Author: Bieler, T.R.
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SUPCAV002 Ex-Situ Investigation of the Effects of Heating Rate on the Recrystallization in Rolled Polycrystals of High-Purity Niobium 1
  • Z.L. Thune, N. Fleming, C. McKinney, E.M. Nicometo
    MSU, East Lansing, Michigan, USA
  • S. Balachandran
    NHMFL, Tallahassee, Florida, USA
  • T.R. Bieler
    Michigan State University, East Lansing, Michigan, USA
  Funding: US Dept. of Energy award DE-SC0009960
The consistent production of high-purity niobium cavities for superconducting radiofrequency (SRF) applications is crucial for enabling improvements in accelerator performance. Recent work has shown that dislocations and grain boundaries trap magnetic flux which dissipates energy and degrades cavity performance. We hypothesize that the current heating rate used in production is too slow and therefore facilitates recovery rather than recrystallization. Recovery, unlike recrystallization, does not reduce the number of geometrically necessary dislocations (GNDs) that are strongly correlated to trapped magnetic flux. Using excess high-purity niobium saved from the production of a cavity, the material was divided into two groups and rolled to ~30% reduction with half rolled parallel to the original rolling direction, and the other half rolled perpendicular. To examine the effect of heating rate, samples were encapsulated in quartz tubes and placed into either a preheated furnace or a cold furnace to allow for heat treatments at different rates. Then using ex-situ electron backscatter diffraction (EBSD) mapping, the extent of recrystallization was determined.
DOI • reference for this paper ※  
About • Received ※ 22 June 2021 — Revised ※ 31 August 2021 — Accepted ※ 16 November 2021 — Issue date ※ 20 February 2022
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TUPFDV001 Effect of Heating Rate on Recrystallization in Rolled Multicrystals of Pure Niobium 396
  • T.R. Bieler, D. Kang
    Michigan State University, East Lansing, Michigan, USA
  • R.R. Desconocido, M.T. Sanchez
    UPM, Madrid, Spain
  • N. Fleming, C. McKinney, Z.L. Thune, K. Zheng
    MSU, East Lansing, USA
  • A.A. Kolka
    Niowave, Inc., Lansing, Michigan, USA
  Funding: Supported by US Dept. of Energy award DE-SC0009960.
The performance of niobium cavities in superconducting radiofrequency particle accelerators requires nearly defect-free inner surfaces. While methods to obtain smooth inner surfaces are established, the role of metallurgical defects on superconducting performance is also important, as defects such as grain boundaries and dislocations are known to trap flux that dissipates energy and reduces efficiency. Variable microstructure and texture gradients may account for the observed variability in cavity performance, so it is hypothesized that the texture and microstructure gradients originate from the large grain size of ingots, whose influence is not completely erased in the process of making sheet metal. To examine the evolution of microstructure and texture gradients, the crystal orientations present in a cylindrical cap rolled to ~90% reduction were heat treated. Initial crystal orientations were measured before rolling, and before and after slow and rapid heating rate vacuum heat treatments.
DOI • reference for this paper ※  
About • Received ※ 23 June 2021 — Revised ※ 22 February 2022 — Accepted ※ 04 May 2022 — Issue date ※ 16 May 2022
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