纳米Ni薄膜在摩擦过程中塑性行为的分子动力学模拟

用分子动力学模拟研究了金刚石压头在Ni晶体薄膜上的摩擦过程和薄膜塑性变形行为的纳观机制.结果表明:在摩擦过程中,穿晶层错和棱形位错环是纳米薄膜结构传递塑性变形的两种载体,纳米薄膜晶界捕获位错阻滞了塑性变形向薄膜晶界下方材料中传播.摩擦过程中易在较薄的薄膜表面和薄膜晶界之间产生穿晶层错,穿晶层错的产生增加了薄膜蓄积塑性变形的能力,从而抑制材料表面摩擦力在黏滑过程中的振荡幅度;在比较厚的薄膜中不易生成穿晶层错,在摩擦过程中位错环依次向体材料发射,并与晶界反应,湮灭于晶界,黏滑动摩擦响应与单晶相似.由于不同厚度薄膜塑性变形产生的位错结构不同,使得在摩擦过程中亚表面微结构的演化亦不同.     

Atomistic simulation of stacking faults in (001), (010), and (100) planes of cementite

Molecular-dynamics method was used to study γ surfaces for the (001), (010), and (100) planes of cementite. Displacement vectors corresponding to stable stacking faults have been determined. The energy of these stacking faults has been calculated by the molecular-dynamics and ab initio methods. The energy of unstable stacking faults, which characterizes the tendency of a material to plastic relaxation, has been estimated. The reactions of the splitting of perfect dislocations have been suggested; the possibility of the propagation of stacking faults in the planes under consideration is discussed.     

A QUENCHING INDUCED PLANAR DEFECT OF MARTENSITE IN Cu-Zn-Al ALLOY

A new kind of planar defect named non-basal plane stacking faults has been studied by usingtrace analysis and lattice image techniques.It has been found that they are located in(125)_M plane of martensite lattice.By lattice image observation,the non-basal plane stack-ing faults are argued to be induced by two incomplete basal plane stacking faults and may bethe results of rapid quenching.The defects are regarded as anomalous structures ofmartensite.     

Direct evidence for Suzuki segregation and Cottrell pinning in MP159 superalloy obtained by FEG(S)TEM/EDX

The effects of Suzuki segregation on the plastic flow behaviour of MP159 alloy deformed at high temperature and on the resulting dislocation structure have been examined. Elemental concentration profiles across both stacking faults and slip bands have been measured in a FEG TEM in nano-probe using the line scanning mode and EDX. It was found that Suzuki segregation resulted in continuously serrated plastic flow for deformation at temperatures from 450–670 °C and at a slow strain rate such as 1.0×10⁻⁴/s. TEM examination showed an increased dissociation width for dislocations and larger and more stacking faults after deformation at high temperatures as compared with those after deformation at room temperature. This can be interpreted as being due to the reduction of stacking fault energy by Suzuki segregation and/or Cottrell pinning. The elemental concentration profiles across stacking faults and slip bands showed that Mo and Al were more often found than other solutes to segregate to stacking faults and slip bands. Occasionally, the segregation of Ti and Nb could also be detected at stacking faults and slip bands.     

淬火在Cu—Zn—Al合金马氏体中引入的一种新缺陷

本文报道了Cu-Zn-Al合金中18R马氏体里存在的一类新的面缺陷。实验结果的统计表明,在由高温直接淬入冰水中的样品里,马氏体中常出现非基面上的层错。衍衬分析证明,这种层错常位于(125)_M面上。通过对晶体结构和处理条件分析,提出了这种层错出现的机制,并给出了层错的结构模型。     

钢中马氏体的层错亚结构

研究发现淬火钢马氏体中存在层错亚结构具有重要理论意义。应用JEM-2100高分辩透射电镜观察35CrMo、2Cr13、W6Mo5Cr4V2等钢的马氏体时,发现存在层错亚结构。层错亚结构条纹微细,间距约为数纳米,且往往与位错伴生。层错的成因是在晶格重构过程中,因新旧相错配度等因素造成晶格错排时形成的。认为切变机制不能解释马氏体层错亚结构的成因。     

Characterization of stacking faults on basal planes in intermetallic compounds La5Ni19 and La2Ni7

Stacking faults on basal planes as well as intergrowth structures in La₅Ni₁₉ and La₂Ni₇ have been characterized by high-resolution transmission electron microscopy (HREM) and electron diffraction. Most stacking faults observed in La₅Ni₁₉ are of the inter-block-layer type in which the faulting in the stacking sequence of block layers occurs without changing the number of unit layers in each block layer. In contrast, most stacking faults observed in La₂Ni₇ are of the intra-block-layer type in which the faulting occurs in the number of unit layers in block layers, although some stacking faults of the inter-block-layer type are also observed. Intergrowth structures are commonly observed in as-cast alloys with compositions corresponding to La₅Ni₁₉ and La₂Ni₇. The implication of these results for increasing the hydrogen absorption rate of LaNi₅-based alloys without significantly affecting hydrogen absorption/desorption properties described in terms of pressure-composition isotherms are discussed.     

