Point Defect Production Under High Internal Stress Without Dislocations in Ni and Cu

Kiritani et al. have observed a large number of small vacancy clusters without dislocations at the tip of torn portions of fcc metals such as Au, Ag, Cu and Ni. Small vacancy clusters, rather than dislocation cell structures, have also been observed after high-speed compressive deformation, suggesting the possibility of plastic deformation without dislocations. In this paper, in order to investigate the mechanism of deformation without dislocations, change in formation energy of point defects under high internal stress was estimated by computer simulation. Elastic deformation up to - 20% strain was found to provide a remarkable lowering of formation energy of point defects. For example, when Ni is subjected to elastic strain, the formation energy of an interstitial atom decreases to 40% that without strain and the formation energy of a vacancy decreases to 51% that without strain. The number of point defects formed under thermal equilibrium during deformation was evaluated. The number was judged to be insufficient for explaining the formation of vacancy clusters as observed in experiments.     

Evolution of the concentration of point defects at the tip of a crack

The evolution of the distribution of interstitial impurity atoms in the plastic zone around the tip of a tension crack is analyzed. The transport of point defects is determined by: 1) the hydrostatic component of the elastic stress at the crack tip, created by the superposition of the elastic fields of the crack and dislocations; 2) the elastic field of moving dislocations (“sweeping out” of interstitial impurity atoms); 3) the dislocation-driven transport of point defects present in the dislocation cores. The contributions of each mechanism of transport of point defects to the crack tip are calculated over the entire time from the start of loading of a sample containing a crack until an equilibrium distribution of plastic deformation is established after the cessation of loading. Numerical calculations are carried out for interstitial hydrogen atoms dissolved in an α-Fe crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1580–1585 (September 1997)     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

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.     

Nonequilibrium Diffusional Phase Transformations in Alloys Induced by Migration of Grain Boundaries and Dislocations

Physics of the Solid State - The main scenarios of nonequilibrium diffusional transformations induced by moving defects (dislocations, grain boundaries) in alloys under severe plastic deformation...     

Structural defects in molecular crystals based on heterospin copper complexes

Jumplike changes in the microhardness, sample dimensions, and parameters of the EPR spectrum were observed in molecular Cu(hfac)₂L^Et crystals undergoing a phase transition. Defects that appear upon plastic deformation (e.g., dislocations) and paramagnetic defects were revealed. The latter defects are likely breaks in polymer chains and can serve as spin marks for investigating the magnetic state of the crystal lattice.     

Structure-mechanical approach to surface tension of solids

Some problems of applying the Lippmann equation to adsorption studies on solid electrodes are shortly reviewed. A novel nonthermodynamic approach to consider the role of elastic and plastic deformation of electrode surfaces during adsorption is proposed. The extremely thin electrode surface layers affected electrically and mechanically by adsorbate are supposed to be free of dislocations because of volume discrepancy. The nearest structure-mechanical analogs of such layers are the whisker crystals whose side surface could have one- and two-dimensional defects, but have no active dislocations. Like whiskers, surface metal layers should possess a high ultimate strength close to the theoretical one and a purely elastic deformation. Affected only by adsorbate, the surface electrode layer should be considered as absolutely elastic body, whose plastic deformation is impossible, i.e. the Lippmann equation and other equations containing terms of plastic deformation cannot be used in thermodynamics of the solid metal surface.     

Geometry of crystal structure with defects. II. Non-Euclidean picture

The following point of view is geometrically formulated and its consequences examined: the lattice of a crystalline body with a continuous distribution of dislocations can be locally described as an ideal lattice in non-Euclidean space. The types of distribution of dislocations are described by the classification of three-dimensional real Lie algebras. The influence of point defects and the elastic deformation field on the geometry of the material structure of a crystalline body with dislocations is examined. The case where a crystal with dislocations reacts as a body with internal rotational degrees of freedom is discussed.     

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Der Einfluß von Gitterstörungen auf die Übergangstemperatur zur Supraleitung von Thallium

The residual resistance of very pure Thallium wires was increased by plastic deformation at liquid Helium temperatures. The accompanying changes of the transition temperature to superconductivity were measured. With gradually growing resistance ratio the transition temperature decreases by some hundredths of a degree, reaches a minimum and increases up to the value of the undeformed specimen. This is a behaviour as observed for many other metals in dilute solid solution experiments, removing Tl from its role as an exception. Annealing the specimens at relative low temperatures will remove preferably point defects. This gives the possibility to study the influence of different types of lattice defects on the transition temperature. The results seem to indicate that point defects tend to increase the transition temperature, whereas extended imperfections, as e.g. dislocations, will decrease it.     

