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

J.-F Croteau,E Pai Kulyadi,C Kale,D Siu,D Kang,A.T Perez Fontenla,E García-Tabarés Valdivieso,T.R Bieler,P Eisenlohr,K.N Solanki,D Balint,P.A Hooper,S Atieh,N Jacques,E Cantergiani

doi:10.1016/j.msea.2020.140258

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.

Highlights

A minimum of two tests were performed for each orientation, but only one curve is presented for orientations that were highly repeatable
Since plasticity in niobium single crystals is related to the thermal activation of disloca tions gliding on slip planes, the anisotropy results from the activation and interaction of different slip systems
Stress anisotropy was more significant at quasi-static rates, but nearly absent at dynamic strain rates

Summary

Introduction

The fabrication of bulk niobium superconducting radiofrequency (SRF) cavities with improved performance and reduced cost is para mount for enabling future particle accelerators with higher collision energies. The use of large grain niobium, with grains on the order of tens of centimeters instead of polycrystalline niobium blanks with ~50 μm grain size, can reduce the production cost of particle accelerators [1]. Large grain niobium cavities have been fabricated by deep drawing at Jefferson Laboratory [2,3], Deutsches Elektronen-Synchrotron (DESY). Forming of large grain disks with anisotropic mechanical properties results in non-uniform deformation and forming defects such as earing at the equator and iris, but high accelerating gradients were still measured in the SRF cavities [1,2,7]. A quantitative understanding of the mechanical properties of niobium single crystals in different orientations is necessary to predict forming

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