An FIM-atom probe study of the precipitation of copper from lron-1.4 at. pct copper. Part II: Atom probe analyses
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s t u d i e s of p r e c i p i t a t i o n o c c u r r i n g by c l a s s i c a l n u cleation and growth, it is u s u a l l y a s s u m e d that the i n i tial p r e c i p i t a t e has the e q u i l i b r i u m c o m p o s i t i o n of the n u c l e a t e d phase. Hobstetter ~ has pointed out that for c o h e r e n t p r e c i p i t a t e s this a s s u m p t i o n m a y not be valid. Since the i n t e r r a c i a l e n e r g y of the m a t r i x - p r e c i p i t a t e b o u n d a r y depends on the p r e c i p i t a t e c o m p o s i t i o n 2 the height of the a c t i v a t i o n b a r r i e r for n u c l e a t i o n will be a function of both n u c l e u s size and c o m p o s i t i o n . The l a r g e r the s u p e r s a t u r a t i o n , the g r e a t e r will be the dev i a t i o n f r o m the e q u i l i b r i u m c o m p o s i t i o n of the bulk p r e c i p i t a t e phase. E x p e r i m e n t a l c o n f i r m a t i o n of a v a r i a b l e n u c l e u s c o m p o s i t i o n has not b e e n p o s s i b l e in the p a s t s i n c e no t e c h nique e x i s t e d that was capable of m e a s u r i n g the composition of the p r e c i p i t a t e s d u r i n g the v e r y e a r l i e s t stages o]~ f o r m a t i o n . With the F I M - a t o m probe 3'4 such a t e c h n|que is now a v a i l a b l e and the c o m p o s i t i o n of p r e c i p i tate p l a t e l e t s s with t h i c k n e s s e s of l e s s than 10 -7 cm have a l r e a d y b e e n s u c c e s s f u l l y d e t e r m i n e d with it. In P a r t I of this study it was shown that p r e c i p i t a t e s as s m a l l as 8.~ diam could be o b s e r v e d with the f i e l d - i o n m i c r o s c o p e in F e - l . 4 at. pct Cu alloys aged at 500~ The p h a s e s in e q u i l i b r i u m in this alloy at 500~ should be n e a r l y p u r e i r o n , (~, and n e a r l y p u r e copper, ~. T h e r e a r e no s t a b ~ i n t e r m e d i a t e phases and the c o m p o s i t i o n of this alloy is well outside the c a l c u l a t e d c h e m i c a l spinodal b o u n d a r y . As was d i s c u s s e d by Hornbogen 33 and L a h i r i , CIiandra, Schwartz, and Fine 34 the p r e c i p i tate p a r t i c l e s l e s s than 50A which a r e i n v i s i b l e in the electron microscope are most probably coherent. Therm o d y n a m i c c a l c u l a t i o n s p r e d i c t (see l a t e r section) that c o h e r e n t p r e c i p i t a t e nuclei in this s y s t e m at 500~ should c o n t a i n l e s s solute than the e q u i l i b r i u m E phase. The p u r p o s e of this work was to t e s t this p r e d i c t i o n ' a n d to f u r t h e r d e m o n s t r a t e that the F I M - a t o m p r o b e is c a pable of m e a s u r i n g the c o m p o s i t i o n of p r e c i p i t a t e s of extremely small dimensions.
APPARATUS The F I M - a t o m probe used in this investigation is similar to the one described by Brenner and McKinney. 4 Essentially it consists of a field-ion microscope coupled to a time-of-flight m a s s spectrometer. Rs principle of operation is
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