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11th International Conference on Through-life Engineering Services - TESConf2022 8-9 November 2022, Cranfield UK The undulatory behaviour is a unique type of mechanical response that was recently observed for metallic glass foils in geometric confinement. It is manifested when normal load is applied on the top of an arc-shaped thin foil of metallic glass; the foil then deforms elastically and its shape changes by progressively increasing the number of formed sinusoidal arcs. This behaviour results from a combination of successive elastic bending and buckling events and can be utilized for developing novel types of non-linear springs. In this work, the undulatory behaviour of a Ni-Fe-Si-B-Mo metallic glass foil has been systematically studied and compared with that of the previously reported Fe-Cr-Si-B foil. The results indicate that the alloy composition and the foil thickness can significantly affect the load required for the formation of the harmonic undulations. The initial geometry of the formed sinusoidal arc including its amplitude and boundary length, can also be used to tune the load and displacement response of the foils. Upon unloading, the foil returns to its initial shape, as long as the loading remains in the elastic deformation range of the metallic glass. The findings suggest that the undulatory behaviour of thin metallic glass foils can be potentially exploited for a wide range of engineering applications including micro-springs, sensors, actuators, and shock absorbers. DMG Mori

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.