# Presentation: 259905

## Mechanical Properties and Surface Functionality of RRR Nb 

- Nb for SRF cavities needs to be formable
- BCC metals are highly formable with deep drawing
- due to high R values from {111} // sheet normal
- SRF cavities must be welded together and be dimensionally stable
- Welds have different mechanical and surface properties from parent material
- Surface properties are crucial for function
- Etching to make smooth surface depends on etching rates of different crystal orientations, forms ledges
- Surface crystal orientations matter

- H. Jiang and T.R. Bieler,  Michigan State University

## Weld has a large grain size with surface grooves or ledges,grain boundaries  to solidification pool ridges

- _Grain Size_6 mm wide weld fusion zone ~ 1 m Heat Affected Zone up to 40 mm away~ 100 m > 80mm away ~ 50 m, same as parent material

- 6 mm

- 198 mm

- Weld has a large grain size with surface grooves or ledges,grain boundaries  to solidification pool ridges

## Orientation Imaging Microscopy shows microstructure and texture information togetherTexture in weld is similar to parent material

- 5 mm from center of weld
- (2 mm from edge of fusion zone)

- 10 mm from fusion zone

- Unaffected Sheet

- [001] // ND
- on surface
-  IPF OIM Viewpoint
-  is along
-     ND
- [111] // ND
-   in center
-       TD

## Engineering Stress-Strain shows degraded tensile properties in weld specimen

- Maximum stress is 140 MPa, 10-20 MPa less than normal material
- Specimens all failed between center and edge of weld

##     Three Weld Creep specimens (Transverse Direction)1.  Loaded directly to 	45 MPa     =0.018 	      2 hr2.  Loaded / Unloaded 37- 43 MPa    =0.008 	  600 hr3.  Step Loaded           37- 45 MPa    = 0.02 	  	  800 hr 

-     Three Weld Creep specimens (Transverse Direction)1.  Loaded directly to 	45 MPa     =0.018 	      2 hr2.  Loaded / Unloaded 37- 43 MPa    =0.008 	  600 hr3.  Step Loaded           37- 45 MPa    = 0.02 	  	  800 hr

- Different strains imply that one side deformed more than the other side
- Dislocation unlocking occurred to allow large strain increments at constant load
- Not very dimensionally stable!

## Room temperature weld creep differs in details between rolling and transverse directions, but gets to a similar strian at the same stress  

- Room temperature weld creep differs in details between rolling and transverse directions, but gets to a similar strian at the same stress

- 35MPa

- 37MPa

- 1.1e-8/s

- 1200 hr 

- 37MPa

## Commercially available 2 mm recrystallized Nb sheet has strong <111> fiber texture, and some <100> fiber on surface – Desirable? Yes and No

- <111> // ND texture is good for deep drawability for steels, also good for deep drawability for Nb (yes).
- Would understanding of cold rolling and recrystallization texture evolution for interstitial free (IF) or extra-low carbon steels be transferable to pure niobium?
- Different surface grain orientations etch differently, causing grain boundary ledges (no) that are centers for electron emission
- <100> // ND grains have lower work function for undesirable electron emission

- H. Jiang, *et al.*, 2003 Particle Accelerator Conference, IEEE, p. 1359.

## Can we control surface texture?  Maybe...Tapered Slab Rolling Experiment

- Rolling Direction

- 2

- After rolling, sliced into strips

- As-received Nb (RRR=150) tapered specimens made from 12 mm plate
- Unidirectional path cold rolling
- Reversed path cold rolling
- without lubrication at room temperature
- A strip was annealed for 1 hour in evacuated quartz tube
- Pole figures were measured from surface, interior of both rolled specimen and annealed specimens
- OIM data was collected for through thickness measurement

## Lots of Data!!Textures are not very strong, < 6 x random...

- Lots of Data!!Textures are not very strong, < 6 x random...

## In Steels, most crystal orientations are in a particular COD section (2= 45o) along  or  fibers.  In this same section, similar changes are observed in 80% rolled Nb, but this is a minority of Nb crystal orientations

- In Steels, most crystal orientations are in a particular COD section (2= 45o) along  or  fibers.  In this same section, similar changes are observed in 80% rolled Nb, ***but this is a minority of Nb crystal orientations***

- Initial           Rolled    Annealed

-   Rolled      Annealed
- -Initial        -Rolled

- In the ***center***,
- After rolling, the  fiber deceased,   fiber increased
- After annealing, the   fiber becomes very strong

- Reversed
- Path
- Uni-
- directional Path has stronger 

- COD sections                   Difference from prior state

## On the surface, ,  fibers in the (2= 45o) COD section change less due to 80% rolling and annealing

- On the surface, ,  fibers in the (2= 45o) COD section change ***less*** due to 80% rolling and annealing

- Initial          Rolled    Annealed

-   Rolled      Annealed
- -Initial        -Rolled

- After rolling,  fiber unchanged, slight  fiber increase
- After annealing, the   fiber weakened

- COD sections                     Difference from prior state

- Reversed
- Path
- Uni-
- directional Path has weaker 

## Processing path affects texture!

- Max 3.4
- x random

- Max 7.7
- x random

- Reversed Rolling has weaker texture, gradients in grain size, texture
- Unidirectional Rolling has stronger [111] texture, more uniform grain size, fewer gradients

- 1.4
- mm

## Continuing questions

- Are there processing paths to obtain best formability ***and ***surface property conditions?
- Are Nb welds dimensionally stable?
- Is Nb like steel or not?
- “Single crystal” cavities ...
- Will they be soft due to no grain boundaries?
- Will they recrystallize inhomogeneously after forming and heat treatment?
- Will they be dimensionally stable?

## Lots of Data!!Textures are not very strong, < 6 x random...