Al–Pd–Mn icosahedral quasicrystal: deformation mechanisms in the brittle domain

The extreme brittleness of Al–Pd–Mn quasi-crystalline alloys over a wide range of temperatures drastically restricts investigation of their plastic deformation mechanisms over a small high-temperature regime. Recently, plastic deformation of Al–Pd–Mn quasicrystal has been achieved in the brittle domain (20?≤?T?≤?690°C) using specific deformation devices, which combined a uniaxial compression deformation or a shear deformation with a hydrostatic pressure confinement (0.35–5?GPa). Results of these experimental techniques, which provide various deformation conditions giving rise to a range of Al–Pd–Mn plastic features in the brittle domain, are discussed. On this basis, we propose that low and intermediate temperature plastic properties of Al–Pd–Mn are controlled by non-planar dislocation core extensions specific to the non-periodic structure.     

Application of synchrotron radiation to the study of the internal structure of natural regular and irregular diamonds

The internal structure of regular and irregular diamond crystals of the Snap Lake deposit of the Slave province (Canada) is studied using the Laue-SR synchrotron method. The crystals under study were classified into regular and irregular diamonds according to IR spectroscopy data. It is shown that irregular diamonds, in contrast to regular, underwent plastic deformation during the postgrowth period. Plastic deformation by slip or spinel-law twinning is observed for diamonds with insignificant nitrogen concentrations. For most studied crystals with high concentrations of platelets (B’ defects), irregular misorientations of local regions of a deformed crystal, such as faults and kinks, are characteristic. The interaction of dislocations formed during plastic deformation, with the dislocations surrounding the platelets, causes destruction of the latter at high P-T parameters typical of the upper mantle.     

Plastic flow macrolocalization in aluminum and the Hall-Petch relation

The parameters of plastic deformation macrolocalization are compared to the parameters of the Hall-Petch relation for the flow stress in polycrystalline aluminum samples with a grain size of 0.008–5.000 mm. Two types of the dependence of the localized plastic deformation autowave length on the grain size and two versions of hardening according to the Hall-Petch relation are found in the grain size range under study. The boundary between these versions is shown to correspond to d ≈ 0.1 mm for both cases. A relation between localized plastic flow patterns and the Hall-Petch relation is revealed.     

Plastic analysis of crack problems in three-dimensional icosahedral quasicrystalline material

Due to their complexity, the basic plastic properties of all quasicrystalline materials are essentially unknown [M. Feuerbacher, C. Thomas, K. Urban, Plastic behaviour of quasicrystalline materials, in Quasicrystals: Structure and Physical Properties, H.R. Trebin, ed., Wiley/VCH, Berlin, 2003]. Plastic deformation of cracks in icosahedral quasicrystals have been analyzed in a strict and systematic form and the crack tip opening displacement (CTOD) and size of the plastic zone around the crack tip was determined exactly. CTOD is suggested as a parameter of plastic fracture for quasicrystalline materials. The present work may also provide a novel methodology for plastic analysis of quasicrystals.     

Crystallographic variant selection of martensite during fatigue deformation

Metastable austenitic stainless steels are prone to form deformation-induced martensite under the influence of externally applied stress. Crystallographic variant selection during martensitic transformation of metastable austenite has been investigated thoroughly with respect to the interaction between the applied uniaxial cyclic stress and the resulting accumulated plastic strain during cyclic plastic deformation. The orientation of all the Kurdjomov–Sachs (K-S) variants has been evaluated extensively and compared with the measured orientation of martensite with their corresponding interaction energies by applying the elegant transformation texture model recently developed by Kundu and Bhadeshia. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed. It has been found that both the applied uniaxial cyclic stress and the accumulated plastic strain are having strong influence on crystallographic variant selection during cyclic plastic deformation. Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

Accumulation of dislocations and thermal strengthening of alloys having an L12 superstructure

