Surface size effects under plastic deformation of microcrystals and nanocrystals

The size effects associated with the crystal surface as an effective sink for moving dislocations in a thin crystal and as a barrier for these dislocations in the presence of a high-strength film or a special hardened layer on the surface, which favor the accumulation of dislocations in the crystal, have been considered theoretically in terms of the kinetic equation for the density of dislocations concentrated in the crystal in the critical lengths of single-ended (unipolar) dislocation sources. The theoretical results have been illustrated by the experimental data available in the literature for microcrystals and nanocrystals of copper and aluminum. It has been found in accordance with these data that the dependence of the yield stress ??_2% of the crystal on the crystal transverse size D has the form ??_2% ?? D ^?0.75 when there is a free crystal surface for the escape ofthe dislocations and ??_2% ?? D ^?0.5 when there is a high-strength layer on the lateral surface of the crystal..     

Plasticity and strength of FCC metals with a nanotwinned submicrograined structure

The plasticizing and hardening effects associated with the existence of nanotwins with a density 1/λ (where λ is the average size (thickness) of nanotwin lamellae) in a submicrograined structure of fcc metals have been theoretically discussed in the framework of the dislocation-kinetics approach. The strength of the nanotwinned submicrocrystalline structure, which is increased as compared to the initial submicron structure, is determined, as in the case of nanograin boundaries, by the action of nanotwin boundaries as sources and barriers for moving dislocations that provide the normal Hall-Petch effect for the flow stress σ ∼ γ^−1/2. The inverse Hall-Petch effect σ ∼ γ ^p (where p > 0), as in the case of nanograin boundaries, is associated with the dislocation absorption by the twin boundaries. The related increased strain-rate sensitivity of the flow stresses is responsible for the significant increase in the uniform strain (from 2–3 to 8–9% in the case of nanotwinned copper) during tension of the specimens with nanotwinned submicrograined structures with retaining a high strength of the material.     

Detection of (<Emphasis Type="Italic">C</Emphasis> + <Emphasis Type="Italic">A</Emphasis>)-type dislocation self-blocking in magnesium

The self-blocking effect predicted theoretically and then observed in intermetallides is established for a pure metal, namely, for magnesium. This effect can be observed only for magnesium single crystals whose axis is parallel to the c axis and whose yield stress behavior σ _y (T) has a temperature anomaly. For such single crystals, the self-blocking of the (c + a)-type edge dislocations is established during pyramidal slip of type II. The self-blocking is proved by dislocation extension along the preferred direction without external stress. In this case, the á 1[`1]00 ñ \left\langle {1\bar{1}00} \right\rangle directions appear preferred. TEM images of (c + a) dislocations extended along the preferred directions are presented. It is demonstrated that two effects – temperature anomaly of σ _y (T) and dislocation self-blocking – have the common nature: a two-valley potential relief of the dislocation. A model of two-valley relief of the (c + a) dislocations in Mg is proposed.     

Structural state of Ge/Si heterosystems with (001), (111), and (7 7 10) interfaces

It is shown for (111) and (001) interfaces that at an identical degree of strain relaxation in semi-conductor epitaxial films, the ratio of distances D between neighboring dislocations is D ₍₁₁₁₎/D ₍₀₀₁₎ = 1.5. This allows us to establish that dislocation interface (7 7 10) contains partial 90° Shockley dislocations lying in three directions of 〈110〉.     

Dislocation unpinning model of acoustic emission from alkali halide crystals

The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constantτ _s for surface annihilation of dislocations and the pinning timeτ _p of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.     

Evolution of dislocation structure during deformation of γ-irradiated LiF crystals

The effect of γ irradiation on the mechanical characteristics and dislocation structure of slip bands in LiF crystals is studied at doses D⩽7.3×10⁸ R. Irradiation causes a substantial increase (up to a factor of 30) in the yield stress τ _y of the crystals, with τ _y ∝ D ^0.4 in the first approximation. The deformation shear increases in the slip bands of irradiated crystals, as do the densities of the screw and edge dislocation components, while the dislocation mean free paths decrease. Irradiation also raises the probability of twinning cross slip for screw dislocations. The observed effects are assumed to be related to the formation of a different kind of defects in the irradiated crystals, primarily clusters of implanted atoms. Fiz. Tverd. Tela (St. Petersburg) 39, 1072–1075 (June 1997)     

Interaction of shockley partial dislocations with reacting forest dislocations

Dislocation combinations formed as a result of interaction between a glissileBσ,d Shockley partial dislocation with reacting undissociated forest dislocations are considered. The value of the parameters characterizing the strength <k> of the dislocation combinations, the probability β_r of their formation, and the interaction intensity α_r of the reacting dislocations are determined for an orientation of the [100] deformation axis of an FCC single crystal for all components of the dislocation loop. Tomsk State Architectural-Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–8, May, 1997.     

