Phase-Field Reaction-Pathway Kinetics of Martensitic Transformations in a Model Fe3Ni Alloy

A three-dimensional phase-field approach to martensitic transformations that uses reaction pathways in place of a Landau potential is introduced and applied to a model of Fe3Ni. Pathway branching involves an unbounded set of variants through duplication and rotations by the rotation point groups of the austenite and martensite phases. Path properties, including potential energy and elastic tensors, are calibrated by molecular statics. Acoustic waves are dealt with via a splitting technique between elastic and dissipative behaviors in a large-deformation framework. The sole free parameter of the model is the damping coefficient associated to transformations, tuned by comparisons with molecular dynamics simulations. Good quantitative agreement is then obtained between both methods.Comment: 4 pages, 3 figure

Mechanism for the {\alpha} -> {\epsilon} phase transition in iron

The mechanism of the {\alpha}-{\epsilon} transition in iron is reconsidered. A path in the Burgers description of the bcc/hcp transition different from those previously considered is proposed. It relies on the assumption that shear and shuffle are decoupled and requires some peculiar magnetic order, different from that of {\alpha} and {\epsilon} phases as found in Density-Functional Theory. Finally, we put forward an original mechanism for this transition, based on successive shuffle motion of layers, which is akin to a nucleation-propagation process rather than to some uniform motion.Comment: 6 pages, 5 figure

Cavitation in compressible visco-plastic materials

International audienceDuctile metals subjected to a high strain rate tensile loading are known to break by coalescence of voids. An analytical solution for the evolution of the cavity is proposed for compressible visco-plastic materials. Comparisons with FE calculations are proposed

On probabilistic aspects in the dynamic degradation of ductile materials

Dynamic loadings produce high stress waves leading to the spallation of ductile materials such as aluminum, copper, magnesium or tantalum. The main mechanism used herein to explain the change of the number of cavities with the stress rate is nucleation inhibition, as induced by the growth of already nucleated cavities. The dependence of the spall strength and critical time with the loading rate is investigated in the framework of a probabilistic model. The present approach, which explains previous experimental findings on the strain-rate dependence of the spall strength, is applied to analyze experimental data on tantalum.Comment: 28 pages, 13 figures, 3 table

Probabilistic-deterministic transition involved in a fragmentation process of brittle materials: Application to a high performance concrete

International audienceDynamic loadings produce high stress waves leading to the fragmentation of brittle materials such as ceramics, concrete, glass and rocks. The transition from single fragmentation (under quasi static loading condition) to multiple fragmentation (dynamic loading) is discussed and a damage model based upon the fragmentation analysis is applied to analyze edge-on-impact of a high performnace concrete

SPAWN: An Iterative, Potentials-Based, Dynamic Scheduling and Partitioning Tool

International audienceMany applications of physics modeling use regular meshes on which computations of highly variable cost can occur. Distributing the underlying cells over manycore architec-tures is a critical load balancing step that should increase the period until another step is required. Graph partitioning tools are known to be very effective for such problems, but they exhibit scalability problems as the number of cores and the number of cells increases. We introduce a dynamic task scheduling approach inspired by physical particles interactions. Our method allows cores to virtually move over a 2D/3D mesh of tasks and uses a Voronoi domain decomposition to balance workload among cores. Displacements of cores are the result of force computations using a carefully chosen pair potential. We evaluate our method against graph partitioning tools and existing task schedulers with a representative physical application, and demonstrate the relevance of our approach

Localisation of plastic deformation in stretching plates: microstructure effects

The incidence of microstructural characteristics on the localization of plastic deformation in stretching plates in the form of necking was investigated by performing polycrystal plasticity simulations: crystal aggregates were generated by a Voronoї tessellation algorithm and crystal orientations were randomly distributed among the grains. The most outstanding results of this analysis, detailed in [1], are presented here. During dynamic extension, a transition is displayed between an initial grain scale strain organization towards the development of much larger patterns and a final stage during which one or a few localization zones develop drastically. The influence of the initial strain distribution on the final necking framework is enhanced when the number of grains in the plate thickness is reduced or the loading rate is moderate. Additionally, the sensitivity of the different stages of the localization process to texture has been highlighted: the co-existence of both favorably and unfavorably oriented grains raises the initial strain heterogeneity and triggers macroscopic localization at shorter times

Approche probabiliste de la fragmentation de matériaux fragiles sous chargement dynamique

International audienceDynamic loadings produce high stress waves leading to the fragmentation of brittle materials such as ceramics. The main mechanism used to explain the change of the number of fragments with stress rate is an obscuration phenomenon. After the presentation of a probabilistic approach, the evolution of the number of nucleated flaws and a damage model are derived.Une sollicitation dynamique ou par explosions de matériaux fragiles conduit à leur fragmentation. Dans cette note, un modèle de fragmentation dynamique pour matériaux fragiles est proposé sur une base probabiliste. Un modèle d'endommagement est également discuté. Le principal mécanisme invoqué pour expliquer la variation en nombre de fragments est un phénomène d'occultation des défauts entre eux

Dislocation Core Structure at Finite Temperature Inferred by Molecular Dynamics Simulations for 1,3,5-Triamino-2,4,6- trinitrobenzene Single Crystal

The dislocation core structures and elastic properties of the insensitive energetic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) are investigated as a function of pressure and temperature. A new method is proposed to compute the generalized stacking fault surfaces (noted γ-surfaces) and the complete second-order elastic tensor at finite temperature through molecular dynamics (MD) simulations. The energy landscapes in the two glide planes are shown to be similar between 0 and 300 K, thus leading to almost no modification on the dislocation evolution. A spreading of the dislocation cores over a hundred Burgers vectors is observed along the [100] and [010] directions for the edge and screw dislocations at 0 and 300 K, showing that dislocations should exhibit a very low friction for these glide systems at ambient pressure. For pressures varying between 0 and 10 GPa, the γ-surfaces' energy barriers that drive the width of partial dislocations follow the increase of shear elastic constants within the considered glide planes, thus limiting the changes of the dislocation core structure