Structure Of Stacking Faults Research Articles

Broad-Temperature Thermoelectric Figure of Merit Enhancement in Unconventional n-Type Bi2Te2.3Se0.7 Alloys.

Bi2Te2.7Se0.3-based alloys are conventional n-type thermoelectric materials for solid-state cooling and heat harvest near room temperature; high thermoelectric performance over a wide temperature range and superior mechanical properties are essential for their use in practical thermoelectric devices. In this work, we demonstrated that decent thermoelectric performance can also be realized in an unconventional composite with a nominal composition of Bi2Te2.3Se0.7 since the emergence of a Bi2Te2Se phase with Se ordered occupation could induce an enlargement of the electronic band gap. Follow-up Cu/Na codoping could generate a dynamic optimization of carrier concentration, significantly broadening the temperature range of high thermoelectric performance. Further B incorporation and annealing treatment resulted in obvious grain refinement and stacking fault structures, which help pushing the ultimate maximal figure of merit up to ∼1.3 at 423 K with an average value of ∼1.2 at 300-573 K. This work might provide insights for further research on bismuth tellurides and other thermoelectric materials.

Nanosecond laser annealing processed surface-structured hyperdoped silicon for efficient near-infrared detection

Surface-structured engineering of hyperdoped silicon can effectively facilitate the absorption of sub-bandgap photons in pristine single-crystal silicon (sc-Si). Here, we conducted different annealing approaches of ordinary thermal annealing (OTA) and nanosecond laser annealing (NLA) on modification of titanium-hyperdoped silicon (Si:Ti) surface structure, to achieve efficient near-infrared detection. It is presented that both OTA and NLA processes can improve the crystallinity of Si:Ti samples. In detail, atomic-resolved STEM characterization illustrates that NLA treatment will further eliminate the amorphous phase on Si:Ti surface to varying degrees. While one-dimensional periodic stacking fault structure of 9R-Si phase is formed at the surface of sc-Si and embedded in the Si matrix during the OTA process, which reveals the seamless interface of 9R-Si/sc-Si along with [11¯0] direction. Due to the high sub-bandgap light absorption and good crystal structure, the Si:Ti photodetector after NLA treatment with an energy density of 2.6 J cm-2exhibited the highest responsivity, reaching 151 mA W-1at 1550 nm even at a low operating voltage of 1 V. We assume the performance enhancement of NLA processed Si:Ti photodetectors can be attributed to two aspects, the one is NLA can reduce the recombination of photo-generated charge carriers in amorphous surface layer by improving crystallization, and the other is that NLA process can weaken the diffusion of titanium impurities due to the extremely rapid heating and cooling rates. This study presents prospects towards surface-structured silicon in infrared light detection.

Microstructure and Properties of Mg-Gd-Y-Zn-Mn High-Strength Alloy Welded by Friction Stir Welding.

Mg-Gd-Y-Zn-Mn (MVWZ842) is a kind of high rare earth magnesium alloy with high strength, high toughness and multi-scale strengthening mechanisms. After heat treatment, the maximum tensile strength of MVWZ842 alloy is more than 550 MPa, and the elongation is more than 5%. Because of its great mechanical properties, MVWZ842 has broad application potential in aerospace and rail transit. However, the addition of high rare earth elements makes the deformation resistance of MVWZ842 alloy increase to some extent. This leads to the difficulty of direct plastic processing forming and large structural part shaping. Friction stir welding (FSW) is a convenient fast solid-state joining technology. When FSW is used to weld MVWZ842 alloy, small workpieces can be joined into a large one to avoid the problem that large workpieces are difficult to form. In this work, a high-quality joint of MVWZ842 alloy was achieved by FSW. The microstructure and properties of this high-strength magnesium alloy after friction stir welding were studied. There was a prominent onion ring characteristic in the nugget zone. After the base was welded, the stacking fault structure precipitated in the grain. There were a lot of broken long period stacking order (LPSO) phases on the retreating side of the nugget zone, which brought the effect of precipitation strengthening. Nano-α-Mn and the broken second phase dispersed in the matrix in the nugget zone, which made the grains refine. A relatively complete dynamic recrystallization occurred in the nugget zone, and the grains were refined. The welding coefficient of the welded joint exceeded 95%, and the hardness of the weld nugget zone was higher than that of the base. There were a series of strengthening mechanisms in the joint, mainly fine grain strengthening, second phase strengthening and solid solution strengthening.

