SRF2017 - List of Authors (Kang, D.)

Paper	Title	Page
THPB025	A Crystal Plasticity Study on Influence of Dislocation Mean Free Path on Stage II Hardening in Nb Single Crystals	783
	T. Maiti, A. Chakrabarty, P. Eisenlohr MSU, East Lansing, USA T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA
	Funding: Financial support from the Department of Energy through grant DE-SC0009962 is gratefully acknowledged. This work was supported in part by MSU through computational resources provided by the ICER. Constitutive models based on thermally-activated stress-assisted dislocation kinetics have been successful in predicting deformation behavior of crystalline materials, particularly in face-centered cubic (fcc) metals. In body-centered cubic (bcc) metals, success has been more or less limited, owing to ill-defined nature of slip planes and non-planar spreading of 1/2\hkl<111> screw dislocation cores. As a direct consequence of this, bcc metals show a strong dependence of flow stress on temperature and strain rate, and violation of Schmid law. We present high-resolution full-field crystal plasticity simulations of single crystal Niobium under tensile loading with an emphasis on multi-stage hardening, orientation dependence, and non-Schmid behavior. A dislocation density-based constitutive model with storage and recovery rates derived from Discrete Dislocation Dynamics is used to model strain hardening in stage II. The influence of dislocation mean free path and initial dislocation content on stage II hardening is simulated and compared with in-situ tensile experiments.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB025
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THPB027	Characterization of Microstructural Defects in SRF Cavity Niobium using Electron Channeling Contrast Imaging	792
	M. Wang, T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: Research supported by DOE/OHEP contract DE-SC0009962 Although the quality factor of niobium cavities has improved, performance variability arises from microstructural defects such as dislocations and grain boundaries that can trap magnetic flux, block heat transfer, and perturb superconducting currents. Microstructural defect evolution is compared in four samples extracted from a 2.8 mm thick large-grain niobium slice, with tensile axes chosen to generate desired dislocation structures during deformation. The four samples are 1) as-extracted, 2) extracted and annealed, 3) extracted and then deformed to 40% strain, and 4) extracted, annealed at 800 °C 2 hours, and deformed to 40% strain. Electron Channeling Contrast Imaging (ECCI) was performed on all samples to characterize initial dislocation density, dislocation structure evolution due to annealing and deformation, and related to the mechanical behavior observed in stress-strain curves. The orientation evolution and geometrically necessary dislocation (GND) density were characterized with electron backscattered diffraction (EBSD) maps. Fundamental understanding of dislocation evolution in niobium is necessary to develop models for computational cavity design.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB027
Export •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

A Crystal Plasticity Study on Influence of Dislocation Mean Free Path on Stage II Hardening in Nb Single Crystals

783

T. Maiti, A. Chakrabarty, P. Eisenlohr
MSU, East Lansing, USA
T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA

Funding: Financial support from the Department of Energy through grant DE-SC0009962 is gratefully acknowledged. This work was supported in part by MSU through computational resources provided by the ICER.
Constitutive models based on thermally-activated stress-assisted dislocation kinetics have been successful in predicting deformation behavior of crystalline materials, particularly in face-centered cubic (fcc) metals. In body-centered cubic (bcc) metals, success has been more or less limited, owing to ill-defined nature of slip planes and non-planar spreading of 1/2\hkl<111> screw dislocation cores. As a direct consequence of this, bcc metals show a strong dependence of flow stress on temperature and strain rate, and violation of Schmid law. We present high-resolution full-field crystal plasticity simulations of single crystal Niobium under tensile loading with an emphasis on multi-stage hardening, orientation dependence, and non-Schmid behavior. A dislocation density-based constitutive model with storage and recovery rates derived from Discrete Dislocation Dynamics is used to model strain hardening in stage II. The influence of dislocation mean free path and initial dislocation content on stage II hardening is simulated and compared with in-situ tensile experiments.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB025

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

THPB027

Characterization of Microstructural Defects in SRF Cavity Niobium using Electron Channeling Contrast Imaging

792

M. Wang, T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: Research supported by DOE/OHEP contract DE-SC0009962
Although the quality factor of niobium cavities has improved, performance variability arises from microstructural defects such as dislocations and grain boundaries that can trap magnetic flux, block heat transfer, and perturb superconducting currents. Microstructural defect evolution is compared in four samples extracted from a 2.8 mm thick large-grain niobium slice, with tensile axes chosen to generate desired dislocation structures during deformation. The four samples are 1) as-extracted, 2) extracted and annealed, 3) extracted and then deformed to 40% strain, and 4) extracted, annealed at 800 °C 2 hours, and deformed to 40% strain. Electron Channeling Contrast Imaging (ECCI) was performed on all samples to characterize initial dislocation density, dislocation structure evolution due to annealing and deformation, and related to the mechanical behavior observed in stress-strain curves. The orientation evolution and geometrically necessary dislocation (GND) density were characterized with electron backscattered diffraction (EBSD) maps. Fundamental understanding of dislocation evolution in niobium is necessary to develop models for computational cavity design.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB027

Export •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)