DECHEMA

About: DECHEMA is a nonprofit organization based out in Frankfurt am Main, Germany. It is known for research contribution in the topics: Corrosion & Oxide. The organization has 756 authors who have published 1307 publications receiving 25693 citations.

Strengthened Cr-Si-base alloys for high temperature applications

Abstract: The microstructural development of Cr Si alloys with Cr ≥ 89 at.% has been studied. As well as silicon, up to 2 at.% germanium, molybdenum, and platinum were used as alloying elements. All alloys consist of only two phases, Crss and A15. The phase fraction of primary A15 precipitates present in the arc melted condition and fine secondary A15 precipitates formed after a heat treatment (100 h at 1200 °C) were determined. EPMA and SEM measurements show that the alloying elements partition in different ways: Molybdenum is homogeneously dissolved in both phases, while platinum, germanium, and silicon predominantly act as A15 phase formers. Additionally, molybdenum refines the A15 precipitates, germanium increases the amount of secondary precipitates, and platinum coarsens the microstructure. The lattice parameters of both phases were determined using XRD. The results were found to be in accordance to the elemental partitioning behavior of the constituent phases and can be correlated to the respective covalent atomic radii of the respective alloying element. Microhardness measurements confirmed the alloy's ability of precipitation hardening. Using nanohardness measurements the A15 phase was found to be around 18GPa harder compared to Crss offering a way to design mechanical properties depending on alloying element additions, A15 phase fraction, and distribution.

The Role that Interacting Failure Mechanisms Have on the Lifetime of APS-TBC under Oxidizing Conditions

Abstract: Bond coat (BC) oxidation as well as bond coat depletion of Al are still believed to be the major degradation mechanisms with respect to the lifetime of thermal barrier coating (TBC) systems. For more than one decade the influence of oxidation behaviour on the service life of components was described by simply the thermally grown oxide (TGO) thickness. However, it has recently been shown that the adherence of the top coat is also an important parameter for the durability of the system. Fracture mechanics in conjunction with finite element modelling (FEM) has also provided valuable insight with regards to the mechanical breakdown and failure of the thermal barrier coating systems. In this study the top coat lifetime is described as being limited by both “mechanical” failure of the top coat and bond coat depletion of Al. The empirical results are introduced by considering three spallation cases, namely, thermal fatigue failure, thermal ageing failure and Al depletion failure. Top coat and TGO degradation due to thermal fatigue is characterized by in-situ acoustic emission analysis, where the data were acquired during cyclic oxidation testing. Degradation due to thermal aging and TGO growth is characterized by four-point bend testing of isothermally preoxidized samples. This four-point bend test also uses acoustic emission analysis for the purpose of determining the critical strains of the top coat and TGO, where the critical strains for the onset of delamination and the onset of through cracking are found. Bond coat depletion of Al is modelled by considering the diffusion of Al into both the TGO and substrate. The diffusion model results are compared to the measured TGO growth kinetics and the Al concentration profile measured with an electron beam micro probe. Top coat spallation occurs when the Al content within the bond coat reaches a critical value. The three mechanisms and the measured results are embedded into a lifetime prediction model. This model is applied to both isothermal and cyclic oxidation samples. The results are compared to measured life times up to 5000 hr in the temperature range of 950°C through 1150°C. Introduction Spallation Cases Here the spallation of the APS-TBC top coat is classified into three cases, namely, Case 1 thermal fatigue failure, Case 2 thermal ageing failure and Case 3 Al depletion failure, which are depicted in Fig. 1. Cases 1 and 2 are considered to be, more or less, “mechanical” failures, where case 3 is considered to be a type of “chemical” failure. This classification produces a conceptual framework that separates “mechanical” and “chemical” spallation mechanisms, which can in turn be modeled separately when the appropriate measurement data is available. In all three cases bond coat oxidation and TGO growth are considered part of the top coat spallation mechanisms. The three mechanisms and the measured results are embedded into a lifetime prediction model. This model is applied to both isothermal and cyclic oxidation samples, where the preliminary modelling equation for “mechanical” failure can be found in reference [1]. The preliminary modelling equations for Al depletion are provided below. Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 729-736 doi:10.4028/www.scientific.net/MSF.461-464.729 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-11/03/20,15:01:57) Case 1 Thermal Fatigue Failure Case 2 Thermal Ageing Failure Case 3 Al Depletion Failure top coat top coat TGO TGO TGO top coat Fig. 1. Spallation Cases: Case 1) Thermal Fatigue Failure; Case 2) Thermal Ageing Failure and Case 3) Al Depletion Failure. Case 1 Thermal Fatigue Failure (cyclic oxidation). The sample on the left of Fig. 1 failed during thermal cyclic oxidation. The oxidation test was at 1100°C and during each cycle the sample experienced one hour at temperature, also during each cycle the sample was cooled to room temperature and remained at room temperature for at least 30 minutes. This type of cycle will produce the maximum thermal expansion mismatch strains (ε=∆α∆T). The macroscopic delamination crack seen in the figure, which plays a critical role with regard to top coat spallation of the smaller oxidation sample, lies mostly within the yttria stabilised zirconia (YSZ) top coat with some of the cracking extending into the TGO. The sample spalled after 277cy, which is 277hr at temperature. Here it is proposed that this sample failed due to a thermal cyclic fatigue mechanism, where the details will be made more clear below. Additionally, bear in mind that the TGO thickness is about 6.7μm and is mostly Al2O3, which indicates that the bond coat is not critically depleted of Al. Case 2 Thermal Ageing Failure (isothermal oxidation). The sample in the middle of Fig. 1 failed after isothermal oxidation. It was tested at 1100°C for 1000hr and experienced only one cooling step. The macroscopic delamination crack seen in this photo again passes through the YSZ top coat with some of the cracking extending into the TGO. In contrast to the case 1 sample, the case 2 sample lived three times longer. The life time of cycled TBC ́s in many cases is shorter than that of isothermally exposed specimens, which is viewed as thermal cycling produces a thermal cyclic fatigue mechanism that can dominate the life time of the sample. The degradation of the top coat under isothermal condition is likely to be caused by sintering of the YSZ and TGO growth. The TGO in the case 2 sample is about 9.3μm, which is about 38% thicker than the case 1 sample. The composition of the TGO is again mostly Al2O3, which is again an indication that the bond coat is not critically depleted of Al. Case 3 Al Depletion Failure (isothermal or cyclic oxidation). The samples of case 1 and case 2 show the crack path passing through the top coat, consequently, the mechanical properties of the YSZ and how the mechanical reliability of the top coat degrades with the respective thermal exposure are suspected to be of crucial importance. The sample on the right side of Fig. 1 (spallation case 3) is showing something completely different when compared to cases 1 and 2. The case 3 sample was isothermally oxidized at 1050°C for 5000hr, where it spalled after cooling. The crack face passes only through the TGO. The TGO is >30μm thick and is composed of Al2O3 with a large amount of Ni (Co,Cr)-spinels. The formation of the spinels and the TGO thickness, which should be 13.1μm thick when calculated from the Al2O3 oxidation kinetics data, is a strong indication that the bond coat is depleted of Al. Consequently, case 3 is classified as Al depletion failure. 730 High Temperature Corrosion and Protection of Materials 6

