The influence of grain size and stress on the morphology of a 300-grade maraging steel
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IN a p r e v i o u s
i n v e s t i g a t i o n , one of the authors (A.G.) noted that a m a r k e d change in m a r t e n s i t e growth m o r phology o c c u r r e d as a r e s u l t of h o m o g e n i z a t i o n and g r a i n growth in a 300-grade m a r a g i n g steel. 1 With an i n c r e a s e in g r a i n s i z e , a change f r o m a blocky to a s t r i n g e r type of s t r u c t u r e was obtained. T h i s change can be s e e n in the sequence of optical p h o t o m i c r o g r a p h s shown in Fig. 1. F i g s . 2 and 3 contain e l e c t r o n p h o t o m i c r o g r a p h s of c a r b o n r e p l i c a s of blocky and s t r i n g e r m a r t e n s i t e s t r u c t u r e s , r e s p e c t i v e l y . M a r t e n s i t e in such s t e e l s h a s g e n e r a l l y been shown to have a blocky a p p e a r a n c e , t y p i c a l of m a r t e n s i t e o b s e r v e d in F e - N i alloys c o n t a i n i n g about 6 to 29 pct Ni. 2 E l e c t r o n t r a n s m i s s i o n s t u d i e s 2-5 have shown that this m a r t e n s i t e c o n s i s t s of p l a t e l e t s in which the shape d e f o r m a t i o n is a c c o m m o d a t e d by a s h e a r i n g p r o c e s s other than that of twinning. Although the i n dividual m a r t e n s i t e units (platelets or laths) a r e r e a d i l y v i s i b l e in the c a r b o n r e p l i c a s for the s t r i n g e r type s t r u c t u r e , they cannot be detected in the r e p l i c a for the blocky s t r u c t u r e obtained from a fine a u s t e n i t e grain size. To a c c o u n t for the two d i f f e r e n t m o r p h o l o g i e s , a s t r e s s - c o n s t r a i n t model was p r o p o s e d for the t r a n s f o r m a t i o n of the f i n e - g r a i n a u s t e n i t e . H e r e , the d i r e c PAUL C. MAXWELL,formerly Summer Employee, University of California Lawrence Radiation Laboratory, Livermore, Calif., is Graduate Student, Department of Materials Science, Stanford University, Stanford, Calif. ALFRED GOLDBERG is Research Metallurgist,University of California Lawrence Radiation Laboratory. JOHN C. SHYNE, formerly consultant, University of California Lawrence Radiation Laboratory, is Professor of Materials Science, Department of Materials Science, Stanford University. Manuscript submitted January 13, 1970. METALLURGICALTRANSACTIONS
tion of growth of a platelet is c o n s i d e r e d to be affected by c o n s t r a i n t s developed at a twin or g r a i n b o u n d a r y . As a growing p l a t e l e t a p p r o a c h e s a g r a i n b o u n d a r y , back s t r e s s e s a r e c r e a t e d at the b o u n d a r y which, in t u r n , nucleate a new platelet that then grows with a t r a n s f o r m a t i o n s h e a r d i r e c t i o n opposite to that of the p r e v i o u s platelet. In this m a n n e r , a packet of p a r a l l e l p l a t e l e t s form in which the s h e a r d i r e c t i o n of a d j a c e n t p l a t e l e t s a l t e r n a t e by 180 deg. Fig. 4(a) s c h e m a t i c a l l y depicts this growth s e q u e n c e , which g e n e r a l l
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