Atomic rearrangements in ordered fcc alloys during neutron irradiation
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s e r v i n g the e f f e c t s of the " t h e r m a l s p i k e " , a concept now l a r g e l y d i s c r e d i t e d which would cause d i s o r d e r to o c c u r by the t h e r m a l a g i t a t i o n in the v i c i n i t y of a ne u t ro n k n o c k - o n . H o w e v e r , p o s i t i v e e v i d e n c e was obtained fo r l o c a l i z e d d i s o r d e r i n g and was p r e s e n t e d as e v i d e n c e f o r a t h e r m a l spike. B l e w i t t and cow o r k e r s 2 at ORNL a few y e a r s l a t e r c o r r e c t l y deduced that an i n i t i al d e c r e a s e in r e s i s t i v i t y in the o r d e r e d a l l o y s b o m b a r d e d by S i e g e l was the r e s u l t of o r d e r ing by enhanced diffusion f r o m the m i g r a t i o n of r a d i a tion c r e a t e d v a c a n t l a t t i c e s i t e s . T h e s e w o r k e r s then d e v i s e d an e x p e r i m e n t 3 u s i n g p a r t i a l l y o r d e r e d CuaAu to c o n c l u s i v e l y show f o r the f i r s t t i m e that r a d i a t i o n enhanced diffusion o c c u r r e d . Since that t i m e a g r e a t d e a l has been l e a r n e d about r a d i a t i o n d a m a g e and the e x p e r i m e n t s have b e c o m e c o r r e s p o n d i n g l y m o r e s o p h i s t i c a t e d . T h i s p a p e r will d e s c r i b e t h r e e d i f f e r e n t e x p e r i m e n t s w h e r e the u t i l i zation of o r d e r e d a l l o y s has p r o v i d e d unique i n f o r m a tion on i m p o r t a n t c o n c e p t s of r a d i a t i o n d a m a g e . Th e f i r s t s et of e x p e r i m e n t s we s h a l l d e s c r i b e c o n c e r n s r e s u l t s f r o m the t h e r m a l n e u t r o n i r r a d i a t i o n of o r d e r e d Ni~Mn at 5 K. F r o m t h e s e e x p e r i m e n t s c o m e s p e r h a p s the m o s t d i r e c t e v i d e n c e f o r the e x i s t e n c e and t y p i c a l M. A. KIRKis Assistant Metallurgistand T. H. BLEW1TTis Senior Scientist, Materials ScienceDivision,ArgonneNationalLaboratory, Argonne,IL 60439. This paper is based on a presentation made at a symposiumon "Radiation Induced AtomicRearrangementsin Orderingand ClusteringAlloys"held at the annual meetingof the AIME,Atlanta, Georgia,March 7 to 8, 1977, under the sponsorship of the PhysicalMetallurgyand NuclearMetallurgyCommittees of The MetallurgicalSocietyof AIME. METALLURGICAL TRANSACTIONSA
magnitude of r e p l a c e m e n t c o l l i s i o n s e q u e n c e s , a r a d i a tion d a m a g e m e c h a n i s m i m p o r t a n t to the u n d e r s t a n d i n g of many e x p e r i m e n t a l r e s u l t s and t h e o r e t i c a l m o d e l s in the f i e l d . A s e c o n d set of e x p e r i m e n t s on the s a m e alloy, but under f i s s i o n n e u t r o n b o m b a r d m e n t at 5 K, will then be d e s c r i b e d . Building upon what we l e a r n e d f r o m the t h e r m a l n e u t r o n i r r a d i a t i o n s (with a v e r a g e r e c o i l e n e r g y of 450 eV), fast n eu t r o n i r r a d i a t i o n s (with t y p i c a l r e c o i l e n e r g y of 20 k
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