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e t a l . , '2 R o t h m a n and P e t e r s o n , '3 and B e y e l e r and A d d a . z4 On the b a s i s of t h i s a g r e e m e n t it a p p e a r s t h a t , a f t e r the s u b t r a c t i o n t e c h n i q u e h a s b e e n a p p l i e d , r e g i o n II c o r r e s p o n d s to l a t t i c e d i f f u s i o n . A s is a p p a r e n t the d i f f u s i o n c o e f f i c i e n t s d e r i v e d f o r r e g i o n III a r e b a d l y s c a t t e r e d . T h e i r m a g n i t u d e r e l a t i v e to t h o s e f o r r e g i o n II c o u p l e d w i t h t h i s s c a t t e r s u g g e s t that t h e y r e p r e s e n t d i s l o c a t i o n - - i n f l u e n c e d d i f f u s i o n . T h e o b s e r v e d v a r i a t i o n s a r e p r o b a b l y due to v a r i a t i o n s in o p e r a t i v e d i s l o c a t i o n d e n s i t y . T h e a b o v e r e s u l t s i n d i c a t e that l a t t i c e d i f f u s i o n c a n in f a c t be d i r e c t l y m e a s u r e d by s e r i a l s e c t i o n i n g in the c a s e of s e l f - d i f f u s i o n in c o p p e r d o w n to 400~ H o w e v e r , to a c h i e v e t h e s e r e s u l t s in the p r e s e n t c a s e it w a s n e c e s s a r y to s u b t r a c t a w a y a d i f f u s i o n c o m p o n e n t due to the p r e s e n c e of s h o r t - c i r c u i t i n g d i s l o c a t i o n s . A p p l i c a t i o n of a s i m i l a r s u b t r a c t i o n t e c h n i q u e to o t h e r c a s e s of n e a r - s u r f a c e s e l f - d i f f u s i o n in the n o b l e m e t a l s t u r n s out to be not n e a r l y s o s u c c e s s f u l : the r e a s o n f o r t h i s i s n o t k n o w n .

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(I04/T)K -I Fig. 2--Arrhenius plot of lattice and dislocation-influenced self-diffusion coefficients for copper, indicated .0 and x r e spectively, compared with downward extrapolations of other high-temperature self-diffusion data. A p l o t of e r f c - l [ A ( x ) / A ( O ) ] v s x c o r r e s p o n d i n g to d i f f u s i o n f o r 12 h a t 423~ is s h o w n in F i g . 1. It c l e a r l y s h o w s t h a t t h e r e a r e t h r e e r e g i o n s of d i f f u s i o n . T h e s e t h r e e r e g i o n s c a n be s e p a r a t e d f r o m e a c h o t h e r f o r the p u r p o s e of c a l c u l a t i n g the c o r r e s p o n d ing d i f f u s i o n c o e f f i c i e n t s by s u p p o s i n g that the p r i n c i p l e of s u p e r p o s i t i o n h o l d s in the p r e s e n t c a s e , T h i s a l l o w s a s i m p l e s u b t r a c t i o n t e c h n i q u e to be e m p l o y e d s i m i l a r to that d e s c r i b e d by H a r d i n g ) ~ T h e p r o c e d u r e i s c a r r i e d out on the A ( x ) v s x p r o f i l e by f i r s t e x t r a p o l a t i n g the l o n g e s t r a n g e c o m p o n e n t b a c k to x = 0, and s u b t r a c t i n g t h i s a s though i t r e p r e s e n t s c o m p l e t e l y i n d e p e n d e n t d i f f u s i o n o v e r the f u l l r a n g e of p e n e t r a t i o n . T h e s a m e p r o c e d u r e is t h e n r e p e a t e d f o r the n e x t c o m p o n e n t . T h e c a l c u l a t e d r e g i o n II is a l s o s h o w n in F i g . 1 and c l e a r l y i l l u s t r a t e s the c h a n g e in s l o p e r e l a t i v e to the c o r r e s p o n d i n g u n t r e a t e d d a t a for t h i s r e g i o n , R e g i o n I is o b v i o u s l y v e r y p o o r l y d e f i n e d in the c a s e s h o w n in F i g , 1. H o w e v e r , in o t h e r c a s e s it w a s s u f f i c i e n t l y w e l l d e f i n e d to e n a b l e e f f e c t i v e d i f f u s i o n c o e f f i c i e n t s to be e v a l u a t e d . T h u s , at 526~ it w a s ~10-16 c m 2 / s , w h i c h is to b e c o m p a r e d w i t h the p u b l i s h e d r e s u l t ~1 f o r the d i f f u s i o n of c o p p e r in c o p p e r o x i d e w h i c h is ~10 - ~ c m 2 / s a t t h i s t e m p e r a t u r e . It w o u l d s e e m d i f f i c u l t , t h e r e f o r e , to r e c o n c i l e r e g i o n I w i t h d i f f u s i o n in a s i m p l e s u r f a c e l a y e r of c o p p e r o x i d e . Of the e i g h t d i f f u s i o n a n n e a l i n g s c a r r i e d out t h r e e y i e l d e d s u f f i c i e n t l y e x t e n s i v e r e g i o n I I ' s to e n a b l e d i f f u s i o n c o e f f i c i e n t s to be e v a l u a t e d . T h e s e a r e p l o t t e d in F i g , 2 and a r e c l e a r l y not i n c o n s i s t e n t w i t h e x t r a p o l a t i o n s of h i g h - t e m p e r a t u r e d a t a due to K u p e r 364 VOLUME 4, JANUARY 1973

