain. In addition, a large number of Σ3 twins formed in this region. Consequently, microstructure refinement is dominated by dynamic recrystallization (DRX), while nucleation is accelerated by dissolution and spheroidization of δ ferrites and evolution of Σ3 twin boundaries. The Fe–Mn–Cr–N steel fabricated at 600 rpm-50 mm/min had the best comprehensive mechanical properties, i.e., its YS, UTS and El changed to 400.9, 649.0 MPa and 60.7 pct from 211.4, 421.2 MPa and 62.4 pct of the original as-casted Fe–Mn–Cr–N steel, respectively. The improvement in the YS was mainly attributed to solution strengthening (σSS = 114.4 MPa) and boundary strengthening (σGB = 189.5–238.1 MPa). [ABSTRACT FROM AUTHOR]

The effect of grain boundary retardation by annealing twins in pure copper has been experimentally and theoretically investigated. A model that describes the influence of annealing twins on the migration of grain boundaries in pure metals has been proposed. The retarding force from the twins has been demonstrated to be analogous to the Zener retarding force created by incoherent fine particles. An equation has been derived to calculate the retarding force induced by annealing twins. The force has been demonstrated to be inversely proportional to the size of the twins and directly proportional to their volume fraction. The simulation results have been compared with the experimental results. A satisfactory correlation between the theoretical and experimental results has been achieved. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation was applied to investigate the interaction between precipitate and 10 1 ¯ 2 twin in Mg-Al alloy in this work. Three sets of simulation including twin nucleation, propagation and growth affected by precipitate were performed. The results show the introduction of precipitate has little effect on neither the homogeneous twin nucleation at random site inside the grain nor the heterogeneous nucleation at grain boundary. During twin propagation, the blocking effect of precipitate to the twin tip depends on its size. The twin tip can bypass the precipitate when the precipitate is not long enough. When the length of precipitate is much larger than the thickness of twin, the twin tip will be completely blocked by the precipitate with elastic bending occurring at the junction. During the twin growth, it is found that the precipitate plays not only an obstacle but a source for the twinning dislocations gliding along the twin boundary. The blocking effect of precipitate on twin growth shows a size effect. The influences of width and thickness on precipitation hardening are significantly greater than that of length. [ABSTRACT FROM AUTHOR]

The synergic effects of scanning pattern types and subsequent heat treatment on the room- and elevated-temperature tensile properties of powder bed fusion-laser beam additively manufactured Hastelloy X Ni-based superalloy are investigated. The samples were built via bi-directional and chessboard scanning patterns. The as-built microstructures comprised dislocation and solidification cells embedded within the columnar grains. After applying heat treatment at 1175 °C for various soaking times, the as-built microstructures were gradually modified via recrystallization resulting from the residual stresses. The recrystallization kinetics equations for both bi-directional and chessboard samples were derived. It was found that the recrystallization occurred via the twinning-assisted nucleation mechanism and was promoted through bulging. The chessboard and bi-directional samples had similar driving forces; however, the recrystallization fraction in the chessboard sample was higher than that of the bi-directional sample due to differences in the grain's micro-texture. It was observed that the heat treatment cycle had no significant influence on the room- and high-temperature strength; by contrast, it interestingly improved the room- and high-temperature ductility due to reduced dislocation density and formation of twin boundaries. In addition, the heat-treated chessboard sample displayed superior hot tensile ductility than the bi-directional ones, owing to more promotion in recrystallization. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we study the roles of geometry, boundary conditions, and nanotwinned structure on the mechanical response of Cu ⟨ 110 ⟩ cylindrical nanopillars under tensile loading. Specifically, we investigate the stress–strain responses, deformation mechanisms, and defect evolution in nanopillars of varying height-to-diameter ( H / D ) ratios, capped with rigid layers to resemble hard coatings, and with microstructures consisting of columnar grains with and without nanoscale coherent twin boundaries (CTBs). Our results indicate that the disk-shaped nanopillars (DPs) with low H / D ratios exhibit high yield strength, significant triaxial tension stress, and pronounced plastic deformation. This deformation is accompanied by high defect density near the Cu/rigid layer interfaces, driven by dislocation pile up from multiple activated slip systems. Conversely, the deformed rod-shaped nanopillars of large ratios exhibit localized plastic deformation characterized by necking phenomena. Our results reveal the dominant role of Shockley type dislocations in plastic deformation, particularly in DPs, which display higher dislocation densities due to reduced dislocation annihilation events at free surfaces and pile up at interfaces. Notably, deformed DPs exhibit a unique vacancy-induced nanovoid nucleation mechanism at interfaces, resulting from the coalescence of vacancies generated during shearing of jogs. Additionally, the results demonstrate that dislocations glide between CTBs forming wide stacking faults, or traverse CTBs in a zigzag pattern as perfect dislocations, confirming a recently hypothesized cross-slip-like transmission in a recent experimental observation. Our simulations indicate that nanometer-spaced CTBs in Cu ⟨ 110 ⟩ nanopillars act as weak barriers to dislocation motion during tensile loading, leading to only a marginal enhancement of mechanical properties. [ABSTRACT FROM AUTHOR]

