The role of partial grain boundary dislocations in grain boundary sliding and coupled grain boundary motion

We study the process of grain boundary sliding through the motion of grain boundary dislocations, utilizing molecular dynamics and embedded atom method (EAM) interatomic potentials. For a Σ = 5 [001]{310} symmetrical tilt boundary in bcc Fe, the sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. While the homogeneous nucleation of these grain boundary dislocations requires shear strain levels higher than 7%, preexisting grain boundary dislocations are shown to glide at applied shear levels of 1.5%. The glide of the dislocations results in coupled motion of the boundary in the directions parallel and perpendicular to itself. Finally, interstitial impurities and vacancies were introduced in the grain boundary to study the effects on the sliding resistance of the boundary. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not. Both hydrogen and C interstitials stop dislocation glide whereas vacancies do not. A detailed study of the dynamic properties of these dislocations is also presented.     

Surface and interior measurements of grain boundary sliding during creep

Measurements of grain boundary sliding have been made on polycrystalline specimens of Magnox AL80, a magnesium-0.78 wt% aluminium alloy, at successive strains during creep at 200° C under a stress of 2800 psi. Three independent methods were used to determine the strain due to sliding ( _gb) at the surface and two to determine _gbin the interior of the specimens. The one direct method of measuring _gbin the interior used oxide markers introduced by extruding a composite billet. The values of _gbobtained from the offsets in these interior markers were found to agree with those given by the three sets of measurements made on the surface, but not with those from the indirect method for the interior which relies on the measurement of grain strain via grain shape changes.     

非连续增强金属基复合材料剧烈塑性变形行为研究进展

综述了非连续增强金属基复合材料剧烈塑性变形(SPD)行为的研究进展,系统阐述了等径弯曲通道变形(ECAP)、高压扭转(HPT)、多向锻造(MF)、累积叠轧(ARB)和循环挤压压缩(CEC)5种SPD的加工原理和方法。集中介绍了这些方法在铝基、镁基、铜基和钛基等金属基复合材料方面应用的研究进展。重点介绍了金属基复合材料SPD的微观组织演化和变形力学行为,详细阐明了金属基复合材料SPD机制以及超细晶形成机理,指出了金属基复合材料在SPD中存在的深层次问题及发展趋势,展望了利用SPD方法制备超细晶非连续增强金属基复合材料的应用前景。     

A model of grain boundary sliding during deformation

A model of slip at grain boundaries is proposed based on the concept of local melting of the grain boundary under intense external influence and by analogy with ultrathin lubrication. A discontinuous regime of slip along the grain boundaries similar to the stick-slip process in thin lubricants is described.     

Compression creep behaviour of steel fibre reinforced cement composites

The paper presents the results of an experimental investigation to determine the influence of steel fibre reinforcement on the creep of cement matrices under compression. Creep tests were carried out at a number of applied stress-strength ratios ranging between 0.3 to 0.9. Melt extract and hooked steel fibres were used at volume fractions ranging between 0 and 3% by volume of a mix. Three types of cement matrices were used namely cement paste, mortar and two mix proportions of concrete. The results indicate that steel fibres restrain the creep of cement matrices at all stress-strength ratios, the restraint being greater at lower stresses and at higher fibre contents. Steel fibres are effective in restraining only the flow component of creep of cement matrices, the delayed elastic component being unaffected. The reduction in creep of cement pastes, due to fibre reinforcement, is much greater than that for mortar or concrete matrices. Mathematical expressions are given for the creep of steel fibre reinforced cement matrices.

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Résumé Cet article présente les résultats d’une étude expérimentale en vue de déterminer l’influence du renforcement de fibres d’acier sur le fluage de matrices de ciment en compression. On a exécuté les essais de fluage à un certain nombre d’intervalles correspondant à des rapports contrainte/déformation allant de 0,3 à 0,9. On a utilisé des fibres d’acier dans des proportions se situant entre 0 et 3% par volume de mélange, et on s’est servi de trois types de matrices de ciment: pate de ciment, mortier et béton selon deux dosages. Les résultats montrent que les fibres d’acier empêchent le fluage des matrices de ciment dans tous les rapports de contrainte/déformation, l’effet étant plus important pour les contraintes faibles et les teneurs en fibres plus élevées. Les fibres d’acier n’agissent qu’en s’opposant à la déformation plastique des matrices de ciment sans que l’élasticité différée en soit affectée. La diminution du fluage des pates de ciment due au renforcement des fibres est beaucoup plus importante que pour le mortier ou les matrices de béton. On donne des formules mathématiques pour le fluage des matrices de ciment renforcé de fibres d’acier.

     

Effect of dopants on alumina grain boundary sliding: implications for creep inhibition

We investigate by means of periodic density functional theory the mechanism of grain boundary sliding along the α-alumina Σ11 tilt grain boundary. We identify minimum and maximum energy structures along a preferential sliding pathway for the pure grain boundary, as well as for grain boundaries doped with a series of early transition metals, as well as barium, gadolinium, and neodymium. We predict that the segregation of those dopants results in a considerable increase in the grain boundary sliding barrier. Grain boundary sliding occurs by a series of bond breaking and forming across the grain boundary. Our results suggest that the presence of large cations inhibits the regeneration of bonds during sliding, which results in a decrease in total number of bonds across the grain boundary interface, thereby raising the barrier to sliding. Trends in predicted grain boundary sliding energies are in good agreement with recently measured creep activation energies in polycrystalline alumina, lending further credence to the notion that grain boundary sliding plays a dominant role in alumina creep.     

