Papers by Author: Tomáš Záležák

Abstract: A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

Abstract: This paper presents a 3D discrete dislocation dynamics (DDD) model describing dislocation processes in crystals subjected to loadings at high temperatures. Smooth dislocations are approximated by short straight segments. Every segment is acted upon by a Peach-Koehler force obtained by summing up forces from all dislocation segments and a force due to the applied stress. The model addresses interactions between individual dislocations and rigid precipitates. The model is applied to a migration of low angle tilt boundaries (LATBs) characterized by different initial dislocation density and constrained by precipitates of different sizes. The calculations showed that, for applied shear stresses σ_xz lower than a certain threshold σ_crit.(h), the LATB is inhibited by the precipitate field. For σ_xz above σ_crit.(h), the LATB passes through the precipitate field. Some combinations of σxz and h lead to a decomposition of the LATB. The LATBs thus may evolve in three distinct modes depending on the initial microstructure. The threshold stress behaviour is known from creep tests of dispersion-strengthened NiCr alloys [1]. Furthermore, the critical stresses obtained from our calculations are below Orowan stresses for corresponding particle distribution. This behaviour has been also reported in creep experiments [1].

Abstract: A 3D model is presented that addresses an evolution of flexible dislocation lines at high temperatures. The model is based on the linear theory of elasticity. A smooth dislocation line is approximated by a piecewise curve composed of short straight dislocation segments. Each dislocation segment is acted upon by a Peach-Koehler force due to a local stress field. All segment-segment interactions as well as an externally applied stress are considered. A segment mobility is proportional to the Peach-Koehler force, temperature-dependent factors control climb and glide motion of the segments. The potential of the model is demonstrated in simulations of simple high temperature processes including interactions of dislocations with secondary particles.