Terms Of Dislocation Mechanisms Research Articles

On the physical basis for the creep of ice: the high temperature regime

AbstractThis work quantifies the increased temperature sensitivity of the constitutive behavior of ice with proximity to the melting point in terms of dislocation mechanics. An analysis of quasistatic and dynamic cyclic loading data for several ice types leads to the conclusion that high temperature (e.g. T ≥ −8°C) behavior is the result of a thermally induced increase in the number of mobile dislocations rather than an increase in the activation energy of dislocation glide or the introduction of a new deformation mechanism. The relationship between dislocation density and temperature is quantified and the model is shown to adequately represent the published minimum creep rate vs stress data for isotropic granular freshwater ice for −48 ≤ T ≤ −0.01°C.

Crystal plasticity theory during the 1950s and 1960s

In the field of crystal plasticity, the two decades 1950–1969 were a time of great activity, in which the foundations were laid for our present understanding of the work-hardening of face-centred cubic metals in terms of dislocation mechanisms. Decisive developments were the ‘invention’ of the Frank–Read source, the recognition of the importance of the specific energy of stacking faults, and the (at least partial) resolution of the controversy between macroscopic stress–strain measurements, X-ray results, ferromagnetism and the study of slip lines and slip bands on the one hand, and the observation of dislocations by transmission electron microscopy of thin foils prepared from plastically deformed macroscopic specimens on the other hand. The understanding of the dislocation mechanisms dominant at very large plastic strains was and still is limited, however. It is argued that here a holistic approach based on concepts such as energy dissipation and self-organization is more promising than the mechanistic approach based on concepts successfully developed in the two decades covered in the paper.

Reserach on Rheology of Biodegradable Soy Protein Materials

The aluminum metal matrix composites (Al MMCs) reinforced by SiC particles with different sizes (25 nm, 150 nm and 3.5 μm) were fabricated by powder metallurgy technique, and the microstructure and tensile properties of the Al MMCs were investigated. When the volume fraction of SiC particles is fixed to be 5 %, the Al MMCs reinforced by nanosized and submicron particles exhibit much higher ultimate tensile strength and yield strength, and much lower ductility compared with those of the non-reinforced aluminum. However, for the MMC reinforced by 3.5 m SiC particles, almost no strengthening effect is found. The strengthening effect of the Al MMCs is analyzed in terms of dislocation mechanism. Moreover, the tensile fracture surface shows that the damage mechanism of the Al MMCs can be changed as the size of SiC particles is changed.

Effect of SiC Particle Size on Microstructure and Tensile Behavior of Aluminum Matrix Composites

The aluminum metal matrix composites (Al MMCs) reinforced by SiC particles with different sizes (25 nm, 150 nm and 3.5 m) were fabricated by powder metallurgy technique, and the microstructure and tensile properties of the Al MMCs were investigated. When the volume fraction of SiC particles is fixed to be 5 %, the Al MMCs reinforced by nanosized and submicron particles exhibit much higher ultimate tensile strength and yield strength, and much lower ductility compared with those of the non-reinforced aluminum. However, for the MMC reinforced by 3.5 m SiC particles, almost no strengthening effect is found. The strengthening effect of the Al MMCs is analyzed in terms of dislocation mechanism. Moreover, the tensile fracture surface shows that the damage mechanism of the Al MMCs can be changed as the size of SiC particles is changed.

The deformation stage II of face-centered cubic crystals: Fifty years of investigations

Abstract This article critically reviews progress in the understanding of strain hardening during the deformation stage II of pure face-centered cubic crystals since its discovery in the mid-1950s. A wealth of models attempted explaining why stage II exhibits a linear slope on the base of specific dislocation configurations and interactions associated with short- or long-range internal stresses. Slip trace observations and more recent investigations on dislocation avalanches led to the identification of intermittent elementary slip events that are now investigated in terms of dislocation mechanisms. Numerical estimates showed that the overcoming of junctions and locks accounts for most of the flow stress and the hardening rate, as assumed by the present forest models. A multiscale analysis of stage II is outlined with emphasis on the modeling of dislocation mean free paths by dislocation dynamics simulations.

Micro-strains in cold rolled Cu–Nb nanolayered composites determined by X-ray line profile analysis

X-ray line profile analysis was used to investigate micro-strains in sputter-deposited Cu–Nb nanolayered composites, with individual layer thickness of 75 nm, in the as-deposited and cold rolled conditions. The measured anisotropic line broadening in these strongly textured foils was interpreted in terms of dislocations. A very high dislocation density, of the order of 10 16 m −2, was inferred from the X-ray data in the as-deposited foil. No significant change in dislocation density was noted after cold rolling to around 150% elongation in the rolling direction. Furthermore, the preferred orientation of the Burgers vectors, both before and after rolling, for the majority of the dislocations was parallel to the interface. The apparent lack of dislocation storage beyond the initial density after large plastic strains is discussed in terms of dislocation mechanisms.

