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.     

Nanoscratch-induced deformation behaviour in B4C particle reinforced ultrafine grained Al alloy composites: a novel diagnostic approach

We report on the novel application of nanoscratch characterization to provide insight into the plasticity mechanisms responsible for the behaviour of composites. Accordingly, we conduct deformation characterization with nanoscratch testing (DCNT) to study the deformation behaviour of two B₄C reinforced ultrafine grained Al alloy tri-modal composites with average B₄C particle sizes of ~1–6?μm and ~500?nm, respectively. To highlight the type of mechanistic information revealed in a DCNT study of composites, we concentrate on the influence of B₄C particle size on deformation mechanisms.     

Dislocation motion in high strain-rate deformation

We present a systematic investigation of dislocation motion, dislocation interactions, and the collective behaviour of dislocations in high strain-rate deformation. Based on results from three-dimensional dislocation dynamics simulations, we find that employing the accurate, full-dynamics, equation of motion (i.e. that includes inertial effects) significantly changes the predictions of microstructural evolution and the macroscopic response compared to the commonly used overdamped equation of motion (i.e. with no inertial effects), especially at high strain rates (10³–10⁶ s^?1). While we find that inertial effects cannot be neglected, the net velocities are not high enough that ‘relativistic’ effects are important. We also present results on the effects of high strain rates on single-crystal deformation, which show good agreement with experimental trends, including increased hardening with increasing strain rate.     

Dislocation dynamics in cyclic plastic deformation

The paper presents a theoretical investigation of the slip avalanches (so-called strain bursts) which occur in single-glide-orientated face-centered cubic or hexagonal close-packed metals during stress-amplitude-controlled cyclic plastic deformation. The study is based on a model of the dynamics of dislocations that has been developed in a companion paper (Part I). It is shown that this model allows for a quantitative treatment of the strain-burst phenomenon. In particular, the scaling relations between different strain-burst-characteristic parameters which have been found by experiment are connected to the evolution of the dislocation microstructure and thus find a natural explanation.     

Dislocation cross-slip mechanisms in aluminum

We have systematically studied dislocation cross-slip in Al at zero temperature by atomistic simulations, focusing on the dependence of the transition paths and energy barriers on dislocation length and position. We find that for a short dislocation segment, the cross-slip follows the uniform Fleischer (FL) mechanism. For a longer dislocation segment, we have identified two different cross-slip mechanisms depending on the initial and final positions of the dislocation. If the initial and final positions are symmetric relative to the intersection of the primary and cross-slip planes, the dislocation cross-slips via the Friedel–Escaig (FE) mechanism. However, when the initial and final positions are asymmetric, the dislocation cross-slips via a combination of the FL and FE mechanisms. The leading partial folds over to the cross-slip plane first, forming a stair-rod dislocation at the intersection with which the trailing partial then merges via the FL mechanism. Afterwards, constrictions appear asymmetrically and move away from each other to complete the cross-slip via the FE mechanism.     

Dislocation motion in anisotropic multilayer materials

Line integral forms for the elastic field of dislocations in anisotropic, multilayer materials are developed and utilized in Parametric Dislocation Dynamics (PDD) computer simulations. Developed equations account for interface image forces on dislocations as a result of elastic modulus mismatch between adjacent layers. The method is applied to study dislocation motion in multilayer thin films. The operation of dislocation sources, dislocation pileups, confined layer slip (CLS), and the loss of layer confinement are demonstrated for a duplex Cu/Ni system. The strength of a thin film of alternating nanolayers is shown to increase with decreasing layer thickness, and that the maximum strength is determined by the Koehler barrier in the absence of coherency strains. For alternating Cu/Ni nanolayers, the dependence of the strength on the duplex layer thickness is found to be consistent with experimental results, down to a layer thickness of ≈10nm.     