Interplay between parameter variation and oxide structure of a modified PAA process

Abstract: In the present work, the robustness of an anodizing process was tested. Important anodizing parameters such as the anodizing potential, the dwell time at the anodizing potential, and the bath temperature were varied. The influence of the parameter variation on the microstructure of the oxide layers was investigated by SEM. It could be shown that the pore diameter as well as the layer thickness reacts very sensitively on the alternating parameters. The barrier layer thickness clearly depends on the anodizing voltage but is independent on the dwell time. Copyright © 2013 John Wiley & Sons, Ltd.

Detection of the microbial activity of aerobic heterotrophic, anoxic heterotrophic and aerobic autotrophic activated sludge organisms with an electrochemical sensor

Abstract: The microbial activity of aerobic heterotrophic, anoxic heterotrophic and aerobic autotrophic microorganisms in biological wastewater treatment was determined by means of an electrochemical bioactivity sensor. The development of the sensor resulted in a system which can determine the microbial activities that are relevant for effective wastewater treatment. The signals of the sensor system are proportional to the substrate degradation and it can show inhibiting effects on the biomass. The most important advantages of the system are: it is independent of O2 consumption, the three most important types of metabolic activities in wastewater technology can be measured with one sensor, furthermore the measurement is suitable for automation and it is on-line. The result is a potential for the optimization of processes based on microbial activity.

Die Oxydation von Eisen bei hohen temperaturen in Inertgasen mit kleinen Sauerstoffgehalten

Abstract: Eisen oxydiert bei Temperaturen > etwa 800°C und bei linearen Stromungsgeschwindigkeiten des Gases zwischen etwa 5 und 12 cm/sec in N2-O2-, Ar-O2- und He-O2-Gemischen mit Sauerstoffgehalten < 1% nach einem linearen Zeitgesetz. Geschwindigkeitsbestimmend ist unter diesen Bedingungen der Antransport des Sauerstoffs aus der Gasphase an die Oberflache der nur aus FeO bestehenden Zunderschicht. Die Konstante des linearen Zeitgesetzes ist bei laminarer Stromung proportional dem Sauerstoffgehalt des Gases, proportional der Quadratwurzel aus der Stromungsgeschwindigkeit des Gaseskehrt, umgekehrt proportional der Quadratwurzel aus der Viskositat des Gases und praktisch unabhangig von der Temperatur. Einige Schlusfolgerungen fur die Praxis werden diskutiert. Oxidation of iron at high temperatures in inert gases with small oxygen contents At temperatures > about 800°C and linear flow velocities of the gas between about 5 and 12 cm/s the oxidation of iron in N2-O2, Ar-O2 and He-O2 mixtures containing < 1% oxygen follows a linear kinetic law. Under such conditions the rate controlling step is the transport of oxygen from the gas phase to the surface of the scale layer which consists exclusively of FeO. The constant of the linear kinetic law under laminar flow conditions is proportional to the oxygen content of the gas and to the square root of the gas flow velocity, and inversely proportional to the square root of the gas viscosity, while it is practically independent from temperature. Some conclusions with reference to practical applications are presented.