On the nature of the influence of an electric current on the magnetically stimulated microplasticity of Al single crystals

A large increase in dislocation mobility in Al single crystals in a static magnetic field in the absence of mechanical loading of the samples is observed when a dc electric current of low density (10⁵–10⁶ A/m²)is additionally passed through the samples. Apparently, the role of the current reduces to depinning of dislocations from strong pinning centers on the surface of the crystal as a result of surface electromigration of defects. This interpretation is supported by the fact that in samples whose surface is insulated by a layer of lacquer the passage of a current through the volume of the crystal does not change the ordinary dislocation mobility level in a magnetic field. It is hypothesized that surface electromigration of defects, which frees dislocations and unblocks dislocation sources, also plays a key role in the physical mechanism of the long-ago discovered macroplastification of metals upon the passage of an electric current through them. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 788–793 (25 May 1998)     

Specific features in the generation and motion of dislocations in silicon single crystals doped with nitrogen

The specific features in the generation and motion of dislocations are investigated in Si: N single crystals grown by the Czochralski method. The motion of dislocation loops is analyzed by the four-point bending technique in the temperature range 500–800°C. The dislocation loops are preliminarily introduced into the samples with the use of a Knoopp indenter at room temperature. It is found that doping with nitrogen leads to a considerable increase in the critical stress of the onset of dislocation motion from surface sources (indentations) and in the stress of the generation of dislocations from internal sources. The velocity of dislocation motion in Si: N crystals is less than that in undoped crystals (under comparable loads). The hardening effect of nitrogen is explained by the fact that nitrogen promotes the decomposition of a solid solution of oxygen in silicon during postcrystallization cooling.     

Evolution of dislocation structures having various orientations in strained single crystals of the alloy Ni3Ge

The dislocation structure of strained single crystals of Ni₃Ge with various orientations is investigated by electron microscopy. The evolution of the dislocation structure parameters is studied as a function of the degree of strain, temperature, and orientation of the single crystals. Analysis of the experimental dependences of the yield stress on the density of dislocations leads to certain conclusions about how various mechanisms for dislocation drag make temperature-dependent contributions to the deforming stress, and about the nature of the thermal hardening of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 40, 672–680 (April 1998)     

A crystal plasticity finite element model for flow stress anomalies in Ni3Al single crystals

This paper presents a dislocation density-based non-Schmid constitutive model to address the anomalous thermo-mechanical behaviour of the L1₂ intermetallic single-crystal Ni₃Al. Ni₃Al is used as a strengthening precipitate (γ′ phase) in Ni-based superalloys. Addressing such anomalous behaviour by accounting for temperature-dependent flow stress and hardening evolution, as well as orientation-dependent tension–compression asymmetry, is necessary for modelling superalloys across a range of temperatures. While hardening in cube-slip systems results from statistically stored dislocations (SSDs), hardening in octahedral slip systems is due to both SSDs and cross-slip dislocations (CSDs). The constitutive model incorporates hardening evolution due to SSDs and CSDs. Experimental data for Ni₃Al-type single crystals, available in the literature, are used to calibrate material parameters. Subsequently, results of crystal plasticity FEM simulations are compared with experimental data for several orientations under constant strain rate and creep loading conditions for a wide range of temperatures. The model is able to correctly predict the response of L1₂ intermetallic single crystals including features of anomalous flow stress and non-Schmid yield behaviour.     

The size effects upon shock plastic compression of nanocrystals

For the first time a theoretical analysis of scale effects upon the shock plastic compression of nanocrystals is implemented in the context of a dislocation kinetic approach based on the equations and relationships of dislocation kinetics. The yield point of crystals τ_y is established as a quantitative function of their cross-section size D and the rate of shock deformation as τ_y ~ ε^2/3 D. This dependence is valid in the case of elastic stress relaxation on account of emission of dislocations from single-pole Frank–Read sources near the crystal surface.     

Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E _a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.     

