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The in situ analysis of the damage evolution in a metal Matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol% Al2O3 MMC after T6 heat treatment.
In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no Observation of damage is possible with radiographic absorption-based imaging methods.
Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the Initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images.
For the first time, X-ray refraction techniques are proven for the identification of void formation in Ti-6Al-4V parts produced by selective laser melting. The topology and volume fraction of pores are measured in samples produced with different laser energy density. Unique X-ray refraction methods identify different kinds of defects, characteristic to the regions below and above the Optimum laser energy density, namely unprocessed powder (unmolten powder particles, balling effect, and Fusion defects) from empty keyhole pores. Furthermore, it is possible to detect small inhomogeneities (voids or cracks) with sizes below the spatial resolution of optical microscopy and X-ray computed tomography.
Microstructure characterisation of advanced materials via 2D and 3D X-ray refraction techniques
(2018)
3D imaging techniques have an enormous potential to understand the microstructure, its evolution, and its link to mechanical, thermal, and transport properties. In this conference paper we report the use of a powerful, yet not so wide-spread, set of X-ray techniques based on refraction effects. X-ray refraction allows determining internal specific surface (surface per unit volume) in a non-destructive fashion, position and orientation sensitive, and with a nanometric detectability. We demonstrate showcases of ceramics and composite materials, where microstructural parameters could be achieved in a way unrivalled even by high-resolution techniques such as electron microscopy or computed tomography. We present in situ analysis of the damage evolution in an Al/Al2O3 metal matrix composite during tensile load and the identification of void formation (different kinds of defects, particularly unsintered powder hidden in pores, and small inhomogeneity’s like cracks) in Ti64 parts produced by selective laser melting using synchrotron X-ray refraction radiography and tomography.
The water management in proton exchange membrane fuel cells (PEMFC) is strongly influenced by the design of the gas diffusion layers (GDL). Limiting current measurements in small-scale cells operating at high stoichiometries are useful to determine the oxygen transport resistance. The oxygen transport resistance increases, once water condenses inside the GDL. In this study a new electrochemical method for voltage loss estimation of GDL induced oxygen transport losses are presented. This new method, referred to as “transient limiting current” (TLC), is compared with the literature method. TLC allows a direct estimation of oxygen transport resistance at an arbitrarily conditioned state. This study also presents a case study of liquid water visualization of a PEM fuel cell with varying GDLs types.
With the help of quasi in-situ synchrotron X-ray computed tomography and time resolved radiography measurements we investigate appearance and distribution of liquid water inside the GDLs under limiting current conditions.
Using SXRR to Probe the Nature of Discontinuities in SLM Additive Manufactured Inconel 718 Specimens
(2020)
The utilization of additive manufacturing (AM) to fabricate robust structural components relies on understanding the nature of internal anomalies or discontinuities, which can compromise the structural integrity. While some discontinuities in AM microstructures stem from similar mechanisms as observed in more traditional processes such as casting, others are unique to the AM process. Discontinuities in AM are challenging to detect, due to their submicron size and orientation dependency. Toward the goal of improving structural integrity, minimizing discontinuities in an AM build requires an understanding of the mechanisms of formation to mitigate their occurrence. This study utilizes various techniques to evaluate the shape, size, nature and distribution of discontinuities in AM Inconel 718, in a non-hot isostatic pressed (HIPed) as-built, non-HIPed and direct age, and HIPed with two step age samples. Non-destructive synchrotron radiation refraction and transmission radiography (SXRR) provides additional information beyond that obtained with destructive optical microscopy. SXRR was able to distinguish between voids, cracks and lack of melt in, due to its sensitivity to the orientation of the discontinuity.
Porosity in additively manufactured materials, such as laser powder bed fusion Ti-Al6-V4, can play an important role in their mechanical performance. Not only the total porosity but also the shape/morphology of the individual pores need to be considered. Therefore, it is necessary to determine the distributions of different defect types (especially fusing defects and keyhole pores) and their dependence on process parameters. We show that synchrotron X-ray refraction radiography allows analysis of large samples (up to several millimeters) without compromising the detectability of submicrometer defects. Correspondingly, a classification tool is introduced that is able to quantitatively distinguish defects such as keyhole pores and binding defects with a confidence level of 94 %, even when the shape cannot be discerned because of limited spatial resolution.
