SRF2015 - List of Authors (Kang, D.)

Paper	Title	Page
MOPB018	Introduction of Precisely Controlled Microstructural Defects into SRF Cavity Niobium Sheets and Their Impact on Local Superconducting Properties	120
	M. Wang, T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA D.C. Larbalestier, A. Polyanskii, Z-H. Sung ASC, Tallahassee, Florida, USA P.J. Lee NHMFL, Tallahassee, Florida, USA
	Funding: Research supported by DOE/OHEP (contract number DE-FG02-09ER41638 at MSU and DE-SC0009960 at FSU) and the State of Florida. When SRF cavity shapes are formed from Nb sheets, the metallurgical processing introduces microstructural defects such as dislocations and low-angle grain boundaries that can serve as pinning centers for magnetic flux that may degrade cavity performance. Therefore, the relationship between magnetic flux behavior and microstructural defects in carefully strained SRF Nb sheet was investigated. Laue X-ray and EBSD-OIM crystallographic analyses of large grain ingot slices were used to characterize microstructural defects and then predict which grains and sample orientations will produce desired model defects due to tensile deformation. Grain orientations were chosen to favor specific slip systems, which generate dislocations with special angles with respect to the sample surface. Nb bicrystals were also prepared to investigate the effects of grain boundaries on flux pinning. The generated defect structures were confirmed by OIM and TEM. Cryogenic magneto-optical imaging was used to directly observe the penetration of magnetic flux into the deformed Nb. These model samples have deformation that is similar to that expected in typical cavity forming processes.
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MOPB045	Study of Slip and Deformation in High Purity Single Crystal Nb for Accelerator Cavities	191
	D. Kang, D.C. Baars, T.R. Bieler Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA A. Mapar, F. Pourboghrat MSU, East Lansing, Michigan, USA
	Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638. High purity Nb has been used to build accelerator cavities over the past couple decades, and there is a growing interest in using ingot Nb as an alternative to the fine grain sheets. Plastic deformation governed by slip is complicated in body-centered cubic metals like Nb. Besides the crystal orientation with respect to the applied stress (Schmid effect), slip is also affected by other factors including temperature, strain rate, strain history, and non-Schmid effects such as twinning/anti-twinning asymmetry and non-glide shear stresses. A clear understanding of slip is an essential step towards modeling the deep drawing of large grain ingot slices, hence predicting the final microstructure/performance of cavities. Two groups of single crystals, with and without a prior heat treatment, were deformed to 40% engineering strain in uniaxial tension. Differences in flow stresses and active slip systems between the two groups were observed, likely due to the removal of preexisting dislocations. Crystal plasticity modeling of the stress-strain behavior suggests that the non-Schmid effect is small in Nb, and that the deep drawing process might be approximated with a Schmid model.
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MOPB054	An Investigation of Correlations Between Mechanical and Microstructural Properties of High Purity Polycrystalline Niobium	219
	Z. Zhao, T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638. An understanding of the relationship between mechanical and functional properties, and processing history is essential in order to manufacture polycrystalline niobium cavities with consistent performance. The crystallographic texture (preferred crystal orientation) and microstructure in polycrystalline sheet varies considerably, so identifying its influence on properties is needed to achieve a better understanding of how to control properties of high purity niobium. Samples extracted from many lots produced by Tokyo Denkai and Ningxia sheet were examined. Through-thickness texture of the undeformed niobium samples was measured using electron backscattered pattern mapping. Texture is identified with pole figures, orientation distribution function, and grain misorientation relationships. Stress-strain tests were done to identify ultimate tensile stress, elongation, 0.2% yield strength, and hardening rate. From tests on many lots, there is no clear trend between the mechanical and material properties in high purity niobium and correlations between various microstructural and mechanical properties show significant scatter and few apparent correlations.
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MOPB057	Crystal Plasticity Modeling of Single Crystal Nb	228
	A. Mapar, F. Pourboghrat MSU, East Lansing, Michigan, USA T.R. Bieler, D. Kang Michigan State University, East Lansing, Michigan, USA C. Compton FRIB, East Lansing, Michigan, USA
	Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638. Deformation behavior of niobium (Nb) is not thoroughly studied, although it is widely used in manufacturing superconducting cavities. This deficiency of knowledge limits the predictibality in raw material properties for fine grain sheets, which are less anisotropic and easier to deform uniformly than large grain sheets. Studies on modeling and simulation of deformation of Nb are also limited. Therefore design of a new manufacturing procedure becomes a costly process, because models predicting the deformation of Nb are not accurate. A polycrystal is an aggregate of single crystals. Tensile tests were performed on single crystal with different orientations, to study the deformation behavior of Nb. A number of crystal plasticity models were developed, calibrated and finally used to predict the deformation of single crystal tensile samples. This study compares the predictions of these models. This provides a foundation for physically realistic polycrystal deformation models.
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Paper

Title

Page

Introduction of Precisely Controlled Microstructural Defects into SRF Cavity Niobium Sheets and Their Impact on Local Superconducting Properties

