Altering the time cycle of heat treatment by controlling grain boundary and subboundary structure
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T H I S p a p e r s e e k s b a s e s f o r a l t e r a t i o n s in t h e t i m e c y c l e o f h e a t t r e a t m e n t s d e s i g n e d to p r o d u c e diffusional p h a s e t r a n s f o r m a t i o n s . P a r t i c u l a r a t t e n t i o n is d i r e c t e d to t h o s e t r a n s f o r m a t i o n s in w h i c h n u c l e a t i o n at g r a i n a n d / o r s u b g r a i n b o u n d a r i e s in t h e m a t r i x p h a s e is i m p o r t a n t . T h e kinetics of such a transformation have l o n g b e e n r e c o g n i z e d as b e i n g subject to m a r k e d c h a n g e s s i m p l y by a l t e r i n g t h e a v e r a g e g r a i n o r subgrain size. T h e classical explanation for this is that the rate of nucleation per unit v o l u m e of s a m p l e is directly proportional to the boundary area. This a s s u m e s that the morphology, rate of nucleation per unit grain b o u n d a r y or subgrain b o u n d a r y area, g r o w t h kinetics, and of c o u r s e the identity of the precipitate crystals all r e m a i n u n c h a n g e d throughout the transformation. Alterations in grain and subgrain size, h o w e v e r , are likely to be a c c o m p a n i e d by c h a n g e s in the s p e c t r u m of boundary structures present. T h e central t h e m e of this p a p e r is that these structures can exert m a j o r effects upon all of the factors customarily a s s u m e d to be unchanged. (Since the role of b o u n d a r y structure in determ i n i n g w h e t h e r the equilibrium o h a s e or a transitional p h a s e is nucleated has been discussed in a current review, ~ this p r o b l e m will not be considered here.) W h e n the s p e c t r u m of b o u n d a r y structures is biased by texturing a n d / o r polygonization of the m a t r i x phase, substantial alterations are possible not only in the time cycle of the heat treatment but also in the microstrue-
H. B. AARON and H. I. AARONSON are associated with the Metallurgy Department, Scientific Research Staff, Ford Motor Company, Dearborn, Michigan. This paper is based upon a presentation made at a symposium on Altering the Time Cycle of Heat Treatment, held at the Philadelphia meeting of T h e Metallurgical Society of AIME, October 14, 1 9 6 9 , under the sponsorship of the IMD Heat Treatment Committee. METALLURGfCAL
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t u r e , a n d t h u s i n the m e c h a n i c a l a n d p h y s i c a l p r o p erties. A t t h e present t i m e , most o f t h e knowledge available o n the influence of g r a i n and s u b g r a i n boundary s t r u c ture upon diffusional transformations concerns t h e morphology and identity of precipitates; growth kinetics d a t a h a v e b e e n o b t a i n e d only a t d i s o r d e r e d g r a i n b o u n d a r i e s , w h i l e i n f o r m a t i o n on n u c l e a t i o n k i n e t i c s i s o n l y qualitative. Fortunately, the qualitative observations a r e both e x t e n s i v e a n d i n s t r u c t i v e b e c a u s e they e n a b l e i n i t i a l t e s t s t o be m a d e of a v a i l a b l e t h e o
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