Browsing by Author "Jorge Junior, A. M."

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    Friction-induced phase transformations and evolution of microstructure of austenitic stainless steel observed by operando synchrotron X-ray diffraction
    (Elsevier, 2022-05-22) Emurlaev, K.; Bataev, I.; Ivanov, I.; Lazurenko, D.; Burov, V.; Ruktuev, A.; Ivanov, D.; Rosenthal, M.; Burghammer, M.; Georgarakis, Konstantinos; Jorge Junior, A. M.
    A materials’ structure and its evolution due to friction play a crucial role in understanding wear and related processes. So far, structural changes caused by friction are mostly studied using ex situ destructive characterization techniques, such as microscopy of post-mortem the prepared specimen by polishing and etching techniques. In this paper, the structural changes of AISI 321 austenitic stainless steel (ASS) during frictional loading were observed by the nondestructive operando method based on synchrotron X-ray diffraction (XRD). Although the martensitic transformation in AISI 321 steel starts at ca. -187 °C, frictional loading induces γ -(ε, α′) transformation in this alloy at room or even higher temperatures. The ε-martensite formation is observed only for a relatively short time. Subsequently, a mechanically-mixed layer (MML), composed mainly of the α′ phase, forms at the sample’s surface. Using XRD peak profile analysis, we observed the accumulation of dislocations, their ordering, and/or stress field shielding before and after phase transformations. The steady-state conditions are reached after ca. 69 friction cycles manifested in reaching the threshold values of the size of the coherent scattering regions (CSRs) and dislocation density in γ and α′ phases. For a better understanding of structural evolution, the microstructure of the sample was studied by scanning electron microscopy (SEM) after the experiment. The structure of the MML, its delamination, the formation of vortices, and carbide crushing are discussed.
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    Novel micro-flat springs using the superior elastic properties of metallic glass foils
    (Elsevier, 2017-01-23) Yousfi, M. A.; Panagiotopoulos, N. T.; Jorge Junior, A. M.; Georgarakis, Konstantinos; Yavari, A. R.
    A thin metallic glass foil of 100 mg mass forming a sinusoidal arc behaves as non-conventional flat micro-spring withstanding loads 105 times higher than its load. Upon a normal load applied on the top of the arc, the foil deforms elastically leading to sinusoidal wavy patterns of higher order. The lifespan of the novel spring is higher than conventional low cycle springs and can potentially be further improved by eliminating surface and edge preparation induced defects. This unique behavior of metallic glass foils has the potential to revolutionize the field of springs and can be exploited for numerous applications.
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    A novel operando approach to analyze the structural evolution of metallic materials during friction with application of synchrotron radiation
    (Elsevier, 2020-06-30) Bataev, I. A.; Lazurenko, D. V.; Bataev, A. A.; Burov, V. G.; Ivanov, I. V.; Emurlaev, K. I.; Smirnov, A. I.; Rosenthal, M.; Burghammer, M.; Ivanov, D. A.; Georgarakis, Konstantinos; Ruktuev, A. A.; Ogneva, T. S.; Jorge Junior, A. M.
    In this study, we describe an experimental setup and a new approach for operando investigation of structural evolution of materials during wear and friction. The setup is particularly suited for testing various friction pairs, including those in which both rubbing bodies are made of metals. The developed device allows circumventing the problems related to significant scattering of X-rays produced by metals and makes it possible using “real samples” in synchrotron beamlines operating in reflection mode. To demonstrate the capabilities of the device and the proposed new approach, an iron-based massive sample was subjected to thousands of friction cycles using a cemented carbide pin. The material was probed with synchrotron X-ray radiation within a few milliseconds after leaving the friction zone. The results of the microstructural and structural analysis, as well as results obtained from diverse mathematical models, allowed us to evaluate several features, including gradual accumulation of defects, microstructural refinement, dislocation density changes, surface layer oxidation, as well as several other phenomena caused by the dry sliding friction process. Mainly, it was possible to conclude that the process of wear occurred due to the cooperative action of oxidation and plastic deformation, which began during the first cycle of frictional interaction and was manifested in increasing the dislocation density, whose type was changed gradually during testing. The number of defects quickly reached a threshold value and subsequently fluctuated around it due to periodically repeated processes of defect accumulation and stress relaxation resulting from material wear. It was also observed that friction led to the quick formation of a mechanically mixed layer, consisting of the sample material and a mixture of two types of iron oxide – hematite and magnetite. The delamination of this layer was probably the primary wear mechanism.