Mechanism of Bcc twinning: Shear or shuffle?
- PDF / 533,763 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 110 Downloads / 257 Views
Mechanism of Bcc Twinning:
Shear or Shuffle.'? ROMAN J. WASILEWSKI A L T H O U G H e x t e n s i v e s t a c k i n g faults, c o m p a r a b l e to those f o r m e d in fcc m e t a l s , have not b e e n o b s e r v e d in pure bcc e l e m e n t s , it has b e e n c o m m o n l y accepted that f a u l t s in the l a t t e r can f o r m on {112} p l a n e s . T h i s view, i n f e r r e d f r o m the g e o m e t r y of the c o h e r e n t {112} b o u n d a r i e s of idealized, l a m e l l a r twins, f u r t h e r s u g g e s t s that the n e c e s s a r y t r a n s l a t i o n of an atom plane by a / 6 (111) (or the e q u i v a l e n t t r a n s l a t i o n by a / 3 (111)) m u s t o c c u r in the d i r e c t i o n of t w i n n i n g s h e a r . 1-4 A g e o m e t r i c a l l y p o s s i b l e s t a c k i n g fault obtained by a s i m i l a r t r a n s l a t i o n in the opposite s e n s e s was d i s c u s s e d by Silcock, B and shown to be c l o s e l y r e l a t e d to the bcc hcp t r a n s f o r m a t i o n p r o p o s e d e a r l i e r . 7 ROMAN J. WASILEWSKIis Senior Technical Advisor, Department of Process and Physical Metallurgy,Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio. Manuscript submitted Febrtlary 10, 1970. METALLURGICALTRANSACTIONS
In the a b s e n c e of 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 the e x i s t e n c e of extended stacking faults in the bcc lattice, the a s s u m p t i o n of {112} as the p r e f e r r e d fault plane is not justifed. An o b s e r v a t i o n of {310} f a u l t s in n i o b i u m has been r e p o r t e d , 8 and the p o s s i b l i t y of {110} as fault p l a n e s suggested by v a r i o u s i n v e s t i g a t o r s . 9-'s Recent c a l c u l a t i o n s by Vitek i n d i c a t e that slip d i s l o c a t i o n s a r e u n l i k e l y to d i s s o c i a t e into extended s t a c k i n g faults; that the p r o b a b l e d i s s o c i a t i o n is l i m i t e d to a few atom d i s t a n c e s ; and that it o c c u r s on both {112} and {110} p l a n e s , le It is, t h e r e f o r e , r e a s o n a b l e to p o s t u l a t e that the a t t a i n m e n t of s t a b l e l a r g e r f a u l t s - - s u c h as, e . g . , c o h e r e n t twin b o u n d a r i e s - - r e q u i r e s o v e r c o m i n g an e n ergy b a r r i e r f r o m the " i n c i p i e n t d i s s o c i a t i o n " of an existing slip d i s l o c a t i o n . If so, then t h e r e is no obvious r e a s o n why the {112} faults should f o r m in p r e f e r e n c e to those in {110}. In the p r e s e n t c o m m u n i c a t i o n , f u r t h e r a r g u m e n t for the f e a s i b i l i t y of f o r m a t i o n of the l a t t e r is p r e s e n t e d , b a s e d on the g e o m e t r y of the " h a r d - s p h e r e " lattice. The two types of fault, c o r r e s p o n d i n g to a • (111) t r a n s l a t i o n of one p a r t of the c r y s t a l r e l a t i v e to the other and viewed c o n v e n t i o n a l l y on {110}, a r e shown in Fig. 1. The " t w i n n i n g " fault 1-
Data Loading...
