Abspact-The decrease of temperature ramp rate during heat treatments of the internal tin process causes an increase of the critical current density at low and medium magnetic fields (4-16"). This paper investigates the metallurgical origin of this benefit by considering the effects of a change of heating rate on the phase transformations of bronze and on the filament recrystallization. The low temperature formation of Nb3Sn is important for the critical current density of this material. This is explained by the composite microstructure evolution. I. INTRODUCITONNumerous studies have shown the importance of heating rate during the thermal treatments of Nb3Sn wires manufactured by the intemal tin process. A decrease of the ramp rate is generally beneficial for the non-Cu critical current density Jc at low and medium magnetic fields (4-16T) E13 - [3]. For instance, the authors have measured a 10-15% increase of Jc (for example from 2090 to 2325A/mm2 at 4.2K and 6T) with a diminution of heating rate from 360 to 6OC/h. Moreover, the practical significance of a low rate must be underlined especially for the treatments of large coils.The purpose of this work is to investigate the metallurgical effect of heating rate in order to explain the Jc variations. Two kinds of phenomena must be considered. First, what is the influence of ramp rate on the nature and kinetics of the phases transformations of bronze, and thereby, on both the chemical composition of the matrix and the Nb3Sn reaction? Secondly, what is the relationship between the grain size of the Nb3Sn compound and the microstructure of the Nb filaments? This correlation is suggested by the literature data. Some workers have found orientation relationships between Nb3Sn grains and Nb plane substrate addition, it is clear that the formation of Nb3Sn occurs on the Nb-side of the Nbbronze interface [5]. It must also be emphasized that a nucleation event generally occurs heterogeneously, which suggests the importance of the dislocation substructure at least of the filaments for the Nb3Sn grain size. ((1oo)Nb // (1oo)Nb.jSn and [011]Nb// [011]Nb3Sn) [4].In a wire at its final size before thermal treatment. In this composite the central tin-cored subelement consists of 270 filaments with the Nb-4at%Ta chemical composition, and it is surrounded by 6 identical subelements. These subelements are separated from the extemal stabilizer of pure copper by a Nb barrier lined with a Ta layer. At the 0.78 mm final wire size, the true deformation of the 5 micrometer-diametered filaments is at least of 6. In the active core of the composite inside the Ta diffusion barrier, the volume fractions of Sn, Cu and filaments are equal to 16.2%, 58.55% and 25.25%, respectively.The samples were 10 cm long parts of the final wire and they were vacuum encapsulated in quartz tubing before thermal treatments. Two sorts of heat treatments differing by the temperature ramp rate (6 or 60°C/h) were carried out. These heating rates were monitored up to several peak temperatures between 220 and 680°C, an...

Two different Nb,Sn strands have been processed a t GEC ALSTHOM using the internal tin process. Conductors were composed with seven sub-elements, made u p with NbTa7,5wt% filaments, surrounded by a Nb/Ta diffusion barrier to isolate the core from the copper shell. Effects of 9 different heat treatments (HT) on superconducting properties such as critical current densities, critical temperatures, critical magnetic fields and hysteresis losses have been investigated. The use of heat treatments adapted to the strand design showed a n increase in critical current up to 48 % with only a slight increase in the level of hysteresis losses. The low variation of losses is correlated to filament locations and bridging distributions across the conductor section. The critical current enhancement is achieved under a more complete transformation of the filaments into Nb,Sn and also by the refinement of Nb,Sn grains controlled by particular conditions of germination. As a result, critical current density Jc,,, = 800 A h " a t 12 T with hysteresis losses Q,,, cu *, -, = 490 mJ/cm3 have been measured.

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Nh3Sn strand for high field fusion magnets has been developed at AISA within the framework of a NET (Next European Torus) contract. The Phase I m 3 S n wire was manufactured using the internal tin aim of this work is to manufacture by an industrial internal tin process a high process developed by Intermagnetics General Corporation critical current density NhjSn strand with low losses in pulsed field (IGC) with components supplied by them, ~h~ approach allowed us to devote our major efforts, during this-initial phase. to developing process Parameters (die series, acid applications (& 3 T).Manufacturing results and wire performances are reported in this paper which also includes discussions on the kinetics of bronze formations at different chrome-coated strand.

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