Influence of the Peierls relief on size effects in plastic deformation of micro- and nanocrystals of body-centered cubic metals

The influence of the Peierls relief and Peierls stresses on the size effects in micro- and nanocrystals of metals with a body-centered cubic (bcc) lattice has been theoretically discussed in the framework of the dislocation-kinetic approach. It has been found that, as compared to micro- and nanocrystals with a facecentered cubic (fcc) lattice, for the bcc crystals, the exponent n in the power law σ ～ D ^?n (where σ is the flow stress and D is the transverse size of the crystal) is the smaller, the higher is the critical temperature T _c above which the Peierls relief ceases to control the motion of dislocations in the bcc metals. The specific features of the influence of the Peierls relief on the coefficient of strain-rate sensitivity of the flow stresses in microcrystals of the bcc lattice have also been discussed. The theoretical results have been illustrated by the experimental data available in the literature.     

Effect of grain size dispersion on the strength and plasticity of nanocrystalline metals

The effect of the dispersion of the grain size distribution on the yield stress, ultimate stress, and uniform strain of nanocrystalline metals is analyzed theoretically. It is shown that, as the grain size dispersion increases, the degree of grain boundary hardening (Hall-Petch effect) of nanocrystalline materials decreases, the onset of the grain boundary softening (inverse Hall-Petch effect) shifts to smaller nanograin sizes, and the uniform strain at which necking occurs increases.     

纳晶铜晶粒尺寸对热导率的影响

用热压烧结法制备得到纳晶铜块体. 用激光法测定了不同温度下制备得到的纳晶铜块体的热导率, 并建立卡皮查热阻模型对样品热导率进行模拟. 通过对比, 模拟结果与实验数据基本一致. 随着热压烧结温度的升高, 纳晶铜晶粒尺寸也随之增大. 在900和700 ℃其热导率分别达到了最大和最小值且所对应的热导率分别为200.63和233.37 W·m^-1·K^-1, 各占粗晶铜块体热导率的53.4%和60.6%. 验证了纳晶铜热导率在一定的晶粒尺寸范围内具有尺寸效应, 随着晶粒尺寸的减小, 热导率逐渐减小.     

Distinctive features of plastic strain localization under severe plastic deformation of metals

Mechanical testing is performed and the structure of zirconium and aluminum predeformed by ∼450% using multiaxial forging (MAF) and equal channel angular pressing (ECAP) is investigated. Tensile loading tests of the severely deformed specimens exhibited their tendency to necking, with the ductility of the material in the neck, however, being superior to that in the neck of initial coarse-grained specimens. The results of the experiments imply that a fundamental stage of plastic flow of solids under severe plastic deformation (SPD) is the formation of cellular-banded structure and strain localization in the fine-grain bands. This considerably retards further deformation-induced refinement of the structure by SPD, and also results in the rapid formation of a fracture neck in the materials with this structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 43–49, November, 2007.     

The effect of grain size on the deformation resistance of nickel

The effect of the grain sizel of commerical nickel on the lower yield point, _y, and flow stress, _f , has been investigated. From the relationship between _y andl ^–1/2 and between _f andl ^–1/2, and also by extrapolation, the parameters ₁( _i ^f ) and k_y(kf), which occur in Petch's well-known expression, were determined. It was found that the values of these parameters depend on the previous history of the samples. It is suggested that the more marked dependence of the deformation resistance of nickel on grain size arising from certain thermal treatments is due to the segregation of impurities to the grain boundaries. It is shown that this is in accord with the presence of grain-boundary hardening and with its dependence on quenching temperature.     

Grain growth and collective migration of grain boundaries during plastic deformation of nanocrystalline materials

A theoretical model is proposed for the collective migration of two neighboring grain boundaries (GBs) in a nanocrystalline material under applied elastic stress. By analyzing the change in the energy of the system, it is shown that GBs can remain immobile or migrate toward each other depending on the values of the applied shear stress and misorientation angles. The process of GB migration can proceed either in a stable regime, wherein the GBs occupy equilibrium positions corresponding to a minimum of the energy of the system under relatively small applied stress, or in an unstable regime, wherein the motion of GBs under relatively high stress is accompanied by a continuous decrease in the system energy and becomes uncontrollable. The stable migration of GBs leads to a decrease of the grain bounded by them at the cost of growth of the neighbor grains and can result in complete or partial annihilation of the GBs and the collapse of this grain. Unstable migration leads either to annihilation of GBs or to passage of them through each other, which can be considered as the disappearance of the grain and nucleation and growth of a new grain.     

Nature of high-angle grain boundaries in metals subjected to severe plastic deformation

The evolution of structural parameters of deformation fragments and dynamically recrystallized grains during severe plastic deformation in a Bridgman chamber as the number of complete revolutions in torsion at room temperature increases is analyzed using transmission electron microscopy and electron backscattered diffraction. It is found that the formation of a significant amount of high-angle grain boundaries in the structure is caused not by deformation fragmentation but by the low temperature dynamic recrystallization.     