This paper discusses the dislocation-accumulation mechanism in alloys having an L1₂ superstructure, which is associated with the formation of Kira-Wilsdorf barriers and the retardation of superdislocations during plastic deformation. A model of the dislocation-accumulation kinetics during plastic deformation is constructed, on the basis of which a mathematical model is formulated for the thermal and deformation strengthening of single crystals of alloys having the L1₂ superstructure. The results of numerical calculations based on the model are compared with the experimentally observed regularities of the deformation and thermal strengthening of single crystals of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 41, 454–460 (March 1999)     

Strain-Induced Disruption of Long-Range Order due to Generation of Superdislocations in L12-Superlattice Alloys

A disruption mechanism for the long-range atomic order in alloys with the L1₂ superlattice is examined. The disruption is caused by an accumulation of superdislocations upon plastic deformation. The rate of increase of the antiphase-boundary area resulting from multiplication of superdislocations is estimated for the cases where the spacing between superpartial dislocations is either constant or strain-dependent. Equations are derived for the rate of change of the degree of long-range order as single-crystalline L1₂-superlattice alloys are subject to deformation.     

Structure and peculiarities of nanodeformation in Ti–Zr–Ni quasi-crystals

Ti–Zr–Ni samples with a substantial predominance of icosahedral quasicrystalline phase were produced by the melt-spinning technique. Their structure and mechanical properties were studied by X-ray diffraction and nanoindentation methods. The quasicrystalline phase was found to have a primitive lattice with the quasicrystallinity parameter a _q = 0.5200–0.5210?nm. Quasicrystalline deformation behaviour under nanoindentation versus phase composition and structure is discussed in comparison with single crystal W–12?wt%?Ta. The estimated elastic modulus E of the quasicrystalline phase shows no correlation with the element composition. The nanohardness was shown to increase with increasing quasicrystalline-phase perfection. Load–displacement curves of Ti–Zr–Ni quasicrystals (QCs) show stepwise character with alternation of elastic and plastic sections. Such non-uniform plastic flow in QCs might be caused by the localization of plastic deformation in shear bands. The non-uniformity of the plastic deformation increases with the increasing quasicrystalline phase perfection.     

Allotropic forms of carbon in the Invar Fe–Ni–C alloy before and after plastic deformation by upsetting

The effect of cold plastic deformation by upsetting (e = 1.13) on structure and hybridised bonds of carbon in the fcc Invar Fe-30.9%Ni-1.23% C alloy was studied by means of X-ray phase analysis and X-ray photoelectron spectroscopy. Carbon precipitates along grain boundaries and inside of grains in the alloy after annealing and plastic deformation were revealed. The presence of mainly sp²- and sp³-hybridised C–C bonds attributing to graphite and amorphous carbon as well as the carbon bonds with impurity atoms and metallic Fe and Ni atoms in austenitic phase were revealed in the annealed and deformed alloy. It was shown for the first time that plastic deformation of the alloy results in partial destruction of the graphite crystal structure, increasing the relative part of amorphous carbon, and redistribution of carbon between structural elements as well as in a solid solution of austenitic phase.     

Deformation of germanium under the joint action of an electric current and a magnetic field

The specific features in the behavior of deformation characteristics of low-ohmic p-type germanium single crystals subjected to different types of combined plastic deformation and the anisotropy of the electrical resistance of specimens in the longitudinal and transverse directions have been investigated. Both the acceptor and donor actions of dislocations have been observed in the motion of charge carriers along the direction of compression of the specimen. Under conditions of the joint action of a weak magnetic field and a combined plastic deformation, a decrease in the macroplasticity effects has been revealed. Anisotropy of the electrical resistance of p-Ge specimens in the longitudinal and transverse directions has been found. A possible explanation of the observed effect is given.     

Defects with deep levels induced by plastic deformation and electron irradiation in El2-free GaAs

Abstract

Defects with deep electronic energy levels induced by electron irradiation at room temperature or plastic deformation at 450°C in GaAs in which grown-in EL2 defects are previously eliminated by heat-treatment are investigated by means of measurements of the optical absorption and the Hall effect. Thermal stabilities of the induced defects are studied by tracing the changes mainly in the absorption specturm due to isochronal annealing. The absorptions both in deformed and irradiated specimens are mostly photo-unquenchable. Therefore, the defects induced by above two procedures are identified not to be EL2. Semi-insulating or n-type specimens convert to p-type by plastic deformation or electron irradiation, showing that high densities of acceptors are generated by the above two procedures.     