1.A.J. Mortlock: Phys. Status Solidi, 1970, vol. 2, p. K85. 2. A. D. LeClaire:Phil. Mag., 1962, vol. 7, p. 141. 3. W. Rupp, V. Ermert, and R. Sizman,Phys. Status Solidi, 1969,vol. 33, p. 509. 4. A. Gainotti, and L. Zecchina: Nuovo Cimento B, 1965, vol. 40, p. 295. 5. J. L. Whitton and G. V. Kidson: Can. J. Phys., 1968, vol. 46, p. 2589. 6. H. M. Morrison and V. Yuen: Can. J. Phys., 1971, vol. 49, p. 2704. 7. R. G. Vardiman and M. R. Achter: Trans. TMS-AIME, 1969, vol. 245, p. 1969. 8. C. T. Lai and H. M. Morrison: Can. J. Phys., 1970, vol. 48, p. 1548. 9. T. Anderson and G. S@rensen: Can. J. Phys., 1968, vol. 46, p. 483. 10. B. C. Harding: Phil. Mag., 1967, vol. 16, p. 1039. 11. W. J. Moore, Y. Ebisuzaki, and J. Sluss: J. Phys. Chem., 1958, vol. 62, p. 1438. 12. A. Kuper, H. Letaw, L. Slifkin, F. Sonder and C. T. Tomizuka: Phys. Rev., 1954, vol. 96, p. 1224. 13. S. J. Rothman and N. L. Peterson: Phys. Status Solidi, 1969, vol. 35, p. 305. 14. M. Beyelerand Y. Adda: J. Phys., 1968, vol. 29, p. 345.

Environmental Hydrogen Embrittlement of an or- fl Titanium Alloy: Effect of Hydrogen Pressure HOWARD

G. NELSON

IN r e c e n t

s t u d i e s 1'2 e n v i r o n m e n t a l h y d r o g e n e m b r i t t l e m e n t of a - ~ t i t a n i u m w a s i n v e s t i g a t e d a t a c o n s t a n t h y d r o g e n p r e s s u r e n e a r 1 a t m a s a f u n c t i o n of a l l o y microstructure, test displacement rate, applied stress i n t e n s i t y , and t e m p e r a t u r e . A l l m i c r o s t r u c t u r e s s t u d i e d w e r e found to be s u s c e p t i b l e to g a s e o u s h y d r o g e n e m b r i t t l e m e n t ; h o w e v e r , f o r a g i v e n s e t of t e s t c o n d i t i o n s , the d e g r e e of s u s c e p t i b i l i t y w a s o b s e r v e d to be s t r o n g l y d e p e n d e n t on the f o r m of the m i c r o s t r u c ture. Microstructures which contained a continuous a - p h a s e m a t r i x w i t h a f i n e , d i s p e r s e d /3 p h a s e in the HOWARD G. NELSON is Research Scientist, NASA-Ames Research Center, Moffett Field, Calif. 94035. Manuscript submitted June 26, 1972. METALLURGICAL TRANSACTIONS

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(b) Fig. 1--Representative microstructures of the titanium alloy (6A1-4V); (a) solution treated at 830~ for 40 rain, water quenched, and 510~ age for 12 h, (b) solution treated at 1037~ for 40 min, stabilized at 704~ for 1 h and 593~ for 1 h, and air cooled. Kroll's etch.