Serrated flow phenomenon has been widely observed among different metals due to the potential influence on their applications. Uniaxial tensile tests of nickel-based superalloy GH536 were carried out at loading rates of 0.06, 0.60, 3.60, and 36.00 mm·min−1 at room temperature, respectively. The tensile stress–strain curves demonstrate repetitive and discontinuous yielding behavior, namely serrated flow. It is observed that the stress–strain curves were dominated by type B serration, and original annealing twin boundaries (TBs) were distorted to various degrees under comparatively different lower tensile rates. In contrast, the TBs almost disappear, and the type B serrations become smoother and regular under the higher loading rates. This phenomenon can be attributed to the interactions between TBs and dislocations during tensile deformation. Coupled effects of the mobile and immobile dislocations illustrate the unsteady amplitude of serrations observed in stress–strain curves. Transmission electron microscope images of tested pieces reveal the interaction of dislocation and TB, with dislocation tangling around the TBs. [ABSTRACT FROM AUTHOR]

Modifying the gain orientation is considered a crucial method for enhancing the corrosion resistance of Mg alloys. This study aims to improve the corrosion properties of AZ31 alloy sheet through pre-stretch deformation, specifically compression (6.8%) along the extrusion direction (ED). To eliminate the twin structure, a recrystallization annealing process was performed after the pre-stretch deformation. To investigate the corrosion performance of Mg alloys, we conducted electrochemical and hydrogen evolution tests, which provided insights into the influence of microstructure on the corrosion resistance of Mg alloys. The results indicate that the corrosion resistance was altered by the presence of twin grain boundaries that formed after pre-stretch deformation. Furthermore, the recrystallized texture with//ED orientation exhibited the highest corrosion resistance, underscoring the significant role played by grain orientation. [ABSTRACT FROM AUTHOR]

The high Sc-alloyed Al–Sc sputtering target is applied to produce Sc-doped AlN (Al1−xScxN) thin films through reactive magnetron sputtering. The sputtering process demands Al–Sc sputtering targets with fine and stable grain sizes featuring random orientations. In this work, laser treatment with varying scanning speeds is applied to the Al-(10 at.%, 20 at.%) Sc alloys, and the microstructure is characterized using the electron backscatter diffraction (EBSD) method and transmission electron microscopy (TEM). The results indicate that the laser treatment enhances the solubility of Sc in the matrix; however, the nucleation crystallography during solidification differs among these alloys. Epitaxial growth is observed in Al–20Sc, remaining a large grain size with different scanning speeds. In contrast, Al–10Sc exhibits refined grain sizes, reduced to about 2 μm compared to 200 μm in the as-casted alloy. Laser treatment generates more than 40% twin boundaries, leading to the randomization of the texture and refinement of the grain size through twin variants. This study demonstrates the feasibility of grain boundary engineering for Al–10Sc sputtering targets through laser scanning. [ABSTRACT FROM AUTHOR]