120

M. Wang, T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA
C. Compton
FRIB, East Lansing, Michigan, USA
D.C. Larbalestier, A. Polyanskii, Z-H. Sung
ASC, Tallahassee, Florida, USA
P.J. Lee
NHMFL, Tallahassee, Florida, USA

Funding: Research supported by DOE/OHEP (contract number DE-FG02-09ER41638 at MSU and DE-SC0009960 at FSU) and the State of Florida.
When SRF cavity shapes are formed from Nb sheets, the metallurgical processing introduces microstructural defects such as dislocations and low-angle grain boundaries that can serve as pinning centers for magnetic flux that may degrade cavity performance. Therefore, the relationship between magnetic flux behavior and microstructural defects in carefully strained SRF Nb sheet was investigated. Laue X-ray and EBSD-OIM crystallographic analyses of large grain ingot slices were used to characterize microstructural defects and then predict which grains and sample orientations will produce desired model defects due to tensile deformation. Grain orientations were chosen to favor specific slip systems, which generate dislocations with special angles with respect to the sample surface. Nb bicrystals were also prepared to investigate the effects of grain boundaries on flux pinning. The generated defect structures were confirmed by OIM and TEM. Cryogenic magneto-optical imaging was used to directly observe the penetration of magnetic flux into the deformed Nb. These model samples have deformation that is similar to that expected in typical cavity forming processes.

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

MOPB045

Study of Slip and Deformation in High Purity Single Crystal Nb for Accelerator Cavities

191

D. Kang, D.C. Baars, T.R. Bieler
Michigan State University, East Lansing, Michigan, USA
C. Compton
FRIB, East Lansing, Michigan, USA
A. Mapar, F. Pourboghrat
MSU, East Lansing, Michigan, USA

Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638.
High purity Nb has been used to build accelerator cavities over the past couple decades, and there is a growing interest in using ingot Nb as an alternative to the fine grain sheets. Plastic deformation governed by slip is complicated in body-centered cubic metals like Nb. Besides the crystal orientation with respect to the applied stress (Schmid effect), slip is also affected by other factors including temperature, strain rate, strain history, and non-Schmid effects such as twinning/anti-twinning asymmetry and non-glide shear stresses. A clear understanding of slip is an essential step towards modeling the deep drawing of large grain ingot slices, hence predicting the final microstructure/performance of cavities. Two groups of single crystals, with and without a prior heat treatment, were deformed to 40% engineering strain in uniaxial tension. Differences in flow stresses and active slip systems between the two groups were observed, likely due to the removal of preexisting dislocations. Crystal plasticity modeling of the stress-strain behavior suggests that the non-Schmid effect is small in Nb, and that the deep drawing process might be approximated with a Schmid model.

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MOPB054

An Investigation of Correlations Between Mechanical and Microstructural Properties of High Purity Polycrystalline Niobium

219

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

Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638.
An understanding of the relationship between mechanical and functional properties, and processing history is essential in order to manufacture polycrystalline niobium cavities with consistent performance. The crystallographic texture (preferred crystal orientation) and microstructure in polycrystalline sheet varies considerably, so identifying its influence on properties is needed to achieve a better understanding of how to control properties of high purity niobium. Samples extracted from many lots produced by Tokyo Denkai and Ningxia sheet were examined. Through-thickness texture of the undeformed niobium samples was measured using electron backscattered pattern mapping. Texture is identified with pole figures, orientation distribution function, and grain misorientation relationships. Stress-strain tests were done to identify ultimate tensile stress, elongation, 0.2% yield strength, and hardening rate. From tests on many lots, there is no clear trend between the mechanical and material properties in high purity niobium and correlations between various microstructural and mechanical properties show significant scatter and few apparent correlations.

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reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)

MOPB057

Crystal Plasticity Modeling of Single Crystal Nb

228

A. Mapar, F. Pourboghrat
MSU, East Lansing, Michigan, USA
T.R. Bieler, D. Kang
Michigan State University, East Lansing, Michigan, USA
C. Compton
FRIB, East Lansing, Michigan, USA

Funding: This work was supported by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-FG02-09ER41638.
Deformation behavior of niobium (Nb) is not thoroughly studied, although it is widely used in manufacturing superconducting cavities. This deficiency of knowledge limits the predictibality in raw material properties for fine grain sheets, which are less anisotropic and easier to deform uniformly than large grain sheets. Studies on modeling and simulation of deformation of Nb are also limited. Therefore design of a new manufacturing procedure becomes a costly process, because models predicting the deformation of Nb are not accurate. A polycrystal is an aggregate of single crystals. Tensile tests were performed on single crystal with different orientations, to study the deformation behavior of Nb. A number of crystal plasticity models were developed, calibrated and finally used to predict the deformation of single crystal tensile samples. This study compares the predictions of these models. This provides a foundation for physically realistic polycrystal deformation models.

Export •

reference for this paper to ※ BibTeX, ※ LaTeX, ※ Text, ※ RIS/RefMan, ※ EndNote (xml)