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Single crystal and polycrystalline niobium and OFE copper for SRF cavities applications : mechanical characterization at low to high strain rates and microstructural investigations

Abstract : Manufacturing of superconducting radiofrequency (SRF) cavities with high performances is paramount to increase the collision energy in new particle accelerators. The use of high-speed sheet forming techniques, such as electro-hydraulic forming, can be beneficial, but requires a detailed understanding of the mechanical properties of the materials being deformed and the consequence on their microstructure. This thesis focuses on the characterization of high-purity niobium single crystals, polycrystalline niobium sheets, and polycrystalline OFE copper sheets. The results from this study are separated in two parts. In Part I, the characterization of niobium single crystals focused on the mechanical properties in tension and compression at strain rates of 10-4 to 103 s-1 and on the microstructure (analyzed using SEM, EBSD, TEM, and nanoindentation) of the deformed specimens. The effect of crystal orientation, strain rate, and loading direction on the mechanical properties, the crystal rotation, and the dislocation substructures are presented. In Part II, the forming limit diagram (FLD) of polycrystalline niobium sheets and OFE copper sheets were measured at a quasi-static strain rate. The FLDs of those materials should provide important data for manufacturers using conventional techniques, such as deep-drawing and spinning. The second part also presents the mechanical properties of electron beam (EB) welded polycrystalline niobium sheets and OFE copper sheets deformed in tension and compression at strain rates of 10-3 to 103 s-1.
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Submitted on : Thursday, April 21, 2022 - 3:54:19 PM
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  • HAL Id : tel-03648489, version 1

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Jean-François Croteau. Single crystal and polycrystalline niobium and OFE copper for SRF cavities applications : mechanical characterization at low to high strain rates and microstructural investigations. Materials and structures in mechanics [physics.class-ph]. ENSTA Bretagne - École nationale supérieure de techniques avancées Bretagne, 2021. English. ⟨NNT : 2021ENTA0006⟩. ⟨tel-03648489⟩

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