Single Glide Research Articles

Al-Zn-Mg-Cu 合金単結晶の硬さと応力-ひずみ曲線

The present paper describes the variations in mechanical properties of Al-Zn-Mg-Cu alloy single crystals with various aging times. The cylindrical single crystals of 7475 aluminum alloys were produced at 1033 K by the Bridgman method, where the composition of initial material and the shape of carbon mold were modified for the growth of quaternary crystals. Specimens appropriate for tensile and hardness testing were obtained from the single crystal rod homogenized at 823 K for 900 ks using a spark-cutting method. Subsequently, they were aged at 393 K for 3.6, 86.4, 900 and 2880 ks after quenching in ice water from 773 K. In the stress-strain curves of the alloy single crystals, the yield stress increased remarkably with the increase of aging time. Also, a decrease of elongation for the aged specimens was seen accompanying a decrease in the rate of work hardening after yielding. Moreover, some serrations occurred particularly in the final plastic stage on the curves. Hardness of the alloy single crystals agreed with those of polycrystals due to the occurrence of multiple slips during loading. However, the alloy single crystals exhibited a marked decrease in yield strength in comparison to the polycrystals. In single crystals, a single glide was found to occur. If we compare the increase of critical resolved shear stress (CRSS) derived from two different theories of precipitation hardening with the experimental value, the present results were roughly in agreement with the theoretical values except for the specimen aged for 3.6 ks. This fact indicated that the maximum strength of Al-Zn-Mg-Cu alloy single crystals was achieved in the transition from the mechanism in which dislocations cut through the precipitated particles to the bypass mechanism of Orowan with the growth in particle size.

Void-induced cross slip of screw dislocations in fcc copper

Pinning interaction between a screw dislocation and a void in fcc copper is investigated by means of molecular dynamics simulation. A screw dislocation bows out to undergo depinning on the original glide plane at low temperatures, where the behavior of the depinning stress is consistent with that obtained by a continuum model. If the temperature is higher than $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the motion of a screw dislocation is no longer restricted to a single glide plane due to cross slip on the void surface. Several depinning mechanisms that involve multiple glide planes are found. In particular, a depinning mechanism that produces an intrinsic prismatic loop is found. We show that these complex depinning mechanisms significantly increase the depinning stress.

Hardness and Stress-Strain Curves of Al-Zn-Mg-Cu Alloy Single Crystals

The present paper describes the variations of the mechanical properties of Al-Zn-Mg-Cu alloy single crystals with various aging times. The cylindrical single crystals of 7475 aluminum alloys were produced at 1033 K by the Bridgeman method, where the composition of initial material and the shape of carbon mold were modified for the growth of quaternary crystals. The specimens proper for tensile and hardness testing were obtained from the single crystal rod homogenized at 823 K for 900 ks using spark-cutting method. Subsequently, they were aged at 393 K for 3.6, 86.4, 900 and 2880 ks after quenching in ice water from 773 K. In the stress-strain curves of the alloy single crystals, the increase of yield stress and the decrease of elongation with an increase of aging time are seen together with a decrease in the rate of work hardening. Moreover, some serrations occur especially in the final plastic stage on the curves. Hardness of the alloy single crystals agrees with those of polycrystals due to the occurrence of multiple slips during loading. However, the alloy single crystals exhibit a marked decrease of yield strength in comparison with the polycrystals, which results from a single glide occurring in the single crystals. If we compare the increase of critical resolved shear stress (CRSS) derived from two kinds of theories of precipitation with the experimental value, both of them are roughly in agreement except for the specimen aged for 3.6 ks. The obtained results indicate that the maximum strength of Al-Zn-Mg-Cu alloy single crystals is achieved in the transitional process between the mechanism of particle cutting and the Orowan mechanism. [doi:10.2320/matertrans.MER2008178]

A Fundamental Model of Aluminum Single Crystal Behavior with Physical Description of Kinematic Work Hardening

In this paper, we present a fundamental model of FCC single crystal behaviour at room temperature: this model includes kinematic work hardening derived from the elementary description of the collective dislocations density evolution during cyclic loading. This kinematic work hardening is then coupled with the isotropic work hardening mechanism. Using this original model, a simulation of a tensile test on a single crystal sample is carried out in the case of an initial crystal orientation that promotes single glide even at rather large strains. The evolution of resolved shear stresses on the primary and secondary slip systems are interpreted by means of the interaction between the evolution of isotropic and kinematic work hardening variables. The evolution of the model state-variables including applied resolved shear strain, dislocation densities, and critical shear stresses are represented as functions of the evolution of crystalline orientation during plastic deformation.

