Phase equilibria in the metal-rich side of the Ta-N system
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I N T E R S T I T I A L ordering in the metals of group V A is well documented. Ordered structures have been observed in the systems V-N, x V-O, a Nb-O, s Ta-C, 4 Ta-O, B and Ta-N. 5'6 These structures consist of longrange-ordered arrays of the small carbon or nitrogen or oxygen atoms in the interstitial voids of the metal structure. The presence of the interstitial sublattice not only expands the metal lattice but m a y also change its symmetry. In the ordered phase V,6N, for example, the vanadium lattice changes from bcc to bct.* An early X-ray investigation of the T a - N system suggested the presence of interstitially ordered phases v and subsequent work has borne this out. However, m a n y of the basic facts about this system are still in question, particulary in tantalum-rich alloys (< 20 at. pct N) and below 1000~ The nature of the phase equilibria in this region as well as the structure of the phases themselves are still uncertain. The present work deals with these questions and describes several precipitates that were observed in this system. Their structures are discussed, and a partial phase diagram is presented. EXPERIMENTAL
PROCEDURE
Tantalum strips of 125 or 250 ~t thickness were Joule heated in a high vacuum chamber and reacted with known quantities of Na gas (< 10 ppm total impurity content). At reaction temperatures of about 1600 ~ to 1700~ virtually all the Nz charge enters the foil when the resulting alloy composition is 5 at. pct N or less. For compositions above this and at reaction t e m p e r a tures of about 2000~ the vapor p r e s s u r e of N2 gas in equilibrium with the alloy is appreciable. The vapor p r e s s u r e s observed agree well with the data of Gebhardt et a/fl By taking into account the gas remaining in the reaction chamber due to the vapor p r e s s u r e , accurate compositions could be determined in all cases. Also the values of the heating current and voltage needed to maintain the reaction temperature yielded the resistivity of the alloys at temperature. The values are consistent with a resistivity increment R. H. GEILS, formerly Graduate Student, Union College, Schenectady, N. Y. 12308, is now Research Scientist, Xerox Research Center, Palo Alto, Calif. D. I. POTTER is Assistant Professor, Department o f Mechanical Engineering, Union College. Manuscript submitted October 17, 1972. METALLURGICAL TRANSACTIONS
of 4 . 8 / z - o h m - c m / a t . pet. The value at 4.2~ is 5.1.9 The supplier's analysis of the tantalum stock used in this study is given in Table I. For a particular alloy, the reaction temperature was chosen so that the solubility was not exceeded and so that homogenization was achieved in an hour or less. Temperatures were measured using an optical p y r o m eter which had been calibrated with a W - W , 26 pct Re thermocouple. The high-temperature solubility data used was that of Gebhardt et al. s and the homogenization times were estimated using the diffusion data of Powers and Doyle. t~ The actual times used were in excess of these estimates by 2 to 4 times
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