Interfacial toughness evolution under thermal cycling by laser shock and mechanical testing of an EB-PVD coating system

One of the major challenges for coatings on superalloys is to keep adherence during aging, where damage is mostly driven by thermal cycling. On the other hand, the methodology of the evaluation of the interfacial toughness should be consistent with in service loading. Recently, the use of LAser Shock Adhesion Test (LASAT) has shown its capability for both ranking different coating solutions and evaluating the evolution of a given coating as a function of aging [1-2]. The intent of this paper is to demonstrate the ability of LASAT to reproduce damage mechanisms observed under quasi-static in plane mechanical testing and to propose a general methodology to assess interfacial toughness evolution based on LASAT measurements. The material chosen in this study is a partially Y2O3 stabilized EB-PVD zirconia layer coating deposited by Electron Beam – Physical Vapor Deposition (EB-PVD) onto a first generation Ni base superalloy. Aging has been performed using thermal cycling under laboratory air. Degradation of the coating system due to ageing is quantitatively assessed by LASAT and accompanied by different microstructural analysis methods. For LASAT, if laser flux is below a threshold, no delamination occurs. When increasing laser flux above this threshold, a systematic sequence is observed: i) delamination without buckling of the ceramic layer, ii) delamination and buckling, iii) partial cracking of the ceramic layer, and iv) spallation [1-2]. These different states are also achieved in compressive quasi-static testing and assessed by means of local strain measurement using digital image correlation technique [3]. Aging is evaluated through the evolution of both the delamination and the buckling behavior induced by the LASAT method or critical strain at ceramic spallation under compressive static load. Please click Additional Files below to see the full abstract

Crack morphology in a columnar thermal barrier coating system

For high temperature application, EB-PVD ceramic layers are commonly used as thermal barrier coating. During thermal transients, the thermal expansion mismatch between coating and substrate drives failure of the TBC mainly by interfacial cracking. Laser Shock Adhesion Test (LASAT) provides stresses at the ceramic/metal interface enabling controlled interfacial cracking [1-2]. For achieving a clear understanding of the influence of local morphology on interfacial toughness, this study aims at characterizing the 3D morphology of a crack at the interface between metal and an EB-PVD TBC having a columnar structure. Please click Additional Files below to see the full abstract

Probing deeper: Determining the 3D gamma-gamma'-microstructure in a Ni-base superalloy by FIB-SEM tomography

Ni-based superalloys consist of gamma- and gamma'-phases which control the high-temperature mechanical properties of these alloys [1]. Their microstructure is typically investigated by 2D imaging methods. Quantitative investigation of the 3D microstructure using 2D image data relies on assumptions and statistical modelling to construct a virtual 3D model [2,3]. Serial sectioning of the material in a Scanning Electron Microscope (SEM) equipped with a Focused Ion Beam (FIB) offers an approach to reconstruct the actual microstructure of a given sample. The morphology is interpolated from a FIB tomography consisting of a large and therefore statistically relevant set of serially acquired images. Preprocessing methods such as selective etching yield contrast enhanced images (Figure 1 a) but are impractical for serial imaging. Sufficient image quality in serial imaging requires the SEM to be operated with retractable detectors between electron source and sample, such as a Circular BackScatter (CBS) detector (Figure 1 b). However, FIB ablation and subsequent SEM imaging require the sample to be tilted facing the ion column, which in turn inhibits retractable detectors to be used for image capturing. In this work, a FIB-SEM setup is implemented that allows for serial sectioning of a Ni-based superalloy with a CBS detector. Here, the sample faces the electron column instead of the ion column. This setup accommodates for a detector to be inserted between electron column and sample, resulting in an angular ablation of the sample. The thus obtained images have high resolution and little noise but are distorted (Figure 1 b). Postprocessing of the image series therefore requires proper alignment and virtual tilt correction. The corrected images (Figure 1 c) are then segmented, and the gamma-gamma'-microstructure can be visualized in 3D representation (Figure 1 d). The 3D microstructural data is then evaluated for morphological features such as phase volume fractions, precipitate sizes, and shape factors. The results can be used to estimate the reliability of microstructural data obtained from 3D stereographic reconstructions from 2D images. References [1] R. C. Reed The Superalloys, Cambridge University Press, 2009 [2] A. Brahme et al. Scripta Materialia, 2006, 55, 75-80. [3] C. Zhang et al. Metallurgical and Materials Transactions A, 2004, 35(7), 1927-193