The microstructure and properties of Mo0.5NbTiZrTax high entropy alloys enhanced by Ta addition

In this study, the Mo0.5NbTiZrTax (x = 0.3, 0.5, 0.7) HEAs obtained by arc melting were controlled by the change of Ta content. The microstructure, the wear properties, and the corrosion resistance of the alloys were studied systematically. The results revealed that the alloys have the BCC (major) phase + BCC (minor) phase structure. At the same time, the increase in Ta content promotes the refinement of grains and the formation of sub-grains with gradually random orientation at grain boundaries but suppresses the randomization of crystal orientation. This also enriched the defect structure inside the alloys, further induced the formation of Zr-rich second phase particles, high-density stacking fault structures, and disordered Zr-rich BCC phases. The increase in Ta content significantly improved the wear properties of the alloys, which is mainly ascribed to the presence of the tough BCC phase and the combined effects of solid solution strengthening, second phase strengthening, fine grain strengthening, lattice distortion effect, and stacking fault strengthening. The electrochemical measurements confirm that the overall corrosion resistance of the alloys shows a trend of first increasing and then decreasing with the increase of Ta content due to the formation of micro batteries between the second phase rich in Zr as the anode and dendrites.

Improvement of thermal shock resistance and hot mechanical properties by FCC/BCC/B2 multiphase strengthened microstructure in laser cladded high-entropy alloy coatings

NiCoFeCrSiAlxCu0.5TiMoB0.4 high-entropy alloy (HEA) coatings were prepared by laser cladding, and the microstructure, phase structure and thermal properties were studied. The results showed that the solid solution phase of the coating matrix changed from FCC + BCC to BCC as the main phases with increasing Al content. Moreover, the enrichment of Ti resulted in the significant formation of a stacking fault structure in the interdendritic region of the dual-phase coatings. The ordered B2 phase precipitated from the BCC structure exhibited high hardness and crack damage resistance. The wear rates of the HEA-Al0.5 and HEA-Al1.5 coatings at 600 °C were 3.78 × 10−5 and 4.35 × 10−5 mm3/(N·m)−1, respectively. The wear mechanisms of HEA coatings were mainly abrasive wear and oxidation wear. Because of the high microhardness of the BCC phase, cracks initiated and propagated on the worn surface, which aggravated the oxidation wear deterioration of the HEA coating. Overall, this study may provide a theoretical basis for the microstructure-property relationships of HEA coatings and contribute to the development of high-performance coatings.

Creep behavior and microstructure evolution of titanium matrix composites reinforced with TiB, TiC and Y2O3

Titanium matrix composites reinforced with (TiB + TiC) and (TiB + TiC + Y2O3) were prepared by induction skull melting. The creep test was carried out at 650 °C and 120−160 MPa, and the microstructure evolution was characterized in detail by XRD, SEM and TEM. The results show that the microstructure of the two composites is basket-weave structure. Under the same creep condition, the composite with the addition of Y2O3 has lower steady-state creep rate. After creep, silicides precipitated at the α/β interface, around dissolved β phases and reinforcements, which have a strong pinning effect on the dislocation movement. Due to the stacking fault structure through TiB whisker, the size of silicides around TiB is significantly larger than that around TiC and Y2O3. Solute-drag creep is the main creep mechanism, and the dislocation movement is also affected by α/β interface, reinforcements and silicides.

