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Influence of atmosphere on small punch testing of P91 steel
Dymáček, Petr ; Dobeš, Ferdinand
The small punch testing (SPT) under constant force condition was performed in various atmospheres: air, hydrogen and argon. The material selected for the study was P91 steel and testing temperature 873 K. The effect of atmosphere on the test results such as time to rupture, minimum deflection rate and other parameters was measured and evaluated. The results show that there is not very strong but definite effect of atmosphere on SPT creep rupture time. The longest times to rupture were obtained in hydrogen atmosphere for whole range of tested forces. However the difference between tests in hydrogen and tests in air or in argon diminished with decreasing force and longer time to rupture.
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Comparison of Creep Behaviour and Microstructural Changes in Electrodeposited Ultrafine-grained Nickel and its Particle-reinforced Nanocomposite
Kvapilová, Marie ; Sklenička, Václav ; Kuchařová, Květa ; Vidrich, G.
On the basic of the creep experimental results the paper reports comparison creep behaviour of electrodeposited ultrafine-grained nickel and its particle-reinforced nanocomposite. The objective of this research was to further improve the knowledge of the creep behaviour of monolithic nickel and to explore the role of nano-sized SiO2 particles in the potential creep strengthening of electrodeposited Ni nanocomposite. The creep behaviour and microstructure of the pure ultrafine-grained nickel and its nanocomposite reinforced by 2 vol.% nano-sized SiO2 particles were studied at temperatures in the range from 293 to 573 K and at the tensile stresses between 100 to 800 MPa. It was found that the creep resistance of the nanocomposite might be noticeably improved compared to the monolithic nickel. Possible creep mechanisms controlling the creep rate during the creep loading were discussed. The Coble creep as creep rate controlling mechanism was founded improbable.
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Creep in NiAl-Mo Eutectics
Dudová, Marie ; Barták, Tomáš ; Kuchařová, Květa ; Dlouhý, Antonín
We report on creep in NiAl-Mo ternary eutectic and NiAl intermetallic, having respective nominal compositions Ni-45.5Al-9Mo and Ni-45.2Al (at.%). These alloys were directionally solidified in a high-temperature optical floating zone furnace. The eutectic alloy exhibited a well-aligned rod-like microstructure, consisting of NiAl matrix and 14% (by volume) continuous Mo-fibres oriented along a [001]-crystal growth direction. Cylindrical [001]-oriented specimens were loaded in compression at temperatures in a range 1073 – 1173 K. Formidable strengthening by regularly distributed fine fibres (typical diameter, 400 nm) was observed which resulted in minimum creep rates of the NiAl-Mo eutectic that were seven orders of magnitude lower than the corresponding minimum creep rates of the NiAl matrix. Preliminary microstructural investigations suggested that the strengthening effect is due to an interaction between Mo fibres and subgrain boundaries that form in the course of creep. This interaction leads to an increase of dislocation density in the vicinity of fibres and to an efficient redistribution of stresses in the microstructure.
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Compressive Creep Tests of An Aluminium Powder Composite Reinforced by Ceramic Particles
Dvořák, Jiří ; Král, Petr ; Balog, M. ; Simančík, F. ; Sklenička, Václav
The aim of this work is to study of a composite mixture of aluminium powders, to analyze of composite microstructure and mostly to describe the creep behaviour of composite in compression. The experimental material was based on Al powder matrix (Al 99%) about the particle size of less than 200 µm. This powder was mixtured with silicon carbide particles (SiC) having the size about 10 µm. The Al composite powder was prepared via conventional direct hot-extrusion (DE) at temperature 450°C. The microstructural investigations were performed using OM and SEM. It was found that DE process exhibits some breaking of carbide particles to size of range from 0.5 to 5 µm. Creep tests in compression were conducted at temperatures 423 -623 K and at applied stresses in the range from 60 to 100 MPa. Consequently, for an evaluation of controlling creep mechanisms an activation energy of creep and the stress exponent of the creep rate were determined.
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INFLUENCE OF ECAP TECHNIQUE ON CREEP BEHAVIOUR OF POWDER ALUMINIUM PROCESSED BY DIRECT EXTRUSION
Dvořák, Jiří ; Král, Petr ; Balog, M. ; Simančík, F. ; Sklenička, Václav
The Al powder was prepared by cold isostatic pressing at the pressure of 200 MPa. The granulometric powder fraction of a particle size less than 400 µm was compacted via conventional direct hot-extrusion (DE) at temperature 450°C. This material was then subjected to ECAP technique up to 4 passes. The microstructural investigations were performed using OM and SEM equipped with an EBSD unit. Creep tests in tension were conducted at temperature of 473 K and at applied stresses in the range from 30 to 60 MPa. Some creep tests in compression were performed on material just after DE. The paper presents results of study of microstructure and creep behaviour of aluminium powder before and after ECAP. It was found that ECAP method does not lead to an additional marked refinement of grain size. However, in dependence on number of ECAP passes ECAP method markedly influences creep behaviour due to development of high-angle grain boundaries during ECAP process.
