|
|
Analysis of the surface layer of the brake disk
Švábenská, Eva ; Roupcová, Pavla ; Podstranská, Ivana ; Petr, M. ; Filip, J. ; Schneeweiss, Oldřich
Our research is focused on the changes in thin surface layer of the brake disk due to the impact of severe plastic deformation and heating by friction process. The aim of this study is to compare structure, chemical and phase compositions of the surface layer in depth up to several hundreds of nanometers in the no affected central part of brake disk and the part which was in contact with brake pads by braking. The basic information on structure and phase composition was obtained by optical and scanning electron microscopy, X-Ray Powder Diffraction and Mössbauer Spectroscopy. The thin surface layer was studied using Conversion Electron Mössbauer Spectroscopy and X-ray Photoelectron Spectroscopy. We explain the observed changes with the knowledge on effects of plastic deformation and/or diffusion of alloying elements in the material of the brake disk.
|
|
|
Compressive creep testing of MoSi2-SiC nanocomposites
Luptáková, Natália ; Ballóková, B. ; Dymáček, Petr
The aim of this study is to investigate the creep resistance of molybdenum disilicide (MoSi2-SiC) based composites with different types of embedded particles. The materials were prepared via powder metallurgy using high temperature controlled reaction sintering (CRS). The creep experiments were performed in uniaxial compression at constant stress in the temperature range from 1273 K (1000 °C) to 1473 K (1200 °C) for applied stress from 50 to 100 MPa. Creep was tested by stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. The applied stress dependences of the creep rate at different temperatures were analyzed in terms of stress exponent (n) and activation energy (Q). Possible rate-controlling mechanisms were suggested.
|
|
|
The Gurson-Tvergaard-Needleman model calibration from sub-size test specimens by size corresponding to the half of Charpy specimen
Stratil, Luděk ; Šiška, Filip ; Hadraba, Hynek ; Dlouhý, Ivo
The paper deals with a utilization of miniaturized tensile specimens for purposes of local approach application from limited amount of test material. The motivation for application of those miniaturized specimens is the utilization of volume of test material corresponding to the half of Charpy specimen. The tensile testing of miniaturized specimens was conducted at room temperature in ductile regime of the Eurofer97 steel followed by detailed fractography and quantitative analysis of micromechanism of damage. The parameters of Gurson-Tvergaard-Needleman micromechanical model of ductile fracture were calibrated. Except for the slight differences in strength and deformation characteristics no other differences between two sizes of tensile specimens were found. The calibrated sets of parameters of GTN model using the same element size were very similar. The application of different calibrated sets of GTN model parameters for simulation of damage in cracked specimen (R-curve) is sensitive to the element size and needs its calibration.
|
|
| |
|
| |
|
|
Applicability of small punch test to determine the material mechanical properties at room and high temperatures
Dymáček, Petr ; Ječmínka, M.
The paper is focused on testing of mechanical properties by small punch test (SPT) and comparison of these properties with mechanical properties obtained by conventional tensile tests of steels P92 and AISI 316L. The yield stress and ultimate tensile stress obtained by SPT at constant deflection rate (SPT-CDR) were evaluated. Values of initial stress and residual stress were evaluated from relaxation small punch test (SPT-R). Mechanical properties obtained by SPT-CDR and SPT-R are compared with mechanical properties obtained by conventional tensile test and relaxation tensile test, respectively. Modified empirical relationships for an assessment of tensile test properties from quantities determined by small punch test are proposed. Good agreement of results of SPT-R and relaxation tensile test is shown by their comparison.
|
|
| |
|
|
CARBON DIFFUSION IN CARBON-SUPERSATURATED 9CR-1MO STEEL: ANOMALOUS TEMPERATURE DEPENDENCE OF CARBON DIFFUSIVITY
Čermák, Jiří ; Král, Lubomír
Carbon diffusion was studied in temperature interval 573 - 1073 K in carbon-supersaturated surface layer of 9Cr–1Mo steel P91 and in model Fe-15Cr binary alloy. Extremely low carbon diffusion coefficient D (by 3 orders of magnitude lower than the value D eq in material with equilibrium carbon concentration) was observed in carburized surface region of Fe-15Cr. Similar results were obtained for P91 above the temperature T n ~ 800 K. However, below T n , the values of D measured in P91 increased and approached the value of D eq . It seems that the non-Arrhenius behaviour of P91 steel around T n may be ascribed to the α → α + α’ phase decomposition. Possible effect of nitrogen upon the low-temperature increase in D in P91 steel was excluded.
|
|
|
NANOSTRUCTURE CHARACTERIZATION OF IN738LC SUPERALLOY FATIGUED AT HIGH TEMPERATURE
Petrenec, M. ; Strunz, Pavel ; Gasser, U. ; Heczko, Milan ; Zálešák, J. ; Polák, Jaroslav
The nanostructure of Inconel 738LC Ni-superalloy strengthened by trimodal γ’ precipitates distribution was investigated after Low Cycle Fatigue (LCF) loading at temperature 700°C. Different microscopic techniques as Scanning Electron Microscope (SEM) equipped with STEM detector, transmission Kikuchi diffraction in the SEM, transmission electron microscope (TEM) in the bright field mode and high resolution transmission electron microscopes in STEM mode were used for the characterization of nanostructure. The characteristic morphology of γ’ precipitates was examined by ex-situ and in-situ Small Angle Neutron Scattering (SANS) at high temperatures. All microscopic techniques indicate that the morphology of γ’ precipitates distributed in the γ matrix as received state corresponds to two types, i.e. large cuboid-like precipitates with the size around 670 nm, and the spherical precipitates with the diameter 52 nm. After the LCF tests at temperature 700°C, the ex-situ SANS measurement yielded additional scattering intensities coming from another small γ’ precipitates with estimated size up to 10 nm.
|
|
|
Initial Stage of Mutual Diffusion of Carbide in Iron
Čermák, Jiří ; Král, Lubomír
Initial stage of mutual diffusion close to carbon/iron interface was investigated in the present work. SIMS technique of concentration profiles enabled to study the carbon redistribution in the very early stages, when the diffusion process occurs – at least partly – in regions, where the carbon concentration exceeds the carbon concentration limit. It was observed that carbon diffusion coefficients are much lower under such conditions, than in Fe matrix with equilibrium carbon concentration. The measured carbon diffusion coefficients approached values reported for carbon diffusion in carbides.
|
guest ::
