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Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications

Abstract : An investigation of the mechanical properties of high-purity niobium single crystals is presented. Specimens were cut with different crystallographic orientations from a large grain niobium disk and uniaxial tensile tests were conducted at strain rates between 10-4 and 103 s-1. The logarithmic strain rate sensitivity for crystals oriented close to the center of a tensile axis inverse pole figure (IPF) is ~0.14 for all strain rates. The strain at failure (ranging from 0.4 to 0.9) is very sensitive to crystal orientation and maximal at ~10-2 s-1 for crystals oriented close to the center of an IPF. The high anisotropy observed at quasi-static strain rates decreased with increasing strain rate. The activation of multiple slip systems in the dynamic tests could account for this reduction in anisotropy. A transition from strain hardening to softening in the plastic domain was observed at strain rates greater than approximately 6 × 10-2 s-1 for crystals oriented close to the center of a tensile axis IPF. Shear bands were observed in specimens with orientations having similarly high Schmid factors on both {110}and {112}slip families, and they are correlated with reduced ductility. Crystal rotations at fracture are compared for the different orientations using scanning electron microscopy images and EBSD orientation maps. A rotation toward the terminal stable [101] orientation was measured for the majority of specimens (with tensile axes more than ~17° from the [001] direction) at strain rates between 1.28 × 10-2 and 1000 s-1.
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Submitted on : Friday, October 2, 2020 - 4:27:32 PM
Last modification on : Friday, October 9, 2020 - 5:58:35 PM

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Jean-François Croteau, Eureka Pai Kulyadi, Chaitanya Kale, Derek Siu, Di Kang, et al.. Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications. Materials Science and Engineering: A, Elsevier, 2020, 797, pp.140258-1 - 140258-15. ⟨10.1016/j.msea.2020.140258⟩. ⟨hal-02956353⟩

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