Abstract
The range of stability and some properties of the A-15 phase in the V-Ga system were investigated. It was found that the A-15 phase forms at temperatures below 1100°C and exists over a composition range of approximately 3.5 V : Ga to 2.1 V : Ga at the maximum temperature of formation investigated. The width of the phase field decreases with decreasing temperature. Accurate values of the composition range could not be established because of the presence of a second phase called the γ phase which forms in the temperature range of 1100°C to 1500°C. The structure of the γ phase has been tentatively identified as hexagonal. Above 1500°C to the melting point, the bcc solid solution of Ga in V is the stable phase and the solubility of Ga in V in this temperature range is at least 33 a/ O Ga in V. The solid solution of Ga in V could be retained at room temperature by cooling from the melt at vanadium compositions of 80% and higher. The A-15 phase could then be precipitated from solid solution by heat treating at temperatures of 700°C to 1050°C. When formed in this manner, the A-15 phase had a superconducting transition temperature comparable to the as-cast stoichiometric material, but the slope of the critical magnetic field versus transition temperature curve was found to be approximately one-half the value of the as-cast stoichiometric material. It is suggested that the presence of the γ phase in the microstructural morphology characteristic of as-cast material is responsible for the observed high value of the slope of the critical field versus transition temperature curves. In fact, the critical field values obtained experimentally for stoichiometric as-cast material may be those for H c3 while for the case of the A-15 phase precipitated from solid solution the critical field values measured are H c2 . The degree of order of the A-15 phase as a function of composition was investigated and the manner of substitution of the excess atoms in off-stoichiometric compositions is described.
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