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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.
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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.
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Comparison of Diffusion Coefficients in Initial Stages of Mutual Diffusion in Layered Structures Carbon/Ferrite and Carbon/Austenite for Chosen Alloys
Čermák, Jiří ; Král, Lubomír
Coefficients of mutual carbon diffusion were measured at initial stages of diffusion for surface thin layer of carbon/ferrite and carbon surface layer/austenite. The aim of the study was to compare the diffusion characteristics obtained at the present strongly non-equilibrium case with those known for carbon mutual diffusion in cases where the carbon concentration does not exceed the solubility limit in the respective substrate. The second goal was to judge, whether the carbon diffusivity is determined by the structure of the substrate matrix (BCC or FCC). As examples of ferrite substrate, pure Fe and chromium steel P91 were chosen, as an example of austenite substrate, AISI316 was taken. It was obtained that – similar to nearequilibrium case – the carbon diffusion measured close to interface C/austenite is significantly lower compared to carbon diffusion near the C/ferrite. Values of carbon diffusion coefficients in cases C/Fe and C/P91 are close one to another. All carbon diffusion coefficients are much lower than those, reported for near-equilibrium conditions; they approach values measured in carbides.
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Measurement of Carbon Diffusion Coeficient - Calibration of Sputter Depth
Král, Lubomír ; Čermák, Jiří
Depth profiling mode of MiniSIMS device was applied to diffusion measurement. Calibration of crater depth was done using shearing interference microscope Zeiss Epival Interphako (ZEI) and confocal mikroskope Olympus LEXT OLS3100 with atomic force microscopy (AFM) modul. The ZEI uses one of the classical interferometric measuring methods. Generally this method leads to results with considerable experimental error. AFM is a very high-resolution type of scanning probe microscopy, with typical resolution of the order of fractions of nanometers, which is more than 1000 times better than the optical diffraction limit. However, the results taken by ZEI were in agreement with much more precise results of surface profiling achieved by AFM. It was found, at given experimental conditions (Fe-rich matrix, Ga + primary ions, 5 kV/3nA, DWT=1), that the sputter rate is some 0.3 nm per a single sputter scan of the crater area (50x50 µm). This depth calibration with SIMS technique can be applied to the study of carbon diffusion in BCC iron as an example. Special technique was developed, which avoids radio-tracer measurements with C-14. Obtained results are lower than the extrapolated values, which may be due to limited solubility of carbon in iron.
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Hydrogen Storage Capacity of Mg/Mg2Ni Eutectic Doped with Elements from Groups XIII and XIV
Čermák, Jiří ; Král, Lubomír
In the present paper, the chemical composition of Mg-8.5 at.% Ni-2.7at.%X alloys (X – elements of the 13th and 14th groups) as prospective hydrogen storage was investigated. Experimental alloys were studied in three states: (i) after stabilization anneal, (ii) after stabilization anneal and hydrogen charging, and (iii) after stabilization anneal, hydrogen charging and hydrogen desorption. It was ascertained that a ratio of concentration of X in two principal phases – Mg2NiH4(-X) and MgH2(-X) – was a decreasing function of the ionization potential of X. The stronger the preference of X for Mg2NiH4-X phase, the higher the hydrogen storage capacity of Mg-Ni-X alloy was. For X = In, the hydrogen storage capacity was slightly higher than the capacity of an un-alloyed binary eutectic Mg-Ni alloy.
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Improvement of Hydrogen Desorption Characteristics of Eutectic Mg/Mg2NiH4 and Mg2NiH4 by In addition
Čermák, Jiří ; Král, Lubomír
Mg-xNi alloys are known as perspective hydrogen storage materials [1,2]. Hydrogen storage capacity and hydrogen desorption rate of binary alloys Mg-Ni are, however, still unsatisfactory and therefore, improvements are sought, e.g., by introduction of a proper catalyst [3]. As candidate elements, transition metals [4] are often used that attract bonding valence electrons and, hence, destabilize the hydrides. However, also elements with weaker bond to H than that of Mg, or even non-hydride forming elements show a beneficial effect upon the hydrogen desorption rate [5,6]. The present investigation continues our previous work [7,8]. Here we investigate the resistance of eutectic Mg/Mg2Ni against the poisoning by residual air and possible ways how to improve the hydrogen desorption kinetics of Mg2NiH4 by doping. As dopants, elements of the 13th and the 14th group were tested that do not form stable hydrides.
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