b o u n d a r i e s w e r e found to be l e s s s e v e r e l y e m b r i t t l e d than m i c r o s t r u c t u r e s c o n t a i n i n g a continuous ~ p h a s e in the b o u n d a r i e s . A d d i t i o n a l l y , b r i t t l e f r a c t u r e in the f o r m e r m i c r o s t r u c t u r e o c c u r r e d p r i m a r i l y by t r a n s g r a n u l a r s e p a r a t i o n a s s o c i a t e d with n o n c l a s s i c a l c l e a v a g e and, in the l a t t e r by i n t e r g r a n u l a r c r a c k i n g . F r o m these s t u d i e s it was s u g g e s t e d that e n v i r o n m e n tal hydrogen e m b r i t t l e m e n t of t i t a n i u m is s i m i l a r to METALLURGICALTRANSACTIONS

slow s t r a i n - r a t e e m b r i t t l e m e n t p r e v i o u s l y o b s e r v e d in h y d r o g e n - c h a r g e d t i t a n i u m a l l o y s 3'4 and, like this f o r m of e m b r i t t l e m e n t , can be explained in t e r m s of r e l a t i v e hydrogen t r a n s p o r t r a t e s within a - p h a s e and /3-phase t i t a n i u m . 3 The p u r p o s e of the p r e s e n t c o m m u n i c a t i o n is to r e p o r t the r e s u l t s of a study a i m e d at q u a l i t a t i v e l y e s t a b l i s h i n g the hydrogen p r e s s u r e d e p e n d e n c e of e m b r i t t l e m e n t for the two m i c r o s t r u c t u r e s p r e v i o u s l y identified to exhibit slight and s e v e r e e m b r i t t l e m e n t , 1 i.e., a continuous a - p h a s e m a t r i x and a c o n t i n u o u s /3-phase m a t r i x c o n t a i n i n g a - p h a s e t i t a n i u m . The m a t e r i a l u s e d was c o m m e r c i a l l y obtained T i 6AI-4V alloy given one of the following two heat t r e a t m e n t s : 1) solution t r e a t e d at 830~ for 40 rain, w a t e r quenched, and aged at 510~ for 12 h; and 2) solution t r e a t e d at 1038~ for 40 m i n , s t a b i l i z e d at 704~ for 1 h and 593~ for 1 h, and a i r cooled. The m i c r o s t r u c t u r e r e s u l t i n g f r o m these heat t r e a t m e n t s a r e shown in Fig. 1. As can be s e e n , the l o w - t e m p e r a t u r e s o l u tion t r e a t m e n t and age, Fig. l(a), r e s u l t e d in p r i m a r y a p h a s e , equtaxed g r a i n s f o r m i n g a continuous m a t r i x with the r e t a i n e d /3 phase finely d i s p e r s e d in the a b o u n d a r i e s . The solution t r e a t m e n t at a t e m p e r a t u r e which was w e l l into the /3 field followed by s t a b i l i z a t i o n , r e s u l t e d in a s t r u c t u r e of c o a r s e , a c i c u l a r a p h a s e , i.e., t r a n s f o r m e d ot n e e d l e s in a /3 m a t r i x , F i g . l(b). T e s t p r o c e d u r e was i d e n t i c a l to that d i s c u s s e d p r e v i o u s l y . 1 T e s t s w e r e conducted at v a r i o u s hydrogen p r e s s u r e s r a n g i n g f r o m 9.06 • 104 N / m 2 to 1.3 N / m 2 at a m o d e r a t e l y slow c r o s s h e a d d i s p l a c e m e n t r a t e , D, of 8.9 • 10 -a m/s on p r e c r a c k e d , t h r e e - p o i n t bend s p e c i m e n s . The s p e c i m e n s did not c o n f o r m to the ASTM t h i c k n e s s guideline for p l a n e - s t r a i n f r a c t u r e and thus s t r e s s - i n t e n s i t y v a l u e s d e t e r m i n e d a r e n o n s t a n d a r d . T h e s e v a l u e s were c a l c u l a t e d f r o m the m a x i m u m load m e a s u r e d on the l o a d - d i s p l a c e m e n t r e c o r d and a r e d e s i g n a t e d as Kscg, the n o n s t a n d a r d s t r e s s - i n t e n s i t y for the i n i t i a t i o n of m e a s u r a b l e s u b c r i t i c a l c r a c k growth. When n e g l i g i b l e s u b c r i t i c a l c r a c k growth o c c u r s , as in a i r , Kscg equals the n o n s t a n d a r d c r i t i c a l s t r e s s - i n t e n s i t y for f a i l u r e , KQ. Fig. 2 shows the r e s u l t s of these t e s t s w h e r e e m b r i t VOLUME 4, JANUARY 1973-365

