A mesoscopic simulation of dislocations and plasticity in b.c.c. crystals at low temperatures is developed and applied to the case of Ta. The thermally activated nucleation of double kinks is taken ...as controlling the mobility of screw dislocations. The resulting temperature and strain rate dependence of the yield stress are investigated and a detailed thermal activation analysis is performed on the output of the simulations. It is shown that within the simplifications made to the input parameters and slip geometry, one is able to simulate realistic dislocation structures and mechanical response. By checking the conformity between the input and output, it is shown that the essential dislocation property (activation energy for screw dislocation motion) can effectively be deduced from the mechanical tests. The mesoscopic simulation provides a potential tool to link atomistic studies at the microscopic scale to the macroscopic modeling of mechanical properties of b.c.c. metals at low temperatures.
Une simulation mésoscopique des dislocations et de la plasticité dans les cristaux c.c. à basse température a été développée. La mobilité des dislocations vis est traitée comme résultant de la germination thermiquement activée des doubles décrochements. La dépendence en température et vitesse de déformation de la limite élastique et une étude détaillée du comportement activé ont été effectuées sur le cristal simulé. Une microstructure de dislocations et un comportement mécanique réalistes sont obtenus, compte tenu des simplifications effectuées sur les paramètres d'entrée et la géométrie du glissement. La conformité entre donnés d'entrée et de sortie de la simulation, montre que la propriété essentielle des dislocations (l'enthalpie d'activation pour le mouvement des vis) peut être déduite des essais mécaniques. Cette simulation mésoscopique constitue potentiellement un outil pour réaliser une connection entre les études atomisitiques et le comportement mecanique macroscopique des métaux c.c. à basse température.
During plastic flow of crystalline solids, dislocations self-organize in the form of patterns, with a wavelength that is inversely proportional to stress. After four decades of investigations, the ...origin of this property is still under discussion. We show that dislocation patterns verifying the principle of similitude can be obtained from dynamics simulations of double slip. These patterns are formed in the presence of long- and short-range interactions, but they are not significantly modified when only short-range interactions are present. This new insight into dislocation patterning phenomena has important implications regarding current models.
A multiscale approach including a finite element framework for polycrystal plasticity is used to model jerky flow, also known as the Portevin–Le Chatelier effect. The local constitutive behavior ...comprises the standard description of the negative strain rate sensitivity of the flow stress in the domain of instability. Due to stress gradients inherent to the polycrystal formulation, the spatial coupling involved in the spatio-temporal dynamics of jerky flow is naturally accounted for in the model, without using any
ad hoc gradient constitutive formulation. For the first time, the static, hopping and propagating band types are recovered in constant strain-rate tests, as well as the temporal properties of the stress serrations. The associated dynamic regimes are characterized and found consistent with recent experimental evidence of both chaos and self-organized criticality in Al–Mg polycrystals.
Dislocation dynamics simulations of multiple slip in f.c.c. crystals lead to the formation of patterned microstructures. The mechanisms participating to dislocation storage and dynamic recovery are ...investigated and discussed. Cross-slip and short-range interactions are found to govern the bifurcation from uniform to ordered microstructures.
The question of the description of the elastic fields of dislocations and of the plastic strains generated by their motion is central to the connection between dislocation-based and continuum ...approaches of plasticity. In the present work, the homogenization of the elementary shears produced by dislocations is discussed within the frame of a discrete-continuum numerical model. In the latter, a dislocation dynamics simulation is substituted for the constitutive form traditionally used in finite element calculations. As an illustrative example of the discrete-continuum model, the stress field of single dislocations is obtained as a solution of the boundary value problem. The hybrid code is also shown to account for size effects originating from line tension effects and from stress concentrations at the tip of dislocation pile-ups.
