abstract: 4.a
Modelling the crack initiation in fatigued 316L by discrete dislocation simulations
CHRISTOPHE
DEPRES,
GPM2, CNRS/INPG, ENSPG, 101 Rue de la Physique BP 46, 38402 St Martin d'heres cedex, France;
CHRISTIAN
ROBERTSON,
CEA Saclay, 91191 Gif sur Yvette, France;
MARC C.
FIVEL,
GPM2, CNRS/INPG, ENSPG, 101 Rue de la Physique BP 46, 38402 St Martin d'heres cedex, France;
MARC
VERDIER,
LTPCM, CNRS/INPG, ENSEEG, 1130 Rue de la Piscine, 38402 St Martin d'heres cedex, France.
In f.c.c. single crystals, fatigue is characterised by strain localization into persistent slip bands. Elimination of dislocations at the free surfaces of the sample creates steps that may remains and accumulate from one cycle to another resulting into localized extrusions and intrusions. This irreversibility character of the behaviour finally creates microcracks leading to damage.
In this presentation the early stages of the formation of the dislocation microstructure in low strain fatigue are analysed using three dimensional discrete dislocation simulations. The simulation box is taken as a polyhedron admitting one single free surface where dislocation lines can escape leaving plastic steps. The simulation is adapted to the case of 316L stainless steel.
The code is validated by comparing both the dislocation microstructures and the associated mechanical behaviour with experiments performed in single slip and double slip loading. In the case of double slip loading conditions, the dislocation microstructure is localised in the form of intense slip bands. A complete and detailed scheme for the microstructure formation is described from simulation snapshots. It appears that the cross-slip plays a crucial role: during the first cycles, cross-slip events spread the plasticity whereas after few cycles it localizes and lock the dislocation structure pinned in the bands. Finally, the relief of the free surface is computed and the evolution of the size of the extrusion versus the cycles is plotted and fatigue life is estimated. The effect of the free surface and the associated image forces on the mechanical behaviour is also studied.
abstract: 4.b
MODELLING OF DISLOCATION-OBSTACLE INTERACTIONS IN METALS EXPOSED TO AN IRRADIATION ENVIRONMENT
DAVID J.
BACON,
University of Liverpool, UK.
Irradiation of metals with high-energy atomic particles such as neutrons and ions creates specific microstructures through which dislocations have to move during plastic flow. Obstacles to glide, such as voids, stacking fault tetrahedra, irradiation-induced precipitates and interstitial dislocation loops, arise in combinations and densities that are not encountered after thermal treatments. Whereas the line tension approximation offers the simplest approach to treatment of strengthening due to dislocation-obstacle interaction, it is deficient in many respects. It is now widely recognised that a multiscale modelling approach should be used to study these interaction processes, wherein the mechanisms and strength parameters of interaction are derived by simulation of the atomic level in order to feed higher-level treatments based on continuum mechanics. Atomic-scale computer models have been developed to provide quantitative information on the influence of stress, strain rate and temperature. In this paper, some critical issues with regard to radiation damage effects are discussed and recent results are reviewed for simulations of both BCC and FCC metals. The stress for dislocations to pass obstacles such as voids and precipitates is analysed with particular reference to zero temperature (0K), for this is the condition that provides for direct comparison with results from continuum mechanics. Other effects that are less amenable to description by elasticity theory, such as the dynamics of drag of loops by gliding dislocations, are also presented. Although some of these processes can be represented within the continuum treatment of dislocations, others cannot, thereby demonstrating the importance of atomic-scale simulation for the advance of understanding of strengthening phenomena.
abstract: 4.c
Statistical dynamics of dislocations in simple models of plastic deformation: Phase transitions and related phenomena
M.-CARMEN
MIGUEL,
Universitat de Barcelona, Barcelona, Spain.
PAOLO
MORETTI,
MICHAEL
ZAISER,
University of Edinburgh, Edinburgh, UK.
STEFANO
ZAPPERI,
INFM & Università "La Sapienza", Roma, Italy.
Statistical treatment of experimental data and numerical models of materials
undergoing plastic deformation reveal important features undergoing this
fundamental process, such as intermittency, avalanches, and scaling relations.
These and some other interesting properties are ubiquitous in the
non-equilibrium dynamics of a wide variety of physical systems, including
amorphous matter (glasses, foams, gels, polymeric melts, and most recently
granular media) and crystalline solids (including vortex lattices). The
dynamics of this wide class of physical systems shares the fact that it is
governed by the presence of kinematical constraints, induced by interactions,
geometry, and/or disorder. For several decades, Statistical Mechanics has been
trying to offer a comprehensive theory that is able to explain the
thermodynamic and kinetic properties of some of these systems, both at
the macroscopic and the mesoscopic level, and that is consistent with the
wealth of experimental observations.
Dislocation assemblies in crystalline solids are yet another beautiful example of these broad class of systems, with several magnitudes playing a decisive role. In this talk we will focus on several aspects concerning the dynamics of dislocation assemblies in simple models of plastic deformation that we believe we can successfully address with the conceptual and technical tools provided by Statistical Mechanics. In particular, we will analyze and discuss the yielding (or jamming) transition between stationary and moving states in these models, the intermittent and globally slow relaxations observed around this transition, the stress-strain relationships measured in the steady regime of deformation, and the strain-rate loops calculated when subject to slowly varying stress cycles.
The interactions between dislocation lines of different type together with
the dynamics constraints which tie the motion of the dislocations to their
slip planes lead to the possibility of forming metastable jammed configurations
even in the absence of any disorder in the material. However, general
dislocation assemblies will be also affected by the presence of
disorder-induced pinning forces. We will also briefly describe the main
implications of this new ingredient in some of their statistical dynamic
properties of experimental interest.
abstract: 4.d
Role of Dislocations on Nano-Scale Mechanical Properties
SUBRA SURESH,
Department of Materials Science and Engineering,
Massachusetts Institute of Technology, USA.
The role of dislocations in influencing the atomic scale and nano-scale deformation, fracture and fatigue response will be explored through a series of systematic experiments on metals and alloys, model material systems, electron microscopy and computational simulations. The presentation will address two aspects of nano-scale mechanical response: (1) deformation, damage evolution and failure at surfaces at the nanoscopic length scale, and (2) mechanisms of deformation and failure in nanostructured metals and alloys. An outcome of this exercise will be the development of strategies and guidelines for the tailoring of microstructures and surfaces for optimizing resistance to plastic yielding, contact damage, fatigue crack nucleation and subcritical fracture.