Debye Screening of Dislocations

Debye-like screening by edge dislocations of some externally given stress is studied by means of a variational approach to coarse grained field theory. Explicitly given are the force field and the induced geometrically necessary dislocation (GND) distribution, in the special case of a single glide axis in 2D, for (i) a single edge dislocation and (ii) a dislocation wall. Numerical simulation demonstrates that the correlation in relaxed dislocation configurations is in good agreement with the induced GND in case (i). Furthermore, the result (ii) well predicts the experimentally observed decay length for the GND developing close to grain boundaries.

Effect of steady-state hydrogen coverage on the evolution of crosshatch morphology during Si1−xGex/Si(001) growth from hydride precursors

Compressively strained Si0.7Ge0.3 layers were grown on Si(001) by gas-source molecular beam epitaxy from Ge2H6/Si2H6 mixtures at 450 °C. The combination of the relatively low growth temperature and high steady-state hydrogen surface coverage, θH=0.52 monolayer, completely suppresses strain-induced roughening and provides extremely flat surfaces with root mean square widths w<1.5 Å for fully coherent layers. These samples were used as the starting point to probe mechanisms that control misfit-dislocation-induced surface roughening (i.e., crosshatch) along 90°-rotated 〈110〉 directions. For film thicknesses t just larger than the critical value for misfit dislocation formation, tc≃1000 Å, surface roughness is dominated by single- and multiple-atomic-height steps generated by the motion of threading dislocations associated with interfacial misfits. The surface steps are preferential H desorption sites and the increase in total step length results in a decrease in θH on terraces as well as at step edges. The latter effect allows a higher adatom crossing probability at ascending steps, leading to the formation of periodic ridges in response to local strain fields associated with misfit dislocation clusters; w increases from 3.1 Å at t=1350 Å (corresponding to strain relaxation R of 1%) to 27 Å at t=4400 Å (R=78%). Simultaneously, the decrease in θH on terraces strongly affects film growth kinetics as the deposition rates increase from 10 Å min−1 with t<tc to ≃60 Å min−1 with t≃1400–4400 Å. Overall, in films with t≲1440 Å (R≲5%), crosshatch is due to surface steps that result from multiple misfit dislocations on single glide planes. At higher film thicknesses (R=22–78%), crosshatch becomes dominated by local strain-induced roughening and leads to periodic ridge formation.

Plastic slip distribution in two-phase laminate microstructures: Dislocation-based versus generalized-continuum approaches

The size-dependent mechanical response of a simple model microstructure is investigated using continuum dislocation-based, Cosserat and strain-gradient models of crystal plasticity. The governing equations and closed-form analytical solutions for plastic slip and lattice rotation are directly compared. The microstructure consists of a periodic succession of hard (elastic) and soft (elastoplastic single-crystal) layers, subjected to single glide perpendicular to the layers. In the dislocation-based approach, inhomogeneous plastic deformation and lattice rotation are shown to develop in the soft channels, either because of bowing of dislocations or owing to pile-up formation. The generalized continuum non-local models are found to be able to reproduce the plastic slip and lattice rotation distribution. In particular, a correspondence was found between the generalized-continuum results and line tension effects; the additional or higher- order balance equations introduced in the non-local models turn out to be the counterparts of the equilibrium equation for bowed dislocations. The relevance and possible physical interpretation of additional or higher-order interface conditions responsible for the inhomogeneous distribution of plastic slip and lattice rotations are discussed.

Dislocation glide and dynamic recrystallization in LiF single crystals

Single crystals of LiF with 〈111〉 orientation have been dynamically recrystallized during steady state of plastic deformation at 673 K. It is shown that localization of single dislocation glide plays an important role in the initiation of continuous dynamic recrystallization. Bands of elongated subgrains of rectangular shape are formed in areas of localized deformation at small strains. Upon subsequent deformation uniform multiple slip occurs and leads to the formation of equiaxed subgrains between bands of elongated subgrains. Further plastic deformation leads to a continuous increase in misorientation of subgrain boundaries and their eventual conversion into high angle boundaries. The mechanisms of the formation of recrystallized grains with different grain boundary configurations and the relationship between grain boundary configuration and character of dislocation slip are discussed.

