Twin Nucleation by Slip Transfer across Grain Boundaries in Commercial Purity Titanium

The role of strain transfer in the activation of deformation twinning at grain boundaries has been characterized in commercially pure titanium deformed in bending. Two different orientations of a textured polycrystal were deformed in bending and were analyzed using electron backscattered diffraction (EBSD) to determine the active slip and twinning systems in the surface tensile region. Prismatic slip and { 10[`1]2 } á [`1]011 ñ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle twinning were the most widely observed deformation modes in both orientations. Nonprismatic slip systems were also activated, most likely to accommodate local strain heterogeneities. A slip-stimulated twin nucleation mechanism was identified for soft/hard grain pairs: dislocation slip in a soft-oriented grain can stimulate twin nucleation in the neighboring hard grain when the slip system is well aligned with the twinning system. This alignment was described by a slip-transfer parameter m′.[24] Twins activated by this mechanism always had the highest m′ value among the six available { 10[`1]2 } á [`1]011 ñ \left\{ {10\bar{1}2} \right\}\left\langle {\bar{1}011} \right\rangle twinning systems, while the Schmid factor, based on the global (uniaxial tensile) stress state, was a less significant indicator of twin activity. Through slip transfer, deformation twins sometimes formed despite having a very low global Schmid factor. The frequency of slip-stimulated twin nucleation depends strongly on the texture and loading direction in the material. For grain pairs having one grain with a large Schmid factor for twinning, nonparametric statistical analysis confirms that those with a larger m′ are more likely to display slip-stimulated twinning.     

Effects of Location of Twin Boundaries and Grain Size on Plastic Deformation of Nanocrystalline Copper

Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems. Both experiments and simulations show that introducing nanoscale twins into nanocrystalline copper is an effective approach to improve strength while maintaining high electrical conductivity. The present study investigated the influence of twin boundary (TB) and grain size refinement on plastic deformation in polycrystalline copper. Molecular dynamics (MD) simulation with embedded-atom method potential for copper was performed to simulate a cell of [011] textured microstructure embedded with four hexagonal grains. Simulation results showed that strength and toughness (i.e., energy per volume absorbed by system up to a certain strain in this study) of copper could be enhanced by introducing TBs within nanocrystalline grains. Nanotwins act as obstacles to dislocation motion that leads to strengthening, as well as sources of dislocation nucleation contributing to the toughness of the materials. The enhancement of the properties is apparently sensitive to the distance between TB and grain boundary (GB); i.e., it exhibits a maximum at an intermediate distance, while it decreases when TBs are far away or very close to GBs. This implies that the volume between TB and GB plays an important role on the plasticity of nanocrystalline copper. Moreover, the deformation behavior in different grains depends on their orientation with respect to the loading direction. The study has also confirmed that grain-size refinement in nanotwinned models would lead to an inverse Hall?CPetch relationship.     

Segregation at Melted Grain Boundaries in Steel

Abstract

Test samples from continuously cast steels have been fractured by grain boundary melting. The faceted fracture surfaces have been examined by Auger spectroscopy. Sulphur has been found to be segregated on the surface of all of the facets examined. The amount of segregation varies appreciably for different facets in a given sample; on average the segregation increases significantly with an increase in sulphur level from 0.014 to 0.024% S. No segregation of P was detected on the fractured surfaces.

Résumé

Des échantillons d'aciers de coulée continue ont été rupturés par fusion aux joints de grains. Les surfaces de rupture à facettes ont été examinées par spectroscopie Auger. On.a trouvé, sur toutes les surfaces des facettes examinées, une ségrégation de soufre. Le montant de la ségrégation varie appréciablement pour différentes facettes d'un même échantillon. En moyenne, l'étendue de la ségrégation augmente quand la concentration en soufre passe de 0.014 à 0.024%. On n'a pas détecté de ségrégation de P sur les surfaces de rupture.     

晶界在超塑性变形中的作用

超塑性材料所具有的微细晶粒组织,意味着材料的单位体积内存在着大量的晶界。本文结合作者及其合作者近几年来所进行的研究工作,详细地论述了晶界在超塑性变形中所起的重要作用。     

Plastic Flow Potential for Cone Region of MRS-Lade Model

The original formulation of the MRS-Lade model, with nonassociated flow rule on the meridian plane in the cone region, has a corner. In order to reduce the computational effort of corner solution algorithms, a modified plastic flow potential for the cone part is found in the literature. This modification may have a nonadmissible flip over of the flow vector in the cone-cap intersection if the plastic flow potential is not correctly defined. Here a corrected plastic flow potential for the cone region is defined to obtain a continuous transition of the flow vector.     