One-unit-cell FeSe films on SrTiO3 substrates are of great interest owing to significantly enlarged pairing gaps characterized by two coherence peaks at ±10 meV and ±20 meV. In-situ transport measurement is desired to reveal novel properties. Here, we performed in-situ microscale electrical transport and combined scanning tunneling microscopy measurements on continuous one-unit-cell FeSe films with twin boundaries. We observed two spatially coexisting superconducting phases in domains and on boundaries, characterized by distinct superconducting gaps ( Δ 1 ~15 meV vs. Δ 2 ~10 meV) and pairing temperatures (Tp1~52.0 K vs. Tp2~37.3 K), and correspondingly two-step nonlinear V ~ I α behavior but a concurrent Berezinskii–Kosterlitz–Thouless (BKT)-like transition occurring at T BKT ~28.7 K. Moreover, the onset transition temperature T c onset ~54 K and zero-resistivity temperature T c zero ~31 K are consistent with Tp1 and T BKT , respectively. Our results indicate the broadened superconducting transition in FeSe/SrTiO3 is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity. The authors study monolayer FeSe via scanning tunneling microscopy and simultaneous micron-scale-probe-based transport. They observe distinct superconducting phases in domains and on boundaries between domains, with different superconducting gaps and pairing temperatures. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion in severely plastically deformed (SPD) CoCrFeMnNi high entropy alloy (HEA) has been studied by applying the tracer diffusion technique. After two passes of equal channel angular pressing (ECAP), three untra-fast diffusion paths in ECAP-processed CoCrFeMnNi HEA were distinguished, which were identified with twin boundaries, high-angle grain boundaries and porosity defects. The enhanced diffusivity is induced by a high strain localization in the vicinity of interfaces in a deformation-induced non-equilibrium state that is correlated with the pronounced deformation twinning due to the low stacking fault energy of the material. [ABSTRACT FROM AUTHOR]

In the two-phase crystal EuRbFe4As4/EuFe2As2 (1144/122), a linear ordering of Abrikosov vortices, uncharacteristic for superconducting pnictides, has been found using the method of decorating with magnetic nanoparticles. The observed chains of vortices directed along crystallographic 〈110〉 axes of the orthorhombic EuFe2As2 phase are explained by pinning of vortices in the superconducting phase 1144 on linear defects associated with the twin boundaries of the non-superconducting 122 phase. [ABSTRACT FROM AUTHOR]

Twinning, on par with dislocations, is critically required in plastic deformation of hexagonal close-packed crystals at low temperatures. In contrast to that in cubic-structured crystals, twinning in hexagonal close-packed crystals requires atomic shuffles in addition to shear. Though the twinning shear that is carried by twinning dislocations has been captured for decades, direct experimental observation of the atomic shuffles, especially when the shuffling mode is not unique and does not confine to the plane of shear, remains a formidable challenge to date. Here, by using in-situ transmission electron microscopy, we directly capture the atomic mechanism of the 11 2 ¯ 1 twinning in hexagonal close packed rhenium nanocrystals. Results show that the 11 2 ¯ 1 twinning is dominated by the (b1/2, h1/2) twinning disconnections. In contrast to conventional expectations, the atomic shuffles accompanying the twinning disconnections proceed on alternative basal planes along 1/6 1 1 ¯ 00 , which may be attributed to the free surface in nanocrystal samples, leading to a lack of mirror symmetry across the 11 2 ¯ 1 twin boundary. Atomic shuffles are important phenomena accompanying the twinning in hexagonal close-packed crystals, but the direct experimental observation on it is lacking. Here, the authors show the shuffling mechanism of {112 ̅1} twinning in rhenium nanocrystals by in situ transmission electron microscopy. [ABSTRACT FROM AUTHOR]

In the two-phase crystal EuRbFe4As4/EuFe2As2 (1144/122), a linear ordering of Abrikosov vortices, uncharacteristic for superconducting pnictides, has been found using the method of decorating with magnetic nanoparticles. The observed chains of vortices directed along crystallographic 〈110〉 axes of the orthorhombic EuFe2As2 phase are explained by pinning of vortices in the superconducting phase 1144 on linear defects associated with the twin boundaries of the non-superconducting 122 phase. [ABSTRACT FROM AUTHOR]

