1. Tube-Type Strand Evolution at 650 C° Nb6Sn5 Morphology Abstract
Phase Evolution and Morphology Studies for Tube-Type Nb3Sn Strands
M2PoB-03
C.J. Kovacs1, M.D. Sumption1, X Peng2, E.W. Collings1
Center for Superconducting & Magnetic Materials (CSMM), Department of Materials Science & Engineering, The Ohio State University, USA
Hyper Tech Research Incorporated, Columbus, Ohio, USA
Tube-Type Strand Evolution at 650 C°
Nb6Sn5 Morphology
Abstract
A model that predicts the coarse-grain and fine-grain A-15 area from initial Cu/Sn ratio was compared to experimental observations made for three Tube-type Nb3Sn strands with varying Cu/Sn ratios reacted at 650C for various times. Short samples were reacted under flowing argon. The coarse/fine grain A-15 ratio was compared as a function of temperature and initial Cu/Sn ratio. SEM-EDS was used to determine the Sn content in the core at times near the completion of 6:5 growth and A-15 growth to inform the model. At the point of maximum 6:5 growth, longitudinal SEM was taken to observe 3-D morphology of the 6:5. Nb Sn
Cu Layer
Unreacted
2 hr
4 hr
8 hr
24 hr
48 hr
100 hr
200 hr
512 hr
Ramp 50C°/hr
Experimental Methods
Fracture images were taken using a Sirion FESEM at 10kV with a spot size of 4.0 in ultra high resolution mode. EDS was performed on the Sirion FESEM at 15kV with a spot size of 4.0 on strands mounted in conductive Bakelite and polished down to 0.05 μm using a Buehler Vibromet. The EDS calibration was verified with a bulk Nb3Sn standard sample received from Dr. W. Goldacker before each measurement session.
Fracture samples were placed in a fracture holder and snapped quickly. LN2 cooling was tried in an attempt to make the samples more brittle for flatter fracture surfaces (total wire and interfilament), but it was found the cooling had no effect and resulted in condensation on the sample which required drying before insertion into the SEM chamber.
Image Analysis
Fracture (Grain-size, radius) and polished (core fraction) images were analyzed using Image J software. Grain-size was determined using the line intercept method and each sample had ~250 grains measured. Radius measurements for microstructure evolution was determined by measuring the area within the outer edge of the layers and determining the equivalent radius assuming the area is a circle. Because it was clear that some filaments were tilted within the fracture sample, it was assumed that all filaments were round and radius measurements were corrected for this tilt. Each sample had the radius measurements averaged for at least 5 filaments.
The composition of phases indentified in the core were measured ~5 times per sample; there was little variation (±1%) in phase composition within samples. The phase fraction of the core was averaged for 10 filaments per sample using Image J software.
Phase Radius, Fine-Grained A-15 size, and Core Composition Evolution
Conclusions
Our proposed model for Tube type phase formation has been compared to experimental results.
Basic agreement is confirmed – but the presence of additional phases in the core at reaction completion will require some refinement/modification.
The Sn% in the Cu-Sn alloy in the core at Nb6Sn5 completion 25 at% -- this is in reasonable agreement with a previously estimated value of 27% -- and indicates that the transition from liquid to solid in the core slows down Sn transport in the Nb.
Various phases were found in the cores at various times after 6:5 completion, including (at longer times) a significant amount of disconnected A15.
It seems that after the 6:5 begins to convert to FG and CG A15, the core evolves separately from the Tube – and certain core reactions lead to various “High Sn-phases”.
Nb6Sn5 grains in these strands are columnar prismatic. Grain morphology affects diffusion growth models.
Strand Evolution at 650 C°- Summary
S9 Core 2hr
Cyan: 4Nb:4Sn:Cu
Dark: 3Cu:Sn
S9 Core 24hr
S9 Core 48hr
Cyan: 3Nb:Sn:0.1Cu
Orange: 4Cu:Sn
Dark: 9Cu:Sn
S9 Core 512hr
Light: 3Nb:Sn:<0.03Cu
Dark: Cu:<0.02Sn
Disperses into core
Decomposes into Nb3Sn
2-phase: Nb3Sn & Cu
2hr
The Model: Phase Formation in Tube-Type Nb3Sn
A theoretical model has been developed that allows phase formation to be predicted. This model takes into account molar volumes, stoiciometry, and the Nb-Sn-Cu phase diagram (extrapolated from 675 C° Nb-Sn-Cu Ternary). Some assumptions of this model are:
Below are equations that predict the phase formation at various times using the unreacted Sn/Cu ratios.
Radii for strand S9 measured experimentally are compared to the model:
The maximum radial growth of 6:5 was the only really accurate measurement . Reasons for discrepancies could be due to the existence of another phase identified in the core that complicated the core composition evolution. This phase (Composition initially 4Nb:4Sn:Cu from EDS) later decomposed into Nb3Sn. This Coarse-Grained Nb3Sn was dispersed throughout the core
Model
Exp
R6:5max (μm)
11.50
11.5
(μ m)
5.44
10.1
RFG (μm)
14.97
17.1
RCG (μm)
9.98
11.2
(μm)
5.51
7.3
24hr
Phases: Sn, Nb, Cu-Sn Bronze, Nb6Sn5, Fine-Grained Nb3Sn, Coarse-Grained Nb3Sn.
Coarse-Grain Nb3Sn nucleates and grows from Nb6Sn5/Fine-Grained Nb3Sn interface and eventually replaces Nb6Sn5.
Outer perimeter of filament is constant and unaffected by shrinkage/expansion or reactions
Sn conc% in the core at the maximum Nb6Sn5 radius is 27% (EDS measurements ~32% ±1%)
Sn doesn’t diffuse through the Nb barrier (no Sn-leakage)
48hr
Acknowledgements
This work was supported by the U.S. Department of Energy, grant DE-FG02- 95ER40900 and DE-SC
512hr
13-17 June 2011