Microstructure of hot-pressed α-Sic after creep experiments at high temperature

Abstract

Transmission electron microscopy analyses of hot-pressed α-Sic deformed at 1600°C have shown the activation of the basal plane with glide dislocations dissociated into Shockley partials. Loop nucleations have been frequently observed along the dislocation lines and a climb mechanism is proposed to explain, the experimental analyses.     

A microscopic theory of Andrade creep

A microscopic theory is developed which gives the exact expression for Andrade creep when work hardening is small, as observed in practice. The microscopic model is that of the forest theory of work hardening and in which glide dislocation lines require supplementation of the applied stress, in order to cut their way through the forest obstacles. The loads shed on neighbours when individual obstacles give way play a major role in this, and thermal activation releases blocked avalanches of yielding to enable them to continue to grow.     

Microstructure of icosahedral Al-Pd-Mn quasicrystals deformed at room temperature in an anisotropic confining medium

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.     

Application of electron channelling contrast to the investigation of strain localization effects in cyclically deformed fcc crystals

The dislocation substructure and the glide activity in cyclically deformed nickel single crystals have been studied using the channelling contrast of backscattered electrons in a scanning electron microscope. Dislocation arrangements which arise in the saturation region after cyclic loading at room temperature and at one elevated temperature are considered. The electron channelling contrast technique is shown to be a useful instrument for the qualitative and quantitative characterization of the dislocation pattern on a macroscopic and a mesoscopic scale. The correlation between the specific dislocation structure in persistent slip bands PSBs and the localized glide activity of PSBs and PSB macrobands are considered.     

The flow stress contribution of the dislocation 'forest' in bcc lattices

The athermal flow stress contribution by junction reactions of a glide dislocation with a 'forest' of density rho is usually written as tau = alpha mu b rho ^1/2. The strength coefficient alpha has been computed for the bcc lattice by virtual displacement of triple nodes following the method of Puschl et al. (1982, Physica status solidi (a), 74, 211). In the isotropic approximation, the values for glide dislocations with 0, 30, 60 and 90^o character are as alpha = 0.18, 0.21, 0.19 and 0.22 respectively. A comparison with values calculated previously by Frydman is made, and good agreement is found when differences in the cut-off radii of the linear elastic solution are accounted for.     

Yield drop in high-strain-rate superplastic deformation of ZrO2-30 vol% MgAl2O4 spinel composite

The origin of a sudden yield drop in a tetragonal ZrO₂ dispersed with 30 vol% MgAl₂O₄ spinel composite has been examined. The present ZrO₂-spinel composite exhibits yield drop in superplastic flow at high strain rates of 0.2s^?1 or greater, where the flow behaviour is characterized by a stress exponent of about 3.5 and a grain-size exponent of about 1.0. Experimental examination suggests that a sudden increase in the mobile dislocation density within spinel grains is responsible for the yield drop.     

Evaluation of the threshold stress for creep deformation in a 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 polycrystal

The origin of the increase in the stress exponent from n , 2.0 to with decreasing stress in 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 has been examined. The present data show that the increase in the n value arises from the existence of a threshold stress that depends on the grain size and temperature. Careful examination of earlier creep data confirms that evaluation of the threshold stress is sensitive to the accuracy of the creep data and the value of n chosen for the compensation of the data. Inspection of the present results and some recent observations of the deformed microstructure suggests that the threshold stress is associated with intragranular dislocation motion.     

Surface effect on the screw dislocation mobility over the Peierls barrier

The effect of the image force on the Peierls stress (τ _p ) of a screw dislocation below a free surface is studied via a self-consistent semidiscrete variational Peierls–Nabarro model. The consequence of reduction in elastic energy and increase in stacking fault energy by the presence of the free surface is found to additively increase the Peierls stress (τ _p ). This model gives a physical interpretation of the same tend observed in a recent molecular dynamic study, while previous continuum analysis predicted the opposite.     

Phase field microelasticity theory of dislocation dynamics in a polycrystal: Model and three-dimensional simulations

A three-dimensional multidislocation system in a polycrystal under applied stress is treated as a particular case of the phase field microelasticity theory of multivariant stress-induced martensitic transformations in polycrystals. This approach reduces the problem of the evolution of a dislocation system to a solution of the nonlinear integrodifferential Ginzburg-Landau equation. In this formalism, the elastic interaction between dislocations and the elastic coupling between grains are taken into consideration through exact analytical solution of the elasticity problem. The dislocation reactions, such as multiplication and annihilation, are taken into account automatically. The dislocations are 'free' to choose the optimal evolution path. Examples of three-dimensional computer simulations are considered.     

Homogeneous nucleation of dislocation loops under stress in perfect crystals

We present an analysis and results on the homogeneous nucleation of a dislocation loop under stress in a perfect crystal. By using a variational boundary integral method in the Peierls-Nabarro framework, we have determined the saddle-point configurations of embryonic dislocation loops and their associated activation energies under stress levels up to the ideal shear strength. The high-energy barriers under the usual levels of applied shear stresses, differing markedly from the ideal shear strength, confirm the widely held view that thermal motion should play no role in such nucleation. The result provides means for more definitive solutions of fundamental problems involving homogeneous nucleation of dislocation loops and has significant implications for models based on the mechanism of nucleation of dislocations from a perfect crystal.     

The annihilation by glide of prismatic loops in walls

Two mechanisms are conjectured for the non-diffusional refinement of prismatic dislocation loops. These mechanisms offer an alternative to the widely accepted concept of spontaneous disintegration below some critical dipole height. In one, a loop array is refined by reaction with a mobile dislocation having the same Burgers vector. Loop shrinkage then depends on the geometry of the array and on the position of the plane of incidence. In principle, any loop array can be refined via this mechanism provided that all the loops have the same sign. An alternative mechanism may take place in dense walls formed of randomly arranged dipolar loops of both signs. As the wall is densified by impacting dislocations that push pre-existing loops on their glide prism against each other, refinement occurs conservatively when two loops of opposite sign come into contact.     