Subsurface defect evolution and crystal-structure transformation of single-crystal copper in nanoscale combined machining

ABSTRACT

In the paper, molecular dynamics simulation is applied to study the evolution and distribution of subsurface defects during nanoscale machining process of single-crystal copper. The chip-removal mechanism and the machined-surface-generative mechanism are examined through analysis of the dislocation evolution and atomic migration of the workpieces. The findings show that under different stresses and temperatures, the difference of the binding energy leads to a zoned phenomenon in the chip. Owing to elastic deformation, some of the dislocations could be recovered and form surface steps; moreover, the work hardening of the workpiece can be achieved on account of generation of twin boundaries, Lomer-Cottrell dislocations, and stacking fault tetrahedra (SFT) by plastic deformation. A process of evolution of an immobile dislocation group containing stair-rod dislocations into SFT is discovered, which is different from the traditional Silcox-Hirsch mechanism. Furthermore, a growth oscillation phenomenon, which corresponding stacking fault planes growth and retraction during the formation of the stable SFT, is discussed.     

Extended defects in crystals: examples in powder processing for ancient cosmetics

The article presents briefly the nature and the properties of extended defects in relation to the light they shed on the history of ancient objects and their conservation. Chemical reactions, phase transitions and plastic deformation of materials, which take place during the fabrication of objects, leave their imprints under the form of specific crystalline imperfections such as grain boundaries, twins and dislocations. Observations of the microstructure of objects from the cultural heritage can reveal many details about ancient civilisations and technologies. Besides optical microscopy, these observations rely on various dedicated techniques based on X-rays and electron microscopy to identify extended defects, viz. grain boundaries, twins, and dislocations. Defects due to powder processing of lead compounds have been studied recently. Such compounds have been used for centuries in the fabrication of cosmetics. PACS 61.72.Dd; 61.72.Lk; 61.72.Mm; 81.20.Wk     

NMR-NQR studies of structural phase transitions

Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.     

Effect of contamination with iron on the electron-beam-induced current contrast of extended defects in multicrystalline silicon

The effect of contamination with iron on the recombination activity of extended defects in multicrystalline silicon has been studied by the electron-beam-induced current (EBIC) technique. It has been shown that this process does not lead to the appearance of EBIC contrast of the ??3 and ??9 grain boundaries. It has been revealed that iron diffusion results in a significant increase in the contrast of dislocations introduced by plastic deformation and of traces behind the dislocations in single-crystal silicon, while the dislocation contrast in multicrystalline silicon remains practically unchanged.     

Generation of point defects in alloys with theL12 superlattice

The generation of point defects by screw superdislocations in the act of forming a shear zone in single crystals of alloys with the L1₂ superlattice is examined. The fraction of mobile jogs occurring in superpartial dislocations is estimated. A balance equation for point-defect concentrations in this kind of alloy under quasi-static deformation is derived. A mathematical model for work-hardening of these alloys, involving balance equations for dislocations and point defects, is constructed. Tomsk State University of Architecture and Building. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 2000.     

Simulation of the process of deformation-induced destruction of long-range order in alloys with an L12 superstructure

Plastic-deformation-induced destruction of long-range order in alloys with an L1₂ superstructure is considered. A mathematical model is suggested which takes into account the following mechanisms that lead to the destruction of long-range order: generation of superdislocations, generation of single dislocations, multiplication of antiphase boundaries (APBs) upon the conservative motion of dislocations, multiplication of APBs upon dislocation climb, formation of APB tubes on superdislocations, generation of point defects, and thermal ordering. A mathematical model of deformation strengthening and long-range order destruction with allowance for the change in the type of shear-forming dislocations from superdislocations to single dislocations is formulated.     

