MSU Ingot Niobium Investigations

Transcription

1 MSU Ingot Niobium Investigations Thomas R. Bieler D. Kang, D. Baars, S. Chandrasekaran, A. Mapar G. Ciovati, N.T. Wright, F. Pourboghrat, C. Compton, J. Murphy, G.R. Myneni Michigan State University University of Nevada Reno Thomas Jefferson National Accelerator Facility Work funded by DOE/OBES

2 Overview Ingot characterization Characterization of a formed half cell from ingot slice Internal defect quantification, i.e. dislocations Lattice curvature necessarily correlated with dislocations Characterization of a large grain tube Fabricated from welded polycrystal tube Deformation of single crystals Crystal plasticity modeling Thermal conductivity in single and bi-crystal samples Effects of deformation and heat treatment Hydrogen Summary

3 BCC dislocations can move in 4 directions on as many as 48 slip systems* Dislocations enable large plastic strains by facilitating shear on slip systems there are 48 ways to do this, but most think that 24 of them are responsible for most of the deformation 4 groups, 12 associated with each <111> direction (connects opposites corners) One group illustrated here [-1 1 1] (1 1 0) [-1 1 1] (1 0 1) [-1 1 1] (0-1 1) [-1 1 1] (1-1 2) [-1 1 1] (-1-2 1) [-1 1 1] (2 1 1) [-1 1 1] (1-2 3) [-1 1 1] (2-1 3) [-1 1 1] (-2-3 1) [-1 1 1] (-1-3 2) [-1 1 1] (3 1 2) [-1 1 1] (3 2 1) * FCC: 6 directions in 12 systems

4 Ingot slice features, dislocation substructure Ingots show huge or variable grain size, dislocation entanglements in asreceived ingot (ECCI image) and substantial orientation gradients (less than about 10 ), Scan area of 2-4 mm 2

5 Laue camera used to measure orientations of several ingots Crystal orientations measured with a Laue camera Indexed using semi-automatic software ~30 cm Vertical lines are scars from end milling ~35 cm CBMM Ningxia Heraeus From measured orientations, maps are drawn using orientation imaging software

6 Characterization of several ingots show no trends in crystal orientation or grain shapes 5 different ingots from different manufacturers have no common orientation trends crystallization during e-beam refining is a random process Bi-axial stress Schmid factor maps for different slip systems some ingot slices deform more uniformly than others CBMM {110} {112} CBMM Ningxia Heraeus. both...

7 Single/multi crystal cavities fabricated by welding two half cells grain boundaries are visible Material in Equator / Weld Experiences 1. Hoop Compression, radial tension 2. Bending + unbending 3. Biaxial stretching (not balanced) Single cell multicrystal cavities made at MSU and J-Lab show effects of grain boundaries, irregular deformation MSU single cell cavity grain boundary ridge visible and easily felt with fingers, and cups have ears

8 Undeformed single crystals can be welded together gracefully (sometimes) Center piece of sliced ingot cut in two, flipped, welded, the weld was clean between two crystal orientations, but new orientations developed at triple junctions Recrystallization features on either side of equator weld in deformed heat affected zone of a finished large-grain cavity Recrystallization in HAZ Weld 1mm Weld Direction Baars et al. Transactions Applied Superconductivity 17, June 2007 B B B Weld Direction A A A B B B B

9 Not surprisingly, welding deformed single crystals leads to recrystallization Two tensile deformed single crystals were welded together Parent orientations in grips have white prism orientation Black prisms show crystal orientations after weld Side F ~(111)[1-10] Weld Side C ~(101)[10-1] 400 m

10 Some commentary on substructural state 3.1mm 2mm WELD 3mm 8.1mm Near shoulder, cold end, some shear bands are evident Recrystallization front; very blue grains have few dislocations within, but along grain boundaries, there is higher dislocation density Substantial dislocation substructure present in recovered grain at recrystallization front Near shoulder, cold end, some shear bands are evident

11 Metallurgical characterization of large grain single cell fabrication study at J-Lab Slices H1, H2, H3, H4 examined, with different heat treatment and etching histories, before / after joining together, etc. Instead of machining off excess, rings were EDM cut at iris and equator, to provide material that could be examined microstructurally. These pieces were characterized with OIM to identify local orientation gradients. Thickness was measured in various locations to identify effects of crystal orientation and grain boundaries. Cavity performance was assessed mediocre until barrel polishing was done ~ microns removed, then it performed well.

12 Thickness, mm Thickness, mm Forming a half cells leads to irregular ears in edge influenced by grain boundaries, thickness issues Sharp Ear C Sharper Ear B D A No obvious trends between crystal orientation and thickness H1 was significantly thicker, had a thick spot at GB that grabbed die E F Biggest Ear Radial A Radial B Radial C Radial D Radial E Radial F Radial A Radial B Radial C Radial D Radial E Radial F Iris --- Position Equator Iris --- Position Equator --

13 Unlike a weld, a large grain piece from equator/iris region before and after heat treatment shows no recrystallization after 800 C furnace anneal (or a 1000 C anneal either) Note surface damage extending into surface by ~100 µm Slow heating prevents recrystallization, leaves geometrically necessary dislocations in place Local Average Orientation identifies regions with more/less defects Grain Orientation Local Orientation Gradient Grain Orientation Local Orientation Gradient Asdeformed Asdeformed anneal GB Equator GB 4mm anneal GB Iris GB Local Average Misorientation (orientation gradient), measures geometrically necessary dislocation density

14 Quantification of LAM measurements Results after heat treatment are ~ same Average of 6 depth traces in iris and equator rings in 6 different locations indicates effects of surface-die interactions, i.e. the damage layer is quantified, µm deep

15 Hot spot correlated with thick GB spot on thick half cell side (no issue on other side!) After H1-H2 cavity was fabricated, etched, the performance was not so good. 1400ºC anneal did not help. Optical inspection of hot spot location was at location where etch pits at grain boundary were located. After 1400ºC anneal Barrel polishing 100 µm led to excellent performance.