(5) Fig. 3 - - M i c r o s c o p i c h y d r o g e n - i n d u c e d c r a c k i n g o b s e r v e d in the t i t a n i u m alloy (6A1-4V) h a v i n g a c o n t i n u o u s /3-phase m a t r i x with a c i c u l a r c~-phase p l a t e l e t s ; (a) t e s t e d at a h y d r o g e n p r e s s u r e of 9.06 • 104 N / m 2. (b) t e s t e d at a h y d r o g e n p r e s s u r e of 1.3 x 10 t N / m 2. K r o l l ' s etch.

tlement, as defined as the r a t i o of K s c g in hydrogen to K~ in a i r , is plotted as a function of the hydrogen p r e s s u r e . It is seen f r o m this figure that hydrogen at a p r e s s u r e near 1 atm e m b r i t t l e s both s t r u c t u r e s , but has a g r e a t e r effect on the Ti-6A1-4V alloy with an a c i c u l a r ~ (continuous ~) m i c r o s t r u c t u r e than on that with an equiaxed, p r i m a r y a (continuous a) m i c r o s t r u c t u r e . Additionally, as the hydrogen p r e s s u r e is d e c r e a s e d , the d e g r e e of e m b r i t t l e m e n t observed in the s p e c i m e n s having an a c i c u l a r m i c r o s t r u c t u r e is d e c r e a s e d until at a hydrogen p r e s s u r e of a p p r o x i m a t e l y 1 N / m e no e m b r i t t l e m e n t was o b s e r v e d . In c o n t r a s t to these o b s e r v a t i o n s , in s p e c i m e n s with a p r i m a r y a m i c r o s t r u c t u r e the degree of e m b r i t t l e ment a p p e a r s to be unaffected by a d e c r e a s e in the env i r o n m e n t a l hydrogen p r e s s u r e down to at l e a s t 1.3 N / m 2, the lowest p r e s s u r e of this study. The m i c r o s c o p i c f r a c t u r e path of hydrogen-induced 366 VOLUME 4, JANUARY 1973

