Unidirectional transformation of Fe-0.8C-Co alloys: Part I. Process per structure relationships and the significance of

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p e a r l i t e r e a c t i o n , p a r t i c u l a r l y in f e r r o u s a l loys, has b e e n the s u b j e c t of e x t e n s i v e e x p e r i m e n t a l r e s e a r c h c o n c e r n i n g the m e c h a n i s m and k i n e t i c s of f o r m a t i o n of l a m e l l a r s t r u c t u r e s . 1 It is well e s t a b l i s h e d that the p e a r l i t e i n t e r l a m e l l a r spacing, ~, is r e l a t e d to the growth r a t e , V, and the u n d e r c o o l i n g ,

AT, viz., X = ~(V, AT)

ll]

but that in an e x p e r i m e n t only V or AT can be fixed. U n t i l r e c e n t l y , however, i s o t h e r m a l t r a n s f o r m a t i o n e x p e r i m e n t s have always b e e n employed, which cont r o l the t e m p e r a t u r e but n e c e s s i t a t e m e t a l l o g r a p h i c m e a s u r e m e n t of both growth r a t e s and i n t e r l a m e l l a r s p a c i n g s of r a n d o m l y n u c l e a t e d p e a r l i t e c o l o n i e s . The most f r e q u e n t method of m e t a l l o g r a p h i c a l l y det e r m i n i n g the growth r a t e is to m e a s u r e l i n e a r l y the r a d i i of the l a r g e s t p e a r l i t e nodules (rma x). The p r o b l e m s of this method may be e n u m e r a t e d as follows. i) The difficulty of locating r max, e s p e c i a l l y at s h o r t r e a c t i o n t i m e s . To give equal p r o b a b i l i t y of locating rrnax in all s p e c i m e n s , an a r e a i n v e r s e l y p r o p o r t i o n a l to t r a n s f o r m a t i o n t i m e m u s t be searched. 2 ii) Nodule i m p i n g e m e n t o c c u r s at long r e a c t i o n times. iii) H i g h n u c l e a t i o n r a t e s give p e a r l i t e s l a b s at the a u s t e n i t e g r a i n b o u n d a r i e s and a r a n d o m s e c t i o n through a s l a b does not give a t r u e m e a s u r e of its thickness. iv) Nodule growth r a t e a p p e a r s to be a function of B. G. MELLOR, formerly Research Student, Department of Metallurgy and Materials Science, University of Cambridge, U. K., is now Royal Society Overseas Research Fellow, Departamento de Metalurgia, UniversidadComplutense de Madrid, Spain, and D. V. EDMONDS is Royal Society Warren Research Fellow, Department of Metallurgy and Materials Science, University of Cambridge. Manuscript submitted March 20, 1975. METALI.URGICALTRANSACTIONSA

t r a n s f o r m a t i o n t i m e . 2 To o v e r c o m e (ii) it is p o s s i b l e to m e a s u r e the a v e r a g e growth r a t e , -V, given by

-~_ 1 dX Af dt

[21

w h e r e X is the volume f r a c t i o n of p e a r l i t e , Af the uni m p i n g e d s u r f a c e a r e a of p e a r l i t e p e r unit v o l u m e and t the t i m e , but this method, 2 i n v o l v i n g both l i n e a l and a r i a l m e a s u r e m e n t s is even m o r e l a b o r i o u s than d e t e r m i n i n g r m a x. By m e a s u r i n g the v a r i a t i o n of nodule size d i s t r i b u t i o n as a function of t i m e it is a l s o p o s s i b l e to d e t e r m i n e the growth r a t e avoiding (iv) above, 3 but