Metallurgical processing, structure and superconducting transition temperature of V 3 Ga
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F OR a variety of superconducting applications, materials are required with high critical temperatures, magnetic fields, and currents. Many intermetallic phases of several structural types (A15, Laves, sigma and sodium-chloride) as well as bcc alloys, Nb-Ti and Nb-Zr, qualify as high-field superconductors. Among these materials the intermediate phases with the AI5 structure possess the highest superconducting transition temperatures determined to date. V3Ga is particularly attractive both as a practical superconductor and as a research material for investigating the role of structure in determining the superconducting properties. As a practical material, VzGa formed in composite wires of Cu-Ga and V-Ga alloys at this laboratory remained superconducting at current densities of 106 amps//cm 2 in transverse magnetic fields as high as 10 Tat4.2K.I The advantages of the V-Ga system in studies of the structural effects on the superconducting properties derive from the nature of the phase relations involving V3Ga. This portion of the equilibrium diagram defined principally by the work of van Vucht e t al2 is shown in Fig. 1. The broad compositional range of the A15 phase spans the stoichiometric V3Ga composition. Also, the A15 phase is formed by a transformation from a bcc phase, rather than a peritectic reaction common to many other systems forming superconducting A15 phases. As a consequence the severe envelopment segregation of the peritectic is not encountered in the cast structures, but the broad liquid + bcc phase field does engender proB. N. DAS, J. E. COX, and R. A. MEUSSNER are with Material Sciences Division, and R. W. HUBER, presently retired, was with Electronics Technology Division, Naval Research Laboratory, Washington, DC20375. Manuscript submittedMay 25, 1976. METALLURGICAL TRANSACTIONS A
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50 40 50 60 GALLIU M (ot-%) Fig. 1--The V-Ga phase equilibria involving the /3 AI5 phase as reported by van Vucht et al 2 nounced dendritic segregation in alloys a c r o s s the entire A15 composition r a n g e . However, as the results of this r e s e a r c h show, this dendritic segregation can be eliminated by an homogenizing treatment at high temperature within the bcc phase field. This paper reports the results of a systematic study of the interrelations a m o n g the metallurgical processing, the resultant microstructures, and the superconducting transition temperatures of the V-Ga A15 phase. Eleven alloys, spanning the entire cornVOLUME 8A, APRIL 1977-541
position r a n g e of the A15 field and each of the adjacent two-phase fields, were a r c - m e l t e d and cast as 13 mm diam r o d s . The structures of t h e s e rods were examined, and the effects of high-temperature homogenization, transformation, and annealing on the m i c r o structure and superconducting transition temperat u r e s were evaluated. Lattice p
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