This paper deals with the performance of sintered nano-silver bonds used as wide-bandgap power module die attachment technology. The paper specifically explores the fine-scale microstructures of highly porous sintered attachments under power cycling to provide a deeper understanding of the significance of porosity as a reliability-related microstructural parameter. Attachments prepared at 220°C using a pressure of 6 MPa for 1 s (parameters known to generate approximately 50% porosity from previous work) and subsequently subjected to 650,000 power cycles between 50°C and 200°C are assessed. A correlative workflow integrating x-ray computed tomography, focused ion beam (FIB) and electron backscatter diffraction (EBSD) data is applied to merge meso- and nanoscale microstructural features to illuminate the degradation mechanisms. The as-sintered Ag layer has a high volume of heterogeneously distributed pores, and consists of randomly oriented equiaxed grains whose sizes vary depending on the local density of the region sampled. Power cycling promotes grain growth and the loss of twin boundaries, and these changes are more pronounced within more dense regions of the Ag attachment. In contrast, the copper substrate appears to undergo some grain refinement, with deformation twins visible within finer-grained zones during power cycling. Cracks, which appear to start off within the Ag layer, propagate across the Ag-Cu boundary and transgranularly through fine-grained regions within the copper with little tortuosity. These observations are discussed within the context of reliability behaviour. [ABSTRACT FROM AUTHOR]

Low-angle grain boundary and twin significantly affect precipitation behavior. Twins were introduced into AZ91 magnesium alloys by pre-rolling. And then, aging treatment was carried out in pre-rolled AZ91 magnesium alloys to investigate the precipitation behavior. The results show that the low-angle grain boundary and the extension twin provide the nucleation position for second phase, while double twin with low-elastic strain energy is not favored for the segregation of Al atom, causing that the precipitate hardly nucleates. Second phase firstly precipitates in the low-angle grain boundary and the twin boundary, followed by precipitating inside the twin. The low-angle grain boundary-induced lath-shaped second phase is attributed to dislocation, while the twin-induced precipitate is mainly affected by dislocation and stacking fault. From thermodynamic perspective, the morphology of twin-induced precipitate firstly displays spherical shape and then grows into lath-like shape. [ABSTRACT FROM AUTHOR]

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases. Phase engineering of 2D transition metal dichalcogenides enables the investigation of emerging physical properties. Here, the authors report a phase selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2 thin films from MoSe2 nanoribbons, showing Tomonaga-Luttinger liquid behaviour in the CrSe2 mirror twin boundaries. [ABSTRACT FROM AUTHOR]

High or medium- entropy alloys (HEAs/MEAs) are multi-principal element alloys with equal atomic elemental composition, some of which have shown record-breaking mechanical performance. However, the link between short-range order (SRO) and the exceptional mechanical properties of these alloys has remained elusive. The local destruction of SRO by dislocation glide has been predicted to lead to a rejuvenated state with increased entropy and free energy, creating softer zones within the matrix and planar fault boundaries that enhance the ductility, but this has not been verified. Here, we integrate in situ nanomechanical testing with energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) and directly observe the rejuvenation during cyclic mechanical loading in single crystal CrCoNi at room temperature. Surprisingly, stacking faults (SFs) and twin boundaries (TBs) are reversible in initial cycles but become irreversible after a thousand cycles, indicating SF energy reduction and rejuvenation. Molecular dynamics (MD) simulation further reveals that the local breakdown of SRO in the MEA triggers these SF reversibility changes. As a result, the deformation features in HEAs/MEAs remain planar and highly localized to the rejuvenated planes, leading to the superior damage tolerance characteristic in this class of alloys. High and medium-entropy alloys have shown excellent mechanical performance, yet the role of short-range order (SRO) on these properties has been unclear. Here, the authors demonstrate that the reduction of SRO by deformation leads to rejuvenation, explaining their remarkable damage tolerance. [ABSTRACT FROM AUTHOR]

