NSCL-RIA-1 Measured properties of High RRR Niobium

Chris Compton National Superconducting Cyclotron Laboratory

Tom Bieler, Ben Simkin, Susheel Jadhav Materials Science and Mechanics

Michigan State University East Lansing, Michigan 48824

August 9, 2000 Background The National Superconducting Cyclotron Laboratory (NSCL) is initiating a superconducting radio frequency (SRF) cavity research and development program including the construction and testing of SRF cavities for the acceleration of charged particles. The cavities are formed using high purity niobium sheet metal that has been properly processed and formed to eliminate impurities and to develop a fine, recrystallized grain structure. In order to operate and hold the desired electrical fields needed for acceleration, the properties of the niobium material must have a high residual resistivity ratio (RRR) value. RRR is the ratio between the resistivity of the material at room temperature (298° K) and the resistance at superconducting temperature (~ 4° K). This RRR value directly depends on the impurity levels of the parent niobium ingot. The niobium sheets are formed into the desired cavity shape using a die and a hydraulic press. The fine, recrystallized grain structure allows the niobium sheet to be formed into the cavity shape without tearing. Purpose The NSCL, in collaboration with the Materials Science and Mechanics (MSM) department at Michigan State University, have initiated a study of high RRR niobium material. This study is to determine property values of the high RRR niobium in order to establish a reference point to cross check future supplies of high RRR material. The high RRR niobium sample was supplied by Wah Chang-Teledyne, see Appendix A, and rated with a RRR of 492. The initial study looked at two areas, tensile properties and optical microscopy. From the tensile tests, a stress-strain curve can be measured from which tensile properties can be found and compared. The optical work was used to determine whether the materials were properly recrystallized, the average grain size, and the uniformity of the grains. In addition to the high RRR niobium, a lower grade niobium sample was also studied for comparison. The lower grade niobium was supplied by Rembar, and has a 99.99% purity level. The RRR value of the low-grade sample was not measured, but based on the purity level, it can be compared to reactor grade niobium, which has RRR values ≈ 30.

Procedure The tensile tests were preformed on a 4200 series Instron testing machine, model number 43K2. Three samples were taken from each of the two grades of niobium. Each set of three samples were machined out of the parent sheets, at 0°, 90°, and 45° as shown in Figure 1. The samples were machined into the standard “dog bone” shape, with a gage length of 2 inches, and gage width of 0.5 inches as shown in Figure 1, [3]. The tensile test was preformed at a strain rate of 0.1 in/min. Data was collected by the tensile tester and put into a database.

Figure 1: Niobium tensile sample layout and dimensions.

The samples, for the optical work, were mounted and polished using two different methods. One group of samples was polished using mechanical polishing techniques. The samples were polished with a 180 grit SiC grinding belt, followed by wet hand grinding with 240, 320, 400, and 600 grits. They were then polished with a 600 grit SiC suspension in water on a polishing wheel. This was followed by a 5 µm SiC suspension, then a 0.3 µm Al2O3 suspension. The etch was 10ml hydrofluoric acid+10ml nitric acid+30ml lactic acid for approximately 40-60 seconds. The second group was electro-polished in 90%H2SO4+10%HF at 0° C using a tungsten electrode at a potential of 15V. The samples were prepared before polishing by immersing them in a 70%HNO3+30%HF bath (chilled to ≈0°C) for approximately 1 hour. A final etch of 45 seconds in the HF+HNO3+lactic acid was then used to bring out the grain boundaries The mounted, polished samples were then viewed under a Nikon Eclipse ME 600 optical microscope operated in reflectance mode with digital image capabilities.