纳米多层膜“软"相结构参量对硬度的Hall-Petch表征

采用直流磁控溅射方法制备了不同调制比的Ni/Al纳米多层膜,利用X射线衍射技术和纳米压入连续刚度法分析了薄膜微结构及塑性变形的尺度依赖性.实验结果表明,尽管调制比有所不同,多层膜的硬度与“软"相的微结构特征参量随调制波长减小具有相似变化规律,说明多层膜的变形机制对“软"相的微结构约束存在敏感性.随着薄膜特征尺度的减小,为统一多层膜中晶界和膜界两种强化机制,提出一个与“软”相相关的表征参量r(r＝L_sub/d,L关键词： 纳米多层膜 塑性变形 调制波长 Hall-Petch关系     

Influence of the transverse size of samples with micro- and nano-grained structures on the yield and flow stresses

The effects of reduction in the strength and deviation from the Hall-Petch relationship under plastic deformation of specimens with micro- and nano-grained structures with decreasing size of their cross section have been considered theoretically. The analysis is based on the kinetic equation for the dislocation density, which takes into account that the surface of the specimen serves as both the source and the sink for dislocations, whereas the grain boundaries are barriers limiting the mean free path of dislocations. It has been found that, when the ratio of the transverse size of the specimen D to the grain size d becomes less than 3, in the dependence of the yield stress on the size of the specimen there appears a minimum as a result of the increase in the number of near-surface grains that exhibit a weak resistance to plastic deformation due to the withdrawal of dislocations through the external surface of the fine-dimensional specimen. The minimum of the strength in the range d < D < 3d is a consequence of the competition and nonlinear interaction of the size factors D and d.     

Phase transformations and thermal stability of the structure of Ti<Subscript>3</Subscript>Al intermetallic compound at deuteration and plastic deformation

The structural transformations in Ti₃Al intermetallic compound at deuteration with concentrations x = 1.2 and 1.7, heating at 100–400°C, and shear deformation under pressure have been studied. It is established that at a given deuterium concentration deuterides with fcc and orthorhombic lattices are formed; under severe shear deformation, nanocrystalline and amorphous (or close to amorphous) deuterides arise. The reasons for the structure amorphization at deuteration and subsequent plastic deformation are discussed.     

Evolution of dislocation density in a hot rolled Zr–2.5Nb alloy with plastic deformation studied by neutron diffraction and transmission electron microscopy

Abstract

The evolution of dislocation density and microstructure of a hot rolled Zr–2.5Nb alloy under compressive plastic strain, at room temperature, was analysed using neutron diffraction and transmission electron microscopy (TEM). The dislocation densities of type 〈a〉, 〈c + a〉 and 〈c〉 dislocations at different plastic strains in the elastic–plastic transition regime and plastic regime have been measured by diffraction line profile analysis (DLPA). TEM microstructure characterization revealed the operation of different slip systems. It has been found that slip of type 〈a〉 dislocations contributed to most of the plastic strain at the early stage of deformation, and strong pyramidal 〈c + a〉 slip did not occur until the deformation was fully plastic. Unambiguous evidence of basal slip occurring at room temperature in Zr is provided. Loading along a plate direction with more basal poles favoured the operation of basal and pyramidal slip. Dislocation features including relative edge:screw character of 〈c + a〉 dislocations are shown to be different under tension and compression loading, providing a mechanistic driver for the previously observed asymmetry in critical resolved shear stress for 〈c + a〉 slip.     

