Fatigue behavior of aluminum-copper alloy exhibiting a duplex dendritic structure
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Fatigue Behavior of Aluminum-Copper Alloy Exhibiting a Duplex Dendritic Structure G. W. S W A R T Z B E C K A N D T. Z. K A T T A M I S
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d u p l e x d e n d r i t i c s t r u c t u r e c o n s i s t s of c o a r s e d e n d r i t e s e m b e d d e d i n a f i n e f e r n l i k e d e n d r i t i c m a t r i x . 1,2 Such a microstructure, o b s e r v e d i n r h e o c a s t a l l o y s a'4 and sometimes in continuously cast and die-cast all o y s , 5-7 i s g e n e r a t e d w h e n a f i r s t s l o w s o l i d i f i c a t i o n stage, during which substantial coarsening and coalesc e n c e of t h e g r o w i n g s o l i d o c c u r s , i s f o l l o w e d b y a s e c ond rapid solidification stage. Alloys cast in the solidliquid region exhibit a duplex microstrueture. The present investigation was undertaken to determine how casting in the solid-liquld region might aff e c t t h e m e c h a n i c a l b e h a v i o r of t h e m a t e r i a l . T h i s b e h a v i o r w a s e v a l u a t e d o n t h e b a s i s of f a t i g u e s t r e n g t h which is known to be intimately related to the distribut i o n of a l l o y i n g e l e m e n t s w i t h i n t h e m a t r i x . T h u s , f a tigue crack growth rate in a high strength aluminum a l l o y w a s s h o w n to i n c r e a s e w i t h a m o u n t of c o p p e r d i s solved in the matrix, s Also, precipitate free zones
G. W. SWARTZBECK, formerly Graduate Student, Department of Metallurgy, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, is now with Westinghouse Electric, Pittsburgh, Pa. T. Z. KATTAMIS, Associate Professor, Department of Metallurgy, Institute of Materials Science, University of Connecticut, Storrs, Conn. 06268. Manuscript submitted May 17, 1973. METALLURGICAL TRANSACTIONS
formed adjacent to the grain boundaries in age-hardenable aluminum alloys were found responsible for poor f a t i g u e p e r f o r m a n c e ; a-xs t h e s e z o n e s a p p e a r e d t o b e p r e f e r r e d s i t e s f o r t h e f o r m a t i o n of s l i p b a n d s a n d u l t i m a t e l y n u c l e a t i o n of c r a c k s . A l - 4 . 5 w t p c t Cu a l l o y w a s m e l t e d i n a c l a y - g r a p h i t e crucible coated with zircon wash and allowed to cool down. During cooling the solid nucleated and the solid plus liquid mixture was homogenized by vigorous handstirring. The system was appropriately degassed, following standard aluminum melting procedures. At
644.5~ 1.5~ below the liquidus temperature the slurry was cast into a copper mold coated with amorphous carbon. Several plates (6 by 3 by 1 in.) were prepared. For comparison purposes some plates were cast at 690~ 49~ above the liquidus temperature in order to avoid the formation of duplex dendritic structure. Reversed bending cantilever type specimens, Fig. I, were machined from the plates. The specimens were solutionized for 6 h at 535~ water quenched, aged for 1 h at 375~ and finely polished on 600 silicon carbide paper before testing. All specimens were tested in air at room
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