SRF2017 - List of Authors (Bieler, T.R.)

Paper	Title	Page
TUPB095	Modeling the Hydroforming of a Large Grain Niobium Tube With Crystal Plasticity	616
	A. Mapar MSU, East Lansing, Michigan, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA J.E. Murphy University of Nevada, Reno, Reno, Nevada, USA F. Pourboghrat Ohio State University, Columbus, Ohio, USA
	Current SRF cavities are made from fine grained polycrystalline niobium half-cells welded together. Hot spots are commonly found in the heat-affected zone, making seamless hydroformed cavities attractive. Large grain cavities usually perform as well as fine grain cavities, often having a higher Q, presumably due to fewer grain boundaries. Large grain Nb forms non-uniformly, which introduces problems in manufacturing. A model that could realistically predict the deformation response of large grain Nb could facilitate the design of large grain hydroformed tubes. To this end, a crystal plasticity model was developed and calibrated with tensile stress-strain data of Nb single crystals. A seamless large grain tube was made from rolling a fine grain sheet into a tube, welded, and heat treated to grow large grains. The heat treatment resulted in a large grain tube with a single grain orientation in the center. The tube was hydroformed until it cracked. The hydroforming process was simulated with the crystal plasticity model, which was able to predict the deformed shape of the tube, the location of the crack and other localized areas with heterogeneous strain.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB095
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THPB002	Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations	741
	P. Garg, I. Adlakha, K.N. Solanki Arizona State University, Tempe, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA
	It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB002
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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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THPB026	Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium	787
	M. Wang, T.R. Bieler Michigan State University, East Lansing, Michigan, USA S. Balachandran, P.J. Lee NHMFL, Tallahassee, Florida, USA S. Chetri, A. Polyanskii ASC, Tallahassee, Florida, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida. High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB026
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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
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THPB061	Effect Of Dislocations On the Thermal Conductivity Of Superconducting Nb	886
	P. Xu, N.T. Wright MSU, East Lansing, Michigan, USA T.R. Bieler Michigan State University, East Lansing, Michigan, USA
	Funding: This work is funded by DOE and OHEP through grant number DE-FOA-0001438. The thermal conductivity of Niobium (Nb) often experiences a local maximum (a phonon peak) at a temperature between 1.8 and 3 K. While the magnitude of the phonon peak has been shown to be related to the dislocation density and may be influenced by manufacturing processes, little has been discussed as to the temperature at which the peak occurs. In examining these phenomena, it has been determined that more explicit accounting of phonon–dislocation scattering in a popular model better represents the thermal conductivity at temperatures colder than 3 K. Scaled sensitivity coefficients show this term to have similar influence as the phonon-electron and phonon-boundary scattering terms. Results using the enhanced model also show an apparent threshold of dislocation density below which there is little contribution to the thermal conductivity of Nb.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB061
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Paper

Title

Page

Modeling the Hydroforming of a Large Grain Niobium Tube With Crystal Plasticity

616

A. Mapar
MSU, East Lansing, Michigan, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
J.E. Murphy
University of Nevada, Reno, Reno, Nevada, USA
F. Pourboghrat
Ohio State University, Columbus, Ohio, USA

Current SRF cavities are made from fine grained polycrystalline niobium half-cells welded together. Hot spots are commonly found in the heat-affected zone, making seamless hydroformed cavities attractive. Large grain cavities usually perform as well as fine grain cavities, often having a higher Q, presumably due to fewer grain boundaries. Large grain Nb forms non-uniformly, which introduces problems in manufacturing. A model that could realistically predict the deformation response of large grain Nb could facilitate the design of large grain hydroformed tubes. To this end, a crystal plasticity model was developed and calibrated with tensile stress-strain data of Nb single crystals. A seamless large grain tube was made from rolling a fine grain sheet into a tube, welded, and heat treated to grow large grains. The heat treatment resulted in a large grain tube with a single grain orientation in the center. The tube was hydroformed until it cracked. The hydroforming process was simulated with the crystal plasticity model, which was able to predict the deformed shape of the tube, the location of the crack and other localized areas with heterogeneous strain.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-TUPB095

Export •

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

THPB002

Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations

741

P. Garg, I. Adlakha, K.N. Solanki
Arizona State University, Tempe, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA

It is known that formation and growth of Nb hydride degrades superconducting radio frequency (SRF) properties of Nb cavities and the treatments that reduce H concentration improve quality factor. Recently it is has also been shown that addition of N through doping or infusion improves the quality factor. Thus, we probe role of N addition in Nb on hydride precipitation and stability through first principles calculations & compared with coupon samples. In presence of N, energetic preference for H to occupy interstitial sites in the vicinity of N is reduced. Furthermore, presence of N forces H to occupy interstitial octahedral site instead of a tetrahedral site. The thermodynamic stability of hydride is decreased in the presence of N in Nb.The quantum insights using charge transfer and density of states show a strong tendency of N to accumulate charge, thereby decreasing the bond strength of neighboring Nb and H atoms. These atomic scale results explain the lesser tendency of surface hydride formation in SRF Nb cavities in presence of N. These results are consistent with metallographic examination of N-treated Nb coupons, which show suppressed hydride formation near N-treated surface.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB002

Export •

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

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

THPB026

Investigation of the Effect of Strategically Selected Grain Boundaries on Superconducting Properties of SRF Cavity Niobium

787

M. Wang, T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
S. Balachandran, P.J. Lee
NHMFL, Tallahassee, Florida, USA
S. Chetri, A. Polyanskii
ASC, Tallahassee, Florida, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: Research supported by DOE/OHEP contracts DE-SC0009962, DE-SC0009960, NSF-DMR-1157490, and the State of Florida.
High purity Nb is commonly used for fabricating SRF cavities due to its high critical temperature and its formability. However, microstructural defects such as dislocations and grain boundaries in niobium can serve as favorable sites for pinning centers of magnetic flux that can degrade SRF cavity performance. In this study, two bi-crystal niobium samples extracted from strategically selected grain boundaries were investigated for the effect of grain misorientation on magnetic flux behavior. Laue X-ray and EBSD-OIM crystallographic analyses were used to characterize grain orientations and orientation gradients. Cryogenic Magneto-Optical Imaging (MOI) was used to directly observe magnetic flux penetration at about 5-8 K. Flux penetration was observed along one of the grain boundaries, as well as along a low angle boundary that was not detected prior to MOI imaging. Hydride scars on the sample surface after MOI were examined using atomic force microscopy (AFM) analysis. The relationships between dislocation content, cryo-cooling, flux penetration and grain boundaries are examined.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB026

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)

THPB061

Effect Of Dislocations On the Thermal Conductivity Of Superconducting Nb

886

P. Xu, N.T. Wright
MSU, East Lansing, Michigan, USA
T.R. Bieler
Michigan State University, East Lansing, Michigan, USA

Funding: This work is funded by DOE and OHEP through grant number DE-FOA-0001438.
The thermal conductivity of Niobium (Nb) often experiences a local maximum (a phonon peak) at a temperature between 1.8 and 3 K. While the magnitude of the phonon peak has been shown to be related to the dislocation density and may be influenced by manufacturing processes, little has been discussed as to the temperature at which the peak occurs. In examining these phenomena, it has been determined that more explicit accounting of phonon–dislocation scattering in a popular model better represents the thermal conductivity at temperatures colder than 3 K. Scaled sensitivity coefficients show this term to have similar influence as the phonon-electron and phonon-boundary scattering terms. Results using the enhanced model also show an apparent threshold of dislocation density below which there is little contribution to the thermal conductivity of Nb.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-SRF2017-THPB061

Export •

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