Dissociation of dislocations in MgAl2O4 spinel deformed at low temperatures

Abstract

When spinel is deformed in compression at 400°C along 〈110〉, the primary slip plane is found to be {111} with cross-slip occurring on a {001} plane. A comparison of weak-beam images of dislocations from both systems indicates that all dislocations which belong to the primary slip plane are dissociated out of the {111} plane independent of the character of the dislocation. It is proposed that deformation occurs by motion of dislocations in their dissociated state and that the partial dislocations actually glide on parallel glide planes. Movement of these dissociated dislocations is then accompanied by a concurrent migration of the stacking fault which takes place by a local shuffling of the cations. A stacking fault energy for conservative dissociation at 400°C on {001} of 530±90mJ m^?2 has been determined from weak-beam images of screw dislocations.     

high-resolution electron microscopy at a (113) symmetric Thermally activated step motion observed by tilt grain-boundary in aluminium

Grain-boundary migration is demonstrated to proceed by lateral propagation of a small step in a (113), [110] symmetric Al tilt grain-boundary. In-situ high-resolution (transmission) electron microscopy (HREM) at 523K allowed the study of atomic-scale detail at video rates during the migration process. The grain-boundary translational states on both sides of the step are identical, which leads to a step dislocation. This defect can move laterally by a combination of climb and glide. Dynamic HREM images indicate considerable atomic agitation within the dislocation core. A detailed temporal analysis of the step movements shows small random displacements of the dislocation core.     

Dynamic properties of edge dislocations decorated by interstitial loops in α-iron and copper

Clusters of self-interstitial atoms (SIAs) in the form of parallel crowdions are created directly in high-energy displacement cascades produced in metals by neutron irradiation. They are equivalent to small perfect dislocation loops and, in isolation in pure metals, undergo fast thermally-activated glide in the direction of their Burgers vector. Their strain field and ability to glide allows long-range interaction with other extended defects. Indeed, dislocations decorated by dislocation loops are commonly observed after neutron irradiation. Dislocations gliding under applied stress also encounter these mobile defects. These effects influence mechanical properties and require further investigation. This paper presents results from an atomic-scale study of copper and α-iron at either 0?K or 300?K. Loop drag and breakaway effects are investigated for an edge dislocation under applied stress interacting with a row of SIA loops below its glide plane. The maximum speed at which a loop is dragged is lower in copper than iron, and the applied stress at which this occurs is also lower. These differences in the dynamics of cluster-dislocation interaction are determined by the atomic structure of the defects and cannot be investigated by continuum treatment.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Creep behaviour of ice single crystals loaded in torsion explained by dislocation cross-slip

Torsion creep experiments are carried out in order to understand the physics of ice plasticity. A dislocation spreading mechanism based on double cross-slip of basal dislocations is proposed to explain the strong plastic anisotropy and the power law relationship between stress and strain rates. The scenario is tested using three-dimensional dislocation dynamics simulations. Numerical investigations give a stress exponent n?=?2.3 in agreement with experimental measurements. This dislocation spreading mechanism sheds a new light on the interpretation of former experimental observations.     

On the ultra-high-strain rate shock deformation in copper single crystals: multiscale dislocation dynamics simulations

Multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper single crystals that have undergone shock loading at high strain rates ranging from 1?×?10⁶ to 1?×?10¹⁰?s^?1. Plasticity mechanisms associated with both the activation of pre-existing dislocation sources and homogeneous nucleation of glide loops are considered. Our results show that there is a threshold strain rate of 10⁸?s^?1 at which the deformation mechanism changes from source activation to homogeneous nucleation. It is also illustrated that the pressure dependence on strain rate follows a one-fourth power law up to 10⁸?s^?1 beyond which the relationship assumes a one-half power law. The MDDP computations are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models.     

Vacancy interaction with glissile interstitial clusters in bcc metals

The interaction between a glissile cluster of self-interstitial atoms and a vacancy has been studied in f -Fe by atomistic modelling and elasticity theory. It was found that vacancies can annihilate only with the cluster edge. Vacancies inside the cluster glide prism do not annihilate with interstitials but affect the cluster dynamic properties. Depending on the cluster size and ambient temperature, these vacancies reduce or even prevent cluster motion. Qualitative differences in the results of atomistic and elasticity theory approaches were found, thereby demonstrating the need for the atomistic approach.     

The Peierls-Nabarro model revisited

We re-examine two important issues within the Peierls-Nabarro model, which are critical in obtaining accurate values for the Peierls stress. The first is related to the sampling scheme (double versus single counting) of the misfit energy across the glide plane and the second is the effect of atomic relaxation on the Peierls stress. We argue that the double-counting scheme is physically more appropriate. An analytical formula is derived for the Peierls stress of dislocations in alternating lattices. The atomic relaxation is shown to play an important role on the Peierls stress for narrow dislocations.     

Modelling of irradiation-induced hardening in metals using dislocation dynamics

A new simulation technique (three-dimensional dislocation dynamics) enabling the capture of a hardening effect in metals due to irradiation is reported. When bombarded with high-energy particles, metals accrue internal damage. In irradiated Pd, for example, damage takes the form of interstitial loops. Such loops are nano-sized and typically have a high number density. The stress field of a loop is given from dislocation theory. It is shown here the hardening is due to the elastic interaction of gliding dislocations with a high number of spatially dispersed interstitial loops. Results are found to correlate well with experiments.