Cleavage planes in lath type bainite and martensite
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Cleavage Planes in Lath Type Bainite and Martensite J. P. NAYLOR
AND
P. R. KRAHE
A p r e v i o u s p a p e r I has shown that the toughness of upper b a i n i t e depends p a r t i a l l y on the size of the b a i n itic packet s i z e ; this p a r a m e t e r b e i n g the d i m e n s i o n of a bundle of elongated p a r a l l e l f e r r i t e p l a t e s . R o b e r t s 2 had p r e v i o u s l y found the s a m e effect in a lath m a r t e n site s t r u c t u r e . Both t h e s e s t u d i e s indicated that the cleavage facet size c o r r e s p o n d e d to the m a r t e n s i t i c and b a i n i t i c packet s i z e s . Other work 3'4 has shown b y etch pit t e c h n i q u e s the p r e s e n c e of s u b - p a c k e t s within the packets where the f e r r i t e g r a i n s have a l m o s t the s a m e o r i e n t a t i o n and a r e s e p a r a t e d only by low angle b o u n d a r i e s , and that f r a c t u r e p r o c e e d s along {100)~. However, e l e c t r o n m i c r o s c o p y has shown 5,6 that the s i z e of a s u b - p a c k e t is l i m i t e d to ~ 5 f e r r i t e laths and that each packet c o n t a i n s m a n y s u b - p a c k e t s with v a r y ing o r i e n t a t i o n s . Another study 7 found that when the f e r r i t e lath b o u n d a r i e s w e r e c l e a r and d i s t i n c t within J. P. NAYLOR is Research Member, Ecole des Mines,Centre des Materiaux, Corbeil-Essones,France. P. R. KRAHE is Professor of Metallurgy, UniversidadeFederal Do Rio Grande Do Sul, Porto Alegre, Brazil. Manuscript submitted August 7, 1974. 5 9 4 - V O L U M E 6A, MARCH 1975
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Fig. i--Kurjumov-Saehs variants from common (111) represented using the method of Speieh and Swann. l~ Variants 1. [ll0ly II [lillt~, 2. [il0]y II [ l l [ ] a , 3. [101]y II [lh]o~, 4. [i01]y II [ i l i ] a , 5. [0il]y II [i-lilts, 6. [011]7 II [i11]~. A represents [001](~, B [010]c~, C [100](~, C- [i00]c~. a packet each lath m a y have one of six p o s s i b l e o r i e n tations, each e q u a l l y p r o b a b l e and c o n s e q u e n t l y the laths have no t e n d e n c y to be in s u b - p a c k e t s . Both t h e s e f i n d i n g s a r e a p p a r e n t l y c o n t r a d i c t o r y with the fact that each cleavage facet owes its s i z e to a b a i n i t e packet and that the lath b o u n d a r i e s s e e m to have little or no effect; for f r a c t u r e to p a s s by {100}~ p l a n e s the d i f f e r ent lath o r i e n t a t i o n s should r e s u l t in high angle d e v i a tions, Fig. 1, and would m e a n that the effect of the b a i n i t i c / m a r t e n s i t i c b o u n d a r i e s could not be d i s t i n guished. To r e s o l v e this a n o m a l y a p r a c t i c a l i n v e s t i gation was u n d e r t a k e n with a 0.1 pct C, 1/2 Mo-B steel. E x p e r i m e n t a l . A s a m p l e of the s t e e l with d i m e n s i o n s 5 x 3 0 x 1 0 0 m m was heated for 12 m i n at 1000~ a n d was quenched at a c o n t r o l l e d r a t e ( t r e a t m e n t A, Ref. 1) u s i n g a w a t e r /
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