An FIM-atom probe study of the precipitation of copper from lron-1.4 at. pct copper. Part I: Field-ion microscopy
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r a m e t e r s , to r e l a t e them to the o b s e r v e d m e c h a n i c a l p r o p e r t i e s , and to i n t e r p r e t them in t e r m s of c u r r e n t precipitation strengthening theories. Part I describes the f i e l d - i o n r ~ i c r o s c o p y of i r o n - c o p p e r alloys and the m o r p h o l o g i c a l changes o b s e r v e d as a r e s u l t of aging at 500~ while P a r t II d e s c r i b e s the c h e m i c a l a n a l y s e s of the p r e c i p i t a t e s obtained by the atom probe. ALLOY AND SPECIMEN PREPARATION The alloy was c a s t in 12.5 m m diam r o d s . After a n n e a l i n g for 18 h at 1000~ it was swaged and drawn to 0.09 m m d i a m w i r e . The wire, wound on a m e t a l f r a m e , was solution a n n e a l e d at 850~ cooled r a p i d l y and then aged for v a r i o u s lengths of t i m e at 500~ Chemical a n a l y s i s indicated a copper content of 1.55 wt pct (1.4 at. pct) with the m a j o r i m p u r i t y being 0.016 wt pct c a r bon. To p r e p a r e the FIM s p e c i m e n the wire was spotwelded u n d e r alcohol to the nickel s p e c i m e n holder. It was then etched in a solution of 10 pct acetic acid and 10 pct HCI at 3 V AC. The etched wire was e x a m i n e d for g r o s s i m p e r f e c t i o n s at m a g n i f i c a t i o n 500 t i m e s p r i o r to i n s t a l l i n g in the FIM c h a m b e r . Apparatus The f i e l d - i o n m i c r o s c o p e used in this i n v e s t i g a t i o n was s i m i l a r to the one d e s c r i b e d by B r e n n e r and McKinney. 2 The s p e c i m e n , mounted on a m a n i p u l a t o r , was cooled to about 30 K using liquid h y d r o g e n g e n e r ated by a m i n i a t u r e J o u l e - T h o m s o n l i q u i f i e r . The s p e c i m e n faced a 7.5 c m diam c h a n n e l - p l a t e i m a g i n g s y s t e m with which n e o n - i o n i m a g e s could be p h o t o g r a p h i c ally r e c o r d e d in l e s s than 1 s. The m e t a l and g l a s s v a c u u m s y s t e m had a r e s i d u a l gas p r e s s u r e of l e s s than 5 • 10 -9 T o r r (6.65 • 10 -7 N/m2). F I E L D - I O N MICROSCOPY OF IRON, C O P P E R AND SOLUTION T R E A T E D ALLOY To p r o d u c e h i g h - r e s o l u t ~ p n i m a g e s of the i r o n - c o p p e r alloy, the m e t a l was f i e l d - e v a p o r a t e d in a h e l i u m - 3 pct h y d r o g e n a t m o s p h e r e b e f o r e the n e o n - i m a g i n g gas was i n t r o d u c e d . It was found helpful to initiate the h y d r o g e n VOLUME 4. OCTOBER 1973-2363
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(b) Fig. 1--(a) Lower yield stress of Fe-l.4 at. pct alloy as a function of aging time at 500~ of heat-treated alloy. Precipitates are first observed by this technique after 9 h aging. p r o m o t e d f i e l d - e v a p o r a t i o n 6 at a t e m p e r a t u r e e s t i m a t e d to be 100 K and to continue e v a p o r a t i o n until the final t e m p e r a t u r e of ~ 3 0 K was r e a c h e d . After switching
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