Papers by Author: Antonín Dlouhý

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.

Abstract: The present study focuses on vacuum induction melting and investment casting of neargamma TiAl intermetallic alloys. The attention is mainly given to a cost-effective melting process in which a primary alloy ingot is re-melted in a ceramic crucible and cast into a ceramic shell mould. Two types of crucibles (based on Al2O3 and Y2O3) are considered. The most detrimental reactions that govern the contamination of the molten alloy with ceramic particles were determined. Results suggest that the crucible wall attack can be considerably limited by using either the Y2O3 (with no SiO2-type binder) or Al2O3 crucibles with a suitable coating. After pouring, a mechanical interaction associated with different thermal expansions of TiAl casts and ceramic shell moulds can result in serious product damage. A simple 1D-1D model of the cooling process was formulated and the heat flow as well as stress states in the cast-mould system were numerically solved. Process parameters (melt superheat, initial mould temperature, cooling kinetics and mould composition) were optimized in order to reduce the stress in the casts. The optimized parameters delimited a processing window in which complex-shaped TiAl castings like turbocharger wheels can be fabricated.

Abstract: The present study investigates stability and motion of low angle dislocation boundaries in an array of precipitates. The model considers discrete dislocations and precipitates that are treated as impenetrable particles. Peach-Koehler forces, which originate due to the combined effect of dislocation-dislocation interactions and the applied stress, act the individual dislocations on. Both, the dislocation glide and the dislocation climb at elevated temperatures are taken into account. Results of the numerical study suggest that a critical applied shear stress (CASS) always exists which separates stable and unstable low angle boundary configurations. Varying particle size, interparticle spacing and density of dislocations in the boundary cause changes of the CASS that are systematically investigated. It is shown that the CASSs can considerably differ from the standard Orowan stress controlling the equilibrium of an isolated dislocation in a given microstructure. This result underlines the importance of long-range dislocation interactions that influence the high temperature strength of the precipitation-hardened alloys.