Hot Topic: Source(s) of Cavity Quench What Causes Surface Pits? Why Do Some Cause Quench? Lance Cooley Fermilab

Crime Scene Investigations – How did pits form? Shape and topography of defects:  Hemispheres – bubbles of gas trapped during solidification, etching at electrolyte bubble edges  Facets – Grain-boundary stress, directional etching  Bumps – Spatter (due to what?)  Other – Incomplete weld? Foreign matter? Location of defects relative to welds:  In re-solidified zone – bubbles  In recrystallized zone – stress, directional attack  At edge of HAZ – dislocation- assisted etching, impurity pockets, machining debris  Elsewhere – “flash” reaction of acid with water droplet Composition of defects  Carbon, oxygen, hydrogen contaminants  (Need more data) Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July 20112

Do we create bubbles, or uncover them? Hemispheres in re-solidified zone - Bubbles formed during welding and became trapped  That is, processing uncovers them  Do bubbles flow along e-beam direction? If so, weld from the inside of the cavity  Local grinding or re-melting may be favored over bulk removal, such as CBP or EP, due to risk of uncovering other bubbles  Is tomography accurate enough to screen welds? Hemispheres in other zones – Corrosion pitting  That is, processing creates them  Sulfuric acid flash-reacting with water droplets  Re-processing should repair them Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July 20113

Can bubbles come from weld-prep etch? 2% H is typical level after 20 µm BCP, based on bulk spectroscopy data With 40 µm grains, then mol H can collect at grain-boundary triple points during heat-up Assuming H is ideal gas, a bubble then can contain 2.5 µJ energy at 3000 K Given surface tension of molten niobium ~2 J/m 2 (NIMS data), the equilibrium bubble radius is approximately 300 µm  This is what is routinely seen! Implication: change the pre-weld etch to a “flash” etch, keep it cold, and perhaps consider a de- gassing bake Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July 20114

Facets and directional attack Welding concentrates dislocations (DLs) & impurities at the HAZ edge, sensitizing metal to pitting  That is, processing creates the defects, and re- processing will continue to create more defects  Grain-boundary junctions and grain orientations with emerging DLs may be troublesome Cold work promotes dislocation-assisted pitting  Therefore, is it prudent to anneal half cells before welding?  800 °C bake produces recovery, but also concentrates DLs into walls via polygonalization Temperature increase during EP promotes fluorine mixing, which breaks down viscous salt film  Supply of fluorine to surface then produces attack where DLs are concentrated Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July 20115

Further deleterious properties of dislocations Dislocations attract hydrogen and bind it  Hydride precipitates are present at room temperature and may remain stable despite 120 C baking Hydrogen bound to DLs expands lattice and promotes further formation of DLs, and hence further uptake of hydrogen  This is a common mechanism of hydrogen embrittlement RF currents induced to flow across precipitates produce dissipation, resulting in Q drop So, if DLs are present at the bottom of etch pits, they provide an inherent loss source  That is, topographical features (such as the pit rim) may be less important for Q limit. Pits without dislocations could be benign. Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July 20116

A pit uncovered by EP August 6, 2010ASC 2010 – Washington D.C. 5MZ-017 As Received After 1 st EP After 2 nd EP Electropolishing of an iris weld reveals (creates?) a nearly hemispherical pit. And, as with other pits, it is at the boundary of the weld bead. Images courtesy of Dmitri Sergatskov, FNAL

Breakdown of viscous salt film by dunking coupon during EP resulted in many large pits along HAZ edge August 6, 2010ASC 2010 – Washington D.C. 5MZ-018

Recovery anneal reduces tendency for attack somewhat, but also seems to concentrate DLs Lance Cooley et al., Fermilab – 2011 CEC/ICMC Presentation M1OrC-069 CW + weld °C 1 cm x 1 cm Weld HAZ CW + weld °C 1 cm x 1 cm Weld HAZ See THPO075 New tool: Laser Confocal Scanning Microscopy

Connection between Q-slope (onset above 100 mT) and small near-surface hydride precipitates! Lance Cooley, Fermilab – PAC11, 31 March A. Romanenko, Fermilab, See THPO008 These are hydrides: Vinnikov and Golubok, Phys. Stat. Sol. 69:631 (1982) and others

Evidence for NbH and Nb 3 H 4 at dislocations B Albee, D Ford et al, THPO060 Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July Black: Raman spectroscopy data – B. Albee et al, IIT Vertical lines – density-functional calculations, D. Ford, FNAL/NWU

Laser melting also produces a HAZ like a weld SEM with orientation imaging shows pile-up of DLs at grain boundaries. Subsequent etching attacks the metal preferentially at these regions  Dzyuba et al, THPO051 Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July

Summary – Hot topic It is plausible that hydrogen bubbles form in the molten weld due to pre-weld etching.  Use “flash” etching, keep cold, minimize H uptake Plausible mechanisms exist for BOTH uncovering pits and creating them during processing.  Maintain efforts on local repairs and processing R&D Dislocations harbor hydrogen and assist chemical attack.  Use baking to recovery or recrystallize  Hydrides produce Q loss – smoking gun: DLs near pit Avoid breakdown of EP salt film (cold EP). If breakdown occurs, it can be tolerated if the metal is not sensitized to attack. Lance Cooley, Fermilab – SRF'11 Hot Topic, 25 July