The effect of a hydrogen preheat-treatment on the oxidation behavior of Ni-Cr-Al-ThO 2 alloys
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Fig. 1--A [22i] diffraction pattern of a bee lattice as affected by single twinning on (115). O Matrix, p r i m a r y twin, and double diffraction due to p r i m a r y twin r e f l e c t i o n s s u p e r imposed on one another. O P r i m a r y twin and double diff r a c t i o n due to p r i m a r y twin s u p e r i m p o s e d . 1. (li0) of m a t r i x and twin 2. (ii0) twin 3. (114) matrix, (330) twin.
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Fig. 2--A [22i] diffraction pattern of a bcc lattice as affected by double twinning. 9 Matrix, p r i m a r y twin, double diff r a c t i o n due to p r i m a r y twin, secondary twin and double diffraction due 1;o secondary twin s u p e r i m p o s e d . [] P r i m a r y twin, s e c o n d a r y twin and double diffraction due to p r i m a r y and s e c o n d a r y twinning. 4, (li0) of matrix and twin 5. (ii0) of twin 6. (114) matrix and (330) twin.
t i o n s . H o w e v e r , s o m e of the twin r e f l e c t i o n s do not have m a t r i x r e f l e c t i o n s s u p e r i m p o s e d on t h e m and t h e s e should give a s i m i l a r type of c o n t r a s t to one another. If the e v i d e n c e p r e s e n t e d by D a s and T h o m a s 1 i s r e e x a m i n e d in the light of above c o m m e n t s it is s e e n that it is not n e c e s s a r y to invoke the i d e a of double twinning. The d a r k - f i e l d i m a g e f r o m F i g . l(c) of Ref. 1 b r i n g s only twins into c o n t r a s t w h e r e a s the r e f l e c t i o n thai is due to m a t r i x , twinning, and double d i f f r a c t i o n - - t h e (Oli) in F i g . l(d) of Ref. 1--does not show much i n t e n s i t y e i t h e r f r o m m a t r i x o r twinning i n d i c a t i n g that the r e f l e c t i o n is c o m m o n to both. Das and T h o m a s c Laim that this l o s s of i n t e n s i t y i s due to double twinning, but t h e r e is no c o n c l u s i v e i n d i c a tion of this in the d i f f r a c t i o n p a t t e r n . We s u g g e s t that this l o s s of i n t e n s i t y could be due to double d i f f r a c t i o n and this is b e t t e r s u p p o r t e d by e x a m i n a tion of the e v i d e n c e in p l a t e m a r t e n s i t e . R e f s . 2 and 3 contain e x a c t l y the s a m e d i f f r a c t i o n p a t t e r n s and m i c r o g r a p h s showing twins in p l a t e m a r t e n s i t e s and the b e s t e v i d e n c e is again f r o m a ( 2 2 t ) m a t r i x zone. D a r k - f i e l d m i c r o g r a p h s a r e taken METALLURGICAL TRANSACTIONS
f r o m s e v e r a l r e f l e c t i o n s ( F i g . 9 of Ref. 2) of which F i g s . 9(c) and 9(e) on one hand and 9(d) and 9 ~ on the o t h e r p r o v i d e s i m i l a r c o n t r a s t e f f e c t s . None of t h e m give the e x p e c t e d d a r k - f i e l d i m a g e s of a l t e r nate m a t r i x and p r i m a r y twin b a n d s ( s e e for e x a m p l e , F i g .
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