A New Structure with Localized sp2 Bonding for Fivefold Twinning in Diamond

Changes in atomic bonding configuration in carbon from sp³ to sp² are known to exist in certain structural defects in diamond, such as twin boundaries, grain boundaries, and dislocations, which have a significant impact on many properties of diamond. In this work, the atomic structure of fivefold twinning in detonation synthesized ultra-dispersed diamonds is investigated using a combination of techniques, including spherical aberration-corrected high-resolution electron microscopy (HREM), HREM image simulations, and molecular mechanics (MM) calculations. The experimental HREM images reveal clearly that the fivefold twinning in diamond has two distinct structures. In addition to the concentric fivefold twins, where the core structure is the intersection of five {111} twinning boundaries, a new extended core structure with co-hybridization of bonding is identified and analyzed in fivefold twinning. The atomic structure forming these fivefold twinning boundaries and their respective core structures is proposed to involve both the tetrahedral sp³ and planar graphitic sp² bonding configurations, in which a co-hybridized planar hexagon of carbon serves as a fundamental structural unit. The presence of this sp²-bonded planar unit of hexagonal carbon rings in general grain boundaries is also discussed.     

The initial stages of ordering in CuAu I and CuAu II

The initial ordering stages of CuAu have been studied by electron microscopy. At 250° C, in the CuAuI stability range, ordering proceeds fast and initially very little twinning is observed. At 390° C, in the CuAuII stability range, a long incubation time is required before CuAull platelets are nucleated. (1 1 0) twinning takes place immediately after the plate formation and is observed down to the tip of the plate. The fine structure of the periodic antiphase boundaries in CuAuII is studied by HREM; it reveals the disorder along the boundary.     

Creep Behavior of High‐Nb TiAl Alloy at 800–900 °C by Directional Solidification

Cold crucible directional solidification Ti44Al6Nb1.0Cr alloy is crept at 800–900 °C. Experimental results show that creep lifetime significantly decreases with the increasing creep temperature. When creeping at 900 °C under 130 MPa, the T_Q twinning is activated in lamellar structures. The T_Q twinning shows a strong dependency on temperature during creep under low creep‐stress and it can overcome α₂ lamellae and transfer into adjacent γ lamellae. The hardening by mechanical twinning and the softening by α₂ lamellar dissolution take place at different zones in lamellar structures and the strain incompatibility between hardening zone and softening zone promotes the microcracks to form in lamellar structures. The deformation characteristic of hard and soft lamellae is studied. Moreover, recrystallization γ phase formed in lamellar structures near colony boundary during creep at 900 °C accelerates the creep failure.

     

Electron microscopic study of digenite-related phases (Cu2−xS)

A study by means of electron microscopy and electron diffraction of synthetic digenite reveals the existence of a series of closely related structures with basic spacings 2a_o, 3a_o, 4a_o, 5a_o and 6a_o, where a_o is the (111) spacing of the cubic close packed sulphur sublattice. In all these structures copper atoms occupy tetrahedral interstices in a cubic close packed matrix of sulphur atoms. By means of high resolution microscopy non-rational spacings are shown to be the result of a mixture of spacings. It is shown that the particular set of systematic extinctions in the diffraction pattern discussed in previous papers, is related to a particular state of disorder and does not imply the presence of twinning.     

Microstructure and mechanical behavior of laser aided additive manufactured low carbon interstitial Fe49.5Mn30Co10Cr10C0.5 multicomponent alloy

Laser aided additive manufacturing(LAAM)was used to fabricate bulk Fe49.5Mn30Co10Cr10C0.5 interstitial multicomponent alloy using pre-alloyed powder.The room temperature yield strength(σy),ultimate tensile strength(σUTS)and elongation(εUST)were 645 MPa,917 MPa and 27.0％respectively.The as-built sample consisted of equiaxed and dendritic cellular structures formed by elemental segregation.These cellular structures together with oxide particle inclusions were deemed to strengthen the material.The other contributing components include dislocation strengthening,friction stress and grain bound-ary strengthening.The high εUTS was attributed to dislocation motion and activation of both twinning and transformation-induced plasticity(TWIP and TRIP).Tensile tests performed at-40℃and-130℃demonstrated superior tensile strength of 1041 MPa and 1267 MPa respectively.However,almost no twinning was observed in the fractured sample tested at-40℃and-130℃.Instead,higher fraction of strain-induced hexagonal close-packed(HCP)ε phase transformation of 21.2％were observed for fractured sample tested at-40℃,compared with 6.3％in fractured room temperature sample.     

