Field ion microscope observations of an incoherent twin segment
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an earlier paper Hren indicated the value of the field ion microscope in analyzing the atomic configuration at an incoherent twin boundary. I In that report the shape of the twin could not be determined since its least dimension was greater than the field of view of the microscope. This paper will report observations of a blade-like twin whichterminated in the crystal within the field of view. The twin boundary was found to be incoherent in spite of its apparent good atomic fit. Twin elements have been identified directly from symmetry elements and contrast at the boundaries in the field ion micrographs. The twin appeared during an investigation of an iron base alloy containing 1680 ppm of tungsten. Its existence was probably due to the polishing procedure by which fine pointed specimens are prepared for the FIM. Wires, 0.004 in. diam, were lacquered near one end and immersed just below the surface of a 10 pct HCI electrolytic polishing bath held at 0°C. Preferential polishing took place at the meniscus so that the lacquered piece eventually dropped off. During dissolution the stress rate increased rapidly, and the stress a t s e p a r a t i o n w a s g r e a t e r than 10s p s i . W h i l e t h e s e c o n d i t i o n s a r e c o n d u c i v e t o t w i n n i n g i n a - F e ,~ t h e probability of observing t w i n s d e c r e a s e s with the extent o f p o l i s h i n g a f t e r t h e b r e a k . O b s e r v a t i o n s of t w i n s in t h i s a l l o y a r e t h e r e f o r e i n f r e q u e n t . I m a g e s p r e s e n t e d h e r e a r e not w e l l d e v e l o p e d i n t h e vicinity o f high index p l a n e s . O t h e r s h a v e o b s e r v e d t h i s e f f e c t a n d h a v e a t t r i b u t e d it t o f i e l d e t c h i n g i n the p r e s e n c e o f h y d r o g e n J T h e h y d r o g e n was unavoidable, h o w e v e r , a s s p e c i m e n r u p t u r e p r e v e n t e d the u s e o f i m a g i n g g a s e s w i t h h i g h e r i o n i z a t i o n e n e r g i e s in e i t h e r p u r e o r m i x e d s t a t e s . F o r t u n a t e l y , t h e low i n d e x p l a n e s a r e the p l a n e s of i m p o r t a n c e a n d t h e s e w e r e d e v e l o p e d sufficiently for limited i m a g e interpretation. T h e twin is shown in Fig. 1 as it a p p e a r e d a f t e r n i n e t y o n e ( i 0 1 ) p l a n e s h a d b e e n r e m o v e d by f i e l d e v a p o r a t i o n . P o o r d e v e l o p m e n t of the h i g h e r i n d e x planes i n h i b i t s v i s u a l l o c a t i o n o f t h e twin at best i m a g i n g v o l t a g e , F i g . l ( a ) . H o w e v e r , by r a i s i n g t h e o p e r a t i n g v o l t a g e slightly t h e i m a g e w a s d e f o c u s e d in a c cordance with local ionization characteristics, outlining t h e t w i n n e d s e g m e n t , Fig. l(b).* Periodically f i e l d *Three (701) planes were removed by field evaporation between Figs. l(a) and l(b); not enough t o shift the boundary noticeably. R. D. FRENCH is Research Metallurgist, Metals Division, Army Ma-
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