Enhanced ductility in an aluminum-4 Pct magnesium alloy at elevated temperature
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A. A Y R E S
A c o n s i d e r a b l e e n h a n c e m e n t of the t e n s i l e d u c t i l i t y in a c o m m e r c i a l A1-4 pct Mg a l l o y is o b s e r v e d d u r i n g d e f o r m a t i o n at e l e v a t e d t e m p e r a t u r e s (up to 250~ and slow s t r a i n r a t e s . T o t a l e l o n g a t i o n s of ~175 pct at 250~ w e r e o b t a i n e d c o m p a r e d to 27 pct at a m b i e n t t e m p e r a t u r e . Much of t h i s d u c t i l i t y was a r e s u l t of l a r g e i n c r e a s e s with t e m p e r a t u r e in the p o s t u n i f o r m o r diffuse n e c k i n g s t r a i n . M e a s u r e m e n t s of s t r a i n r a t e s e n s i t i v i t y , m , a s a function of s t r a i n , s t r a i n r a t e , and t e m p e r a t u r e showed that m n e a r f r a c t u r e was l i n e a r l y r e l a t e d to t o t a l e l o n g a t i o n . The m e c h a n i s m s c o n t r o l l i n g m in t h i s A14 pct Mg a l l o y w e r e d y n a m i c s t r a i n aging a t the l o w e r t e m p e r a t u r e r a n g e and d y n a m i c r e c o v e r y at the h i g h e r t e m p e r a t u r e s , m was found to be a function of s t r a i n only when the r e l a t i v e f r a c t i o n of d y n a m i c r e c o v e r y was g r e a t e r than ~35 pct. A c o m p a r i s o n of m a s m e a s u r e d in p u r e a l u m i n u m and in the c o m m e r c i a l A1-4 p c t Mg a l l o y s u g g e s t s that Mg a d d i t i o n s can s i g n i f i c a n t l y i n c r e a s e m d u r i n g d y n a m i c r e c o v e r y .
ALLOYING
m e t a l s to i n c r e a s e the y i e l d s t r e n g t h u s u a l l y r e s u l t s in a d e c r e a s e in d u c t i l i t y . T h i s is a p p a r e n t in a l u m i n u m s h e e t a l l o y s that a r e s i m i l a r in s t r e n g t h to m i l d s t e e l but exhibit only about o n e half of the t e n s i l e e l o n g a t i o n to f r a c t u r e (~22 pct c o m p a r e d to 45 pct). B e c a u s e of t h i s r e d u c e d d u c t i l i t y , f a b r i c a t i o n of a l u m i n u m p a r t s i s m o r e difficult. Although f u t u r e a l l o y i n g d e v e l o p m e n t s m a y i m p r o v e d u c t i l i t y s o m e w h a t , it a p p e a r s u n l i k e l y that the t o t a l e l o n g a t i o n can be doubled in an a l u m i n u m a l l o y without an u n a c c e p t a b l e l o s s of s t r e n g t h . D e f o r m a t i o n at e l e v a t e d t e m p e r a t u r e o f f e r s a p o t e n t i a l f o r i n c r e a s i n g d u c t i l i t y without d e c r e a s i n g the s u b s e q u e n t a m b i e n t t e m p e r a t u r e y i e l d s t r e n g t h . Quite l a r g e e l o n g a t i o n s (~1000 pct) have b e e n a c h i e v e d in s o m e a l u m i n u m s u p e r p l a s t i c a l l o y s ~ at c e r t a i n s t r a i n r a t e s , 4, and t e m p e r a t u r e s , T. Howe v e r , t h e s e d u c t i l i t y m a x i m a tend to o c c u r within n a r r o w r a n g e s of t e m p e r a t u r e and s t r a i n r a t e and only for e x t r e m e l y fine g r a i n s i z e s . It w
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