slow c r a c k growth was also examined. In s p e c i m e n s having an equiaxed, p r i m a r y ot m i c r o s t r u c t u r e , growth at a l l p r e s s u r e s investigated a p p e a r e d to be p r i m a r i l y t r a n s g r a n u l a r through the ~ - p h a s e g r a i n s accompanied by a minor amount of i n t e r g r a n u l a r growth. In s p e c i mens having an a c i c u l a r ~ m i c r o s t r u c t u r e , a change in hydrogen p r e s s u r e was observed to have an effect on m i c r o s c o p i c f r a c t u r e path. F i g s . 3(a) and 3(b) a r e p h o t o m i c r o g r a p h s of typical subsurface c r a c k s obs e r v e d in a c i c u l a r s p e c i m e n s f r a c t u r e d in hydrogen at 9.06 • 104 N / m 2, F i g . 3(a), and at 1.3 • 101 N / m 2, Fig. 3(b). It is seen that at the higher hydrogen p r e s s u r e , cracking occurs in an i n t e r g r a n u l a r manner along p r i o r ~ and t r a n s f o r m e d c~ p l a t e l e t boundaries; however, at the lower hydrogen p r e s s u r e , c r a c k i n g is p r i m a r i l y t r a n s g r a n u l a r through the p r i o r ~ g r a i n s and a c r o s s the t r a n s f o r m e d a p t a t e l e t s . The r e s u l t s of the p r e s e n t study support the p r e vious suggestion that environmental hydrogen e m b r i t tlement of ot-~ phase titanium is controlled by the r a t e p r o c e s s e s involved in the competition of i n t e r g r a n u l a r cracking along the a / ~ boundaries and t r a n s g r a n u l a r cracking a c r o s s the a g r a i n s . 1'2 When a continuous network of ~ phase is p r e s e n t in the m i c r o s t r u c t u r e , a " s h o r t - c i r c u i t " t r a n s p o r t path e x i s t s which enables hydrogen to p e n e t r a t e m o r e deeply into the titanium lattice. F o r this s t r u c t u r e , Fig. l(b), as the hydrogen p r e s s u r e is d e c r e a s e d , the s o l i d - s o l u t i o n hydrogen concentration in the ~ phase near the c r a c k tip surface is lowered through the n o r m a l e q u i l i b r i u m r e l a t i o n s , the c o n c e n t r a t i o n - g r a d i e n t induced h y d r o gen t r a n s p o r t in the ~ phase is d e c r e a s e d , l e s s h y d r o gen is able to p e n e t r a t e into the m i c r o s t r u c t u r e , and e m b r i t t l e m e n t is d e c r e a s e d , Fig. 2. In m i c r o s t r u c t u r e s containing a continuous m a t r i x of ot p h a s e , F i g . 1 (a), a " s h o r t - c i r c u i t " hydrogen t r a n s p o r t path does not exist. Hydrogen must, then, i n t e r a c t d i r e c t l y with a - p h a s e titanium initially forming a thin, continuous hydride l a y e r on the clean ~ titanium s u r f a c e . Under such conditions further hydride growth will be volume diffusion limited in a manner s i m i l a r to the Wagner theory of oxidation? Under the conditions of the p r e s e n t study where the hydrogen p r e s s u r e is much g r e a t e r than the hydride d i s s o c i a t i o n pressure,8'~ further hydride growth will be n e a r l y independent of hydrogen p r e s s u r e , 5 F i g . 2. The observed change in f r a c t u r e path in the continuous ~-phase m i c r o s t r u c t u r e , F i g . 3, is consistent with the idea of competing p r o c e s s e s . At higher p r e s s u r e s i n t e r g r a n u l a r cracking occurs because h y d r o gen can be r e a d i l y t r a n s p o r t e d in the ~ phase and can i n t e r a c t with the ~ phase at the boundaries in a m a n ner s i m i l a r to that proposed by C r a i g h e a d e t al. 3 At low p r e s s u r e s , however, the enhanced hydrogen t r a n s p o r t afforded by the ~ phase is no longer p r e s e n t . E m b r i t t l e m e n t , however, will be l e s s than that obs e r v e d in a continuous ot m a t r i x m i c r o s t r u c t u r e b e cause c r a c k s propagating through the ~ phase a r e blunted by the ~ phase at the b o u n d a r i e s . Even at low p r e s s u r e s , then, e m b r i t t l e m e n t will be controlled by hydrogen t r a n s p o r t in the fl-phase titanium, F i g . 2. 1. H. G. Nelson, D. P. Williams, J. E. Stein: Met. Trans., 1972, vol. 3, pp. 469-75. 2. D. P. Williams and H. G. Nelson: Met. Trans., 1972, vol. 3, pp. 2107-13. 3. C. M. Craighead, G. A. Lenning, and R. I. Jaffee: AIME Trans., 1956, vol. 206, pp. 923-28. METALLURGICAL TRANSACTIONS

4. D. N. Williams,F. R. Schwartzberg,and R. I. Jaffee: Trans. ASM, 1960,vol. 52, pp. 183-90. 5. P. Kofstad:High-Temperature Oxidation of Metals, John Wileyand Sons,Inc., New York, 1966. 6. A. D. McQuillan:Proc. Roy. Soc., 1950,vol. A204, pp. 309-25. 7. T. A. Giorgiand F. Ricca:Nuovo Ciemento Supp., 1967,voh 5, pp. 472-82.