For CdZnTe crystals containing extended defects – coherent and incoherent twin boundaries, as well as lamellar systems of twins, we measure the propagation patterns of surface acoustic waves (SAW) in the gigahertz range excited by a sharply focused femtosecond laser pulse. Comparison of the calculated SAW radial velocities with experimental data allows us to determine the local crystallographic orientation of crystal regions with defects. We discover strong anisotropic scattering of SAW at twin boundaries and show that the structure of the twin boundary can be determined from the pattern of SAW scattering. Also, we demonstrate the non-reciprocal SAW propagation in lamellar twin systems, as well as quasi-waveguide SAW propagation along the lamellar twin layer. [ABSTRACT FROM AUTHOR]

Bulk bismuth has a complex Landau spectrum. The small effective masses and the large g-factors are anisotropic. The chemical potential drifts at high magnetic fields. Moreover, twin boundaries further complexify the interpretation of the data by producing extra anomalies in the extreme quantum limit. Here, we present a study of angle dependence of magnetoresistance up to 65 T in bismuth complemented with Nernst, ultrasound, and magneto-optic data. All observed anomalies can be explained in a single-particle picture of a sample consisting of two twinned crystals tilted by 108° and with two adjacent crystals keeping their own chemical potentials despite a shift between chemical potentials as large as 68 meV at 65 T. This implies an energy barrier between adjacent twinned crystals reminiscent of a metal- semiconductor Schottky barrier or a p-n junction. We argue that this barrier is built by accumulating charge carriers of opposite signs across a twin boundary. [ABSTRACT FROM AUTHOR]

Twinning is an essential mode of plastic deformation for achieving superior strength and ductility in metallic nanostructures. It has been generally believed that twinning-induced plasticity in body-centered cubic (BCC) metals is controlled by twin nucleation, but facilitated by rapid twin growth once the nucleation energy barrier is overcome. By performing in situ atomic-scale transmission electron microscopy straining experiments and atomistic simulations, we find that deformation twinning in BCC Ta nanocrystals larger than 15 nm in diameter proceeds by reluctant twin growth, resulting from slow advancement of twinning partials along the boundaries of finite-sized twin structures. In contrast, reluctant twin growth can be obviated by reducing the nanocrystal diameter to below 15 nm. As a result, the nucleated twin structure penetrates quickly through the cross section of nanocrystals, enabling fast twin growth via facile migration of twin boundaries leading to large uniform plastic deformation. The present work reveals a size-dependent transition in the nucleation- and growth-controlled twinning mechanism in BCC metals, and provides insights for exploiting twinning-induced plasticity and breaking strength-ductility limits in nanostructured BCC metals. Deformation twinning in body-centered cubic (BCC) metals is generally believed to be controlled by twin nucleation, but facilitated by rapid twin growth once the nucleation energy barrier is overcome. Here, the authors demonstrate both nucleation- and growth-controlled twinning modes in BCC Ta nanocrystals depending on the crystal size. [ABSTRACT FROM AUTHOR]

In this study, a new composition of HEA ((Fe50Mn30Co10Cr10)91Al9, at.%) was designed, and the microstructure evolution and tensile behavior of hot-rolled (Fe50Mn30Co10Cr10)91Al9 (at.%) high-entropy alloy (HEA) were investigated after annealing at various temperatures. The HEA exhibited a three-phase structure of the FCC phase, BCC phase, and a small amount of HCP phase after hot rolling and annealing. Annealing at lower temperatures led to a higher fraction of BCC phase and sub-crystalline grains in the recovery state, making the HEA have high strength. After annealing at high temperatures, the fraction of FCC phase increased. The fraction of recrystallized grains and twin boundaries in the HEA also increased, improving the ductility of the HEA. The HEA exhibited a multistage strain-hardening mechanism during deformation, which was rarely seen in traditional HEAs. The relationship between the microstructure and mechanical properties was discussed from the lattice frictional stress, solid solution strengthening, phase fractions, grain sizes, and dislocation. [ABSTRACT FROM AUTHOR]