Stress-Strain plot of Nb Samples

-500

50100150200250300350

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Strain(mm/mm)

Stre

ss(M

Pa)

0 HG90 HG45 HG45 LG90 LG0 LG

An average grain size for the samples was determined by using the intercept method [2]. In this method, lines of equal distance are drawn onto the microstructure images. The number of grains that are intercepted by each line are counted. The average number of grains intercepted is then determined by taking the counted grains per line and averaging among all the lines. The line length is then divided by the average grains per line. The average grain diameter is then found by dividing by the magnification power at which the microstructures were analyzed. Results The tensile tests showed the high RRR niobium to have a lower 0.2% yield and maximum stress values with corresponding higher elongation values than the low RRR material as shown in Figures 2-4 and Table 1. This is consistent with the concept of the higher purity material having the lower the stress values. All the samples in each grade showed similar stress-strain values independent of angle. The largest variance was found in the high RRR samples. For the high RRR, the elongation value for the 90° sample was ~ 13 % below the other two angles, while all the low RRR samples angles were within < 4%.

Figure 2: Stress-Strain curves showing all samples from both grades of niobium. High Grade (HG) sample thickness = 3 mm (.11 inches) with RRR value of 492. Low Grade

(LG) sample thickness = 0.838 mm (0.031 inches), with a RRR value ~ 30.

Stress-Strain plot of High-Grade Nb Samples

-50

0

50

100

150

200

250

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Strain(mm/mm)

Stre

ss(M

Pa)

0 HG90 HG45 HG

Stress-Strain plot of Low-Grade Nb Samples

-50

0

50

100

150

200

250

300

350

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Strain(mm/mm)

Stre

ss(M

Pa)

45 LG90 LG0 LG

Figure 3: Stress-strain curves showing samples from high-grade niobium

Figure 4: Stress-strain curves showing samples from low-grade niobium.

Table 1: Shows the results of the tensile test for the three samples of the two grades. High grade (HG) has a RRR value of 492, low grade (LG) has a RRR value of ~30.

Niobium Samples 0.2% Yield (MPa) Max. Stress (MPa) Elongation (%)HG 0° 77.67 202.6 66.28 HG 90° 73.12 198.7 57.34 HG 45° 70.84 200.1 66.38 LG 0° 202.4 320.1 50.42 LG 90° 194.8 327.4 52.48 LG 45° 157.11 318.11 50

The optical work showed recrystallization had occurred throughout most of the grain structure in both samples, as shown in Figures 5-8. In some areas, both samples did show slightly elongated grains that could be attributable to less than full recrystallization. Future texture tests could be done to determine the actual percent of recrystallization. The microstructure of both samples showed a heterogeneous mixture of grain sizes. The different sizes appeared to be formed in bands. There would be an area of smaller grains, then as one moved across the microstructure, one would see bands of larger grains as shown in Figure 6. The different sizes could be the result of areas with localized stress and different nucleation rates during the recrystallization process. The average grain size was determined in each of the two grades of niobium. Due to the bands of different grain sizes in each grade, the average grain size was determined for the large grain areas and for the small grain areas as shown in Table 2.

Table 2: Average grains sizes found in the two niobium grades. Large Grains Small Grains

High Grade Nb 59 µm 29 µm Low Grade Nb 57 µm 32 µm

Figure 5: Microstructure of high-grade niobium, small grain sizes (28 µm)

Figure 6: Microstructure of high-grade niobium, showing banding. Right edge of structure small grain sizes (25 µm), on left large grains (60 µm).

Figure 7: Microstructure of low-grade niobium, showing small grain size (31 µm).

Figure 8: Microstructure of low-grade niobium, showing large grain size (59 µm).