Dislocation decoration and raft formation in irradiated materials

Experimental observations of dislocation decoration with self-interstitial atom (SIA) clusters and of SIA cluster rafts are analysed to establish the mechanisms controlling these phenomena in bcc metals. The elastic interaction between SIA clusters, and between clusters and dislocations is included in kinetic Monte Carlo (KMC) simulations of damage evolution in irradiated bcc metals. The results indicate that SIA clusters, which normally migrate by 1D glide, rotate due to their elastic interactions, and that this rotation is necessary to explain experimentally-observed dislocation decoration and raft formation in neutron-irradiated pure iron. The critical dose for raft formation in iron is shown to depend on the intrinsic glide/rotation characteristics of SIA clusters. The model is compared with experimental observations for the evolution of defect cluster densities (sessile SIA clusters and nano-voids), dislocation decoration characteristics and the conditions for raft formation.     

Dislocation structure and deformation hardening of bcc metals

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 23–34, March, 1991.     

Dislocation diffusion mechanism in Pd/Ag crystals

In epitaxial Pd/Ag crystals, dislocations in the palladium film have a large influence on the effective diffusion coefficient. In silver the diffusion coefficient hardly depends on the dislocation density. With a dislocation density of 10¹⁰m⁻² the diffusion coefficient in silver is larger than in the palladium, and at 773 K it is 10⁻¹⁸ m²/s. At a dislocation density of 3·10¹² m⁻², the diffusion coefficient in the palladium becomes larger than in silver, and at 773 K it is 3·10⁻¹⁸ m²/s. It is most likely that diffusion in silver takes place via the lattice, while in palladium it occurs at mobile dislocation sites. State Technical University, Samarsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 116–118, August, 1996.     

Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1?1?0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress–strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.     

Bound diffusion of ultrafine particles in generalized anharmonic potential

Bound diffusion of ultrafine particles of ferric hydroxide precipitated inside the polymeric cross-linkage degree the experimental spectra reveal a striking broadening up to hundreds of mm/s. This is a consequence of the crossover from the diffusion of a Brownian oscillator to free diffusion in restricted volume, the spatial displacements of the particles depending on the cross-linkage and changing from 0.1 to 0.6 Å.     

The role of surface and volume diffusion in the growth of TGS single crystals

Msasurements of the growth rates of {110} and {001} faces of triglycinsulphate (TGS) as functions of supersaturation of the solution are analyzed on the basis of the surface-diffusion model of Burton, Cabrera and Frank. Approximate values of the free activation energies of dehydratation, surface diffusion and incorporation of TGS molecules into the crystal lattice are determined. It is shown that surface diffusion is responsible for the low growth rate of {001} faces; in the case of {110} faces, this mechanism is less important at higher values of supersaturation than volume diffusion. This fact is discussed on the basis of results obtained from measurements of the influence of hydrodynamical conditions on the growth of the prismatic {110} faces of TGS crystals.List of symbols T temperature - k Boltzmann constant - KT/h atomic frequency factor - a shortest distance between growth units in the crystal - x ₀ distance between nearest kinks in a step on the surface - x _s mean quadratic displacement of growth units on the surface - volume of the growth unit in the crystal - unit free energy of the growth unit in the crystal - N ₀ number of growth units in the solution per unit volume - N _s0 number ot growth units in the adsorbed layer per unit volume - thickness of the volume-diffusion layer - thickness of the adsorbed layer of growth units on the crystal surface - d characteristic length in semi-empirical formulas for determination of - c ₀ retardation factor in the case ofx _s x ₀ - retardation factor which depends on the rate of penetration of growth units into kinks in the crystal surface, and expresses deviations from the ideal BCF function - number of active growth spirals - G _h activation free energy of dehydratation - G _a activation free energy of desorption - G _s activation free energy of surface diffusion - G _k activation free energy of penetration of growth units into kinks in the crystal surface - D _v volume-diffusion coefficient - D _s surface-diffusion coefficient - Re Reynolds number - Sc Schmidt number - Gr Grashoff number - C first constant in the BCF function - ₁ second constant in the BCF function - mean distance between equilibrium positions of growth units in the adsorbed layer     