Study on fracture strain of discontinuously reinforced metal matrix composites

The fracture strain of discontinuously reinforced metal matrix composites (DRMMCs) is studied according to a damage mechanics model proposed by McClintock, with a localized crack propagation hypothesis and particle-related crack initiation mechanism. An estimation equation is proposed. The estimated results are verified with the data available in the literature and those measured on some types of DRMMCs obtained by powder metallurgical techniques. The experimental results show good agreement with the estimation model.     

Impression creep behaviour of TiB2 particles reinforced steel matrix composites

Impression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873–973?K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149?kJ?mol^?1 and steel reinforced with 2 and 4?vol.-% TiB₂ was found to be 298 and 338?kJ?mol^?1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application.     

非连续增强钛基复合材料的热处理研究进展

综述了非连续增强钛基复合材料热处理的研究现状,主要分析了热处理工艺对复合材料微观组织与力学性能的影响,提出了当前研究中存在的问题和今后潜在的发展方向.     

Application of electron beam lithography to study microcreep deformation and grain boundary sliding

Electron beam lithography has been employed to study microcreep deformation and grain boundary sliding in pure copper. Fine electron-sensitive microgrids of an alloy of palladium and gold were developed on the surface of rectangular specimens. Interrupted creep tests were carried out at 723 K at two stress levels in an argon atmosphere. Creep strain and grain boundary sliding were determined by forming Moire fringes in a scanning electron microscope as well as from the displacement of the grid lines. Local distribution of creep strain inside the grains was found to be non-uniform. Grain boundary sliding exhibited a wavy behaviour.     

超塑性形变中的晶界形态变化及再结晶的作用

为了研究晶界形态及动态再结晶在超塑性形变中的作用,采用光学显微镜、扫描电镜、透射电镜,对硬铝ＬＹ１２ 的超塑性形变过程进行了观察、分析．提出金属材料的超塑性主要依靠晶界流态化区的粘性变形来实现,动态再结晶不是超塑性的一种机制．     

Fracture and fatigue of discontinuously reinforced copper/tungsten composites

The strength, toughness and resistance to cyclic crack propagation of composites consisting of copper reinforced with short tungsten wires of various lengths have been studied and the results compared with the behaviour of continuously reinforced composites manufactured by the same method, i.e. by vacuum hot-pressing. It has been found that whereas the resistance to fatigue crack growth of continuously reinforced composites is very similar to that of continuous Al/stainless steel composites reported elsewhere, the addition of short fibres completely changes the mode of fracture, and no direct comparisons are possible. In effect, short fibres inhibit single crack growth by causing plastic flow to be distributed rather than localized, and although these composites are much less strong than continuous fibre composites, they nevertheless have much greater fatigue resistance. The fracture toughness of the composites is thought to be derived simply from the separate contributions of matrix and fibre plastic flow and, in composites containing fibres near to the critical length, from the very substantial work of fibre pull-out.     

Hot workability and processing maps of aluminium based discontinuously reinforced composites

Abstract

Hot deformation behaviour under tensile conditions of Al6061/20%Al₂O₃ , Al2618/20%Al₂O₃, and Al2618/20%SiC particle reinforced composites and of an unreinforced 6061 aluminium alloy was investigated in the temperature range 350–500°C and in the strain rate range 10^–3-10^–1s^–1. Processing maps were generated from the obtained data and the domains of optimal hot workability were identified for the different materials. Microstructural analyses on deformed specimens allowed it to be stated that dynamic recovery was the main restoration mechanism for the composites investigated. The concurrent evaluation of ductility and flow stress data also suggested that differences in reinforcement properties can play a significant role on property definition.     

Dry sliding wear behaviour of fly ash particles reinforced AA 2024 composites

AA 2024 alloy has been melted and cast in a permanent cast iron mould in the form of 18 mm ?? fingers. The synthesis of AA2024 alloy ? 5wt.% fly ash composite was made by stir cast technique. A uniform distribution of fly ash particles in the matrix phase was obtained. Good bonding between the matrix and reinforcement was also achieved. Dry sliding wear behavior of the alloy and the composite has been investigated using a pin-on-disc wear tester. The investigation was carried out at a fixed sliding velocity of 2.0 m/s, track diameter of 60 mm and load ranging from 0.5 kgf to 1.5 kgf (4.9?C14.7 N). SEM studies were carried out to assess the wear behavior of the alloy and the composite. The composite showed better wear resistance than the base alloy for the lower loads. However, for the higher loads and longer sliding distances, the wear in the composite was extensive due to the existence of fractured and dislodged fly ash particles in the alloy matrix.     

原位自生非连续增强钛基复合材料的研究现状与展望

从基体和增强相选取、制备方法、微观结构、界面、氧化特性及力学性能方面综述了目前原位自生非连续增强钛基复合材料的发展状况, 结合本文作者的课题研究重点介绍了反应热压法(RHP) 制备钛复合材料的工艺以及所制备复合材料显微组织和力学性能。提出了当前研究中存在的问题和今后潜在的发展方向。