Physical and mechanical metallurgy of high purity Nb for accelerator cavities

Bieler, T. R.; Wright, Neil T.; Pourboghrat, Farhang; Compton, C.; Hartwig, K. T.; Baars, D.; Zamiri, Amir; Chandrasekaran, Saravan Kumar; Darbandi, Payam; Jiang, Helen; Skoug, Eric J.; Balachandran, Shreyas; Ice, Gene E.; Liu, W.

doi:10.1103/physrevstab.13.031002

Cited by 45 publications

(35 citation statements)

References 31 publications

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“…As we could not establish a clear correlation between grain orientations and rf behavior, it is not possible to indicate, in absolute terms, a crystal orientation which is more susceptible to pitting. A possible reason for this, as shown in [14], is that slip system activation during forming of the halfcells depends on the crystal orientation, grain boundaries, and orientation gradients within a grain.…”

Section: Analysis and Discussionmentioning

confidence: 99%

Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

Zhao

Ciovati

Bieler

2010

Phys. Rev. ST Accel. Beams

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The performance of superconducting radio-frequency (SRF) resonant cavities made of bulk niobium is limited by nonlinear localized effects. Surface analysis of regions of higher power dissipation is thus of intense interest. Such areas (referred to as ''hotspots'') were identified in a large-grain single-cell cavity that had been buffered-chemical polished and dissected for examination by high resolution electron microscopy, electron backscattered diffraction microscopy (EBSD), and optical microscopy. Pits with clearly discernible crystal facets were observed in both ''hotspot'' and ''coldspot'' specimens. The pits were found in-grain, at bicrystal boundaries, and on tricrystal junctions. They are interpreted as etch pits induced by crystal defects (e.g. dislocations). All coldspots examined had a qualitatively lower density of etch pits or relatively smooth tricrystal boundary junctions. EBSD mapping revealed the crystal orientation surrounding the pits. Locations with high pit density are correlated with higher mean values of the local average misorientation angle distributions, indicating a higher geometrically necessary dislocation content. In addition, a survey of the samples by energy dispersive x-ray analysis did not show any significant contamination of the samples' surface. The local magnetic field enhancement produced by the sharp-edge features observed on the samples is not sufficient to explain the observed degradation of the cavity quality factor, which starts at peak surface magnetic field as low as 20 mT.

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Section: Analysis and Discussionmentioning

confidence: 99%

Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

Zhao

Ciovati

Bieler

2010

Phys. Rev. ST Accel. Beams

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“…Why it is absent in the CW samples could indicate that the dislocations are effectively dispersed in the CW samples, whereas they are clustered in the A6 and A8 samples. Since 800 °C is not sufficient to begin recrystallization, the net effect of the annealing may be to cluster dislocations into geometric bands, thereby concentrating them in certain regions [15]. The concentrated dislocation bands might actually make the metal more susceptible to pitting, not less.…”

Section: Discussionmentioning

confidence: 99%

“…Such pockets might lead to local stress and high material removal rate, which could occasionally result in the formation of a pit with faceted edges, as is also observed. Likewise, dislocations can be concentrated at the HAZ border [6], making those regions susceptible to dislocation-assisted pitting, as is well known for niobium [7,8]. It should be noted that cavity equatorial welds are not stress relieved, and that cold work is not removed from pieces prior to pre-weld acid etching or welding.…”

Section: Introductionmentioning

confidence: 99%

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Cooley¹,

Hahn²,

Hicks³

et al. 2012

AIP Conference Proceedings

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Ongoing studies at Fermilab investigate whether dislocations and other factors instigate pitting during cavity electropolishing (EP), despite careful processing controls and the inherent leveling mechanism of EP itself. Here, cold-worked niobium coupons, which exhibited increased tendencies for pitting in our past study, were annealed in a high vacuum furnace and subsequently processed by EP. Laser confocal scanning microscopy and special defect counting algorithms were used to assess the population of pits formed. Hardness measurements indicated that annealing for 2 hours at 800 °C produced recovery, whereas annealing for 12 hours at 600 °C did not, as is consistent with known changes for cavities annealed in a similar way. The 800 °C anneal was effective in some cases but not others, and we discuss reasons why tendencies for pitting remain. We discuss implications for cavities and continued work to understand pitting.

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“…Large grain ingot slices deform less uniformly than fine grain sheets, leading to problems such as thickness variations and ridges at grain boundaries in deep drawing, which may affect cavity performance. There are ongoing interests in the SRF community in relating the unique forming and metallurgical properties of large grain Nb to cavity performance [4,5].…”

Section: Introductionmentioning

confidence: 99%

Effects of processing history on the evolution of surface damage layer and dislocation substructure in large grain niobium cavities

Kang

Bieler

Compton³

2015

Phys. Rev. ST Accel. Beams

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Large grain niobium (Nb) is being investigated for fabricating superconducting radiofrequency cavities as an alternative to the traditional approach using fine grain polycrystalline Nb sheets. Past studies have identified a surface damage layer on fine grain cavities due to deep drawing and demonstrated the necessity for chemical etching on the surface. However, the origin of and depth of the damage layer are not well understood, and similar exploration on large grain cavities is lacking. In this work, electron backscatter diffraction (EBSD) was used to examine the cross sections at the equator and iris of a half cell deep drawn from a large grain Nb ingot slice. The results indicate that the damage (identified by a high density of geometrically necessary dislocations) depends on crystal orientations, is different at the equator and iris, and is present through the full thickness of a half cell in some places. After electron backscatter diffraction, the specimens were heat treated at 800°C or 1000°C for two hours, and the same areas were reexamined. A more dramatic decrease in dislocation content was observed at the iris than the equator, where some regions exhibited no change. The specimens were then etched and examined again, to determine if the subsurface region behaved differently than the surface. Little change in the dislocation substructure was observed, suggesting that the large grain microstructure is retained with a normal furnace anneal.

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Physical and mechanical metallurgy of high purity Nb for accelerator cavities

Cited by 45 publications

References 31 publications

Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

Characterization of etch pits found on a large-grain bulk niobium superconducting radio-frequency resonant cavity

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Effects of processing history on the evolution of surface damage layer and dislocation substructure in large grain niobium cavities

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