Since the early '60s, injections of a broad-spectrum muscarinic cholinergic agonist, carbachol, into the medial pontine reticular formation (mPRF) of cats have been extensively used as a tool with ...which to study the neural mechanisms of rapid eye movement (REM) sleep. During the last decade, new carbachol models of REM sleep were introduced, including chronically instrumented/behaving rats and "reduced" preparations such as decerebrate or anesthetized cats and rats. The combined results from these distinct models show interspecies similarities and differences. The dual nature, both REM sleep-promoting and wakefulness (or arousal)-promoting, of the cholinergic effects exerted within the mPRF is more strongly expressed in rats than in cats. This strengthens the possibility suggested by earlier central neuronal recordings that active wakefulness and REM sleep have extensive common neuronal substrates, and may have evolved from a common behavioral state. Carbachol studies using different intact and reduced models also suggest that powerful REM sleep episode-terminating effects originate in suprapontine structures. In contrast, the timing of REM sleep-like episodes in decerebrate models is determined by a pontomedullary neuronal network responsible for the generation of an ultradian cycle similar to the basic rest-activity cycle of N. Kleitman. Other presumed species differences, such as the more widespread distribution of carbachol-sensitive sites or the relative failure of carbachol to increase the duration of REM sleep episodes in rats when compared to cats, may be of a quantitative or technical nature. While carbachol and many other neurotransmitters and peptides microinjected into the mPRF evoke, enhance or suppress REM sleep, the most sensitive site(s) of their actions have not been fully mapped, and the nature of the cellular and neurochemical interactions taking place at the sites where carbachol triggers the REM sleep-like state remain largely unknown. Similarly, little is known about the pathways between the mPRF and medial medullary reticular formation, but the existing evidence suggests that they are reciprocal and essential for the generation of both natural and carbachol-induced REM sleep. Studies of the mesopontine cholinergic neurons, which are hypothesized to be the main source of endogenous acetylcholine for the mPRF, need to be extended to neurons of the mPRF and cells located functionally downstream from this important site for REM sleep, or both REM sleep and active wakefulness.
The mutual interactions of dislocation lines containing collinear superjogs are investigated in face-centered cubic crystals by dislocation dynamics simulations in single slip and for various loading ...axes. The examination of the dislocation microstructures, dislocation densities and stress–strain curves allows the origin of work hardening during stage I to be clarified and explains the unusual hardening that is observed experimentally in 0
0
1 crystals at low stresses.
The temporal and spatial features of the Portevin–Le Chatelier plastic instabilities in single and polycrystals of Al–Mg alloys were investigated systematically, with special emphasis being put on ...the character of the statistical distributions of the stress drops. The effect of strain rate, temperature and the microstructural state of the alloy was studied experimentally. It was shown that an experimentally accessible quantity, the flow stress, governs to a large extent the observed correlation between the variation of the type of serrations and of the stress drop distributions. Computer simulations of the Portevin–Le Chatelier effect were carried out using a simple spatial coupling model. It was demonstrated that the salient features of the complex spatio-temporal behaviour observed experimentally for different microstructural states are adequately reproduced by the model. A comparison between the experimental data and the simulation results suggests that the spatial coupling stems from plastic strain incompatibilities.
In the past twenty years, new experimental approaches, improved models and progress in simulation techniques have brought new insights into longstanding issues concerning dislocation-based plasticity ...in crystalline materials. During this period, three-dimensional dislocation dynamics (DD) simulations appeared and reached maturity. Their major objective is to contribute to the multiscale modelling of plastic flow by bridging the gap between atomic-scale studies of dislocation core properties and continuum models. Three-dimensional DD simulations are now becoming accessible to a wide range of users. This book presents to students and researchers in materials science and mechanical engineering a comprehensive coverage of the physical body of knowledge on which they are based and of current unsolved issues that deserve further investigation. The contents include classical studies, which are too often ignored, recent experimental and theoretical advances, and a discussion of selected applications on various topics. As the field is very broad, an extensive bibliography is provided. Chapter 1 presents an introduction to the topics discussed in the ensuing chapters. Chapters 2 and 3 are devoted to crystal plasticity, from elementary core and elastic dislocation properties to the mechanical response. The first of these discusses essentially FCC metals, and the second discusses BCC metals, some transition HCP metals and silicon. Chapter 4 presents basic elements that are common to all three-dimensional DD simulations, as well as characteristic features of each existing code, and Chapter 5 provides a synthesis of applications of DD simulations to large-scale crystals and to size effects in small-scale crystals.
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