国际标准化组织ISO结对协议简介和实践

本文介绍了国际标准化组织ISO结对协议的概念、类型、基本要求和原则,以及达成结对协议的具体步骤。介绍了ISO/TC67/SC2石油、石化和天然气工业设备材料和海上结构技术委员会管道输送系统分委员会并行秘书处运作三年来的一些实践经验和体会。     

Device Physics of Solution‐Processed Organic Field‐Effect Transistors

Field‐effect transistors based on solution‐processible organic semiconductors have experienced impressive improvements in both performance and reliability in recent years, and printing‐based manufacturing processes for integrated transistor circuits are being developed to realize low‐cost, large‐area electronic products on flexible substrates. This article reviews the materials, charge‐transport, and device physics of solution‐processed organic field‐effect transistors, focusing in particular on the physics of the active semiconductor/dielectric interface. Issues such as the relationship between microstructure and charge transport, the critical role of the gate dielectric, the influence of polaronic relaxation and disorder effects on charge transport, charge‐injection mechanisms, and the current understanding of mechanisms for charge trapping are reviewed. Many interesting questions on how the molecular and electronic structures and the presence of defects at organic/organic heterointerfaces influence the device performance and stability remain to be explored.     

Parametric study of stress state development during twinning using 3D finite element modeling

A deformation twinning model which simulates the characteristic twin shear and corresponding grain reorientation has been developed using a 3D finite element method. This model has been used to study how twinning affects the stress state in both the parent grain and twin, and the stress states that are energetically favorable for twinning. The component of shear stress on the twin plane and direction is primarily responsible not only for whether twinning can occur, but also the energetically favorable twin volume fraction. A map predicting twin volume fraction as a function of parent grain deviatoric stress has been developed.     

亚稳β型钛合金中的{332}〈113〉变形孪晶

{332}113变形孪晶是亚稳β型钛合金变形过程中的一种独特变形机制。该类型孪晶具有特殊性质并且对亚稳β型钛合金力学性能具有显著影响。本文总结了{332}113变形孪晶的研究状况和特性,重点介绍了{332}113变形孪晶形成的几种代表性模型。通过分别对这些模型的假设条件以及需要进一步解释和验证的科学问题进行分析,旨在为理解和揭示{332}113变形孪晶的变形机制提供有用的参考信息。     

High-resolution electron microscopy of the twinning and intergrowth in Ca4Al6SO16 and Ca3SrAl6SO16

The microstructures of unhydrated calcium aluminosulphate Ca₄Al₆SO₁₆ and Ca₃SrAl₆SO₁₆ have been studied by high-resolution electron microscopy (HREM). The results showed that twinning and twinned slabs could be introduced taking the [1 1 2] direction as the twin axis so that it seems to be coincident with the law of twinning formed in body-centred cubic structures. A previously reported superlattice with a repeat period twice that of the fundamental structure along the 〈1 1 0〉 direction has also been found in both matrix and twin variants. The close intergrowth of Ca₃SrAl₆SO₁₆ and another phase, possibly Sr₃Al₂O₆ existing as an inclusion between these two twin variants, was determined and clearly revealed by electron diffraction and HREM images. The coherent interphase boundaries and orientation relationship between them can also be deduced.     

The application of the Kossel technique and electron microscopy to the study of the microstructure of Fe-32% Ni martensite crystals

The back-reflection Kossel technique and transmission electron microscopy have been used to investigate the internal structure of Fe-32% Ni martensite crystals. The internal structure of these martensites has been shown to consist in general of interpenetrating bands of fine twins and slip dislocations on the (112) and (101) planes respectively. A small number of plates have been shown to exhibit interpenetrating {112} and {145} twinning. The relevance of complex internal martensite structures to the crystallography of martensitic transformations and to the deformation of internally twinned martensitic phases in general are discussed.     

Combustion synthesis/quasi-isostatic pressing of TiC<Subscript>0.7</Subscript>–NiTi cermets: microstructure and transformation characteristics

TiC_0.7–NiTi cermets were produced by combustion synthesis followed by quasi-isostatic consolidation while the reaction products were still hot and ductile. The TiC_0.7–NiTi cermets were characterized by differential scanning calorimetry, room temperature transmission electron microscopy (TEM), and in-situ TEM (temperature varied during observation). The matrix of the as-synthesized 20NiTi, 40NiTi, and 60NiTi composites contains both R and B19′ martensites at room temperature. No distinct R-phase morphology could be imaged. In the B19′ martensite, [011] Type II twinning, Type I twinning and (001) compound twinning modes were observed as the lattice invariant shear (LIS) of the R-B19′ transformation. The [011] Type II twinning is often reported as the LIS of the B2-B19′ transformation, but this is the first experimental confirmation of its predicted presence as a qualified LIS of the R-B19′ transformation. The (001) compound twinning mode is responsible for the fine structure of the martensite with a wavy morphology. Nanoscale structures with a thickness of 5 nm were obtained inside the twins. Twinning was also observed at the interface with carbide particles, which confirms that some stress relaxation of the elastic mismatch occurs. At room temperature, the matrix of the 80NiTi composite had the R-phase structure, which appeared with a needle-like morphology. Thermal cycling resulted in the suppression of the R-phase transformation. This is the opposite of the behavior observed in un-reinforced NiTi alloys.     