Ceramics and their composites are in general brittle materials because they are predominantly made up of ionic and covalent bonds that avoid dislocation motion at room temperature. However, a remarkable ductile behavior has been observed on newly developed 11 mol.% ceria-stabilized zirconia (11Ce-TZP) composite containing fine alumina (8 vol.% Al2O3) and elongated strontium hexa-aluminate (8 vol.% SrAl12O19) grains. The as-synthesized composite also has shown full resistance to Low Temperature Degradation (LTD), relatively high strength and exceptionally high Weibull modulus, allowing its use in a broader range of biomedical applications. In this study, to deepen the understanding of plastic deformation in Ce-TZP based composites that could soon be used for manufacturing dental implants, different mechanical tests were applied on the material, followed by complete microstructural characterization. Distinct from pure Ce-TZP material or other zirconia-based ceramics developed in the past, the material here studied can be permanently strained without affecting the Young modulus, indicating that the ductile response of tested samples cannot be associated to damage occurrence. This ductility is related to the stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation, analogue to Transformation-Induced Plasticity (TRIP) steels, where retained austenite is transformed to martensite. The aim of this study is to corroborate if the observed plasticity can be associated exclusively to the zirconia t-m phase transformation, or also to microcraking induced by the transformation. The t-m transformed-zones produced after bending and biaxial tests were examined by X-ray refraction and SEM/TEM coupled with Raman. The results revealed that the observed elastic-plastic behavior occurs without extensive microcracking, confirming a purely elastic-plastic behavior driven by the phase transformation (absence of damage).
While structural ceramics usually display a brittle mechanical behavior, their composites may show nonlinearities, mostly due to microcracking. Herein, the stiffness evolution of a sandwich-like laminate of an Al2O3 15%vol. ZrO2 matrix reinforced with Nextel 610 fibers is studied as a function of number of cycles N in tension. The stiffness of the composite degrades with increasing N, indicating microcracking. However, synchrotron X-ray refraction radiography shows that the internal specific surface of such cracks varies differently. A modeling strategy is developed for the calculation of the equivalent stiffness of mixtures (first the matrix and then the sandwich), based on the Voigt and Reuß schemes. The Bruno–Kachanov model is then used to estimate the initial microcrack density in the matrix (due to the thermal expansion mismatch) and the amount of microcracking increase upon cyclic loading. The stiffness in the composite degrades dramatically already after 20 000 cycles but then remains nearly constant. The combination of mechanical testing, quantitative imaging analysis, and modeling provides insights into the damage mechanisms acting: microcrack propagation is more active than microcrack initiation upon cyclic loading, but the second also occurs. This scenario is similar but not equal to previous results on porous and microcracked ceramics.
The morphology of the antennal hearts in the head of Phasmatodea and Embioptera was investigated with particular reference to phylogenetically relevant key taxa. The antennal circulatory organs of all examined species have the same basic construction: they consist of antennal vessels that are connected to ampullae located in the head near the antenna base. The ampullae are pulsatile due to associated muscles, but the points of attachment differ between the species studied. All examined Phasmatodea species have a Musculus (M.) interampullaris which extends between the two ampullae plus a M. ampulloaorticus that runs from the ampullae to the anterior end of the aorta; upon contraction, all these muscles dilate the lumina of both ampullae at the same time. In Embioptera, only the australembiid Metoligotoma has an M. interampullaris. All other studied webspinners instead have a M. ampullofrontalis which extends between the ampullae and the frontal region of the head capsule; these species do not have M. ampulloaorticus. Outgroup comparison indicates that an antennal heart with a M. interampullaris is the plesiomorphic character state among Embioptera and the likely ground pattern of the taxon Eukinolabia. Antennal hearts with a M. ampullofrontalis represent a derived condition that occurs among insects only in some embiopterans. These findings help to further clarify the controversially discussed internal phylogeny of webspinners by supporting the view that Australembiidae are the sister group of the remaining Embioptera.