ZnS/Cd1-xZnxS composite photocatalyst synthesized with excessive strong alkali solution as a hydrothermal solvent: The chemical equilibrium theory of the synthesis process and its influence on photocatalytic performance

The ZnS/Cd1-xZnxS composite photocatalyst was successfully synthesized using a hydrothermal solvent by adding excess hydroxide ions. The ZnS/Cd1-xZnxS composite photocatalyst showed excellent photocatalytic activity for hydrogen production with a hydrogen evolution rate of 21.8 mmol/g−1 h−1, which is the highest photocatalytic activity reported to date among visible-light-driven photocatalysts without loading cocatalysts. According to the theoretical calculation and analysis of the chemical equilibrium in the synthesis process, the synthesized Cd1-xZnxS solid solution contained a certain amount of ZnS. A certain amount of ZnS was also contained in the synthesized Cd1-xZnxS solid solution based on experimental results, thus demonstrating the validity of the theoretical calculation. Experimental results showed that the key factor affecting the photocatalytic activity of hydrogen production for the ZnS/Cd1-xZnxS composite photocatalyst was not the heterojunction formed between ZnS and Cd1-xZnxS but rather the stacking fault structures in the Cd1-xZnxS solid solution. However, more stacking fault structures could be formed in the Cd1-xZnxS solid solution due to the presence of ZnS.

A novel D022 precipitation-hardened Ni2.1CoCrFe0.5Nb0.2 high entropy alloy with outstanding tensile properties by additive manufacturing

ABSTRACT To introduce D022 superlattice (noted as γ′′ phase) precipitation strengthening in additive manufacturing, a design strategy of combining the overall valence electron concentration with the calculation of phase diagrams is proposed, and Ni2.1CoCrFe0.5Nb0.2 HEA is designed. The wall-shaped samples were prepared by AM, and after solution at 1100 °C for 2 h and aging at 650 °C for 120 h, the γ′′ phase with volume of 14% causes yield strength increase by 727 MPa, the yield strength increased dramatically to ∼1005 MPa, the ultimate strength increased dramatically to ∼1240 MPa, the tensile elongation maintained at ∼20%. The high strength results from the precipitation strengthening of the γ′′ phase, and the large ductility are primarily attributed to an evolution of multiple stacking fault structures. The present study will not only promote the development of high-performance HEAs by AM but also provides a pathway for achievement of AM technology industrial applications.

Mechanical Behavior and Constitutive Modeling of the Mg-Zn-Y Alloy in an Electrically Assisted Tensile Test.

The Mg-Zn-Y alloy containing the LPSO phase has excellent mechanical properties and functional application prospects. In an effort to clarify the electrically assisted deformation behavior of the Mg-Zn-Y alloy, electrically assisted tensile tests of Mg98.5Zn0.5Y1 alloy sheets were carried out at different temperatures, current densities, duty ratios, and frequencies. The experimental results showed that, after the pulse current was applied (26.58 A·mm−2), the peak stress of the sample deformed at 200 °C decreased by 8 MPa. The peak stress of the material decreased with the increase in current density. It is noticeable that the changes in duty ratios and frequencies have a small effect on the peak stress and strain. When the current was applied, more recrystallized grains appeared in the alloy and the basal texture was weakened. According to the experimental results, the Arrhenius model was derived based on the Zener–Hollomon parameter. Owing to the appearance of the stacking fault structure (LPSO), the activation energy Q of the Mg98.5Zn0.5Y1 alloy was 389.41 KJ/mol, which is higher than conventional Mg alloys. Moreover, the constitutive equation of the electro plastic effect coupled with temperature and pulse current parameters was established by introducing electrically assisted characteristics. By comparing the experimental and predicted values, the established model can effectively predict the variation trend of flow stress under electrically assisted deformation. Moreover, the constitutive model was incorporated into the UHARD subroutine of ABAQUS software to study the deformation behavior of the Mg98.5Zn0.5Y1 alloy.

Deformation of Copper Nanowire under Coupled Tension-Torsion Loading.

