Calorimetric studies of 7000 series aluminum alloys: I. Matrix precipitate characterization of 7075
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in t h e s e t e m p e r s a r e i d e n t i f i e d , and r e a c t i o n s a c c o m panying heating are clarified.
EXPERIMENTAL Samples of 7075 aluminum alloy were taken from a 7.6 cm (3 in.) thick by 15.2 cm (6 in.) wide rolled bar that was supplied in the highest strength, stress relieved, T651 temper. Individual specimens were overaged by heat treating in an oil bath at 175~ for 9 h. The mechanical properties of these tempers were found to be consistent with expected values for this alloy. 9 C a l o r i m e t r i c m e a s u r e m e n t s w e r e m a d e using a DuP o n t 900 T h e r m a l A n a l y z e r containing a d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r (DSC) p l u g - i n m o d u l e . T h i s t e c h n i q u e h a s been d e s c r i b e d a~ and was p r e v i o u s l y a p p l i e d to a n a l y s i s of 7075 a l u m i n u m a l l o y , s S a m p l e d i s c s of 0.56 c m (7/32 in.) d i a m >~ 0.127 e m (0.050 in.) t h i c k n e s s w e r e p r e p a r e d and e x a m i n e d for each of the two t e m p e r s . To i n c r e a s e the s e n s i t i v i t y of the m e a s u r e m e n t s a high p u r i t y a l u m i n u m d i s c w a s used a s a r e f e r e n c e . Runs w e r e m a d e at a h e a t i n g r a t e of 10~ rain o v e r the r a n g e f r o m r o o m t e m p e r a t u r e to 480~ D r i e d n i t r o g e n (1 • 10-4m3/min) was p a s s e d t h r o u g h the c a l o r i m e t e r to m i n i m i z e o x i d a t i o n . Hot s t a g e 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 used to o b s e r v e the s o l i d s t a t e r e a c t i o n s t h a t a c c o m pany heating. Sample d i s c s of 0.318 c m (0.125 in.) d i a m w e r e thinned by twin j e t - p o l i s h i n g , using a t w o p a r t s m e t h a n o l , one p a r t n i t r i c a c i d e l e c t r o l y t e a t t e m p e r a t u r e s l e s s than - 30~ and a t an a p p l i e d v o l t age of 15 V. S p e c i m e n e x a m i n a t i o n w a s p e r f o r m e d with a P h i l i p s EM 300 e l e c t r o n m i c r o s c o p e , equipped with a hot s t a g e s p e c i m e n h o l d e r with a t e m p e r a t u r e c o n t r o l unit. The s p e c i m e n was c l a m p e d onto a s m a l l f u r n a c e e l e m e n t that w a s i n s u l a t e d f r o m the h o l d e r b y t h r e e z i r c o n i u m oxide s p h e r e s . The f u r n a c e c o n t a i n e d a s m a l l h e a t i n g e l e m e n t and a c h r o m e l - a l u m e l t h e r m o couple.
In the hot stage T E M work, we attempted to duplicate VOLUME 8A, JULY t977- 1177
the 1 0 ~ h e a t i n g r a t e that w a s used for the DSC m e a s u r e m e n t s . However, the t e m p e r a t u r e c o n t r o l unit for the hot s t a g e was not equipped with c o n s t a n t h e a t ing r a t e c a p a b i l i t y . It was n e c e s s a r y to a d j u s t the t e m p e r a t u r e m a n u a l l y ; it w a s a l s o n e c e s s a r y to hold the t e m p e r a t u r e c o n s t a n t f o r a 0.50 to 0.75 m i n p e r i o d b e f o r e p h o t o g r a p h
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