Comparison of physically based constitutive laws used for numerical simulations of plasticity of single crystals

Various microstructural physical mechanisms involving mostly dislocation glides are responsible for plasticity of face centred cubic single crystals. These mechanisms have been pointed out experimentally using at least uniaxial tensile tests. Thus, many constitutive laws have been developed and integrated at the microstructural level to reproduce the experimental evolutions of various parameters (single glide, dislocation density...). More recently, general models have been written in a continuum mechanics framework for finite element simulations. Four main models of this kind are described and applied to the case of uniaxial tensile loading. The selected models present a similar formalism: a flow law plus a hardening law involving a hardening matrix describing the interaction between all the considered glide systems. Each model is integrated using a standard numerical procedure in order to simulate tensile tests. The simulation of a tensile test with a given orientation of the tensile axis is done for each model in order to compare them all together on the same basis. The suitability between the simulated results and experimental evidence is also commented. Finally, the evolution of the components of the hardening matrix is discussed.

Scale transition from single glide bands to macroscopic deformation

By analyzing the spectrum of sources inducing surface nucleation of a glide bands and on the basis of the averaged one-band contribution to deformation of a crystal, the scale transition is realized from the mesoscopic level (glide bands) to the macroscopic one. A model based on this approach is proposed to describe the macroscopic stress-strain curve T- e of a crystal.

Dislocation patterning and conservative recovery in single slip

ABSTRACTPrismatic loops generated by cross-slip are organised in staircase-like strings such that the beginning of a loop corresponds in the screw orientation with the end of its neighbour. Strings constitute nucleation sites for dislocation tangles thus offering an explanation for spontaneous patterning in single glide. A loop string can be partially and even totally eliminated by a single impacting dislocation. In dense walls, loop refinement can also take place conservatively by mutual annihilation of loops of opposite signs.

Material model describing the orientation dependent creep behavior of single crystals based on dislocation densities of slip systems

Abstract A material model is proposed which describes single crystal creep behavior by evolution equations for dislocation densities on individual slip systems. An interaction matrix determines the influence from one glide system to the other. Assuming a face centered cubic crystal, allowing deformation on octahedral glide planes and cube glide planes with a Burgers vector of the type a/2〈110〉, nine independent parameters of the interaction matrix can be distinguished. A parameter check of the nine independent parameters has been carried out, showing the influence of parameters on specific orientations of the load axis. If one assumes dislocation interaction of a glide system only with itself a smooth behavior is predicted with a maximum creep rate for [001] orientation, followed by [011] and [111]. If a strong interaction is assumed, the orientation dependent creep behavior is not at all smooth, instead it shows a sharp drop in creep rates mainly in symmetric positions of the standard orientation triangle. The orientations with highest creep rates are in this case those which favor single glide. Highly symmetric orientations, such as [001], [011] and [111] have strongly decreased stationary creep rates.

First Attempt to Develop A Material Model Describing Single Crystal Creep Behavior

AbstractA material model is proposed which describes single crystal creep behavior by evolution equations for dislocation densities on individual slip systems. An interaction matrix determines the influence from one glide system to the other. Assuming a face centered cubic crystal, allowing deformation on octahedral glide planes and cube glide planes with a Burgers vector of the type a/2 <110>, nine independent parameters of the interaction matrix can be distinguished. A parameter check of the nine independent parameters has been carried out, showing the influence of parameters on specific orientations of the load axis. If one assumes dislocation interaction of a glide system only with itself a smooth behavior is predicted with a maximum creep rate for [001] orientation, followed by [011] and [111]. If a strong interaction is assumed, the orientation dependent creep behavior is not at all smooth, instead it shows a sharp drop in creep rates mainly in symmetric positions of the standard orientation triangle. The orientations with highest creep rates are in this case those which favor single glide. Highly symmetric orientations, such as [001], [011] and [111] have strongly decreased stationary creep rates.

Effect of grain size on wear behaviour of alumina cutting tools

Pure alumina inserts with grain sizes of 0.83, 2.94 and 6.33 μm were produced by slip casting and pressureless sintering. They were used to turn medium carbon steel in dry, continuous cutting conditions, at a cutting speed of 450 m min -1. Examination of the flank face showed that the flank wear land was characterized by long and parallel ridges at short machining times, with the ridges becoming wavier as the grain size of the insert increased. The initial volume flank wear rate increased with increasing grain size of the insert and is consistent with grain spallation being the primary wear mechanism, although the wear rate was moderated by the subsequent smearing of spalled grains onto the tool surface by single glide. After the attainment of a critical width of flank wear land, a transition in wear feature to a two-zone wear feature with spikes nearer the cutting edge and ridges further away occurred; shortly after which the volume flank wear rate decreased appreciably. A similar two-zone wear feature was also observed on the crater of the tool even with short machining times. Although the size of the spikes observed in the crater varied with the grain size of the insert, crater wear rate, expressed as time rate of change of the maximum crater depth, is grain-size independent. This is because the maximum depth of the crater occurred in the region covered with spikes, where material loss is controlled by plastic deformation-induced necking of asperities, a process known to be grain-size independent.