To overcome the trade-off between strength and ductility, a low-cost method comprising room-temperature compression followed by artificial aging and compressive deformation was applied to a commercial AZ61 alloy. The goal of this work was to regulate the microstructure rather than refine the grains. For this purpose, fragmented precipitates decorating the boundaries of narrow twins and stable networks developed by two non-basal slip systems were achieved. With this novel microstructure, a highly improved yield strength of almost 120 MPa was realized with slightly decreased ductility. It is believed that this provides a novel orientation for rationally designing strength and ductility in age-hardenable Mg alloys by tailoring the precipitate scale and dislocation configuration. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulation was employed to test the tension and compression of CoCrFeNiAl high-entropy alloy (HEA) at a strain rate of 109 s−1 in order to examine the development mechanism of tension–compression asymmetry in the HEA as well as the mechanism of its own mechanical characteristics. Molecular dynamics study shows that there is a very obvious tension–compression asymmetry in HEA, and that the HEA in the compressed state has a higher flow stress than that in the stretched state. This phenomenon can be explained by the limitation of grain boundary sliding and the stimulation of dislocation nucleation. In the set simulation unit, when twinning boundary spacing (TBS) is 3.66 nm, the tension–compression asymmetry has a critical minimum. The interaction of the free surface and dislocation activity under different twinning boundary spacing can account for the observed terminations. This is due to the fact that, as the TBS gradually increases, the intragranular deformation mechanism of HEAs switches from partial dislocation nucleation and dislocation crossing and sliding along twin grain boundaries to partial dislocation nucleation and dislocation sliding parallel to the grain boundaries. [ABSTRACT FROM AUTHOR]

Single-crystal face-centered cubic high-entropy alloys (SCF-HEAs) CoCrFeMnNi have gained considerable interest because of their excellent mechanical properties. It is believed that building a proper nano-laminated dual-phase (DP) FCC/HCP structure is one of the most promising strategies to further improve the strength of HEAs. However, the relationship between atomic evolution and its mechanical properties is still unclear. In this work, the mechanical responses of the DP CoCrFeMnNi HEA samples under nanoindentation were investigated using molecular dynamics (MD) simulations. It was found that the indentation depth related to the initiation of plastic deformation in DP-HEA is different from that in SCF-HEA, and the hardness of the former is higher than that of the latter, but lower than that of the HCP structured HEA (SCH-HEA). The relationship between the hardness and dislocation density was described with the Taylor hardening model, and the strengthening effect of the FCC/HCP interface was also compared with that of the twin boundary. The current work could provide insight into the microstructural and interfacial deformation of the DP-HEA, which would help to optimize the strength and plasticity of the HEA composite. [ABSTRACT FROM AUTHOR]

The stratification of Pb nanoislands on the vicinal Si(7 7 10) surface into layers 2 nm thick, due to the quantization of the electronic spectrum, implies the formation of two-dimensional defects separating the layers. Energy balance of energy gain due to the quantum confinement effect and energy consume for a twinning boundary formation was determined using DFT simulations. It was established that established that the twin boundary is formed at the stage of growth of the next layer after the formation of a layer 2 nm thick, in which a standing wave of Fermi electrons is formed. On the surface of a thinner layer, where there is no standing electron wave, a perfect crystal structure grows. The formation of a twin boundary between layers 2 nm thick Pb begins with the formation of a rarefied atomic monolayer of pairs of atoms. [ABSTRACT FROM AUTHOR]

Microstructural defects play a critical role in flourishing various properties of materials. Here in this article, the effect of Ag substitution on the various properties of CdS quantum dots (QDs) is studied. Ag doped CdS QDs are synthesized using chemical co-precipitation, a simple, cost-effective method suitable for mass-production. High resolution transmission electron microscopy (HRTEM) images show dopant induced change in density and types of microstructural defects. The development of multiple nanotwinning at the cost of stacking faults as well as single twin boundary via the substitution of Ag in CdS lattice is noticed. In present study, a strong quantum confinement is observed and confirmed by the crystallite sizes estimated via X-ray diffraction (XRD) and TEM studies. A systematic enhancement in the bandgap and reduction in crystallite size are observed with increasing concentration of dopant. The atomic concentration of element and chemical bonding in CdS QDs are studied using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) respectively. Unlike other elemental dopants, QDs when doped with Ag with the varying concentration exhibit a non-systematic emission behaviour with increasing concentration which is correlated to the microstructural defects and confinement strength. [ABSTRACT FROM AUTHOR]

