Florin Buta

Direct observation of Nb3Sn lattice deformation by high-energy x-ray diffraction in internal-tin wires subject to mechanical loads at 4.2 K

With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads.

Phase Transformations During the Reaction Heat Treatment of Internal Tin Nb

The phase transformations that occur during the reaction heat treatment (HT) of Nb3Sn superconductors depend on the overall elemental composition of the strand subelements. In the case of modern high Jc strands with a relatively low Cu content, liquid phases are present during large temperature intervals and phases that can be detrimental for the microstructural and microchemical homogeneity of the fully reacted strand are formed. We report synchrotron X-ray diffraction measurements during in-situ reaction HT of a state-of-the-art high Jc Nb3 Sn internal tin strand. In this strand, Cu3Sn is formed upon Cu 6Sn 5 decomposition at 415degC, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345degC-575degC, and NbSn2 is present in the temperature interval 545degC-630degC. The formation of voids in the strand subelements has been monitored by synchrotron microtomography during in-situ reaction HT.

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The electromechanical behavior of a Nb3Al wire manufactured according to the RHQT process (rapid-heating, quenching and transformation) has been investigated at magnetic fields between 15 and 19 T at 4.2 K. Of particular interest was the critical current, Ic, as a function of transverse pressure up to 300 MPa and as a function of axial tensile stress. The studied wires are pieces of a 870 m long copper stabilized Nb3Al wire with a rectangular cross section of 1.81 mm × 0.80 mm. It was observed that the critical current at 300 MPa transverse pressure, applied to the narrow side, is reduced to 93%, 90% and 88% of its stress free value at 15 T, 17 T and 19 T, respectively. After unloading from 300 MPa Ic recovers to 94% and 97% at 19 T and 15 T, respectively. A field dependence of the effect is visible above 200 MPa. For completeness, the critical current was also measured under axial tensile strain. The maximum of Ic is at 0.15% applied strain and irreversibility has been observed above 0.26%. Finally a stress versus strain measurement at 4.2 K has been carried out allowing the conversion from axial strain to stress.

Sn Strands With High Sn Content

The high flux of high energy X-rays that can be provided through state-of-the-art high energy synchrotron beam lines has enabled a variety of new experiments with the highly absorbing Nb3Sn superconductors. We report different experiments with Nb3Sn strands that have been conducted at the ID15 High Energy Scattering beam line of the European Synchrotron Radiation Facility (ESRF). Synchrotron X-ray diffraction has been used in order to monitor phase transformations during in-situ reaction heat treatments prior to Nb3Sn formation, and to monitor Nb3Sn growth. Fast synchrotron micro-tomography was applied to study void growth during the reaction heat treatment of Internal Tin strands. The elastic strain in the different phases of fully reacted Nb3Sn composite conductors has been measured by high resolution X-ray diffraction during in-situ tensile tests.

Critical current of a rapid-quenched Nb3Al conductor under transverse compressive and axial tensile stress

The lattice parameter changes in three types of Nb3Sn superconducting wires during uniaxial stress–strain measurements at 4.2 K have been measured by high-energy synchrotron x-ray diffraction. The nearly-stress-free Nb3Sn lattice parameter has been determined using extracted filaments, and the elastic strain in the axial and transverse wire directions in the different wire phases has been calculated. The mechanical properties of the PIT and RRP wire are mainly determined by the properties of Nb3Sn and unreacted Nb. This is in contrast to the bronze route wire, where the matrix can carry substantial loads. In straight wires the axial Nb3Sn pre-strain is strongest in the bronze route wire, its value being smaller in the PIT and RRP wires. A strong reduction of the non-Cu elastic modulus of about 30% is observed during cool-down from ambient temperature to 4.2 K. The Nb3Sn Poisson ratio at 4.2 K measured in the untwisted bronze route wire is 0.35. The present study also shows that the process route has a strong influence on the Nb3Sn texture.

Synchrotron Radiation Techniques for the Characterization of

A study of the critical current of a PIT strand under transverse compressive and axial tensile loads is presented. The conductor is not reinforced and has a proof strength, , of 142 MPa. Although the tensile strain at the maximum of the critical current, , is only 0.14% (low thermal precompression) the strand behaves as expected. However it is particularly sensitive to transverse compressive forces. Even relatively small forces yield to a reduction of the critical current. Qualitatively this degradation may be explained by a non-uniform deformation of filaments, yielding to irreversible micro cracks, and by a reduced of intact filaments. The recovery of upon unloading of the transverse compressive force has also been investigated.

{\rm Nb}_{3}{\rm Sn}

The formation of coarse Nb3Sn grains in Internal Tin (IT) strands has been studied at the example of a prototype strand with high Sn content. Metallographic examination revealed that the comparatively low critical current density (Jc) of this strand is partly due to the formation of a significant fraction of coarse grained Nb3Sn at the periphery of the individual filaments within the subelements. The phase evolution during the reaction heat treatment has been determined in situ by high energy synchrotron X-ray diffraction as well as ex situ by Energy Dispersive X-ray Spectroscopy in a Scanning Electron Microscope (SEM) in order to identify the conditions under which the coarse grains form. Similar to what is observed in the tubular type strands, Nb3Sn coarse grain formation occurs in the filament areas that had first been transformed into NbSn2 and Nb6Sn5, prior to Nb3Sn formation, and it accounts for an estimated Jc reduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn2 that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb6Sn5 formed appears to be hardly influenced by the different heat treatments that have been tested.

Superconductors

The evolution of Nb containing phases during the diffusion heat treatment of three different high critical current Nb3Sn strand types is compared, based on synchrotron X-ray diffraction results that have been obtained at the ID15 beam line of the European Synchrotron Radiation Facility (ESRF). In all strands studied, Nb3Sn formation is preceded by the formation of a Cu-Nb-Sn ternary phase, NbSn2 and Nb6Sn5. As compared to the PIT and Tube Type strand, the amount of these phases formed in the RRP strand is relatively small. In the RRP strand subelements with a fine filament structure Nb3Sn grows more quickly, thereby preventing to a large extent the formation of the other higher tin phases.

Stress distribution and lattice distortions in Nb3Sn multifilament wires under uniaxial tensile loading at 4.2 K

The critical current of a Nb3Al wire (RHQT process) with a rectangular cross section of 1.81 × 0.80 × mm2 has been measured at 4.2 K under transverse compressive loads at fields between 15 T and 19 T. The same wire (HE2432) was previously used for winding an insert coil at NIMS (coil #B) that generated 4.5 T in a background field of 15 T at 4.2 K. In this work the load is applied perpendicularly to the narrow side of the conductor and the corresponding stresses reach 300 MPa. The critical current stays almost constant up to 150 MPa and shows a flat maximum around 75 MPa. At 300 MPa the critical current is reduced to 93%, 90% and 88% of its stress free value at 15 T, 17 T and 19 T, respectively. This indicates a rather small field dependence and significant degradation being seen only above 200 MPa. Depending on the field, unloading from 300 MPa yields to an almost complete recovery of Ic between 94% and 97% of its initial value, respectively. These observations are compared to the behavior of a Nb3Sn bronze wire.