CHARACTERIZATION OF THE INTERDIFFUSION MICROSTRUCTURE, A15 LAYER GROWTH AND STOICHIOMETRY IN TUBE-TYPE Nb3Sn COMPOSITES (original) (raw)
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AIP Conference Proceedings, 2008
Multifilamentary tube-type composites consisting of subelements of Nb-7.5wt%Ta tubes with a simple Cu/Sn binary metal insert were studied in an attempt to improve the performance limits of these conductors. Transport J c values for these multifilamentary strands were found to be in excess of 2200A/mm 2 at 12 T, 4.2 K. In order to further improve these values we performed a study of the reaction systematics, focusing initially on correlating the composition of the A15, the coarse/fine grain ratio, and the transport properties to the original Sn/Cu ratio and heat treatment temperature with the aid of an analytical model. Samples were heat treated at five different temperatures between 650°C-800°C for 250 hours. The extent of A15 growth, the coarse/fine grain area ratios, and the levels of residual phases were then observed as a function of temperature. In addition the temperature variation of the grain size, B c2 , and microchemistry across the A15 were determined.
Aggregation of Nb filaments in Cu–Nb/Sn composite during low-temperature intermetallic growth
Materials Letters, 1998
The solid-state diffusion behavior in the layered Cu–Sn/Cu intermetallic phase system was investigated by using Cu–20 wt.% Nb (filament)/Sn diffusion couple at 400–450°C. Very thin Nb filament is used as an inert marker for studying the Kirkendall effect-like phenomena. Experimental results show that the Nb filaments near the interface of Cu–Nb/Sn are shifting toward the Cu–Nb side and forming a Nb filament aggregating zone with the concentration of Nb up to 27–40 wt.% and a width about 10–15 μm. The study proved that the shift of Nb filament originated from volume expansion caused by the formation of ε phase (Cu3Sn). The convoluting or coarsening of ribbon-like Nb filament resulting from stress relaxation during annealing plays a role of driving force for the aggregation of Nb, making the density of Nb filaments in the aggregating zone dependent on temperature.
IEEE Transactions on Applied Superconductivity, 2011
The formation of coarse Nb 3 Sn 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 (J c ) of this strand is partly due to the formation of a significant fraction of coarse grained Nb 3 Sn 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, Nb 3 Sn coarse grain formation occurs in the filament areas that had first been transformed into NbSn 2 and Nb 6 Sn 5 , prior to Nb 3 Sn formation, and it accounts for an estimated J c reduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn 2 that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb 6 Sn 5 formed appears to be hardly influenced by the different heat treatments that have been tested.
Cryogenics, 1989
Binary and Ti + Mg alloyed NB3Sn wires both containing 19 x 132 and 55 x 588 filaments with diameters ranging between 1 and 5/~m have been produced by the internal Sn diffusion process, starting with cold hydrostatic extrusion. These wires exhibit a regular distribution of Sn cores and Nb filaments through the whole cross-section area. A heat treatment scheme including short preheating and a final reaction at 780°C yields Jc (non-Cu) values of 1.5 x 105 A cm-2 at 10 T for binary Nb3Sn wires. Ti + Mg alloyed Nb3Sn wires reacted according to the same scheme show higher Jc values above 12 T, i.e. 6.1 x 104 A cm-2 at 14 T, 1.8 ix 104 A cm-2 at 18 T. The present Paper includes measurements of Jc at applied fields up to 20 T and Jc as a function of the applied uniaxial strain, ~, at 13.5 T as well as T c after different heat treatments. The analysis was completed by the SEM determination of the A15 grain size with different additives and the measurement of the Sn concentration profiles by means of Auger spectroscopy for filaments located at different matrix regions. The absolute Sn concentration values were determined by EDX. It is found that the simultaneous addition of Ti and Mg to the Sn facilitates Nb3Sn grain growth and leads to finer grains.
Refinement of Nb3Sn grain size by the generation of ZrO2 precipitates in Nb3Sn wires
Applied Physics Letters, 2014
In this letter we demonstrate that if oxygen can be properly supplied to (Nb-Zr)-Sn wires, ZrO 2 precipitates will form during the heat treatment, refining the Nb 3 Sn grain size markedly. Here, a Nb 3 Sn subelement was fabricated in which Nb-1Zr alloy was used, and oxygen was supplied via SnO 2 powder. The results showed that such a design could supply sufficient oxygen to internally oxidize the Zr in the Nb-1Zr alloy, and that the sample reacted at 650 °C had grain sizes of ~45 nm, less than half the size of the grains in present Nb 3 Sn conductors. Magnetic measurements showed that the peak of the pinning force vs.
Materialia, 2019
The alloying of Ti with Cu(Sn) and Nb significantly increases the grain boundary diffusioncontrolled growth kinetics of Nb 3 Sn accompanied with a decrease in the activation energy in the Cu(5.5 at.% Sn, Ti)/Nb and Cu(5.5 at.% Sn)/Nb(Ti) diffusion couples. In either case, the β-(Ti,Nb) precipitates form at the grain boundaries of Nb 3 Sn. On the other hand, the ternary intermetallic phase, Nb 3 Sn 2 Ti 3 , is present in the interior of the Nb 3 Sn phase matrix only when Ti is added to Nb. The pinning forces on the grain boundaries of Nb 3 Sn exhorted by the β-(Ti,Nb) precipitates and related microstructure refinement results in an enhanced growth kinetics of the product phase, Nb 3 Sn. The addition of 0.5 at.% Ti to Cu(Sn) has a stronger influence on the growth kinetics and the activation energy for the growth of Nb 3 Sn compared to 3 at.% Ti to Nb owing to a higher fraction of smaller and equiaxed grains with high angle grain boundaries of Nb 3 Sn. The Ti-free Nb 3 Sn phase layer grows with a weak texture, a commonly observed behavior in other material systems for the product phases grown by diffusion-controlled mechanism in the interdiffusion zone. On the contrary, a very strong crystallographic texture of the Ti-containing product phase, Nb 3 Sn, is reported that has a unique pattern depending on the orientation of the adjacent Nb or Nb(Ti) grains. The Cu atoms segregate to the grain boundaries of Nb 3 Sn over a distance of ~2-5 nm with a depletion of Nb.