Disclination mechanism for plastic deformation of nanocrystalline materials

A model is developed for the plastic deformation of nanocrystalline materials in terms of the evolution of a spatial grid of disclinations located at the triple junctions of grains. Plastic deformation takes place as the result of plastic rotation of grains, the mismatch of whose rotations causes the nucleation of partial disclinations at the junctions of intergrain boundaries. It is shown that the distinctive feature of the mechanical behavior of nanocrystals is a deviation from the Hall-Petch law up to a critical grain size D _cr⩽25 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2023–2028 (November 1997)     

A thermally activated dislocation-based constitutive flow model of nanostructured FCC metals involving microstructural evolution

Abstract

Due to their interface and nanoscale effects associated with structural peculiarities of nanostructured, face-centered-cubic (FCC) ultrafine-grained/nanocrystalline (UFG/NC) metals, in particular nanotwinned (NT) metals exhibit unexpected deformation behaviours fundamentally different from their coarse-grained (CG) counterparts. These internal boundaries, including grain boundaries and twin boundaries in UFG/NC metals, strongly interact with dislocations as deformation barriers to enhance the strength and strain rate sensitivity (SRS) of materials on the one hand, and play critical roles in their microstructural evolution as dislocation sources/sinks to sustain plastic deformation on the other. In this work, building on the findings of twin softening and (de)twinning-mediated grain growth/refinement in stretched free-standing NT–Ni foils, a constitutive model based on the thermally activated depinning process of dislocations residing in boundaries has been proposed to predict the steady-state grain size and simulate the plastic flow of NT–Ni, by considering the blocking effects of nanotwins on the absorption of dislocations emitted from boundaries. It is uncovered that the stress ratio (η_stress) of effective-to-internal stress can be taken as a signature to estimate the stability of microstructures during plastic deformation. This model not only reproduces well the plastic flow of the stretched NT–Ni foils as well as reported NT–Cu and the steady-state grain size, but also sheds light on the size-dependent SRS and failure of FCC UFG/NC metals. This theoretical framework offers the opportunity to tune the microstructures in the polycrystalline materials to synthesise high performance engineering materials with high strength and great ductility.     

Distortion of spores of moss Venturiella under ultra high pressure

In our previous studies on the tolerance of living organisms such as planktons and spores of mosses to the high hydrostatic pressure of 7.5 GPa, we showed that all the samples could be borne at this high pressure. These studies have been extended to the extreme high pressure of 20 GPa by using a Kawai-type octahedral anvil press. It was found that the average diameter of the spores of Venturiella exposed to 20 GPa for 30 min was 25.5 μm, which is 16.5% smaller (40.0% smaller in volume) than that of the control group which was not exposed to high pressure. The inner organisms showed a further extent of plastic deformation. As a result, a gap appeared between the outer cover and the cytoplasm. A relationship has been obtained between the survival ratio and plastic deformation of spores of moss Venturiella caused by the application of ultra high pressure.     

The distribution of plastic deformation during 21 kHz fatigue of copper samples

A new method of investigating the portion of plastic deformation during fatigue tests of copper samples has been developed. Until now plastic deformation could only be estimated as opposed to being directly measured. Cholesteric li-crystals were applied to the sample surfaces before the fatique test and the temperature distribution during deformation was measured by the change in colour of these crystals. This then allows the distribution of the plastic deformation along the sample to be calculated. Film recordings of the colour, and hence the temperature change showed that plastic deformation is spread along the sample in a manner similar to that predicted by theory; plastic deformation is not mainly concentrated in the middle of a specimen.     

Channelling of Plastic Deformation by Interfaces in TiAl Intermetallics

Plastic deformation in the lamellar microstructure of the L1₀ tetragonal phase is strongly affected by special rotational lamellar interfaces on the {111} close packed planes and by general grain boundaries separating the lamellar colonies. The activity of possible deformation modes in Ti-rich TiAl alloys is explained considering in addition to generally known asymmetry of deformation twinning the asymmetry of superdislocation motion. The restrictions imposed on the direction of propagating deformation by the lamellar interfaces are analysed in detail. Even in the case when the transfer of plastic deformation across the interfaces does not occur, the presence of interfaces as strong obstacles to moving dislocations and deformation twins can lead to localisation of strain parallel to the lamellae, to so called channelling of deformation. General grain boundaries can also significantly influence plastic deformation by stress redistribution due to the compatibility stresses arising from the crystal elastic anisotropy and from the anisotropy of plastic deformation.