Thermal stretching of defective nanowires: the coupled effects of vacancy cluster defects,operating temperature,and wire cross-sectional area

Extensive atomistic simulations of the thermal stretching of defective nanowires (NWs) were performed using the embedded-atom molecular dynamics modeling approach. The nucleation and propagation of dislocations are described via quantitative dislocation-based analyses. The investigation focuses on the coupled effects of various vacancy cluster (VC) defects, operating temperature, and wire cross-sectional area on the mechanical properties and plastic deformations of defective NWs. With increasing internal stress of a stretched wire, a rapidly moving dislocation loop that transferred atoms to fill up the original vacancy cluster before the wire yielded was found (i.e. it carried the vacancies away from the inside of the wire and formed a notch at the wire edge). The heterogeneous nucleation of dislocations from the notch site propagated along the {111}〈112〉 partial dislocations and formed stacking faults or perfect dislocations on the {111} activated planes. Simulation results show a decreasing yield strength with increasing VC size for a given wire sectional area and temperature. Quasi-linear decreasing Young’s moduli were observed with increasing operation temperature. For a given operation temperature, NW Young’s modulus increased with increasing NW size. Two typical deformation regimes under various operation temperatures were found: (i) a high-temperature-induced pre-melting phenomenon and a thermal softening effect caused low-stress plastic flow and rapid pillar-necking deformation, and (ii) step-wise glides, slip bands, and cross-slips proceeded along the activated glide planes in the low-temperature hard-brittle structure. These two regimes were thoroughly characterized via the evolutions of microscopic dislocations and the changes of true stress. For operation at high temperatures, the ultra-thin 1/5-type pentagonal ring chains exhibit a relatively robust structure, which can potentially be used as building blocks and components for high-temperature nanoelectromechanical systems (NEMS) devices in the future.     

Structure,impurity composition,and photoluminescence of mechanically polished layers of single-crystal silicon

The introduction of optically active defects (such as atomic clusters, dislocations, precipitates) into a silicon single crystal using irradiation, plastic deformation, or heat treatment has been considered a possible approach to the design of silicon-based light-emitting structures in the near infrared region. Defects were introduced into silicon plates by traditional mechanical polishing. The changes in the defect structure and the impurity composition of damaged silicon layers during thermal annealing (TA) of a crystal were examined using transmission electronic microscopy and x-ray fluorescence. Optical properties of the defects were studied at 77 K using photoluminescence (PL) in the near infrared region. It has been shown that the defects generated by mechanical polishing transform into dislocations and dislocation loops and that SiO₂ precipitates also form as a result of annealing at temperatures of 850 to 1000°C. Depending on the annealing temperature, either oxide precipitates or dislocations decorated by copper atoms, which are gettered from the crystal bulk, make the predominant contribution to PL spectra.     

Characteristics of plastic deformation under the action of ultrasound

Conclusions The process of plastic deformation under the action of ultrasound differs in a number of ways from deformation under static loading; a computer simulation has shown that these differences are attributable to the specific characteristics of the operation of a source of dislocations for a periodic law of variation of the stresses. Among the most significant features of the ultimate dislocation structure are the saturation of the density of defects (attainment of limiting states) and the formation of stable dislocation clusters, which do not produce stress fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 118–128, June, 1982.     

Berkovich indentation-induced deformation behaviors of GaN thin films observed using cathodoluminescence and cross-sectional transmission electron microscopy

Presented in this study are the Berkovich nanoindentation-induced mechanical deformation mechanisms of metal-organic chemical-vapor deposition (MOCVD) derived GaN thin films, investigated by using the cathodoluminescence (CL) and the cross-sectional transmission electron microscopy (XTEM) techniques. The multiple “pop-in” events were observed in the load-displacement (P-h) curves and appeared to occur randomly with increasing the indentation load. These instabilities are attributed to the dislocation nucleation and propagation. CL images of nanoindentation show a very well-defined rosette structures with the hexagonal system, and clearly display the distribution of deformation-induced extended defects/dislocations which affect CL emission. By using focused ion beam (FIB) milling to accurately position the cross-section of an indented area, XTEM results demonstrate that the major plastic deformation is taking place through the propagation of dislocations. The present observations are in support to the massive dislocations activities occurring underneath the indenter during the loading cycle. No evidence of either phase transformation or formation of micro-cracking was observed by using XTEM observations. We also discuss how these features correlate with Berkovich nanoindentation-induced defects/dislocations microstructures. Finally, this study has significant implications for the extent of contact-induced damage during fabrication of GaN-based optoelectronic devices.