Key Properties of Ni?Mn?Ga Based Single Crystals Grown with the SLARE Technique

Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of Ni? Mn? Ga based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of Ni? Mn? Ga, Ni? Mn? Ga? Fe, and Ni? Mn? Ga? Co of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. Ni? Mn? Ga alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as Ni? Mn? Ga with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.     

A decrease in the mobility and multiplication of twinning dislocations in bismuth crystals exposed to constant magnetic field

Simultaneous application of a constant magnetic field and a concentrated load partly suppresses twinning in bismuth crystals by decreasing the mobility and multiplication of twinning dislocations. This significantly decreases both the total twinned volume and the total area of the twin-matrix boundaries.     

High coercive field and magnetization reversal in core-shell cum nanotwin driven Ni/NiO nanospheres

We report here nanotwin-core-shell Ni(core)NiO(shell) spheres of average size 25 nm prepared through polyol method. They exhibit high coercive field at 2 K, sharp peak at approximately 20 K in magnetization curve and magnetization reversal. Interestingly, exchange bias due to antiferromagnetic NiO shell is absent. Among other possibilities, anisotropy variations due to particle size distribution and twinning associated with disorder appear to play an important role. Further, magnetic interactions of twinned bigger spheres, which may also act as superferrimagnetic-like Ni multilayer cores, with superparamagnetic Ni of smaller spheres, might be the additional causes. These nanostructures therefore seem to have potential interest in memory effect.     

Insight from in situ microscopy into which precipitate morphology can enable high strength in magnesium alloys

Magnesium alloys, while boasting light weight, suffer from a major drawback in their relatively low strength. Identifying the microstructural features that are most effective in strengthening is therefore a pressing challenge. Deformation twinning often mediates plastic yielding in magnesium alloys. Unfortunately, due to the complexity involved in the twinning mechanism and twin-precipitate interactions, the optimal precipitate morphology that can best impede twinning has yet to be singled out. Based on the understanding of twinning mechanism in magnesium alloys, here we propose that the lamellar precipitates or the network of plate-shaped precipitates are most effective in suppressing deformation twinning. This has been verified through quantitative in situ tests inside a transmission electron microscope on a series of magnesium alloys containing precipitates with different morphology. The insight gained is expected to have general implications for strengthening strategies and alloy design.     

{111} twinning structure and interfacial energy in nonstoichiometric TiCx with ordered carbon vacancies

Here we report the experimental and theoretical investigations of the {111} twinning structures in nonstoichiometric TiC_0.7 with ordered carbon vacancies. Owing to the ordering of carbon vacancies, a cubic Ti₂C-type ordered phase is formed in nonstoichiometric TiC_x (0.53 ≤ x ≤ 0.81). Via the TEM measurements, the {111}-specific twinning structure with incoherent boundary has been observed in ordered TiC_0.7. The first principles calculations indicate that the presence of ordered carbon vacancies leads to reduction in the energy of {111}_Ti type twinning interface, thus favoring the formation and stabilization of {111}_Ti-specific twinning structure in the ordered TiC_x.     

Microstructure evolution of Mg-3%Al-1%Zn alloy tube during warm bending

Microstructure evolution and deformation mechanisms of AZ31 magnesium alloy tubes during bending have been investigated. Dislocation slip appears to be the main deformation mechanism, along with a few {10–12} [−1011] extension twins at the outer bend radius which undergoes tensile deformation. In contrast, the material in the tube wall at the inner bend radius of the tube, which undergoes compression, deforms mainly by extension twinning. This understanding of deformation mechanisms has explain the optimum bending temperature of 150–200 °C for the AZ31 tubes where both slip and twinning are active.     

A quantitative understanding on deformation twins generation in single-phase austenitic stainless steels

Single-phase austenitic stainless steels (316L) have attracted widespread attention from scientists because of their duplex microstructure. In this paper, to have a quantitative understanding on the microstructure deformation of 316L, a physical model based on dislocation theory and strain gradient theory is established to find out the critical conditions when deformation twins generate. The twinning stress and the stress caused by strain gradients are two factors affecting the deformation twinning process. Numerical simulation results reveal that the twinning stress decreases with the increase of twin spacing and the decreases of volume fraction of twins and the orientation of external shear stress; the stress caused by strain gradients increases with the decrease of matrix grain size.