Simulation and theory of polygonization in single glide

We present the results of numerical simulations of two-dimensional polygonization, the simplest example of dislocation patterning. The simulations are based on the elastic interactions of straight edge dislocations, with a law of motion that combines fast glide with slow climb. The simulated patterns reproduce experimental patterns for Fe-Si quite well. The effects of changing the Peierls stress for glide processes, or immobilizing some of the dislocations, are addressed. Decreasing the Peierls stress and increasing the fraction of mobile dislocations speeds up the pattern formation process. The energy release at short times is dominated by the disorder in walls, and at longer times by wall-spacing effects. We provide insight into the dynamics of wall coarsening via the concept of random dislocation walls. The applicability of continuum flow equations is evaluated. We find that description of the polygonization process in terms of the dislocation configuration tensor is inadequate.

Transitions in wear mechanisms of alumina cutting tools

Alumina inserts were used to turn medium carbon steel bars in dry, continuous cutting conditions at a cutting speed of 450 m min−1. Three wear mechanisms were identified: plastic deformation-induced necking of asperities; grain spallation; and bulk plastic deformation of alumina. The crater wear land was characterized by spikes nearer the cutting edge and ridges further away because the temperatures and compressive stresses were higher nearer the cutting edge. The crater wear-rate, as measured by the maximum crater depth, was controlled by plastic deformation-induced necking, which was also responsible for the formation of the spikes. The flank wear land was characterized by ridges during the initial stage of machining, formed by the spallation of grains and their subsequent smearing by single glide. As the temperatures and compressive stresses on the flank wear land increased with increasing width of the wear land and machining forces, a transition to a two-zone wear feature (TF) characterized by spikes and ridges occurred, similar to that observed on the rake face. Seizure then followed and a transition in wear rate (TR) was observed in that further increases in the width of the flank wear land occurred by bulk plastic deformation of the to material at a much reduced rate. The present work shows that there exists a transition zone (from TF to TR) in flank wear, sepa the rapid wear mechanism of grain spallation from the slower mechanism of bulk plastic deformation, defined by both machining time and a critical flank wear land width.

Stochastic dislocation patterning during cyclic plastic deformation

A dislocation dynamical theory is developed for the formation of dipole dislocation patterns during cyclic plastic deformation in single glide. The stochastic dislocation dynamics adopted is suitable to account, in terms of a fluctuating effective medium, for the effects of long-range dislocation interactions on a mesoscopic scale. The theory can explain the occurrence of a matrix structure and persistent slip bands as a result of evolutionary processes, it gives the intrinsic strain amplitudes and the characteristic wavelength of these structures, and it allows for an interpretation of the structural changes associated with changes of the deformation conditions. Quantitative results are in good agreement with experimental observations.

The dynamics of dislocation dipoles during single glide

The dynamics of dislocation dipoles during single glide

The dynamics of dislocation dipoles during single glide

The dynamics of dislocation dipoles during single glide

Dynamic strain ageing and the boundaries of stable plastic deformation inCu-Mn single crystals

Abstract The boundaries of stable plastic deformation, characterized by critical stresses τi(T), are investigated for single crystals of Cu-2 at.% Mn and compared with those of polycrystals. The single crystals were oriented along [100], i.e. for symmetric multiple glide, or for single glide. Strain rate sensitivity was measured between about 300 K and 500 K and for different basic strain rates to deduce activation parameters. The main result is that the form of τi(T) for the [100]-crystals is very similar to that for polycrystals in all respects, whereas it differs appreciably from that for the single-glide crystals. In all cases two different T intervals, labelled α and β, can be distinguished. The activation parameters obtained are the exponent n of the strain rate and the activation energy Ea of the diffusion process inducing jerky flow at τi. Whereas n always equals 1/3, we find for both the single-glide and the [100]-multiple-glide crystals that E α a=0.9eV in the interval α and E β a=1.6eV in the i...