On the Use of the Scanning Electron Microscope for" Kinetic Studies C. J . VARKER AND K. V. RAVI T H E m o s t widely used methods of o b s e r v i n g kinetic p h e n o m e n a in m a t e r i a l s involve u t i l i z i n g optical or t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y 1'2 for following s t r u c t u r a l c h a n g e s . Optical m i c r o s c o p y is r e s o l u t i o n l i m i t e d and hence i n c a p a b l e of v e r y detailed s t r u c t u r a l observations. T r a n s m i s s i o n microscopy limits studies to v e r y thin f i l m s . In this c o m m u n i c a t i o n we p r e s e n t a technique u t i l i z i n g the s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) for i n v e s t i g a t i n g k i n e t i c p h e n o m e n a in s e m i c o n d u c t o r m a t e r i a l s . The SEM c o m b i n e s the advantage of in s i t u s t u d i e s of bulk m a t e r i a l s with good r e s o l u tion. In p a r t i c u l a r the s t r u c t u r a l changes that take p l a c e when s i n g l e c r y s t a l s i l i c o n s a m p l e s a r e a n n e a l e d at high t e m p e r a t u r e s will be d e s c r i b e d to i l l u s t r a t e the technique. The method of detecting the p r e s e n c e of c r y s t a l lographic defects in s e m i c o n d u c t o r s by u s i n g the SEM was f i r s t u t i l i z e d by L a n d e r e t al. 3 and C z a j a e t al. 4 The SEM is o p e r a t e d in the e l e c t r o n b e a m induced c u r r e n t (EBIC) mode, w h e r e i n the e l e c t r o n b e a m g e n e r ates c h a r g e c a r r i e r s in the s e m i c o n d u c t o r . The b e a m is s c a n n e d in s y n c h r o n i s m with a d i s p l a y CRT and the charge c a r r i e r s a r e detected by a p - n j u n c t i o n within the s e m i c o n d u c t o r . The c u r r e n t g e n e r a t e d at the p - n j u n c t i o n is a m p l i f i e d and m o d u l a t e s the b r i g h t n e s s on the d i s p l a y CRT. The p r e s e n c e of any c r y s t a l l o g r a p h i c defects such as d i s l o c a t i o n s , s t a c k i n g f a u l t s , p r e c i p i t a t e s and so forth in the s e m i c o n d u c t o r will r e s u l t in l o c a l r e c o m b i n a t i o n of the m i n o r i t y c h a r g e c a r r i e r s and hence p r o d u c e c o n t r a s t effects in the i m a g e . F i g . 1 shows a s c h e m a t i c of the t e c h n i q u e . E x p e r i m e n t s w e r e conducted to i n v e s t i g a t e the growth k i n e t i c s of oxidation induced s t a c k i n g faults on s i n g l e c r y s t a l s i l i c o n s u r f a c e s . T h e s e faults have b e e n o b s e r v e d to function as d i s l o c a t i o n g e n e r a t i n g s o u r c e s when a n n e a l e d . 5 When b o r o n is p r e s e n t in the s i l i c o n the faults grow with s u b s e q u e n t a n n e a l i n g by g e n e r a t i n g p a r t i a l d i s l o c a t i o n loops in a m a n n e r analogous to the B a r d e e n - H e r r i n g 6 m e c h a n i s m of d i s l o c a t i o n g e n e r a t i o n . <111> s u r f a c e o r i e n t e d s i l i c o n c r y s t a l s w e r e oxidized at l l 0 0 ~ and s u b s e q u e n t l y b o r o n was diffused into the s i l i c o n through s e l e c t i v e l y etched windows to f o r m shallow (0.5 ~m) p - n j u n c t i o n s in the s i l i c o n . T h e s e s a m p l e s w e r e then a n n e a l e d in C. J. VARKER and K. V. RAVI are Senior Scientist and Manager, respectively, MetallurgicalResearch and Technology Section, Central Research Laboratories, Motorola, Inc., Semiconductor Products Division, Phoenix, Ariz. 85008. Manuscript submitted May 26, 1972. METALLURGICALTRANSACTIONS

Fig. 1--Electron beam induced current display technique.

Fig. 2--Electron beam induced current (EBIC) displays of boron doped, thermally aged silicon. The aging was done at 900~ in an inert ambient for the times shown. Note the apparent nucleation of new defects in conjunction with the growth of previously existing defects. i n e r t a m b i e n t s to force the growth of the faults. O b s e r v a t i o n s of the s t r u c t u r a l changes o c c u r r i n g w e r e made in the SEM u t i l i z i n g the EBIC mode. F i g s . 2 and 3 show the stacking fault growth s e q u e n c e s at two m a g n i f i c a t i o n s . The faults and the a s s o c i ated p a r t i a l d i s l o c a t i o n s function as r e c o m b i n a t i o n c e n t e r s in the s e m i c o n d u c t o r and give r i s e to the o b s e r v e d c o n t r a s t . The growth s e q u e n c e for stacking faults a n n e a l e d at 900~ for d i f f e r e n t t i m e s can be e a s i l y followed. F a u l t growth and d i s l o c a t i o n g e n e r a tion o c c u r s along <110> and <112> d i r e c t i o n s p r o d u c i n g VOLUME4, JANUARY 1973 367