Properties and microstructure of aluminum-copper-magnesium-lithium alloys
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A L u M I N U M - l i t h i u m a l l o y s have two a d v a n t a g e s over c o n v e n t i o n a l a l u m i n u m a l l o y s : f i r s t , they show an i n c r e a s e d e l a s t i c modulus, ~ and secondly, they have a lower d e n s i t y . ~'2 T h e i r c o m m e r c i a l application has b e e n slowed by d i f f i c u l t i e s in producing sound c a s t ings and by the low t o u g h n e s s of the alloys.~ The c a s t ing p r o b l e m s can be o v e r c o m e at some additional cost by the use of a powder m e t a l l u r g y p r o c e s s . T h i s p r o g r a m was d e s i g n e d to m e a s u r e the m e c h a n i c a l p r o p e r t i e s of a l u m i n u m - l i t h i u m powder alloys and, in p a r t i c u l a r , to d e t e r m i n e whether the toughness p r o b l e m s could be o v e r c o m e by the use of r a p i d l y cooled powder p o s s e s s i n g both a fine g r a i n size and r e l a t i v e f r e e d o m f r o m the l a r g e i n t e r m e t a l l i c compounds that f o r m in castings.
E X P E R I M E N T A L TECHNIQUE The powders w e r e produced by m e l t spinning3 in the f o r m of flakes about 1 • 1 m m • 50 p m at the Batelle Columbus Laboratories. This process uses a r e s e r v o i r of m o l t e n m e t a l in a g r a p h i t e c r u c i b l e which is forced u n d e r the p r e s s u r e of an i n e r t gas through a s m a l l o r i fice onto the s u r f a c e of a s p i n n i n g copper wheel d e signed to r a p i d l y chill the m o l t e n m e t a l and b r e a k it up into a s e r i e s of f l a k e s . B e c a u s e of the r e a c t i v i t y of the m o l t e n a l u m i n u m - l i t h i u m alloys the jet of molten m e t a l was p r o t e c t e d f r o m oxidation by a s t r e a m of a r g o n g a s . The a l l o y s e x a m i n e d in this p r o g r a m w e r e n o m i n a l l y A1 2024, A1 2024 + 1.5 Li, and A1 2024 + 3 Li. The c h e m i cal c o m p o s i t i o n s of the hot p r e s s e d and extruded s a m ples a r e given in T a b l e I. A s e r i e s of cast alloys of s i m i l a r c o m p o s i t i o n s were made in the f o r m of 5 c m d i a m c y l i n d r i c a l ingots and used for c o m p a r i s o n p u r p o s e s . The c o m p o s i t i o n s of t h e s e alloys a r e a l s o g i v e n in T a b l e I. The powders were c o n s o l i d a t e d by v a c u u m hot p r e s s i n g to full d e n sity at 755 K and then e x t r u d e d at 730 K with a 10:1 r e d u c t i o n r a t i o to f o r m a flat e x t r u s i o n about 5 m m thick and 4 c m wide. The c a s t ingots were a l s o e x t r u d e d u n d e r the s a m e c o n d i t i o n s . T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y was p e r f o r m e d on the flakes without r e s o r t i n g to e l e c t r o p o l i s h i n g at some a r e a s of the a s D O N A L D W E B S T E R is Staff Engineer, L o c k h e e d Missiles and Space Co., Sunnyvale, CA 9 4 0 8 6 . M a n u s c r i p t s u b m i t t e d April 3, 1979. METALLURGICAL TRANSACTIONS A
r e c e i v e d flake which were thin enough
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