Papers by Author: A. Kroupa

Abstract: In this paper we will try to further clarify the creep-strength degradation of selected advanced creep resistant steels. In order to accelerate some microstructural changes and thus to simulate degradation processes in long-term service, isothermal ageing at 650°C for 10000 h was applied to P91, P92 and P23 steels in their as-received states. The accelerated tensile creep tests were performed at temperature 600°C in argon atmosphere on all steels in the as-received state and after long-term isothermal ageing, in an effort to obtain a more complete description of the role of microstructural stability in high temperature creep of these steels. Creep tests were followed by microstructural investigations by means of both transmission and scanning electron microscopy and by the thermodynamic calculations. It is suggested that microstructural instability is the main detrimental process in the long-term degradation of the creep rupture strength of these steels.

Abstract: Ab initio electronic structure theory has achieved considerable reliability concerning predictions of physical and chemical properties and phenomena. It provides understanding of matter at the atomic and electronic scale with an unprecedented level of details and accuracy. In the present contribution, the electronic structure theory and state-of-the-art ab initio calculation methods in solids are briefly reviewed and the application of the calculated total energy differences between various phases (lattice stabilities) is illustrated on construction of phase diagrams by the CALPHAD (CALculation of PHAse Diagrams) method in systems containing phases with complex structures, as e.g. Laves phases or sigma phase. Particular examples include description of the Laves phases in the Cr-Nb, Cr-Ta and Cr-Zr systems, sigma-phase in the Fe-Cr system and prediction of the phase composition of ternary Fe-Cr-Mo system and super-austenitic steels. It is shown that the utilization of ab initio results introduces a solid basis of the energetics of systems with complex phases, allows to avoid unreliable estimates and extrapolations of Gibbs energies and brings more physics into the CALPHAD method.

Abstract: The methods for the modeling of thermodynamic properties of multicomponent systems are described here. The rules for the creation of a consistent database for multicomponent systems are described generally and documented on the Thermodynamic Database for Lead-free Solders developed in the scope of COST 531 Action. The reassessment of the Sb-Sn system is shown as an example, illustrating the application of consistency rules.