The solubilities of cerium and lanthanum nitride in iron
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l i n e a r a b s o r p t i o n c o e f f i c i e n t ~t, and for good t r a n s m i s s i o n ~ 9t < 2. In this C o m m u n i c a t i o n we s h a l l r e s t r i c t o u t s e l v e s to outlining m e r e l y the p o t e n t i a l i t i e s of the c o m b i n e d X - r a y m e t h o d s for f r a c t u r e s t u d i e s . A m o r e detailed a c c o u n t of the q u a n t i t a t i v e r e s u l t s will be given in a forthcoming technical publication. W e d g e - s h a p e d c r y s t a l s p e c i m e n s into which a V - n o t c h was i n t r o d u c e d w e r e p r e p a r e d for t e n s i l e d e f o r m a t i o n . The c r y s t a l s w e r e v i r t u a l l y d i s l o c a t i o n f r e e , s i n c e none w e r e o b s e r v e d e i t h e r by the e t c h - p i t or by the X - r a y t r a n s m i s s i o n topography method. The c r y s t a l s u r f a c e had a (211) o r i e n t a t i o n , with tttp [01I] d i r e c t i o n p a r a l l e l to the t e n s i l e a x i s , Fig. l(a). F i g . l(b) shows the X - r a y (Lang) topograph taken f r o m a s i l i c o n c r y s t a l s t r e s s e d to f r a c t u r e at 600~ At this low t e m p e r a t u r e of d e f o r m a t i o n the c r y s t a l f r a c t u r e d in a totally b r i t t l e m a n n e r . The c r a c k propagated f r o m the notch N along one of the (111) p l a n e s without any l a t e r a l f o r m a t i o n of a p l a s t i c zone, as evidenced by the fact that the P F r e m a i n e d u n d i s t u r b e d up to the f r a c t u r e s u r f a c e F . A p l a s t i c zone a r o u n d the c r a c k tip was p r o d u c e d , h o w e v e r , when the c r y s t a l was d e f o r m e d with a n o m i n a l applied t e n s i l e s t r e s s of 1.43 kg m m -2 at 700~ as shown in F i g . 2. The n o m i n a l s t r e s s a n is defined by F / A , where F is the applied load and A the m e a s u r e d m i n i m u m c r o s s - s e c t i o n a l a r e a . D e f o r m a t i o n at this t e m p e r a t u r e gave r i s e a l s o to the g e n e r a t i o n of a few d i s l o c a t i o n s o u r c e s far r e moved f r o m the c r a c k tip, such as those shown at the edge opposite to the c r a c k tip at A. The s p r e a d of these p l a s t i c z o n e s p e r p e n d i c u l a r to the t e n s i l e axis and the c o a l e s c e n c e with the zone e m a n a t i n g f r o m the c r a c k tip b e c a m e v e r y p r o n o u n c e d when the c r y s t a l was d e f o r m e d with a n o m i n a l applied t e n s i l e s t r e s s of 0.80 kg m m -2 at 800~ F i g . 3. The d i s l o c a t i o n d e n s i t y of the p l a s t i c zones can be a s s e s s e d f r o m the half-width v a l u e s of X - r a y r o c k i n g c u r v e s taken f r o m these a r e a s . F o r e x a m p l e , the r o c k i n g c u r v e taken f r o m the a r e a 0.25 m m 2 below the c r a c k tip N for the c r y s t a l d e f o r m e d at 700~ F i g . 3, yielded a h a l f - w i d t h v a l u e of 55 sec. of a r c . The h a l f - w i d t h of the r o c k i n g c u r v e p e r t a i n i n g t
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