Defects formed during quenching in Al-4 pct Cu
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L. H A L L
A N D J O H N B. V A N D E R
SANDE
Thin foils of AI-4 pet Cu alloys were examined by transmission electron microscopy in the as-quenched and the aged states in an attempt to clarify the mechanism of the prolonged low-temperature aging phenomenon in these a11oys known as the slow reaction. The asquenched microstructure is found to consist of bands of vacancy clusters or small dislocation loops with helical dislocations sometimes associated with them. It is postulated that these defects are formed by the interaction of vacancy agglomerates and dislocations during the quench. The contrast images associated with the defects are found to vary in radius from approximately 15 to 85A, and in density roughly from 1 to 5 • 10'S/cm s. These results are consistent with the calculations of Okamoto and Kimura, who showed that the slow reaction could best be explained as being caused by supersaturated vacancies in equilibrium with the surface tension of defects of this size. As the slow reaction proceeds, these vacancies are annihilated at permanent sinks, and this disturbance of equilibrium causes the defects to emit vacancies and, hence, shrink and disappear completely in the aged state.
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has been well established that m a n y aluminum alloys, upon quenching from high temperatures and then aging, display a hardening phenomenon caused by the segregation of solute atoms into zones or clusters, referred to as precipitation hardening. '-4 W h e n the process is followed using electrical or calorimetric techniques, it is found that cluster formation proceeds at rates m a n y orders of magnitude greater than it should if it were governed by the diffusion constants obtained by the extrapolation of the high temperature diffusion d a t a a l o n e , x'z'~ F e d e r t g h t 8 and D e s o r b o e t a l z showed i n d e p e n d e n t l y that t h i s e x t r e m e l y r a p i d r e a c t i o n could be e x p l a i n e d m o s t s i m p l y and a d e q u a t e l y b y the h y p o t h e s i s that the s o l u t e diffusion is a f f e c t e d b y v a c a n c i e s , which a r e r e t a i n e d in the s p e c i m e n d u r i n g the quench f r o m htgh t e m p e r a t u r e s . T h e s e s u p e r s a t u r a t e d v a c a n c i e s r e a c t with the s o l u t e a t o m s to f o r m c o m p l e x e s , which then a i d in zone f o r m a t i o n , m o s t p r o b a b l y b y the " v a c a n c y p u m p " m e c h a n i s m p r o p o s e d by G i r i f a l c o and Herman.7 This e n h a n c e d r a t e of c l u s t e r f o r m a t i o n c a u s e d b y v a c a n c y s u p e r s a t u r a t i o n is a p p r o x i m a t e l y 109 t i m e s the e q u i l i b r i u m r a t e at the o n s e t , and the r a t e c o n s t a n t l y d e c r e a s e s with t i m e . *'2 The t i m e d u r i n g which the r a t e i s m a n y o r d e r s of m a g n i t u d e g r e a t e r than the e q u i l i b r i u m value is r e f e r r e d to a s the " f a s t " r e a c t i o n and D e s o r b o et al 2 showed that it i s v i r t u a l l y c o m p l e t
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