Influences of heat treatment and Ti addition on high performance Rod-in-Tube (Nb,Ta)3Sn Strands X. Xu M. D. Sumption C. J. Kovacs E. W. Collings Hyper-Tech Research, Inc. X. Peng Acknowledgements: This work was funded by the US Department of Energy, Division of High Energy Physics, Grant No. DE-FG02-95ER40900 and a DOE Contract Number DE-SC0001558.

Motivation To explore the possibility of using higher reaction temperature and Ti doping to flatten Jc(B) curves to improve low field stability, using Ta-Ti doped strands comparing to Ta-doping only. To explore the effects of HT temperature, time, and Ti doping on Birr, Fp,max, grain size, and 12 T Jc with respect to the (Nb,Ta)3Sn strands. Overall, looking to use Ti or higher Temp HTs to have large high field Jc but stable low field performance.

Samples (c) (a) (b) Heat treatment (HT): pre HT (210˚C/48h+400˚C/72h) + reaction HT (Table 2): Strand Name Reaction Heat Treatment (˚C/h) G1 & R1 615/150,300; 650/50,100; 700/35,50; 750/40; 800/20,30

Low field flux jumps M-μ0H loops on VSM/PPMS: 4.2K, ±14 T applied field ┴ strand axis ramp rate: 13 mT/s Observation 1: The lowest stable fields decreased as the HT temperature increased.

Lowest B of first FJ, and maximum pre-FJ M Observation 1: The lowest stable fields decreased as the HT temperature increased. Observation 2: The highest stable magnetization increases HT temperature increased up to 700C. G1 G1

Lowest B of first FJ, and maximum pre-FJ M Observation 3: Ti added sample stable to lower B Observation 4: Magnetization limit higher for Ti added sample G1 G1

M-H Loop Comparison for R1 and G1

Questions High Temp HT could be expected to reduce low field magnetization, decreasing the field for first flux jump Ti additions could be expected to increase Bc2 and increase grain sizes, also decreasing the field for first flux jump But – why would high temp HT increase the possible magnetization before first FJ? Why also would Ti addition increase the possible magnetization before flux jump?

Reasons for stability increases? Possible reasons: (1) Tc goes up, stored energy goes up Critical temperature Tc Tc increases with reaction temperature from 615 to 750 C. Tc increases with reaction time. Tc also increases by Ti doping. ρ: mass density; C: specific heat; Tb: bath temperature. 2. Increases in Heat capacity C? The linear term term of electronic specific heat, γ, is a function of stoichiometry. According to Flukiger [1], As Sn content is below 24 at.%, γ increases by 1.5 ± 0.1 mJ/K2 at-g per at.% Sn, reaching a maximum of 13.5 mJ/K2 at-g at stoichiometry. Larger HT temperature and time, and Ti doping improve stoichiometry, and thus C. 3. RRR influence on dynamic stability 4. Lower slope Jc vs T at low field [1] Flukiger R, Uglietti D, Senatore C and Buta F 2008 Cryogenics. 48 298-307

Magnetic non-Cu Jc rI rO The magnetic and transport Jcs are consistent (the discrepancy is no more than 4%). Below 700 ˚C, the Jc is maintained above 3000 A/mm2 level. The highest 12 T Jc was achieved on (Nb,Ta)3Sn-615x300h : above 3500 A/mm2. Above 700 ˚C Jc drops with temperature. For (Nb,Ta)3Sn strands, longer HT time led to higher Jc. For optimum HT time, Ti doping was not beneficial to improve 12 T Jc.

Ratio of 3 T to 12 T Jc Arguably a 10% reduction in 3T/12 T Jc with Ti additions Roughly 10% reduction of 3T/12T per 50 C HT

Kramer plots and calculations of Birr and Fp,max By finding a and k values from Jc1/2B1/4(B) plots, we can get: and . This Fp,max is quite consistent with peaks of Fp(B) curves.

Irreversibility fields Birr,K Birr,k increases with HT Temperature. For R1 strands longer duration time led to higher Birr,K values at each HT temperature. Reason: Birr increases with HT time, and then plateaus, similar to grain growth. At above 700 C the Ti doping improves Birr, could be because Ti doping improves stoich-iometry. At below 700 C, the shorter and longer reacted G1 strands had similar Birr, could be because they were both in the plateau regions? Which indicates Ti doping cut short the reaction time to reach the plateau regions. These Birr values were similar with longer reacted R1 strands, perhaps because at low temperature majority of Ti remains in the Cu-Sn core? Muller H and Schneider Th 2008 Cryogenics. 48 323-30

Maximum Pinning force, Fp,max At above 650 C higher HT temperature led to lower Fp,max. The Fp,max between 615 and 650 C are similar. Possible reasons: strands at 615 ˚C had smaller A15 area fractions; smaller grain size at 615 C failed to improve Fp,max a lot? reasons to be addressed later. In the investigated range the influence of HT time over the Fp,max was insignificant. Ti doping caused Fp,max to drop but the influence was not as significant as enhanced HT temperature.

Grain Size Analysis (a) (b) (c) (d) (e) (f) (g) (h) In 615 ˚C strands grains were somewhat columnar towards the cores of the original filaments, with aspect ratios ~ 2.1. Strands at 650 ˚C had aspect ratios of ~ 1.8 while those above 700 ˚C had equiaxed grains (aspect ratios < 1.5). Prolonging reaction time caused only slight grain coarsening, indicating both reaction times in the plateau regions. Schelb W 1981 J. Mater. Sci. 16 2575-82

The correlation between Fp,max and grain size 3. The grain sizes of the longer reacted strands increased with HT temperature exponentially: The calculated activation energies for R1 and G1 strands were 70.9 and 57.8 kJ mol-1, respectively, proving that Ti doping promotes the Nb3Sn grain growth. Correlation between Fp,max and grain size: As grain size was >120 nm, Fp,max increased linearly with the reciprocal of grain size 1/d. As grain size was smaller than 120 nm, the increase of Fp,max with 1/d became much slower. This transition could be due to the change in A15 grain morphology: the grains of the 615 ˚C strands had larger aspect ratios, tending to have higher fractions of low angle grain boundaries, which are perhaps less efficient pins. Similar transition was also observed by Marken.

Conclusions We were interested to investigate Ti additions and HT variations to flatten Jc vs B and increase Nb3Sn stability. We found … Ti additions to NbSn-Ta tilts the Jc vs B (flattens) slightly (3T/12 T ratio drops by 10%) Increases in HT Temp flattens Jc vs B (3T/12T Jc) by 10% per 50 C HT increase Ti additions to NbSn-Ta conductors decreases the field of first FJ Increases in HT Temp decreases the field for first FJ Ti additions to NbSn-Ta Increase the stable limit Magnetization Increases in HT Temp increase the stable limit Magnetization Results 1-4 easy to understand – results 4,5 not so much – may be due to (i) Tc increases (ii) Cp increases, (iii) RRR increases, (iv) Jc vs T changes Birr,K and increases grain size with HT temp are shown as decreases Fp,max. HT temperature above 700 C could reduce Jc severely. Long enough reaction time is required to push the Birr,K of (Nb,Ta)3Sn strands to the ultimate level. A small amount of Ti addition could cut short the duration time to reach the ultimate value, but Ti doping increases grain size by decreasing the formation energy of grain growth. The columnar Nb3Sn grains could be less efficient pins. To improve Fp,max for strands reacted at low temperatures, it is beneficial to decrease Sn content gradient so that the aspect ratios of grains can be reduced.