Combining 2D and 3D characterization techniques for determining effects of HIP-rejuvenation after fatigue testing of SX microstructures

A hot isostatic pressing rejuvenation heat treatment is applied to a CMSX-4 type SX superalloy after it has been subjected to a low cycle fatigue test to rupture. The evolution of microstructural defects, such as pores and cracks which are present after fatigue, has been tracked in 3D by X-ray tomography before and after rejuvenation. From the rejuvenated specimen series of metallographic cross sections were prepared and investigated by scanning electron microscopy for getting complementary 2D information at high resolution. The micrographs were stitched to a panorama which was then matched into the 3D representation of the specimen volume. By combining 3D and 2D data, statistical volume related quantities were achieved while detailed characteristics have been assigned to individual defects present in the 2D panorama micrograph. This technique is in general appropriate for length-scale bridging microstructural investigations. Results of the performed investigations concerning the rejuvenation effect on the microstructure are presented and discussed

Modified zirconium dioxide supports for copper catalysts in the steam reforming of methanol

Zur Katalyse der Methanoldampfreformierung wurden im Rahmen dieser Arbeit Cu-Katalysatoren mit unterschiedlichen Trägermaterialien untersucht. Die Gemeinsamkeit dieser Trägermaterialien war das ZrO2, das entweder im Kationen- oder im Anionengitter modifiziert wurde. Die Reaktionen über bzw. in den Proben wurden in-situ mit Röntgenbeugung, Röntgenabsorptionsspektroskopie und thermoanalytischen Methoden untersucht. Während dieser Experimente wurde die Gasphase mittels Massenspektrometrie und zum Teil auch mittels Gaschromatographie charakterisiert. Zur weiteren ex-situ Charakterisierung der Proben wurden die Transmissionselektronenmikroskopie, die Infrarotspektroskopie, die BET-Methode und die Trägergasheißextraktion verwendet. Die Cu-Katalysatoren mit dem CeO2-ZrO2-Träger, die durch Co-Fällung des entsprechenden Sols auf einem Templat hergestelllt wurden, zeigten bei geringem Cu-Gehalt die höchsten Umsätze. Jedoch wurde bei dieser Probe keine stabile Reaktion erreicht. Die Strukturuntersuchungen ergaben für diese Probe eine feinverteilte, stärker an den Träger gebundene Cu-Phase. In den Proben mit höheren Cu-Gehalten konnte eine weitere Cu-Phase identifiziert werden, die nicht so stark an den Träger gebunden war. Diese zweite Phase führte nach einigen Tagen zu einer stabilen Reaktion. Geringere Umsätze in diesen Proben konnten kleineren Cu0-Oberflächen zugeschrieben werden. Zusätzlich konnte der Einfluß von Spannungen im Cu-Gitter auf den Umsatz erkannt werden. Die Cu-Katalysatoren mit den Zirconiumoxidnitridträgern, die durch Imprägnierung des Trägermaterials hergestellt wurden, zeigten neben unterschiedlichen Umsätzen auch unterschiedliche Reaktionen in der Methanol-Wasserdampf-Atmosphäre. Die Imprägnierung mit Cu-Citrat führte durch die stärkere Verteilung der Cu-Phase und die dadurch größere Cu0-Oberfläche zu einem höheren Umsatz als die Imprägnierung mit Cu-Nitrat. In einem Vergleich zwischen zwei Cu-Katalysatoren mit stickstofffreiem und stickstoffhaltigem Träger wurde gezeigt, daß auch der Träger die auftretenden Reaktionen beeinflußt. Durch die Kalzinierung in Sauerstoff oder ebenfalls durch eine spätere Sauerstoffbehandlung bei erhöhter Temperatur wurde die Zusammensetzung des Trägermaterials verändert, d.h. es bildete sich durch den Ausbau von Stickstoff neues tetragonales ZrO2. Dieses tetragonale ZrO2 hat einen positiven Einfluß auf die stattfindende Reaktion und beeinflußt zusätzlich die Cu-Phase. Im Kupfer konnten dadurch erhöhte Spannungen und ein erhöhter Sauerstoffanteil bestimmt werden. In der hier vorgelegten Arbeit konnte geschlossen werden, daß die durch den Stickstoffeinbau induzierten Sauerstoffleerstellen in der Struktur des Trägermaterials keinen vorteilhaften Einfluß haben. Als