A general mechanism of martensitic nucleation: Part III. Kinetics of martensitic nucleation
- PDF / 977,548 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 329 Views
IN P a r t s
11 and H, 2 a g e n e r a l m e c h a n i s m of m a r t e n s i t i c n u c l e a t i o n was developed w h e r e b y a m a r t e n s i t i c e m b r y o is f o r m e d by a faulting p r o c e s s d e r i v e d f r o m a group of e x i s t i n g d i s l o c a t i o n s . T h e fault e n e r g y p e r unit a r e a , V, a s s o c i a t e d with s u c h a n e m b r y o , n a t o m i c p l a n e s in t h i c k n e s s , can be e x p r e s s e d a s : y = npA(AG
them
+
E str) + 2a(n)
[1]
w h e r e P A is the d e n s i t y of a t o m s in a c l o s e packed p l a n e in m o l e s p e r unit a r e a ; AGchem and E sLr a r e the p a r e n t p r o d u c t c h e m i c a l f r e e e n e r g y d i f f e r e n c e and coherency strain energy, expressed as molar quantities; and a(n) is the p a r t i c l e T m a t r i x i n t e r f a c i a l e n e r g y . T h e c r i t i c a l condition for the s p o n t a n e o u s f o r m a t i o n of a n e m b r y o f r o m a defect was t a k e n as V = 0, and Eq. [1] was used to define the s i z e of the e x i s t i n g defects n e c e s s a r y to a c c o u n t for e m b r y o f o r m a t i o n u n d e r the o b s e r v e d conditions for m a r t e n s i t i c n u c l e a t i o n . We now f u r t h e r r e f i n e our t r e a t m e n t of the c r i t i c a l condition for m a r t e n s i t i c n u c l e a t i o n by exa m i n i n g s m a l l e r s t e p s in the p r o p o s e d f a u l t i n g p r o c e s s , a s well as l a t e r stages in the d e v e l o p m e n t of a m a r t e n s i t i c p a r t i c l e , as p o s s i b l e r a t e l i m i t i n g s t e p s which could u l t i m a t e l y c o n t r o l the o b s e r v e d m a c r o scopic k i n e t i c s of m a r t e n s i t i c t r a n s f o r m a t i o n s .
DISLOCATION
MOTION
If the i n t e r a c t i o n of the p a r t i a l d i s l o c a t i o n s bounding a fault is r e p u l s i v e , the e q u i l i b r i u m s e p a r a t i o n of the d i s l o c a t i o n s b e c o m e s infinite when ~ = 0. However, G. B. OLSON and MORRIS COHEN are Research Associate and Institute Professor, respectively, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. This paper is Part III of a three-part series based on a thesis submitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974. Manuscript submitted February 9, 1976. METALLURGICALTRANSACTIONSA
if a s h e a r s t r e s s To is r e q u i r e d to move the d i s l o c a t i o n s , the c o n d i t i o n of ~ = 0 i s no l o n g e r s u f f i c i e n t to d r i v e the d i s l o c a t i o n s to l a r g e s e p a r a t i o n s . F o r a fault e m b r y o bounded by a n a r r a y of n d i s l o c a t i o n s of B u r g e r s v e c t o r b, the f o r c e p e r unit length of d i s l o c a t i o n a r r a y r e q u i r e d to move t h e a r r a y is t h e n nTob. A fault e n e r g y V e x e r t s a f o r c e p e r unit length on this a r r a y o f - ~ . A c c o
Data Loading...
