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

Abstract

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.

Surface processing for bulk niobium superconducting radio frequency cavities

“…More recently, metallurgical studies on niobium [14] using electron backscattered diffraction to map the niobium grain size, structure and orientation as a function of depth show that the process of rolling a billet of niobium (cut from an ingot) into a thin ∼3 mm sheet preferentially changes the grain structure and orientation near the surface and in the bulk. Similarly, the die forming process leads to differing grain structure in the equators and irises of elliptical cavities [15]. Together these studies indicate that a major contribution to the damage layer in niobium sheet is inherent to the niobium rolling and forming processes.…”

“…The relatively large coherence length, together with the proximity effect [22], is the reason that the supercurrents in niobium move, in most cases, through grain boundaries or other lattice defects with small losses [23]. The size of the niobium grains is determined by the history of the niobium production [14] and the cavity fabrication [15]. For reproducible strength and formability, the grain size for industrially produced bulk niobium sheet is typically specified to be 50 μm or close to ASTM#5 [24].…”

Direct evidence of microstructure dependence of magnetic flux trapping in niobium

“…2 . There is evidence that low deformation strains 42 and up to 70% RR 43 is not sufficient to complete the recrystallization processes in high purity Nb (RRR: residual resistivity ratio > 250). Our study shows that high-temperature heat treatment of 1000 °C/3 h is needed to obtain recrystallization and growth.…”

Deformation mechanisms, defects, heat treatment, and thermal conductivity in large grain niobium

“…4, the LAM map ranges from 0 (blue) to 1.5° (red) over a 40 µm range. Note the greater depth of high LAM values on outside than inside, and in the right grain than the left grain (adapted from [17]).…”

“…Normal direction inverse pole figure orientation map and local average misorientation map of a grain boundary region 1 at the equator in the half cell H2 shown in Fig.5, where the left side is toward radial B and the right side toward radial C. The orientation map perspective is the same as in Fig.4, the LAM map ranges from 0 (blue) to 1.5° (red) over a 40 µm range. Note the greater depth of high LAM values on outside than inside, and in the right grain than the left grain (adapted from[17]).…”