The concept of apparent symmetry rising, opposite to symmetry breaking, was proposed to illustrate the unusual phenomenon that the symmetry of the apparent morphology of the multiply twinned particle is higher than that of its crystal structure. We developed a unique strategy of co-crystallization-driven self-assembly of amphiphilic block copolymers PEO-b-PS and the inorganic cluster silicotungstic acid to achieve apparent symmetry rising of nanoparticles under mild conditions. The triangular nanoplates triply twinned by orthogonal crystals (low symmetry) have an additional triple symmetry (high symmetry). The appropriate crystallization inhibition of short solvophilic segments of the block copolymers favors the oriented attachment of homogeneous domains of hybrid nanoribbons, and consequently forms kinetic-controlled triangular nanoplates with twin grain boundaries. The concept of apparent symmetry rising, opposite to symmetry breaking, was proposed to illustrate the unusual phenomenon that the symmetry of the apparent morphology of the multiply twinned particle is higher than that of its crystal structure. Here the authors develop a strategy of co-crystallization-driven self-assembly of amphiphilic block copolymers and the inorganic clusters to achieve apparent symmetry rising of nanoparticles under mild conditions [ABSTRACT FROM AUTHOR]

Using the molecular dynamics simulation method, nanoindentation simulation was carried out on a variety of nanotwin Cu-Ag alloys with different gradient distribution structures and twin spacing to study the mechanical properties and deformation mechanism of nanotwin Cu-Ag alloys with a grain size of 15 nm. The deformation, failure process, and microstructure evolution of the model during extrusion were defined and analyzed by means of common neighbor analysis (CNA) and dislocation analysis (DXA), and the micro-mechanism of twin spacing and solute atom gradient segregation structure affecting the mechanical properties of nanotwin Cu-Ag alloy was revealed. The results show that different solute atom gradient segregation structures and twin spacing have important effects on the mechanical properties of nanotwin Cu-Ag alloy. Dislocations and stacking faults are more likely to occur within grains with thicker solute concentrations and larger twin boundary spacing. [ABSTRACT FROM AUTHOR]

The solution annealed Inconel 718 alloy was pre-stretched at different status, followed by a two-stage aging treatment and then subjected to medium temperature creep to investigate the creep deformation behavior from the perspective of microstructural evolution. The experimental results indicated that the precipitate morphology and creep resistance of the alloy were slightly changed with the increase in pre-stretching from 0 to 1%. The samples without pre-stretching revealed relatively homogeneous distribution of precipitates and higher volume fraction of Σ3 twin boundaries, resulting in higher creep life compared with the pre-stretched specimens. Furthermore, the creep life decreased by 33.5% as the pre-stretch amount increased to 4%, which could be explained by microtopography change. Specifically, pre-stretching induces precipitation behavior and defects in the alloy, which presented faster coarsening and a monotonous increase in the number of microvoids with increasing pre-stretching value. Microstructural analysis showed that the microvoids and microcracks arising from the δ phase at the grain boundary were responsible for the creep fracture of alloy, and the creep deformation mechanism was microtwinning and dislocation cutting through the γ″ phase, which were present irrespective of the pre-stretching amount. [ABSTRACT FROM AUTHOR]

