A survey of eutectoid decomposition in ten Ti-X systems
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study was u n d e r t a k e n to provide a b e t t e r u n d e r s t a n d i n g of the conditions u n d e r which the b a i n i t e or the p e a r l i t e r e a c t i o n is the d o m i n a n t mode of eutectoid d e c o m p o s i t i o n , and t h e r e b y a c q u i r e a b e t t e r u n d e r s t a n d i n g of the n a t u r e of t h e s e r e a c t i o n s . Bainite is defined h e r e as the product of a n o n l a m e l l a r mode of eutectoid d e c o m p o s i t i o n . 1. T h i s is in c o n t r a s t to *Althoughthere is extensivecontroversyaboutthe definitionof bainite,2the difficultiesthus engenderedcanbe avoidedin the presentcontextif those who prefer a differentdefinitionwillsubstitutethe term "nonlamellareutectoid" whenever"bainite"is usedin this paper. p e a r l i t e which is produced by the l a m e l l a r f o r m of eutectoid r e a c t i o n . I l l u m i n a t i n g the d i f f e r e n c e between the two modes m e c h a n i s t i c a l l y , H i l l e r t 3 d e s c r i b e s p e a r l i t e as b e i n g f o r m e d by the cooperative growth of two p r e c i p i t a t e p h a s e s and b a i n i t e as the r e s u l t of noncooperative growth of these p h a s e s . A t y p i c a l T i - X b a i n i t e is shown in Fig. 1. T h i s was p r o d u c e d by the fl ~ a + TizNi r e a c t i o n in the s y s t e m T i - N i . Note that the i n t e r m e t a l l i c compound p a r t i cles a r e roughly e q u i - a x e d . The hcp a phase c r y s t a l s a r e s e e n to be u s u a l l y plates but in some c a s e s g r a i n b o u n d a r y a l l o t r i o m o r p h s . On the m i c r o s t r u c t u r a l definition of b a i n i t e , 1 the morphology of n e i t h e r phase is i m p o r t a n t as long as both a r e not l a m e l l a r and they do not grow in an e s s e n t i a l l y c o n t i n u o u s l y cooperative manner. Much of the i n f o r m a t i o n p r e s e n t l y a v a i l a b l e on the p e a r l i t e mad b a i n i t e r e a c t i o n s has b e e n obtained f r o m s t u d i e s on s t e e l . U n l e s s the s t e e l is r a t h e r highly alloyed, p e a r l i t e is u s u a l l y the d o m i n a n t product of eutectoid d e c o m p o s i t i o n at higher t e m p e r a t u r e s (say above 550 to 600~ in hypoeutectoid as well as in eutectoid a l l o y s . 4'~ B a i n i t e f o r m s p r i m a r i l y at lower t e m p e r a t u r e s , when the f o r m a t i o n k i n e t i c s of p e a r l i t e b e c o m e , e v i d e n t l y of t h e i r own a c c o r d , less comG. W. FRANTI and H. I. AARONSONare Postdoctoral Research Associate and Professor, respectively, Department of Metallurgical Engineering,MichiganTechnologicalUniversity, Houghton, MI 49931, and J. C. WILLIAMSis Professor, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submitted March 17, 1978. METALLURGICALTRANSACTIONSA
petisive. At the outset of the p r e s e n t investigation, the q u e s t i o n was asked: need p e a r l i t e always be the d o m i nant eute
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