Microstructural changes produced in a multifilamentary Nb-Ti composite by cold work and heat treatment
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I.
INTRODUCTION
THEgeneral effects of the commercial fabrication process on the microstructure of Nb-Ti composite superconductors are well established. Extensive cold drawing with typical area reduction ratios of 10 3-10 5 to 1 are required to reduce the initial extrusion billets to wires with diameters of 1 mm or less. These large deformations produce very high dislocation densities in the Nb-Ti filaments and give rise to the typical sub-band structures observed. These sub-bands run parallel to the drawing axis of the wire and generally have diameters between 35 and 50 nm. 1The Nb-Ti alloys used in superconductor manufacture have compositions in the range Nb 45 to 65 wt pct Ti. These alloys are two-phase at room temperature consisting of Ti-rich a-phase (hcp) and Nb-rich /3-phase (bcc). Decomposition of the metastable, singlephase, Nb-Ti alloy rods used in the initial billet assembly occurs during the relatively low temperature heat treatments at 623 to 673 K (350 to 400 ~ which are given as part of the commercial processing. These temperatures are only about 0.3 to 0.4 TM, and the kinetics and morphologies of the a-Ti precipitation reaction are therefore strongly influenced by the matrix microstructure at the time of heat treatment. Originally it was believed that the lower Ti-content alloys obtained their excellent superconducting properties solely by the production of a very fine sub-band structure with no a-Ti precipitation, 2 while the higher Ti-content alloys exhibited good superconducting properties due to the combination of a fine sub-band structure and t~-Ti precipitates. 3 It is now clear that all alloys of technical interest are optimized by the production of two-phase microstructures. 4 However, the interdependence of the deformation and precipitate microstructures makes it difficult to obtain quantitative relationships between the fabrication parameters and the resulting microstructural and superconducting properties. The superconducting critical current density (Jc) is determined by the interaction between the fluxoid lattice and the
defect microstructure and hence is strongly microstructuredependent. Since the fluxoid separation varies over the range of approximately 10 to 30 nm and the fluxoid diameter is of the order of 10 nm, it is necessary to obtain a quantitative analysis of the microstructure over the same range. The sub-band structure may be characterized by the measurements of the average sub-band diameter from transmission electron micrographs. However, the characterization of the ot-Ti precipitate distributions using transmission electron microscopy (TEM) has proved more of a problem, owing to difficulties in actually imaging the precipitates.4'5 As stated previously, the distribution of the a-Ti precipitates which form during heat treatment must be dependent on the existing Nb-Ti microstructure. Up to three or four heat treatments are carried out between drawing cycles during some commercial fabrication procedures. It is therefore important to understand how the sub-band structures an
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