The V-VO phase system
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A phase diagram is proposed for the V-VO system based on melting point determinations, differential thermal analyses, metallographic observations, and X-ray parametric measurements. A eutectic reaction occurs at 1640°C and 29 at. pct O. The intermediate phases VgO and V20 form peritectoidally at 510° and 1185°C, respectively, while V40 forms by a peritectic reaction at 1665°C. The VO phase melts congruently at 1790°C. The terminal solubility of oxygen in vanadium increases from 3.2 at. pct at room temperature to a maximum of 17.0 at. pct at the peritectic temperature. There is also extensive solid solubility associated with each of the intermediate phases. Two martensite-like phases form in alloys in the composition range 6 to 9 at. pct O upon quenching from above the 510°C peritectoid horizontal.
THE possible
u s e of vanadium alloys a s a structural m a t e r i a l in the liquid-metal-cooled f a s t b r e e d e r r e a c tor has brought about a need for a b e t t e r understanding of the p h a s e relationships b e t w e e n vanadium and oxygen. Vanadium-base alloys are being considered a s a cladding m a t e r i a l for f a s t b r e e d e r reactors b e c a u s e of the favorable n u c l e a r properties of vanadium in the f a s t fission r a n g e , its fabricability and weldability,
good high temperature strength, and relative stability (as compared with stainless steel) toward fast neutron irradiation. A critical factor, however, is the susceptibility of vanadium and vanadium-base alloys to oxygen contamination from either the liquid sodium coolant or the oxide f u e l elements and the resulting degradation of t h e i r m e c h a n i c a l properties. The V-O system h a s been the subject of numerous investigations with contradictory versions of the p h a s e diagram having b e e n proposed. S e v e r a l discrepancies occur in the literature concerning the n u m b e r and composition ranges of the p h a s e s present. Stringer1 r e v i e w e d the literature up to 1964 and proposed the composite p h a s e diagram shown in Fig. 1. During the course of the present investigation Henry et al e published a substantially different version of the vanadiumr i c h portion of this system for temperatures below 1200°C. The (~ solubility limit w a s found to be m u c h g r e a t e r t h a n previously reported, increasing from 1.1 wt p c t (3.4 at. pct) a t 250°C to 3.5 wt p c t (10.4 at. pct)
at 1200°C. They also identified a new phase, designated VgO, whichforms peritectoidally at 510°C and has a b c t structure. The banded microstructure observed by earlier investigatorsa'4 was also studied by Henry who found that a metastable bct phase with a c/a ratio greater than 1.0 was produced in alloys conmining from 2.2 to 3 wt pct (6.7 to 9.0 at. pct) oxygen upon quenching in water from 1000°C. However,upon air quenching or furnace cooling the same alloys contain the bct equilibrium phase, VgO, with a c/a ratio less than 1.0. The purpose of the present study was to clarify the points of disagreement in the earlier work
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