16 The possibility of hydroforming a large grain tube was explored Large grain tube made from welded polycrystalline tube after heat treatment made by Jim Murphy at U. Nevada Reno Grain were grown to very large size, but not a single crystal See unwrapped grain map Center region has only one grain orientation Tube was slightly warped, difficult to achieve seal for pressurizing Deformed heterogeneously, cracked within the large center crystal Still under analysis, will be simulated using crystal plasticity model

17 Crystal orientations chosen strategically to favor {110} vs. {112} slip, single vs. duplex slip. Results support preference for {112} slip; two lower stressed {112} is more favorable than favored {110} Single Crystals taken from (Ningxia) ingot, have highly varying mechanical properties

18 {112} slip almost accounts for all the initial hardening behavior of the 9 specimens Baars, Investigation of active slip systems in high purity single crystal niobium, PhD dissertation Specimen Difference between highest resolved shear stress on primary and secondary {112} slip systems (MPa), and ratio of resolved shear on two highest {112} systems Initial hardening rate X Barely Q Barely S Barely R Slight W Moderate- Low T High U Very High V Moderate- High P High

19 Annealing the samples leads to significant decrease in strength, and more regular deformation characteristics As-extracted curves show some softening in softer orientations with single slip conditions, suggesting unlocking of dislocation tangles. After annealing all curves show lower yield stress and always positive work hardening, and indicating fewer defects present U V P X As extracted from ingot W S Tensile axis orientations White {110} Gray {112} Schmid Factor Contours 0.5, 0.499, 0.49, 0.47, 0.44, 0.40, 0.36, 0.32 R T Q U P V W X Deformed in-situ after 800 C 2hr anneal S Q T R (a) (b)

20 Effect of heat treatment favors operation of {110} slip system (crystal rotations) From analysis of crystal rotations during the tension test, slip on {110} planes dominates, rather that {112} slip in as-received Slip trace features suggest bursts of dislocations occurring at the micron scale Rotations that develop during deformation will evolve in complex ways as dislocations accumulate This information about conditions that affect slip system activity is needed for codes than can predict heterogeneous strain in large grain forming

21 Eng. Stress, MPa Eng. Stress, MPa Crystal plasticity models are able to capture overall trends, imperfectly Different crystal plasticity modeling approaches that monitor relative amounts of slip in different slip directions do a better job than classical hardening models to predict flow behavior calibration based upon two samples has some capability to predict deformation in other orientations P - Experiment P - Classical P - Dynamic T - Experiment T - Classical T - Dynamic U - Experiment U - Classical U - Dynamic W - Experiment W - Classical W - Dynamic Eng. Strain, % P - Experiment P - Diff-Exponential R - Experiment R - Diff-Exponential U - Experiment U - Diff-Exponential W - Experiment W - Diff-Exponential Eng. Strain, %

22 High thermal conductivity minimizes degrading effect of hot spots Different phenomena above and below ~ 3K Phonon peak appears and disappears but dislocation content is a key factor for thermal conductivity How does dislocation substructure affect RRR?

23 Hull & Bacon Introduction to Dislocations, 1984 Dislocation substructure & thermal properties Prior work (Cotts, Northrup, Anderson ) examined phonon transport (dissipation) in LiF crystals with known dislocation substructures When phonon transport direction was parallel to a mobile dislocation segment, phonon was dissipated (phonon converted to random vibration) Phonons travel according to (anisotropic) elastic properties Grain boundaries also dissipate phonons Is the low T phonon-peak killed when there are mobile dislocations that can couple with phonon and disperse its energy? Dislocation walls in Fe If so, then dislocation substructure may need to be managed with respect to crystal orientation to maximize phonon transport So, Recovery or Recrystallization? Subgrain Bdy Subgrain Boundaries in Fe

24 High thermal conductivity minimizes degrading effect of hot spots Different phenomena above and below ~ 3K Phonon peak appears and disappears but dislocation content is a key factor for thermal conductivity How does dislocation substructure affect RRR?

25 Dislocations clearly kill the phonon peak Carefully analyzed thermal conductivity shows that the phonon peak component (k pp2 ) plateaus at about 1000ºC. The decrease in k pp2 is greatest in orientations where multiple slip occurs nearer edge of triangle, which causes greater increases in dislocation content.

26 Introduction of Hydrogen also affects phonon peak, sometimes not reversible Phonon peak was restored Phonon peak not restored q q on side 2

27 Observations and Speculations From making an ingot to final function, dislocations are an omnipresent enabler and suspect, Additional suspects: H, O, impurities, interfaces, magnetic fields, surface energy Dislocations can be removed most effectively by recrystallization; Recovery leaves substructure that is oriented in crystallographic directions Is the perfect cavity a recrystallized single crystal with dislocation segments not lined up in a radial direction?