基于分子动力学的3D打印TiAl纳米多晶合金变形机制及其温度相关性分析

借助透射电镜观察和分子动力学计算,对3D打印Ti-6Al-4V合金的变形行为及其温度相关性进行了系统研究。结果表明,温度在TiAl纳米多晶体变形机制的竞争中起关键作用。当温度低于800 K,平均晶粒尺寸低于8.3 nm的单相TiAl纳米多晶合金首先出现位错运动,且层错保留在晶粒中并形成交错结构。同时,大尺寸晶粒(≥8.3 nm)为位错运动提供了足够的空间,很少在晶粒中形成层错。在双相TiAl+Ti₃Al纳米多晶合金中,层错的交割是低应变(ε<18.0%)TiAl晶粒的主要变形机制,并且Ti₃Al晶粒保持其初始结构。当ε≥18.0%时,Ti₃Al晶粒中的位错开始运动并形成层错交割。当温度高于800 K时,Ti和Al原子处于高能状态,主要的变形机制与具有非晶结构的滑移边界有关。非晶滑移边界及再结晶结构是双相TiAl+Ti₃Al纳米多晶合金组织变形的最重要特征。     

Characterization of planar features in Mg–Y–Zn alloys

The planar features in a Mg–8Y–2Zn–0.6Zr (wt.%) alloy solution-treated at 500 °C for 1 h have been examined using conventional transmission electron microscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. Three types of planar features are detected in the microstructure. The first type, which was previously reported to be an intrinsic stacking fault I₁ bounded by a Frank partial dislocation, is shown to be the 14H precipitate phase that is associated with Shockley partial dislocations. The second type is also a precipitate phase that has a single unit cell height and is associated with Shockley partial dislocations. The third type of planar feature comprises small ribbon-like stacking faults. These stacking faults are determined as intrinsic I₂ type bounded by two Shockley partial dislocations, which is further confirmed by computer simulation. The stacking fault energy associated with the faults is much smaller than that of pure magnesium.     

Microscopic description of plasticity in computer generated metallic nanophase samples: a comparison between Cu and Ni

Simulations are reported on the plastic behavior of two model f.c.c. metals, Ni and Cu, with different stacking fault energies, and average grain sizes in the range of 3–12 nm. A change in deformation mechanism is observed: at the smallest grain sizes all deformation is accommodated in the grain boundaries. At higher grain sizes intragrain deformation is observed. Analysis of the atomic configurations shows that intrinsic stacking faults are produced by motion of Shockley partial dislocations generated and absorbed in opposite grain boundaries. In Cu the stacking faults are observed at smaller grain sizes than in Ni (8 nm in Cu, 12 nm in Ni) which is attributed to the lower stacking fault energy. Shockley partial dislocations appear on slip systems that are not necessarily those favored by the Schmid factor. Atomic displacement analysis shows that deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.     

Effect of nitrogen on stacking fault energy of f.c.c. iron-based alloys

A method is proposed to estimate the stacking fault energies of face-centered-cubic (f.c.c.) iron-based alloys. The segregation of alloying elements to stacking faults and the interaction of substitutional and interstitial alloying elements in solid solution and their effect on stacking fault energy have been taken into account. It is shown that at low nitrogen concentrations (e.g. 0.05 wt%), the stacking fault energy is increased mainly due to the effect arising from the bulk of the alloy. At high nitrogen concentration (e.g. 0.5 wt%), the stacking fault energy is decreased due to the segregation of the alloying elements (mainly nitrogen) on the stacking faults of the alloy. Moreover, it is shown that in nitrogen alloying of f.c.c. iron-based alloys the magnetic contribution of the nitrogen to the stacking fault energy is negligible. The method shows reasonable agreement with existing experimental data.     