Discussion Tensile values and optical images were used to compare the different grades of niobium. The tensile tests showed significant differences between the two niobium grades. The removal of impurities and the recrystallization process of the high RRR niobium had a large impact on the values seen in stress-strain curves as shown in Figure 2. These values were also compared with published stress-strain curves of similar high RRR niobium and cross-referenced with tensile measurements done on the parent niobium material at Wah Chang, prior to shipping, as given in Appendix A. Wah Chang’s values were within a few percent of those obtained in this study. The largest difference between our measurements and Wah Chang’s values was a 14% higher maximum tensile stress. The discrepancy could be based on the different strain rates used (0.1 in/min in this study and 0.005 to 0.05 in/min in the Wah Chang study), as shown in Appendix A. The values obtained from this study were also compared to those found in the literature [1]. In this context, the stress-strain curves as well as the 0.2% yield stress values were found to agree within a few percent. However, the maximum stress values assuming inverse scaling by RRR are in apparent disagreement. The literature value of 140 MPa for a RRR of 250 is in apparent discrepancy with our study value of 200 MPa for a RRR of 492. This circumstance may be due to differences in crystallographic texture and/or sample thickness, but without further investigation this can only be conjecture. The optical work showed the microstructures of both grades to be almost completely recrystallized with equal axial grains dominant. Both grades showed similar grain structures with some bands of large grain growth and areas showing some slight elongation. Conclusions based on these images are hard to derive without knowledge of the process history of the low RRR niobium and high RRR niobium. As mentioned above, texture tests could be performed to gain a better idea on the percent of recrystallization that has occurred in both samples by quantifying the amount of “deformation” and “recrystallization” texture components. The variation in grain size between the two grades was very similar, as shown in Table 2. Both grades displayed areas of small grains as well as areas of large grains. In comparing calculated grain sizes to reference sizes given in published literature, [1], some questions concerning the large grain areas need to be addressed. The literature states that the high RRR materials must have a uniform grain size of < 50 µm, to avoid problems during the cavity forming process [1]. The grain structure in this study clearly showed a non-uniform grain size, and the large grain areas were calculated to be slightly larger than the 50 µm grain size. It is not clear whether this would lead to fabrication problems, but the point should be considered. Conclusion The tensile values obtained in this study compare well with values measured in other studies of similar high RRR niobium. However, some discrepancies with the literature for the maximum stress values were noted. These values give a well-defined benchmark

to cross reference future supplies of niobium materials. The optical examination showed a recrystallized structure, but raised some question as to the uniformity of grain size and the size-banding phenomenon that went along with the grain structure. Grain sizes, in areas, were observed to be slightly higher than documented limits (< 50µm). It is undetermined what effects these differences will make during cavity fabrication. Appendix A WAH CHANG Niobium RRR sheet Heat No.: 503878 SFC No.: 1161860

Ingot Chemistry Analysis: (Results in PPM) Elements 1 2 3 4 5

C <20 <20 <20 <20 20 Fe <30 <30 <30 <30 <30 H <3 <3

Mo <30 <30 <30 <30 <30 N <20 <20 O 40 <40 Si <25 <25 <25 <25 <25 Ta 110 <100 <100 <100 100 Ti <40 <40 <40 <40 <40 W <30 <30 <30 <30 <30

Ingot Hardness Results: Avg.: 44 Ind.: 48 Metallography Test: (Results) Micro No.: CI-437 Percent Recrystallization: (Results) Sample: 100% Grain Size Test: (Results) Sample: 90%-6.5; 10%-8.5 Note: No Grains larger than No. 4 Product RRR Test: (Results) Sample: 492

* RRR Testing performed at Oregon State University Room Temperature Tensile Test: (Results)

* Strain rate 0.005 in/in/min through the 0.2% YS, and 0.05 in/in/min. thereafter Long Trans. Tensile Strength: 24800 Psi 25400 Psi

Yield Strength (0.2%) 12100 Psi 8900 Psi Elongation 64% 64%

References

1. H. Padamsee, J. Knobloch, and T. Hays, “ RF Superconducting for Accelerators”, J. Wiley and Sons, Inc., New York p.105-127 (1998)

2. W. Callister Jr. “Materials Science and Engineering, An Introduction”, 2nd

Edition, J. Wiley And Sons, Inc., New York p.88-89 (1991)

3. American Society for Metals, “Mechanical Testing”, Vol. 8, Metals Handbook 9th Edition, p. 28-30, 553-554 (1985)