wichtige Charakteristika stellten sich eine große, stabile Cu0-Oberfläche und die passenden Oberflächeneigenschaften des Trägermaterials heraus.Copper catalysts with different ZrO2-based supports for the steam reforming of methanol were investigated in this thesis. The ZrO2-based supports varied in the modification of their cation and anion lattice. Reactions over and in the samples were investigated by in situ and ex situ methods. X-ray diffraction, X-ray absorption spectroscopy and thermo analytical methods for in situ characterizations were combined with gas phase analysis by mass spectrometry and gas chromatography. Ex situ characterizations were performed by transmission electron microscopy, infrared spectroscopy, BET method and hot gas extraction. Copper catalysts with the CeO2-ZrO2 support were prepared by co-precipitation of the corresponding sol on a template and showed the highest conversions at low Cu content. However no stable reaction was obtained for this sample. Structural investigations revealed a dispersed Cu phase strongly interacting with the support. In the other samples with higher Cu contents another Cu phase was identified which showed weaker interactions to the support. This second phase yields stable conversion after some days. Lower conversions could be attributed to smaller Cu0 surface areas. Additionally an influence of strain in Cu lattice on the conversion was identified. Copper catalysts with zirconium oxynitride supports were prepared by impregnation and showed both different conversions and different reactions in an atmosphere of methanol and water. Impregnation by Cu citrate leads to a higher dispersion of Cu phase and therefore the larger Cu0 surface area causes a higher conversion compared to impregnation with Cu nitrate. By comparison of two copper catalysts with both nitrogen free and nitrogen containing support it was shown that the occurring reactions are also influenced by the support. Both calcination in O2 containing atmosphere and (a later) treatment with O2 containing gas at higher temperatures causes a change in the composition of the support. In both cases a removal of nitrogen was observed leading to formation of tetragonal ZrO2 as new phase. This new tetragonal ZrO2 makes a positive impact on the occurring reaction and influences the Cu phase by increasing the strain and the residual oxygen content. In this thesis it can be concluded that there is no beneficial influence of the oxygen vacancies in the structure of support material which were caused by nitrogen incorporation. A large and stable Cu0 surface and appropriate surface properties of support material were identified as important characteristics of supported Cu catalysts

Microstructures in a ternary eutectic alloy: devising metrics based on neighbourhood relationships

Ternary eutectics, where three phases form simultaneously from the melt, present an opportunity to study the fundamental science of microstructural pattern formation during the process of solidification. In this paper we investigate these phenomena, both experimentally and by phase-field simulations. The aim is to develop necessary characterisation tools which can be applied to both experimentally determined and simulated microstructures for a quantitative comparison between simulations and experiments. In SEM images of experimental cross sections of directionally solidified Ag-Al-Cu ternary eutectic alloy at least six different types of microstructures are observed. Corresponding 3D phase-field simulations for different solidification conditions and compositions allow us to span and isolate the material parameters which influence the formation of three-phase patterns. Both experimental and simulated microstructures were analysed regarding interface lengths, triple points and number of neighbours. As a result of this integrated experimental and computational effort we conclude that neighbourhood relationships as described herein, turn out to be an appropriate basis to characterise order in pattern