Influence of environment and grain size on magnetic properties of nanocrystalline Mn–Zn ferrite

Nanocrystalline Mn_1−xZn_xFe₂O₄ (0.2?x?0.9) was prepared by mechanical alloying of the concerned oxide precursors and subsequent annealing in air and Ar atmosphere, respectively. Milling and annealing in air produces Zn-ferrites (ZnFe₂O₄) instead of Mn–Zn ferrites as MnO converts to higher oxides at higher oxygen partial pressure and fails to dissolve in the spinel phase. This is confirmed by careful quantitative X-ray diffraction analysis using Rietvelt profile matching and also by the non-saturating paramagnetic nature of the magnetization response with very low saturation level of these spinels milled and annealed in air. On the other hand, single-phase Mn–Zn ferrite results from the identical precursor oxide blend when milling and annealing are carried out under controlled (Ar) atmosphere. The average grain size of the as-milled and annealed powders, measured by Rietvelt refinement, varies between 6–8 and 14–18 nm, respectively. Further investigations performed with Mn_0.6Zn_0.4Fe₂O₄ reveal that a careful selection of annealing parameters may lead to an early superparamagnetic relaxation. Therefore, the blocking temperature can be significantly reduced through proper heat treatment schedule to ensure superparamagnetism and negligible hysteresis at low temperature.     

Structural mechanisms of plastic deformation in nanocrystalline materials

A model of the initial stage of plastic deformation in nanomaterials is proposed. Within this model, the plastic deformation occurs through grain boundary microsliding (GBM). The accommodation processes accompanying the formation of GBM regions are considered. The relationships describing the regularities in the deformation behavior of nanomaterials and the dependence of the flow stress on the grain size are derived, and the temperature dependence of the GBM resistance stress is calculated. It is demonstrated that the results obtained are in good agreement with the experimental data.     

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)     

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size d _opt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity d _opt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.     

Influence of grain size on luminescence properties of micro- and nanopowder Zn2V2O7 vanadate

Micro- and spongiform nanocrystalline Zn₂V₂O₇ compounds were synthesized by hydrothermal and solid-state reaction techniques, and their morphological features were investigated by scanning electron microscopy (SEM). The grain size ranges of the produced powders were 3–15, 0.5–2 μm, and 50–500 nm. The luminescence spectra of these compounds were measured under pulse cathode beam and photoexcitation (200–400 nm). The luminescence decay properties of Zn₂V₂O₇ were studied.It is found that the intensity, maximum position of luminescence spectra and luminescence decay time of Zn₂V₂O₇ samples depend considerably on the grain size of the synthesized powders. The processes of energy relaxation in Zn₂V₂O₇ and the observed size effect on the luminescence properties are also discussed.     

Surface effects in plastic deformation of metals

It is now accepted that the appearance of slip bands on the surface of a plastically deformed metal is evidence that the deformation is not homogeneous but is concentrated on relatively few atomic planes. Recent microscopical experiments have suggested that this conclusion is only valid in the later stages of deformation and that the first fractional per cent of strain is much more nearly homogeneous. Theories to account for both these stages of deformation are examined in the light of microscopical evidence.

The validity of conclusions about internal processes based on experiments on the surface is discussed; it is shown that the surface finish affects not only the appearance of internal processes but also the processes themselves.

In cases where the deformation is not homogeneous the balance of evidence is that it is also not continuous in time: instead, slip on an active slip plane tends to a limit which is reached either gradually or suddenly depending on the nature of the metal and the conditions of stress. The same processes which stop slip on the active planes produce general hardening of the metal. However, slip can restart on or near to former slip planes as a result of mechanisms activated by temperature and stress, and can, in favourable cases, continue until fracture. Therefore slip bands, the sources of hardening, are also places of weakness.     

Modern aspects of plastic deformation in metals

After a historical survey of crystal plasticity the rôle of dislocations in plastic deformation of metals is outlined. The theory of plastic deformation in metal single crystals before the impact of transmission electron microscopy is described. Recent experimental results on glide and workhardening in single crystals of b.c.c. metals are reviewed and explained by a simple dissociation model of the screw-dislocation cores. A relative success of this explanation together with calculations of atomic structure of dislocation cores support the conclusion that different structures of screw dislocation cores are responsible for both differences and similarities between plastic behaviour of f.c.c. and b.c.c. metals. Further developments in the field of metal crystal plasticity by modern experimental techniques (transmission electron microscopy of deformed crystals in the stress-applied state, magnetic studies of dislocations in ferromagnetic crystals) and by atomic calculations of defect configurations are discussed.Na Slovance 2, Praha 8, Czechoslovakia.Invited paper presented on a plenary session of the First European Conference on the Physics of Condensed Matter organized by the Board of the Condensed Matter Division of E.P.S., in Florence on 14–17 September 1971 (Chairman: Prof. S. F. Edwards; Schuster Laboratory, University of Manchester).     

Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

A theoretical model is proposed that describes the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission is driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.     

Influence of impurities on the low-temperature deformation of nanocrystalline copper

The influence of ZrO₂ particles on the low-temperature deformation of nanocrystalline copper produced by strong plastic deformation is investigated using equichannel angular pressing. A comparison is made between the deformation characteristics in tension and compression in the temperature range 4.2–400 K, measured for copper and the composite Cu:0.3 vol. % ZrO₂. It is shown that within 4.2–200 K the yield point σ _sm of the composite is higher than that for copper, attaining 680 MPa at 4.2 K, then the yield points are close in value up to room temperature, and diverge again as the temperature is raised. Possible causes of the dissimilar influence of an impurity on the strength and plasticity characteristics of nanocrystalline copper in various temperature intervals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1639–1641 (September 1998)