Dislocation evolution in titanium during surface severe plastic deformation

Surface mechanical attrition treatment (SMAT) is an innovative technique which can produce nanocrystalline (nc) layers of several tens of micrometers thickness on surfaces of metallic materials. In this work, the grade structures of commercially pure titanium (CP Ti) processed by SMAT was studied intensively, and the microstructure observations indicated that the dislocation evolution could be separated into three steps: (1) formation of dislocation tangles; (2) formation of dislocation bands; and (3) dynamic recrystallization of dislocation bands until the formation of nc Ti.     

Understanding the Li diffusion mechanism and positive effect of current collector volume expansion in anode free batteries

In anode free batteries(AFBs), the current collector acts as anode simultaneously and has large volume expansion which is generally considered as a negative effect decreasing the structural stability of a battery. Moreover, despite many studies on the fast lithium diffusion in the current collector materials of AFB such as copper and aluminum, the involved Li diffusion mechanism in these materials remains poorly understood. Through first-principles calculation and stress-assisted diffusion equations, here we study the Li diffusion mechanism in several current collectors and related alloys and clarify the effect of volume expansion on Li diffusion respectively. It is suggested that due to the lower Li migration barriers in aluminum and tin, they should be more suitable to be used as AFB anodes, compared to copper, silver, and lead. The Li diffusion facilitation in copper with a certain number of vacancies is proposed to explain why the use of copper with a thickness 100 nm as the protective coating on the anode improves the lifetime of the batteries. We show that the volume expansion has a positive effect on Li diffusion via mechanical–electrochemical coupling. Namely, the volume expansion caused by Li diffusion will further induce stress which in turn affects the diffusion. These findings not only provide in-depth insight into the operating principle of AFBs, but also open a new route toward design of improved anode through utilizing the positive effect of mechanical–electrochemical coupling.     

Understanding diffusion in nanoporous materials

Can we predict diffusion behavior of molecules in confinement by looking at the match between the molecule and the structure of the confinement? This question has proven difficult to answer for many decades. As a case study, we use methane and a simple model of ellipsoids to arrive at a molecular picture that allows us to make a classification of pore topologies and to explain their diffusion behavior as a function of loading. Our model is surprisingly simple: regarding a structure as consisting of interconnected ellipsoids is enough to understand the full loading dependence.     

Kinetics of high-temperature deformation of polycrystalline OFHC copper and the role of dislocation core diffusion

The mechanisms of the high-temperature deformation of oxygen-free high-conductivity (OFHC) copper have been evaluated over a wide temperature (300–950°C) and strain rate (0.001–100?s^?1) regime. The stress–strain behaviour in hot compression is typical of the occurrence of dynamic recrystallization with an initial peak in the flow stress followed by a steady state, preceded by oscillations at lower strain rates and higher temperatures. The results are analysed using the kinetic rate equation involving a hyperbolic sine relation of the steady-state flow stress with the strain rate. In the temperature and strain rate range covering 500–950°C and 0.001–10?s^?1, a stress exponent of 5 and an apparent activation energy of 145?kJ/mol were evaluated from this analysis. The power law relationship also yielded similar values (5.18 and 152?kJ/mol, respectively). On the basis of these parameters, the rate-controlling mechanism is suggested to be dislocation core diffusion. The flow stress for the OFHC copper data reported by earlier investigators for different oxygen contents is consistent with the above analysis and revealed that an oxygen content of less than about 40?ppm does not have any significant effect on the core diffusion since it is too low to ‘clog’ the dislocation pipes. At strain rates greater than 10?s^?1 and in the temperature range 750–950°C, the stress exponent is about 3.5 and the apparent activation energy is 78?kJ/mol, which suggests that the plastic flow is controlled by grain boundary diffusion.