The hot deformation behavior and dynamic recrystallization (DRX) of super-austenitic stainless steel were investigated by uniaxial compression in the temperature ranges of 1223 to 1523 K and strain rate ranges of 0.01 to 0.1 s−1. The apparent activation energy for hot deformation was calculated to be 556 kJ/mol by the regression analysis of sine hyperbolic Arrhenius function. The processing map based on dynamic materials modeling was divided into different hot working domains. DRX fractions and grain sizes of each domain were evaluated to understand the relationship between dissipation efficiency, flow instability, and recrystallization. It was observed that the dynamic precipitation of sigma at low-temperature and low-strain rate domain inhibited the development of DRX leading to the flow instability in this domain. DRX at high-strain rate domains was predominantly governed by high nucleation resulting in a finer grain size. DRX at low-strain rate domains was controlled by growth of grains leading to high DRX fraction with larger grain size. By analyzing the DRX mechanisms of different domains, it was found that the softening of austenite was mainly achieved by discontinuous dynamic recrystallization, and continuous dynamic recrystallization only occurred at low-strain rate and high-temperature domain. Moreover, multiple twinning was first observed in dynamic recrystallization, which suggested that the twin boundaries could propagate not only by "growth accident" but also by interactions between pre-existed twin boundaries. [ABSTRACT FROM AUTHOR]

Many researches have shown that nanotwinned structure and graphene reinforcing phase can improve the mechanical properties of aluminum (Al) metal, but the synergistic effect of the two on nanoindentation mechanical response of Al matrix composites is rarely reported. In this research, based on molecular dynamics simulations, the nanoindentation mechanical properties and microscopic deformation mechanisms of graphene/nanotwinned Al matrix composites are studied. The effect of the inclination angle and spacing of the twin boundaries (TBs), the insertion position of the graphene sheet, the graphene coating and the layer number of the graphene sheet on the nanoindentation mechanical properties of the Al matrix composites is analyzed. The nanoindentation mechanical properties of nanotwinned Al strongly depend on the inclination angle of TBs, and the dislocation slip modes mainly includes three types: hard mode I (dislocation pile-up penetration mode), soft mode I (partial dislocation parallel twin boundary slip mode) and hard mode II (dislocation limited slip mode). Moreover, as the twin boundary spacing increases, the dislocation slip mode starts to weaken until the nanoindentation mechanical properties are insensitive to the twin inclination orientation. The insertion of the graphene sheet weakens the indentation mechanical properties of the nanotwinned Al matrix composite, which is attributed to the interaction of the graphene with the dislocations and the subsequent absorption of part of them. The weakening effect depends on the distance between the graphene/Al interface and the plastic zone. What's more, graphene coating can improve the indentation mechanical properties of composites. In addition to the load bearing effect of graphene in the elastic stage, this strengthening effect is also attributed to the combined action of graphene and TBs in the plastic stage, which makes the dense dislocation network widely distributed around the indentation of graphene coating and induces the interfacial strain hardening. [ABSTRACT FROM AUTHOR]

Background: Recent improvements in spatial resolution and measurement sensitivity for high-resolution digital image correlation (HR-DIC) now provide an avenue for the quantitative measurement of deformation events and capturing the physical nature of deformation mechanisms. However, HR-DIC measurements require significant time due to scanning electron image acquisition; such a limitation prevents the widespread use of HR-DIC for material characterization. Objective: Apply a novel SEM acquisition technology to enhance HR-DIC measurements for high throughput applications. Methods: Multi-beam SEM technology is employed to image an entire gauge length at once at high resolution and at nearly a hundredfold acceleration of typical HR-DIC image acquisition, even when automated stage movement and image acquisition are employed. These images were fed into a discontinuity-tolerant HR-DIC software to determine slip localization induced by non-metallic inclusions and grain structure. Results: Slip localization was able to be analyzed to an unprecedented level, with over 210,000 slip bands able to be investigated, with the most intense slip localizing near and parallel to twin boundaries and in the vicinity of non-metallic inclusion clusters. Additionally, secondary slip activation and grain boundary shearing by intense dislocation pileups are observed to reduce slip amplitude near and parallel to twin boundaries. Conclusions: By performing HR-DIC in conjunction with a multi-beam SEM, high-throughput measurements of large field-of-view, high-resolution images were able to be performed in a timely manner. These measurements provided an immense number of slip events for statistical analysis to be performed on to relate to microstructural features. [ABSTRACT FROM AUTHOR]

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length. [ABSTRACT FROM AUTHOR]