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Microstructure and mechanical properties of powder aluminium prepared by severe plastic deformation
Dvořák, Jiří ; Král, Petr ; Balog, M. ; Simančík, F. ; Sklenička, Václav
This paper describes properties of Al 99.7% produced by PM. Powder green were prepared via CIP. The prepared powder about size less than 400 µm was compacted via conventional direct hot-extrusion (DE) at temperature of 450°C. Some extruded material was additionally processed by ECAP. The microstructural investigations were performed using OM and SEM equipped with EBSD unit. Examination of extruded powder compacts showed that during DE initial powder particles deformed into grains about size of 5 m. This material was further subjected to ECAP passes. The mechanical properties of this material were investigated using hardness measurements. In addition, creep tests in tension were conducted at temperature of 473 K and at the different applied stresses. The basic aim of this work is microstructure characterization and description of creep behaviour of powder aluminium prepared by DE. The further aim of this work is the evaluation of ECAP technique effect on mechanical properties of powder Al.
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Different creep behaviour and microstructure of ECAP aluminium with the identical ECAP history
Král, Petr ; Dvořák, Jiří ; Sklenička, Václav ; Svoboda, Milan
Experiments were conducted on extremely coarse - grained Al (99.99%. The subsequent extrusion passes were performed by route A (no rotation between subsequent passes) up to 2 ECAP passes. Creep tests in tension were performed on the as-pressed samples at 473 K and applied stress 15 MPa. It was found that the ultrafine - grained Al specimens with the same ECAP history showed the scatter in the creep properties. In this work the microstructure changes within grains with different crystallographic orientations and near their high - angle boundaries of coarse-grained state in the sample after different ECAP passes were studied. Mutual comparison of the results of microstructural characterization of microstructure of as-received coarse-grained state and the state after different ECAP passes were performed. It is concluded that the scatter in creep properties could be influenced by inhomogeneity of ECAP microstructure.
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Effect of different number of ECAP passes on creep in Cu-Zr alloy
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Kvapilová, Marie ; Sklenička, Václav
A coarse-grained binary Cu-0.2wt%Zr alloy was processed by equal-channel angular pressing (ECAP) in order to evaluate the effect of severe plastic deformation on the thermal stability of resulting ultrafine-grained microstructure after ECAP and on creep behaviour of the pressed alloy. Constant stress creep tests in tension were conducted at temperature 673 K and at the different applied stresses. It was found that the Cu-0.2Zr alloy after the first four ECAP passes exhibited higher creep resistance by comparison with unpressed state. However, the alloy processed by 8 ECAP passes showed faster creep rate than unpressed material under the same loading conditions. The measured value of stress exponent of the creep rate n~6 indicates possibility of an intragranular deformation mechanism as the rate-controlling process in creep of an alloy after 8 ECAP passes. Furthermore, creep behaviour of ECAPed material may be influenced by mesoscopic boundary sliding along shear bands.
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Dimensioning of thick-walled pipe in creep
Zouhar, Michal ; Hutař, Pavel ; Náhlík, Luboš ; Hlavoň, P.
The numerical simulations of the creep in the case of X10CrMoVNb91 steel were performed. Geometry of the objects studied well-fit usual parts of the pipe system. The discrepancies between numerical simulations and usual standard Design By Formula (DBF) used for the pipe system lifetime prediction were found. Therefore, accurate numerical simulations based on creep description of the material behaviour is necessary for the accurate estimation of the service conditions.
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MKP studie protlačovacích zkoušek na miniaturních discích při podmínkách konstantního zatížení
Dymáček, Petr
Comparison of results of creep small punch tests (SPT) on miniaturized discs with results of their modeling by means of finite element method (FEM) is presented. Advanced heat resistant chromium steel P91 was selected for the investigations. In the numerical model, the Norton power-law and the exponential relationships were applied. Parameters of both relationships were derived from stress dependences of minimum creep rate obtained from conventional creep tests at 873 K. The FE model included contact elements taking in account the surface friction between all contact surfaces of the test setup. Various material models from elastic&creep to elastic-plastic&creep were applied in the FE model. Obtained results of the numerical analyses were compared with the experimental results. These computational results allow us to better understand the behavior of the specimen at accelerated creep rates and comprehend some of the principally important conditions of the SPT.
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