Metastable and equilibrium phases in dilute V-N alloys
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D I L U T E a l l o y s of the v a n a d i u m - n i t r o g e n s y s t e m have r e c e i v e d c o n s i d e r a b l e attention, p a r t i c u l a r l y tn r e c e n t y e a r s . At l e a s t t h r e e studies of the t e r m i n a l solubility of the n i t r i d e in the bcc a - p h a s e have been p e r f o r m e d and the lack of a g r e e m e n t between these is significant. Henry e t a ! x and I-r6rz z both d e t e r m i n e d the solvus line at t e m p e r a t u r e s g r e a t e r than 600~ finding different heats of solution (922 J / m o l and 1147 J / m o l , r e s p e c tively) and s o l u b i l i t i e s . At 600~ for example, Henry e t al found the solubility to be 3.8 pet (concentrations in at. pct) while H6rz found it to be 2.6 pct. Cost and Monroe, a using internal friction, studied the solubility in the range 275~ to 575~ They obtained 1310 J / m o l for the heat of solution and a 600~ solubility ( e x t r a p o lated) of about 2.0 pot. Solubility differences such as those d e s c r i b e d above a r e s o m e t i m e s caused by the p r e s e n c e of different p h a s e s in m e t a s t a b l e or stable equilibrium with the t e r m i n a l phase. P e r h a p s a c l a s s i c c a s e in this r e g a r d is that r e p o r t e d by D i j k s t r a , involving the reduction in nitrogen content in the a - p h a s e of the i r o n - n i t r o g e n s y s t e m r e s u l t i n g f r o m the FesN to Fe4N t r a n s f o r m s tton. 4 At l e a s t two different phases a r e known to e x i s t in contact with the a - p h a s e in the V-N s y s t e m , Hahn s has identified the high t e m p e r a t u r e , hop n i t r i d e as VaN , and this has been confirmed by m o r e r e c e n t studtes, x'2 P o t t e r s has shown that VaN exhibits long range o r d e r i n g of the i n t e r s t i t i a l nitrogen atoms and has d e s c r i b e d its morphology. All of these i n v e s t i g a tions a r e c o n s i s t e n t with equilibrium between VaN and the a - p h a s e at t e m p e r a t u r e s above 675~ Potter and A l t s t e t t e r 7 have studied the low t e m p e r a -
ture t r a n s f o r m a t i o n of s u p e r s a t u r a t e d a - p h a s e in the t e m p e r a t u r e range 280~ to 350~ At these t e m p e r a t u r e s , nitrogen is o r d e r e d in a s u p e r l a t t i c e V~6N, which f o r m s within 100 min and r e m a i n s for aging t i m e s in e x c e s s of s e v e r a l hours. Morphologically, V~6N is disc shaped and has a {100) habit plane, s The vanadium atoms f o r m a bct c r y s t a l with c / a > 1. The d i s c s a r e i n t e r n a l l y twinned on the { 110} planes p a r a l l e l to the disc normal, which is the C axis of the metal unit cell. Henry e t a l ~ a p p a r e n t l y o b s e r v e d this phase, which they c a l l e d VtaN, at annealing t e m p e r a t u r e s near 500~ This led them to place a p e r i t e c t o i d horizontal on the phase d i a g r a m . A f u r t h e r f e a t u r e of the V-N s y s t e m is the p r e s e n c e of a r e a c t i o n accompanying coolin
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