Papers by Author: Milan Svoboda

Abstract: The creep behavior of high purity aluminum and copper, Al-0.2wt.%Sc and Cu-0.2wt.%Zr alloys was examined after processing by equal-channel angular pressing (ECAP) with an emphasis on the link between microstructure and creep. The microstructure was revealed by electron backscatter diffraction (EBSD) and analyzed by stereological methods. Representative microstructural parameters were obtained using orientation imaging microscopy and EBSD on the relationship between creep behavior and microstructure.

Abstract: The applicability of the Monkman-Grant relationship was analyzed and validated for ultrafine-grained metallic materials under investigation. A special attention has been given to the creep damage tolerance factor which is defined as the ratio of the strain to fracture to the Monkman-Grant ductility and which describes the coupling between creep deformation and damage based on continuum creep damage approach. It was found, that ultrafine-grained materials generally obey the Monkman-Grant relationship, however, the relationship is especially suitable for materials exhibiting short secondary creep and long tertiary creep stages when dislocation-controlled creep is dominant.

Abstract: The microstructure characterization of Al73Mn23Pd4 and Al73Mn21Pd6 alloys was done after annealing at 900°C for 312 h and subsequent water quenching, as well as after thermal cycling. DTA and EDX/WDX/SEM techniques were used in the investigation. It was found out that the alloys consist of the single ternary T-phase after annealing and water quenching. The DTA experiment confirmed the stability of this phase also at lower temperatures. After DTA, the alloys exhibited double-phase microstructure consisting of the ternary T-phase and probably the icosahedral I-phase. It was proved an incongruent transformation of the ternary T-phase into the liquid and vice versa.

Abstract: A dispersion-strengthened Cu-0.2 wt.% Zr alloy was subjected to equal-channel angular pressing (ECAP) at room temperature for up to 12 passes through route BC using a die having a channel angle of 90°. The microstructural investigations were performed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Tensile creep tests were conducted at temperature 673 K and at the applied stress in the range from 80 to 180 MPa. The migration of boundaries and subsequent grain growth were restricted by Cu9Zr2 precipitates. The study was performed in order to evaluate the effects of severe plastic deformation and precipitation on creep behaviour and microstructure of the pressed alloy. It was found that creep behaviour is strongly dependent on number of ECAP passes. The pressed alloy after up to 4 ECAP passes exhibited a considerable improvement in creep properties in comparison with the unpressed alloy.

Abstract: Experiments were conducted on extremely coarse-grained pure copper to evaluate the effect of equal-channel angular pressing (ECAP) on microstructure evolution in the as-pressed state and after creep exposure using various stereological methods. The microstructure formed by severe plastic deformation is an unusual structure which can be hardly characterized only by the mean grain size especially after low number of ECAP passes. The purpose of this paper is a detailed examination of (sub)boundaries and grain boundaries in the microstructures of the pressed material. The inhomogeneity of deformed microstructures is also evaluated. The detailed description of ECAP microstructures should contribute to the better understanding of mechanical properties of the pressed materials.

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: Significant developments have been made in recent years in the description of microstructure evolution and its effects on the creep behaviour in advanced 9-12%Cr steels. However, data available for assessing the predictability of the creep behaviour are somewhat scarce since creep testing has generally been for constant temperature and load conditions. The present investigation was conducted on three advanced 9-12%Cr martensitic/ferritic steels (P91, P92 and E911) in an effort to obtain more complete description and understanding of the role of degradation processes in high temperature creep during intermittent heating. A comparison between the creep characteristics of non-steady and monotonously loaded creep specimens has revealed no significant deterioration of the creep strength and fracture resistance of the steels P91, P92 and E911 under non-steady loading in power-law (dislocation) creep. The final part of this paper deals with detailed experimental microstructural and fractographic investigations of crept specimens to explain the observed creep behaviour. Special attention is paid to the thermodynamic calculations using the software package Thermo-Calc, that have been used to predict precipitation reactions during intermittent heating of 9-12% chromium steels. The results of the thermodynamic calculations are in a good agreement with experimental data.

Abstract: In the temperature range 600–1000 °C, the effect of material purity on self-diffusion along grain boundaries has been studied in both the pure (Puratronic 99.9945%) and the technical (99.5%) nickel. The penetration profiles were measured by the serial sectioning method using the 63Ni radiotracer. The extensive electron backscatter diffraction (EBSD) analysis was performed on the same samples in order to reveal possible differences in microstructure induced by the impurity content. The obtained microstructure characteristics were further interpreted in terms of the coincidence site lattice (CSL) model.

Abstract: The grain structure of pure aluminium processed by equal channel angular pressing (ECAP) was examined electron backscattered diffraction (EBSD) in the as pressed state and after heating preceding the creep deformation using various stereological methods. Area intensities of grain and subgrain boundaries, length intensities of triple grain and subgrain junctions, structural homogeneity and its thermal stability are strictly dependent on the number of passes.

Abstract: This paper deals with an experimental investigation of the effect of various microdefects induced by equal-channel angular pressing (ECAP) on mechanical and creep properties of ultrafinegrained pure aluminium, an Al-0.2%Sc alloy and copper.