Influence of temperature and strain on the amplitude-dependent internal friction of high-purity aluminum

The dislocation amplitude-dependent friction (ADIF) of high-purity (99.999%) polycrystalline aluminum is investigated in the temperature range 7–300K at vibrational strain amplitudes of 10⁻⁷–10⁻⁴ for samples in the annealed and deformed (by quasistatic, shock, and ultrasonic loading) states. The ADIF is a multistage effect in the indicated temperature and vibration amplitude ranges. Analysis of the amplitude-temperature spectra of the ADIF permits separation of components attributable to: interaction between dislocations, the interaction of dislocations with pinning points, and pure dislocation relaxation (the interaction of dislocations with the Peierls relief). ADIF is observed to depend nonmonotonically on the initial quasistatic strain determined by strain hardening and recovery processes. Fiz. Tverd. Tela (St. Petersburg) 40, 1839–1844 (October 1998)     

Outside dislocations generated from phosphorus implanted silicon layers

The growth, movement and nature of outside dislocation, which propagate from heavily phosphorus (>10¹⁵ ions/cm²) implanted (111), (100), and (110) silicon layers into unimplanted outside regions by a compressive strain induced during 1100° C wet O₂ annealing, are investigated using transmission electron microscopy and x-ray diffraction topography. Outside dislocations are formed, mainly on (111) planes., by the glide motion of dislocation networks formed in implanted layers during early annealing. This results in dislocations extending into the unimplanted areas to different degrees, in the order of, from the largest to smallest, (111), (110), and (100) wafers. In (110) wafers, the [001] oriented dislocations in the implanted regions rise to the surface at the implant and unimplant boundary. On the other hand, the [110] dislocations penetrate into the unimplanted region. Two sets of orthogonal 〈110〉 oriented dislocations generated in (100) implanted wafers behave in the same manner as the [001] dislocations in (110) wafers. Some sources of the compressive strain related to the generation of these dislocations are discussed.     

Mechanism of decrease in the strength of submicron-sized specimens of FCC metals with a nanocrystalline structure

The effect of decrease in the strength of submicron-sized specimens of face-centered cubic (fcc) metals with a nanocrystalline structure and a cross-sectional size D < 5d, as compared to the strength of the specimens with D ? 5d (where d is the grain size), has been considered theoretically on the basis of the dislocation–kinetic equations and relationships. Previously, it has been found that this decrease is caused by the escape of a part of the dislocations through the surface of the specimen under the action of single-pole dislocation sources in grains adjacent to the surface. In this study, it has been shown that the absorption of lattice dislocations by grain boundaries and its accompanying grain boundary sliding lead to a further decrease in the flow stress of specimens, which is equally related to both thin (D < 5d) and thick (D ? 5d) specimens.     

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.     

Modeling of motion of dislocation train through dislocation forest

Modeling methods were used to analyze tie motions of a train of dislocations through a dislocation forest. Differences were found in this motion in the basal plane of hep crystals and in the {11} plane in crystals of the NaCl type. It was established that the process of formation of dislocation loops when dislocations pass through a dislocation forest may be responsible for considerable strain hardening.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 97–103, December, 1979.     

Modeling of Dislocation Generation and Interaction During High-Speed Deformation of Metals

Recent experiments by Kiritani et al. [1] have revealed a surprisingly high rate of vacancy production during high-speed deformation of thin foils of fcc metals. Virtually no dislocations are seen after the deformation. This is interpreted as evidence for a dislocation-free deformation mechanism at very high strain rates. We have used molecular-dynamics simulations to investigate high-speed deformation of copper crystals. Even though no pre-existing dislocation sources are present in the initial system, dislocations are quickly nucleated and a very high dislocation density is reached during the deformation. Due to the high density of dislocations, many inelastic interactions occur between dislocations, resulting in the generation of vacancies. After the deformation, a very high density of vacancies is observed, in agreement with the experimental observations. The processes responsible for the generation of vacancies are investigated. The main process is found to be incomplete annihilation of segments of edge dislocations on adjacent slip planes. The dislocations are also seen to be participating in complicated dislocation reactions, where sessile dislocation segments are constantly formed and destroyed.     

In situ investigation of the effect of a magnetic field on the mobility of dislocations in deformed KCl:Ca single crystals

This paper presents the results of an experimental investigation into the mobility of edge dislocations in KCl:Ca single crystals and the effect of a static magnetic field of 0.3 T on the dislocation mobility. The experiments on the effect of a magnetic field on the dislocation mobility were carried out with the use of a high-resolution (1 ms) method that permits in situ measurements of the sample dipole moment induced by the motion of charged dislocations as the crystal is being deformed. It is found that the starting stress is reduced in a magnetic field and the activation volume for overcoming of point defects by the dislocations is increased. It is further found that the magnetic field increases the rate of motion of the dislocations at the initial stage of deformation (to the point of dislocation multiplication) but has no effect on the mobility in the multiplication stage. Fiz. Tverd. Tela (St. Petersburg) 39, 630–633 (April 1997)