Microstructures of aluminum compressed at various rates and temperatures
- PDF / 3,861,916 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 70 Downloads / 268 Views
S p e c i m e n s of 1100 a l u m i n u m were c o m p r e s s e d u n i f o r m l y to a l o g a r i t h m i c s t r a i n of 0.7 (50 pct r e d u c t i o n ) at c o n s t a n t t r u e s t r a i n r a t e s in a Cam P l a s t o m e t e r . T e s t s were conducted at s t r a i n r a t e s between 220 and 0.1 s e c - x and between 500 ~ and 20~ E x a m i n a t i o n of the m i c r o s t r u c t u r e s by e l e c t r o n m i c r o s c o p y indicated that the d e f o r m e d s t r u c t u r e c o n s i s t e d of d i s l o c a t i o n c e l l s o r s u b g r a i n s . As the t e m p e r a t u r e of t e s t i n g was r a i s e d or the s t r a i n r a t e lowered, the r e s u l t i n g s u b g r a i n s were l a r g e r , and the d i s l o c a t i o n s in the s u b b o u n d a r i e s w e r e a r r a n g e d in m o r e o r d e r l y a r r a y s . The cell s i z e s a r e r e l a t e d q u a n t i t a t i v e l y to the flow p a r a m e t e r s . The dependence of dyn a m i c r e c o v e r y on t e m p e r a t u r e and s t r a i n r a t e is c o m p a r e d to the dependence o b s e r v e d in e x t r u s i o n of a s i m i l a r a l l o y . At high s t r a i n r a t e s at 400 ~ and 500~ p a r t i a l r e c r y s t a l l i z a t i o n occ u r r e d while the s p e c i m e n s were cooling to r o o m t e m p e r a t u r e . M I C R O S T R U C T U R A L studies1- 7 of s e v e r a l m e t a l s , e.g., a l u m i n u m , z-3'5-7 A1-5 pct Mg, 5 and F e - 2 5 pct C r 4 have r e v e a l e d that r e c r y s t a l l i z a t i o n had not o c c u r r e d d u r i n g hot working (the s p e c i m e n s were r a p i d l y cooled i m m e d i a t e l y after d e f o r m a t i o n ) . The i n v e s t i g a t i o n s f u r t h e r r e v e a l e d the f o r m a t i o n of s u b g r a i n s 2-7 which were s i m i l a r to those r e s u l t i n g f r o m r e c o v e r y of the cold worked m e t a l or f r o m r a p i d h i g h - t e m p e r a t u r e c r e e p . T h i s phenomenon h a s been n a m e d " d y n a m i c r e c o v e r y " a s the r e c o v e r y t a k e s place d u r i n g d e f o r m a tion. The s u b s t r u c t u r e is a r e s u l t of m u t u a l a n n i h i l a tion of m a n y d i s l o c a t i o n s and of the r e a r r a n g e m e n t of the r e m a i n i n g d i s l o c a t i o n s into r e g u l a r low-angle b o u n d a r i e s . The r e l a t i v e l y r a p i d r a t e s of c r o s s - s l i p and c l i m b r e q u i r e d for the above phenomenon a r e found only in m e t a l s and a l l o y s which have n a r r o w d i s l o c a t i o n s b e c a u s e the high e n e r g y of t h e i r stacking faults i n h i b i t s d i s s o c i a t i o n . 5'7 The low d i s l o c a t i o n d e n sity developed d u r i n g hot working gives r i s e to a low flow s t r e s s and inhibits the f o r m a t i o n of r e c r y s t a l l i z a tion n u c l e i . 5'7 A l u m i n u m , which e x e m p l i f i e s these c h a r a c t e r i s t i c s , has been studied in t o r s i o n , z'8 e x t r u sion,Z,e,9- zz r o l l i n
Data Loading...