Calculating Dislocation Spacings in Pile-Ups at Grain Boundaries

Dislocation pile-ups near planar grain boundaries have been analyzed employing full anisotropic elastic solutions, single-crystal anisotropic approximations, and isotropic approximations. The calculations were performed for a series of iron-silicon bicrystal configurations and results from the various methods compared. Three of the bicrystal/slip system combinations exhibited repulsive image forces, allowing elastically self-consistent position calculations. Based on these results, either linear elastic or nonlinear effects can dominate the behavior of these pile-ups, depending on specific boundary conditions.     

Prediction of Generation of High- and Low-Angle Grain Boundaries (HAGB and LAGB) During Severe Plastic Deformation

Metallurgical and Materials Transactions A - It is well known that a balance between the generation of low-angle and high-angle grain boundaries (LABG and HAGB) is achieved in materials undergoing...     

Low-Angle Grain Boundaries in Sublimation Grown 6H-SiC Crystals

High-resolution X-ray diffractometry (HRXRD) was used to assess the quality of 6H-SiC crystals grown by sublimation method. The results show the occurrence of low-angle grain boundaries (LB) is close relative to the inclination of the crystal interface. At the central faceted region with 0° inclination the crystal is of high structural perfection. However, at the region close to the facet with less than 5° inclination LB occurs slightly and at the region close to the peripheral polytype ring with more than 5° inclination LB defect occurs heavily. The density of LB can be drastically reduced by decreasing radial temperature gradient that determines the shape of the crystal growth interface.     

铝合金热塑性变形流动应力及其数学模型

采用恒应变速率凸轮式压缩试验机,测定了4种铝合金材料在热状态下的流动应力,分析了应变率、应变速率及变形温度对流动应力的影响规律,通过对多种结构型式流动应力数学模型的回归分析比较,确定了计算精度较高、结构型式较简单、适合于现场计算机在线控制和工程计算的数学模型。     

The Use and Control of Grain Boundaries in Thin Films

Abstract

Polycrystalline thin films are widely used in the electronics industry and in surface protection applications. For most cases, a single crystal film would be preferable but is too costly or simply impossible to achieve. Thus, the grain boundaries are regarded as a necessary evil. We discuss the behavior of grain boundaries during film formation, processing and utilization, illustrating beneficial as well as deleterious effects. Some potential strategies for the control of grain boundary behavior are derived by the application of basic interfacial science.

Résumé

L'utilisation de films finspolycristallins est largement répandue dans l'industrie électronique et dans les applications de protection de surfaces. Dans la plupart des cas, un film monocristallin serait préférable, mais ceci est en fait trop coûteux et bien trop difficile à réaliser, alors les grains sont perçus comme une nécessité malencontreuse. Nous considérons le comportement des joints des grains durant la formation, le traitement et l'utilisation des films, en illustrant tout aussi bien les effets bénéfiques que nuisibles. Nous avons tiré une stratégie potentielle pour le contrôle du comportement des joints de grains par l'application de la science fondamentale des interfaces.     

多晶铜中晶界发射位错的动态电镜观察

介绍了纯铜试样在拉伸过程中晶界发射位错的透射电子显微镜动态观察结果。表明面心立方多晶纯铜晶界中也存在“坎”（Ｌｅｄｇｅ），在应力的作用下，“坎”向晶粒内部发送位错。     

Grain Boundary Curvature in a Model Ni-Based Superalloy

The local grain boundary (GB) curvature in a model Ni-based superalloy was measured experimentally using Dehoff’s tangent count method. The results show that, in materials containing significant amounts of second-phase particles, the curvature parameter, κ, which relates the mean local curvature to the grain size, can adopt far lower values than have been reported previously. It is also shown that the value of κ is not a constant, as is usually assumed, but instead varies both with the volume fraction of second-phase particles and with the holding time during high-temperature annealing. The lowest values for κ were obtained for high particle volume fractions and long annealing times. Because the local boundary curvature constitutes the driving force for grain growth, these observations could help to explain grain growth phenomena in heavily pinned systems.

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Continuum Modeling and Discrete Element Simulations of Elastic-Quasi-Static Granular Flow in a Compressing Slot

The stress generated by the horizontal compression of a vertical column of granular material is investigated. The column is open at the top so that the material is free to flow upward in the slot as it is compressed. This simple geometry has interesting mechanics because both elastic and frictional regimes coexist, and it is also relevant to problems involving the insulating material in cryogenic storage tanks. Two methods are used to investigate this problem: traditional continuum modeling and discrete element simulation. The continuum model assumes that the column consists of a frictional region near the top of the column and a linearly elastic region near the bottom. Analytic solutions are obtained for each region and predictions for the location of the transition are made based on intersections of the two solutions. A discrete element simulation of the same geometry is performed to compare with the results from the continuum model. Various conditions of wall friction and particle stiffness are simulated. Based on the outcome of this comparison, we verify that in a compressing slot there are essentially two distinct regions: a frictional flow region near the top of the column that results in an exponential increase in stress with depth. This eventually saturates leading to a linear-elastic plane strain region. The location of the transition between these two regions depends on the material properties and the state of the compression.     

