抄録
Although the rapid heating and quenching (RHQ) optimization process is a key to achieving high critical current density (Jc) through controlling the microchemistry and grain structure of a bcc supersaturated-solid solution and subsequently transformed (T) Nb 3 Al phases, only the RHQ parameters of wire diameter (dwir e ), wire speed (vwi re ) and heating current (IRHQ) for a given electrode spacing (Lelectrode) have been investigated. A heating time (τ) of a focused wire position was given by Lelectrode/vwir e. Thus, a larger τneedes a smaller vwir e, which also causes a slower wire cooling rate that partly forms the undesirable A15 phase during RHQ treatment. In the present study, an attempt has been made to extend the Lelectrode from 100 mm to 470 mm. This enables both a large τand a relatively large vwir e , which ensures a cooling rate sufficient to obtain the bcc supersaturated solid solution. The Nb/Al precursor used is a single-filament jelly roll (JR) wire in which the matrix (sheath) species is Ta and the Al layer thickness is 600 nm (3-6 times thicker than conventional multifilament JR precursors). A large τresults in an increase in the volume fraction of the bcc supersaturated solid solution. The critical temperature (Tc) and Jc optimized by RHQ treatment for each τare found to increase in proportion to τ. The plastic deformation carried out between RHQ and transformation treatments is effective for enhancing the Jc properties, even for RHQ parameters of large Lelectrod e and τ .
