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The s u b s t r a t e was held in a p i c t u r e f r a m e device, upon which r e s t e d heating e l e m e n t s . T h e r m o c o u p l e s w e r e placed a g a i n s t the unexposed side of the subs t r a t e and s e r v e d to provide a c o n t r o l s i g n a l for the s u b s t r a t e h e a t e r and a m e a s u r e of t e m p e r a t u r e at several positions. The s u b s t r a t e s w e r e p r e c o a t e d with an e v a p o r a t e d l a y e r of copper 0.012 to 0.025 m m thick. A f t e r deposition, the copper l a y e r was r e m o v e d b y i m m e r s i n g the deposit and s u b s t r a t e in a s o l u t i o n of n i t r i c acid. Other r e l e a s i n g t e c h n i q u e s were e x a m i n e d , but the above technique p r o v e d the most s a t i s f a c t o r y . Background p r e s s u r e was m o n i t o r e d with an ion gage and p a r t i a l p r e s s u r e a n a l y z e r . The r a n g e of p r e s s u r e s m a i n t a i n e d d u r i n g these e x p e r i m e n t s was between 13.0 and 0.13 m P a (10 -4 and 10-6 T o r r ) . The c h r o m i u m used in these e x p e r i m e n t s was obtained f r o m MRC C o r p o r a t i o n in the f o r m of a r c - c a s t ingots. All but one of the e x p e r i m e n t s d e s c r i b e d h e r e i n were p e r f o r m e d u s i n g VP G r a d e Cr, the other involving the use of MRC M a r z - G r a d e I o c h r o m e . The c o m p o s i t i o n of t h e s e m a t e r i a l s is given in T a b l e I. C h e m i c a l a n a l y s e s for i n t e r s t i t i a l i m p u r i t i e s were p e r f o r m e d ort the ingot m a t e r i a l and on d e p o s i t e d m a t e r i a l u s i n g i n e r t gas f u s i o n a n a l y s i s . T h i s s e r v i c e was kindly donated by A i r c o Vacuum M e t a l s , Inc., B e r k e l e y , Calif. The conditions for s o m e of the dep o s i t i o n e x p e r i m e n t s a r e given in T a b l e II, along with the c o m p o s i t i o n of the d e p o s i t s . S p e c i m e n s for m e c h a n i c a l t e s t i n g w e r e p r e p a r e d by cutting and g r i n d i n g the f r e e - s t a n d i n g deposits, followed by e l e c t r o p o l i s h i n g . T e n s i l e s p e c i m e n s with gage d i m e n s i o n s 16.5 x 5.0 x 0.8 to 0.2 m m w e r e t e s t e d in a i r at t e m p e r a t u r e s up to 773 K, at a s t r a i n r a t e of 2.5 x 10 -4 s -I. T h r e e - p o i n t bend s p e c i m e n s with d i m e n s i o n s 30 x 5 x 0.8 to 0.2 c m w e r e t e s t e d in vacuum, P = 2.7 to 10.7 m P a (2 to 8 x 10-s T o r r ) at t e m p e r a t u r e s up to 1273 K, at v a r i a b l e s t r a i n r a t e s .

Table I. Chemical Analysis (At. PPM) of Chromium

O T. C. REILEY is with The Oak Ridge National Laboratory, Metals and Ceramics Division,Oak Ridge, TN 37830. W. D. NIX is Professor of Materials Science and Engineeringat Stanford University, Stanford, CA 94305, where T. C. Reiley was formerly Graduate Research Assistant. Manuscript submitted February 19, 1976. METALLURGICALT R A N S A C T I O N S A

VP Grade 3330 (99.7 Cr