Metallic nanowires (NWs) are essential building blocks for flexible electronics, and experience different deformation modes due to external mechanical loading. Using atomistic simulations, this work investigated the deformation behavior of copper nanowire under coupled tension–torsion loading. A transition in both yielding pattern and dislocation pattern were observed with varying torsion/tension strain ratios. Specifically, increasing the torsion/tension strain ratio (with larger torsional strain) triggered the nucleation of different partial dislocations in the slip system. At low torsion/tension strain ratios, plastic deformation of the nanowire was dominated by stacking faults with trailing partial dislocations pinned at the surface, shifting to two partial dislocations with stacking faults as the strain ratio increases. More interestingly, the NW under tension-dominated loading exhibited a stacking fault structure after yielding, whereas torsion-dominated loading resulted in a three-dimensional dislocation network within the structure. This work thus suggests that the deformation behavior of the NW varies depending on the coupled mechanical loading, which could be beneficial for various engineering applications.

A high-entropy alloy with dislocation-precipitate skeleton for ultrastrength and ductility

The introduction of dislocations and precipitates has proven to be the effective methods to improve the mechanical properties of metallic materials and break strength-ductility trade-off. However, it is difficult to obtain a suitable combination of both strategies in the metal materials, that is, the coexistence of high-density dislocations and high-volume-fraction precipitates. Here, utilizing a three-dimensional (3D) printing technique, we have successfully achieved a combination of high-density dislocation structures and high-volume-fraction ductile nano-precipitates in a high-entropy alloy (HEA). This 3D-printed HEA, with a novelty dislocation-precipitate skeleton (DPS) architecture and high-density ductile nano-precipitations wrapped in the DPS, has an ultra-high tensile strength of ∼ 1.8 GPa together with the maximum elongation of ∼ 16%. The ultra-high strength mainly comes from dislocation-precipitation synergistic strengthening, while the large ductility mainly originates from an evolution of multiple stacking fault (SF) structures. The DPS can not only slow down the dislocation movement during the strain process without completely hindering its motion, but more importantly, the DPS still has good structural stability during the deformation, which avoids any premature failure due to stress concentrations at the boundary. The DPS formation promotes the development of the metal-based 3D printing technique in the preparation of the high-performance materials, and it can provide an efficient pathway for further enhancement of the alloy properties.

Stacking-faulted CDO zeolite nanosheets efficient for bulky molecular reactions.

The synthesis of thin zeolite nanosheets beneficial for catalytic reactions, adsorption and diffusion involving bulky molecules remains a great challenge. Herein, we report a unique layered zeolitic nanosheet ECNU-57 with a CDO-type structure that was synthesized by employing small-molecular 1,4-bis(N-methylpyrrolidinium) cations as the organic structure-directing agent (OSDA). The ECNU-57 nanosheets are 50 nm thick along the b-axis and have an open hierarchical porosity with 486 m2 g-1 surface area. HRTEM investigation indicates that the nanosheets exhibit a stacking fault structure along the b-axis with the inclusion of the FER structure since the CDO and FER topologies possess very similar building layers. As a solid-acid catalyst, ECNU-57 exhibits promising performances in the reactions of 1,3,5-triisopropylbenzene cracking and Friedel-Crafts acylation, due to its enhanced mass transfer and more accessible active sites to bulky organic molecules.

The effects of atomic arrangements on mechanical properties of 2H, 3C, 4H and 6H-SiC

As a typical polytypic compound, silicon carbide has various atomic arrangements; however, the role of atomic arrangements in the plastic deformation mechanisms remains unclear. In this article, the relationship between atomic arrangements and mechanical properties in single crystal and polycrystalline SiC is revealed by first-principle calculations and molecular dynamics simulations. We found that the twin boundary can delay the fracture of the single crystal 6H-SiC chemical bonds, could account for its excellent tensile toughness. The plastic deformation of polycrystalline SiC is dominated by dislocation nucleation and propagation under compressive strain, and by intergranular fracture under tensile strain. In addition, when the temperature exceeds 1200 K, the yield stress of polycrystalline SiC gradually stabilizes. Hexagonal SiC (2H, 4H and 6H) have stronger strain resistance than cubic one (3C) due to the hierarchical resistance of twinning and stacking fault structures to external stress. These results can provide insight into the relationship between atomic arrangements and stress response in SiC materials.