6061挤压铝棒晶粒度的控制研究

6061铝合金在挤压过程中,不同的挤压工艺条件对粗晶组织产生不同的影响,通过合理调整6061铝合金挤压温度、挤压系数以及挤压速度可以控制粗晶的产生。     

Damage Susceptibility of Grain Boundaries in HT9 Steel Subjected to High-Temperature Creep

HT9 steel is an attractive ferritic/martensitic steel that is used in components of nuclear and fossil power plants because of its high strength and good swelling resistance. Specific phenomena (such as segregation, voiding, cracking, etc.) are prevalent along grain boundaries since these interfaces act as efficient sources for vacancies. The accumulation of vacancies in grain boundaries may result in intergranular fracture. In this study, HT9 steel was subjected to creep tests at elevated temperature (about 0.5 T _m) and two different creep conditions (where creep lifetimes were about 100 and about 1000?hours, respectively). The grain boundaries in HT9 steel after creep tests were studied by the use of scanning electron microscopy in order to establish the relationship between the grain boundary structure and creep damage. Images and data obtained using electron backscatter diffraction reveal a high susceptibility of high-angle boundaries to creep cavitation, as expected. In addition, the ??3 boundaries are also susceptible to damage under these conditions at a similar or even higher rate as compared with random high-angle boundaries.     

冷轧多晶纯镍中晶界对显微硬度和微观组织结构的影响

纯度为99．9％的多晶镍（晶粒尺寸为0．5～2mm），被轧制到55％形变量，用测试显微硬度的手段研究选定晶粒内部的形变不均匀性．用扫描电镜（SEM）上的背散射电子衬度成像分析晶粒内不同位置的微观组织的变化。结果表明，与轧制方向成较大角度的晶界的影响导致晶粒内形变的不均匀性，造成微观组织及形变储能的不同，宏观上表现为显微硬度的不同。     

Ni-30Cr-1Cu合金晶粒尺寸及晶界沉淀与脆性的关系

用晶界萃取复型技术研究了晶界一次相极少、二次相析出倾向很大的Ni-30Cr-1Cu合金的晶界沉淀规律。采用不同温度固溶后盐水快速冷却以抑制晶界二次相的析出获得了晶界沉淀状态相近的不同尺寸晶粒，结果表明，随单纯晶粒长大，合金的室温冲击韧性、拉伸塑性均有不同程度的升高。于1180℃固溶后以不同速度冷却，结果显示了随冷却速度减慢，晶界M23C6显著增多、分布密度增大；其沉淀特征分别由空冷的精细薄片状向缓冷的颗粒状、棒状、薄膜状乃至枝晶状转变，使晶界在室温脆化，冲击韧性和拉伸塑性显著下降；断裂特征由以穿晶为主的混合型断裂向沿晶脆性断裂转化。     

Behaviors of Lanthanum and Cerium on Grain Boundaries in Carbon Manganese Clean Steel

The behaviors of La and Ce on gram boundaries in carbon manganese clean steel were investigated by high-reso- lution transmission elecetron microscope （HRTEM）, scanning elecetron microscopy（SEM ）, energy dispersive spectrometer （EDS） and X-ray diffraction（XRD） analysis. The existing forms of rare earths （RE） in clean steel were as follows： dissolved in sohd solution, forming inclusion or second phase containing RE （RE-Fe-P, La-P, Fe-La eutectic and Fe-Ce phase）. The dissolved La or Ce segregated at grain boundaries. The segregation of both S and P at gram boundaries was reduced with suitable RE content. The impact toughness of the steel was improved obviously. La and Ce had effecets on purifying molten steel and modifying inclusions in clean steel, whereas with excessive La or Ce, La-Fe-P, La-P and Fe-La eutecetic phase or Ce-Fe-P and Fe-Ce intermetallic compound would form along grain boundaries, causing the impact energy to decrease significantly.     

Effect of Heat Treatment on Solute Concentration at Grain Boundaries in 7075 Aluminum Alloy

The effect of heat treatment on the relative amounts of magnesium, zinc, copper, iron and silicon in grain boundaries of aluminum 7075 alloy was investigated with Auger electron spectroscopy in a scanning Auger microprobe. It was observed that both aging and solution treatments affect the distribution of these elements at grain boundaries. If identical solution heat treatments were employed, the aging treatment determined the solute distribution at the grain boundaries, and for fixed final aging treatment, the temperature of solution anneal determined the solute distribution. Among specimens given a T6 final aging treatment (24 h at 394 K (250 °F)) with prior solution treatment temperatures in the 666 K (740 °F) to 800 K (980 °F) range, 711 K (820 °F) solution treated specimen was found to have the least solute concentration at grain boundaries. The observed behavior is explained on the basis of equilibrium and nonequilibrium segregation of solute elements to grain boundaries occurring on quenching and during aging.