On the strain-hardening of tempered martensitic alloys

The strain-hardening behaviour of a tempered martensitic iron–chromium–carbon model was examined in terms of dislocation mechanics. Tensile tests of this body-centered cubic alloy were carried out from 77 up to 293 K. A specific relationship between the flow stress σ and the dislocation density ρ was considered below the transition temperature ( T < 200 K) where the Peierls lattice friction becomes the single rate controlling mechanism. This relationship is different from the usual Taylor equation, where the flow stress scales with the square root of the dislocation density. This σ– ρ relationship was used in conjunction with an equation characterizing the dislocation evolution with strain to derive a strain-hardening law. The parameters that play a key role in the strain-hardening are the mean dislocation free path and a dislocation annihilation coefficient. The values of these parameters used to describe the strain-hardening law as well as their temperature dependence are found to be consistent with the microstructure.

Temperature dependence of microhardness of tetragonal hen-egg-white lysozyme single crystals

The mechanical properties of high-quality single crystals of tetragonal hen-egg-white lysozyme from hen egg white were investigated by the indentation method. The indentation marks were clearly observed on the crystal surface and elastic recovery did not occur. The micro-Vickers hardness of the crystals in the wet condition was observed to be 20 MPa at room temperature. To investigate deformation, the temperature dependence of the hardness was investigated from 278 to 308 K. The hardness strongly depended on the temperature above 290 K. From these results the deformation of lysozyme single crystals was explained in terms of dislocation mechanisms.

On the mechanical properties of the advanced martensitic steel EUROFER 97

In an effort to better understand the plasticity of tempered martensitic steels, uniaxial tension and compression tests were carried out. The temperature dependence of both the yield stress and the post-yield stress behavior were compared, and it was shown that a good correspondence exists between tensile and compressive behavior. The strain-hardening of EUROFER 97 steel was examined in terms of dislocation mechanics. A simple strain-hardening law was established within the framework of a model accounting for dislocation storage and annihilation. Moreover, an analytical constitutive equation was derived which describes the overall post-yield curve. While the equation predicts the flow stress should reach a saturation stress with strain, this could not be verified by the mechanical tests due to deformation instabilities such as necking in tension or side-slip buckling in compression. Additional tests will have to be devised and applied to investigate these predictions.

Relation Between Microstructures and Constitutive Behavior of Advanced Tempered Martensitic Steels

ABSTRACTIn an effort to better understand the plasticity of the tempered martensitic steels, the strain-hardening of two 7-9Cr steels is examined in terms of dislocation mechanics. It is shown that, over the temperature range investigated (173K-523K), the strain-hardening as a function of stress can be described by an equation taking account the dislocation storage and annihilation. The model of strain-hardening used in this study is based on the original Kocks description of the dislocation density evolution with plastic strain but the heterogeneous distribution of dislocations resulting from the lath boundaries and prior austenite grain boundaries is taking into account. The effect of stress and temperature on the storage and annihilation is discussed. The relation between the determined mean dislocation path associated to the heterogeneous lath dislocation structure and the transmission electron microscope observations performed on the two different steels is outlined.

Attenuation Stress Hysteresis Loops and Low‐Frequency Internal Friction Associated with Thermomechanical Breakaway of Dislocations

AbstractMeasurements of ultrasonic attenuation and velocity changes are performed on 5N aluminium during the application of a low frequency bias stress. The characteristics of the attenuation‐stress Δα = f(σ) loops are studied as a function of the repetition of loading, amplitude σm of loading stress, temperature, and pretreatment of the sample (prestrain, ageing time, or temperature). The results show that at least two kinds of dislocation‐point defect breakaway mechanisms are involved in the experiments. Then a model is developed in order to calculate from the attenuation‐stress hysteretic loop a low frequency internal friction decrement δα: peak events versus amplitude (δα = f(σm)) are thus observed, in good accordance with classical models of internal friction associated with breakaway of dislocations; the activation energy for depinning and the space between pinning points are deduced from these peaks and their evolutions are followed as a function of the pretreatment of the sample. In addition, although the internal friction decrements δα and δ, obtained directly from torsion pendulum, have some similar characteristics, a noticeable difference appears in the decrement levels which is especially explained by the characters of the solicitation in bias stress and torsion pendulum experiments, respectively, one sided and alternative; their effects are discussed in terms of dislocation mechanisms.

Solid Solution Effects and Deformation Micros trueture of Shock-Loaded Iron Manganese Alloys

In general, all BCC transition metals have been observed to twin under appropriate conditions. At the present time various experimental reports of solid solution effects on BCC metals have been made. Indications are that solid solution effects are important in the formation of twins. The formation of twins in metals and alloys may be explained in terms of dislocation mechanisms. It has been suggested that twins are nucleated by the achievement of local stress-concentration of the order of 15 to 45 times the applied stress. Prietner and Leslie have found that twins in BCC metals are nucleated at intersections of (110) and (112) or (112) and (112) type of planes.In this paper, observations are reported of a transmission microscope study of the iron manganese series under conditions in which twins both were and were not formed. High strain rates produced by shock loading provided the appropriate deformation conditions. The workhardening mechanisms of one alloy (Fe - 7.37 wt% Mn) were studied in detail.

On the relation between continuum plasticity and dislocation theories

A theory of elastic-plastic flow based upon dislocation mechanics is presented. The logical structure of the theory parallels the Green-Naghdi plasticity theory, thereby permitting direct interpretation of the classical continuum plasticity assumptions in terms of dislocation mechanics. Thermodynamical restrictions derivable from the Clausius-Duhem inequality are discussed in terms of dislocation motion.