Utilizing local phase transformation strengthening for nickel-base superalloys

Almost 75 years of research has been devoted to producing superalloys capable of higher operating temperatures in jet turbine engines, and there is an ongoing need to increase operating temperature further. Here, a new disk Nickel-base superalloy is designed to take advantage of strengthening atomic-scale dynamic complexions. This local phase transformation strengthening provides the alloy with a three times improvement in creep strength over similar disk superalloys and comparable strength to a single crystal blade alloy at 760 °C. Ultra-high-resolution chemical mapping reveals that the improvement in creep strength is a result of atomic-scale η (D024) and χ (D019) formation along superlattice stacking faults. To understand these results, the energy differences between the L12 and competing D024 and D019 stacking fault structures and their dependence on composition are computed by density functional theory. This study can help guide researchers to further optimize local phase transformation strengthening mechanisms for alloy development.

Spallation Characteristics of Single Crystal Aluminum with Copper Nanoparticles Based on Atomistic Simulations.

In this study, the effects of Cu nanoparticle inclusion on the dynamic responses of single crystal Al during shockwave loading and subsequent spallation processes have been explored by molecular dynamics simulations. At specific impact velocities, the ideal single crystal Al will not produce dislocation and stacking fault structure during shock compression, while Cu inclusion in an Al–Cu nanocomposite will lead to the formation of a regular stacking fault structure. The significant difference of a shock-induced microstructure makes the spall strength of the Al–Cu nanocomposite lower than that of ideal single crystal Al at these specific impact velocities. The analysis of the damage evolution process shows that when piston velocity up ≤ 2.0 km/s, due to the dense defects and high potential energy at the interface between inclusions and matrix, voids will nucleate preferentially at the inclusion interface, and then grow along the interface at a rate of five times faster than other voids in the Al matrix. When up ≥ 2.5 km/s, the Al matrix will shock melt or unloading melt, and micro-spallation occurs; Cu inclusions have no effect on spallation strength, but when Cu inclusions and the Al matrix are not fully diffused, the voids tend to grow and coalescence along the inclusion interface to form a large void.

Increasing high-temperature fatigue resistance of polysynthetic twinned TiAl single crystal by plastic strain delocalization

Polysynthetic twinned (PST) TiAl single crystal possesses great potentials for high-temperature applications due to its excellent combination of strength, ductility and creep resistance. However, a critical property for high-temperature application of such material involving high-temperature fatigue properties remains unknown. Here, the high-temperature high-cycle fatigue performance of PST TiAl single crystal has been studied. The result shows that PST TiAl single crystal can withstand more than 107 cyclic loadings at 975 °C under a stress amplitude of 270 MPa, which is significantly higher than traditional TiAl alloys. Experimental observations and atomistic simulations indicate that the improvement of fatigue resistance is attributed to the plastic strain delocalization in uniform lamellar structure, and the plastic deformation is well-distributed and sufficient in each lamella. Even in the α2 lamella with difficult slippage, a large number of stacking fault structures can be observed. The 〈c + a〉 dislocations in α2 tend to dissociate into a Frank partial with b = 1/6 <22¯03¯>, forming a ribbon of I1 fault which ensures the continuity of deformation.

Deep Insights into the Twinning Mechanism in High-Performance Al Alloys: A Comprehensive First-Principles Study

The twinning mechanism of Al solid solutions is comprehensively investigated via the first-principles method. The relative stability of C and H atoms at tetrahedral and octahedral centers is discussed. Moreover, the interaction energies between solute atoms at different atomic layers and generalized stacking-fault structures are calculated. Results indicate that the stable occupation of C atom exists at the octahedral center rather than at the tetrahedral center. By contrast, the H atom stably exists at the tetrahedral center rather than at the octahedral center. Both atoms can effectively reduce the minimum energy barrier of dislocation nucleation, thereby promoting dislocation nucleation. The C atom more easily promotes dislocation nucleation than the H atom. Furthermore, both atoms are repelled by the stacking-fault plane. However, they are more likely to be segregated in the second neighboring layer of unstable stacking fault, intrinsic stacking fault (ISF), and unstable twinning fault (UTF) structures. Charge density measurements reveal that the twinning process is likely inhibited because of the strong local chemical bonding around the UTF layer from the ISF structure to the UTF structure. High concentrations of both atoms inhibit the twinning deformation at crack tips and grain boundaries, but they have almost no effect on the twinning deformation inside the grains. This study provides deep insights into the twinning deformation mechanism of face-centered-cubic alloy systems.