Development of γ phase stacking faults during high temperature creep of Ru-containing single crystal superalloys

An unusual deformation mode involving the formation of intrinsic stacking faults in the γ matrix of experimental Ru-containing γ–γ′ superalloys with high Co and Re contents during high temperature creep at 950 °C/290 MPa has been observed. The morphology, distribution and dependence of these stacking faults on alloy chemistry has been investigated along with their formation mechanism. Additions of Re and Co substantially decrease the stacking fault energy of the γ matrix. The observed stacking faults in the γ matrix form by the dissociation of a/2〈1 1 0〉 matrix dislocations with Burgers vectors perpendicular to the loading direction in the early stages of creep. The dependence of creep properties on elemental additions that influence stacking fault energy is discussed.     

Direct observation of dislocation dissociation and Suzuki segregation in a Mg–Zn–Y alloy by aberration-corrected scanning transmission electron microscopy

Crystal defects in a plastically deformed Mg–Zn–Y alloy have been studied on the atomic scale using aberration-corrected scanning transmission electron microscopy, providing important structural data for understanding the material’s deformation behavior and strengthening mechanisms. Atomic scale structures of deformation stacking faults resulting from dissociation of different types of dislocations have been characterized experimentally, and modeled. Suzuki segregation of Zn and Y along stacking faults formed through dislocation dissociation during plastic deformation at 300 °C is confirmed experimentally on the atomic level. The stacking fault energy of the Mg–Zn–Y alloy is evaluated to be in the range of 4.0–10.3 mJ m^?2. The newly formed nanometer-wide stacking faults with their Zn/Y segregation in Mg grains play an important role in the superior strength of this alloy at elevated temperatures.     

Atomistic simulation of stacking faults in cementite: Planes containing vector [100]

The method of molecular dynamics (MD) has been used to study γ surfaces in crystallographic planes of cementite containing Burgers vector [100]. Displacement vectors that correspond to stable stacking faults (SFs) and the energies of these SFs have been determined. The energies of unstable SFs, which characterize the tendency of the material toward plastic relaxation, have been estimated. It has been established that, in all these planes except for (013), the dislocations with the Burgers vector [100] are not split. Planes have been found that are most suitable for the slip of these dislocations. The opportunity of the propagation of stacking faults in these planes is discussed; reactions of the splitting of perfect dislocations with the Burgers vector [100] in the plane (013) have been suggested.     

4H-SiC band structure investigated by surface photovoltage spectroscopy

The conduction and valence band structure of high-purity 4H-SiC epilayers have been studied by surface photovoltage spectroscopy (SPS). A comparison between defect-free and single-layer stacking fault affected areas is reported. Electronic transitions, determined by SPS, are in good agreement with ab initio calculations. Electronic transitions and changes in band occupation have been observed in stacking fault rich areas below the band gap. Moreover, stacking faults induce the presence of a split-off band below the conduction band and a modification of the electron density of states in the conduction band always at the M point.     

N对TWIP钢马氏体相变及力学性能的影响

以传统TWIP钢为对比,测试了含N TWIP钢的力学性能,并利用XRD进行物相分析和TEM进行做观结构表征.结果表明,在由fcc或hcp结构向bcc结构马氏体进行相变时,晶体结构中的最大间隙由0.1047 nm降低至0.0725 nm.间隙原子N的存在显著增大bcc结构的晶格畸变能,提高α马氏体切变的阻力,因而强烈抑制α马氏体相变,导致组织中hcp结构ε相含量大幅度增加,提高了TWIP钢的强度,但也降低了钢的塑性.另外,奥氏体平均和区域层错几率的计算及微观组织分析结果表明,形变增加层错的数量,而马氏体相变消耗层错,从而减少层错数量.     

Microstructure Studies of GaInAsSb/GaSb Heterostructure

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HREM study on stacking structure of SiGe/Si infrared detector

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MICROSTRUCTURES OF ULTRAFINE WC POWDERS

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Formation of shear bands and strain-induced martensite during plastic deformation of metastable austenitic stainless steels

Formation of shear bands and strain-induced ε- and α′-martensite phases during tensile deformation of austenitic stainless steels was studied. Stacking fault energies (SFE) of the studied steels were measured by X-ray diffraction. Effects of external stress and SFE on the width of the stacking faults were analysed. An excellent correlation between the calculations and actual microstructures examined by scanning electron microscopy was found. Effect of overlapping of stacking faults on the fault width was discussed. Based on the discussions and experimental results, compositional, temperature and strain rate dependencies of the strain-induced α′-martensite transformation are believed to be governed mainly by the variation in the SFE.