Resolving the stacking fault structure of silver nanoplates.

The stacking fault structure (SFT) is the key to understanding the symmetry breaking of fcc nanocrystals and the origin of two-dimensional (2D) anisotropic growth of nanoplates. After resolving the SFT in Ag nanoplates under aberration-corrected transmission electron microscope (TEM) observations, it is found that there are three basic stacking faults, namely, twinned stacking fault (SF-t), a layer missed stacking fault (SF-m) and a layer inserted stacking fault (SF-i). The SFT is composed of one or a combination of two or all of the three kinds of stacking faults with a total number varying from 4 to 9. It has been demonstrated that the SFT could generate concave faces, step faces and (100) faces in the lateral directions, which provides sites for adding-atoms with a higher coordination number than on the top and bottom flat (111) faces, and results in the anisotropic growth along the 2D direction. Additionally, Ag nanoplates fall into either center symmetry or mirror symmetry when the corresponding number is even or odd. The center symmetry and mirror symmetry with different side face arrangements in turn manipulate the shape evolution to cubes and bipyramids, respectively. Our study provides a comprehensive understanding of the formation and growth of 2D metal nanomaterials.

The theoretical calculation and analysis of the chemical equilibrium in the synthetic process and its effect on hydrogen production performance for sulfide catalysts

In the present study, the authors found that the growth of sulfide catalyst crystals involved the dissolution-recrystallization process, resulting in the formation of stacking fault structured homojunctions through the theoretical calculation and analysis of the chemical equilibrium in the synthetic process for sulfide catalysts. It was also revealed that, compared with the alkali solution, using an alkaline ammonia solution as the hydrothermal solvent could promote more NiS cocatalysts to be distributed on the surface of the sulfide catalysts, leading to the formation of more p-n junctions. Both the p-n junctions and the stacking fault structures effectively promoted the separation of photogenerated charges, leading to a quantum efficiency at 420 nm of up to 74.6%, which is the highest efficiency reported to date among visible-light-driven photocatalysts without noble metal cocatalysts. Research of chemical equilibrium in the synthetic process for the CdZnS solid solution, it was revealed that certain amounts of CdS and ZnS may exist, thus forming a three-phase detached catalyst (CdS and ZnS single-phases and CdZnS solid solution phase). In this way, the structure could improve the hydrogen production performance of the CdZnS solid solution. The results were also confirmed by characterization of the catalysts.

Investigation of flexural behavior and microstructure of Amosic-3 silicon carbide composites under neutron irradiation

SiC/SiC composites prepared by the chemical vapor infiltration process have been investigated under neutron irradiation at approximately 40 °C to doses of 0.03 and 0.2 displacements per atom (dpa) in the China Mianyang Research Reactor. Irradiation effects were characterized by three-point flexural tests, micro-Raman spectra and transmission electron microscopy observations. The results demonstrated that the ultimate flexural stress and flexural modulus decreased remarkably with irradiation doses up to 0.2 dpa. Micro-Raman spectra of the Amosic-3 fiber and SiC matrix revealed that both reduction of Si–C bonds and enhancement of Si–Si bonds were increased from 0.03 to 0.2 dpa. Microstructural observation revealed that the Amosic-3 fiber suffered from grain fragmentation and amorphization. Meanwhile, matrix amorphization is generated preferentially at the site of stacking fault structures. In addition, it suggested that in the current work, up to 0.2 dpa, full amorphization was not completed in the Amosic-3 fiber, PyC and SiC matrix.