Structural changes during the superplastic deformation
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s u l t s in an i n t r i n s i c h a r d n e s s of 77 kg p e r sq m m for the c o p p e r studied. Melting of the o r i g i n a l c o p p e r p o w d e r under n i t r o g e n and annealing at 1000~ under hydrogen r e s u l t e d in a m a t e r i a l with a V i c k e r s h a r d n e s s of 70 kg p e r sq ram. T a k i n g this f i g u r e as the a p p r o x i m a t e i n t r i n s i c h a r d n e s s , the a g r e e m e n t with the i n d i r e c t l y d e t e r m i n e d v a l u e s can be c o n s i d e r e d satisfactory. E x a m p l e No. 2. The a p p l i c a b i l i t y of the m e t h o d to a p a r t i c u l a t e m a t e r i a l is d e m o n s t r a t e d with a c o m m e r c i a l grade a t o m i z e d a l u m i n u m powder. After annealing at 650~ under a r g o n , the - 3 2 5 m e s h f r a c t i o n of the a l u m i n u m powder and a h a r d e n a b l e liquid r e s i n w e r e i n t i m a t e l y m i x e d so that a u n i f o r m c o m p o s i t e of paste type c o n s i s t e n c y was f o r m e d . E n t r a p m e n t of a i r and the p r e s e n c e of p o r o s i t y w e r e m i n i m i z e d by p r e p a r ing the c o m p o s i t e under v a c u u m . F o r the d e t e r m i n a tion of the i n t r i n s i c h a r d n e s s of the r e s i n a l o n e , a r e s i n s p e c i m e n was p r e p a r e d and t r e a t e d in like m a n n e r . The s p e c i m e n s w e r e c u r e d at r o o m t e m p e r a t u r e . A f t e r h a r d n e s s m e a s u r e m e n t s had b e e n taken, c u r i n g was continued at 120~ A f t e r p o l i s h i n g , a n o t h e r set of h a r d n e s s m e a s u r e m e n t s w a s taken. The r e p e t i t i o n of this p r o c e d u r e p e r m i t t e d the d e t e r m i n a t i o n of h a r d n e s s d i s p e r s i o n s as a function of r e s i n h a r d n e s s f r o m a single s p e c i m e n . The r e s u l t s a r e s u m m a r i z e d in Table I and F i g . 4. In F i g . 4, the l o g a r i t h m s of the g e o m e t r i c s t a n d a r d d e v i a t i o n s o r the d i s p e r s i o n s of the c o m p o s i t e s a r e plotted vs r e s i n h a r d n e s s . With i n c r e a s i n g r e s i n h a r d n e s s , the d i s p e r s i o n f i r s t dec r e a s e s and then i n c r e a s e s again. T h i s i n d i c a t e s that the unknown h a r d n e s s of the p a r t i c u l a t e a l u m i n u m powder l i e s in b e t w e e n the e x t r e m e r e s i n v a l u e s . The e x p e r i m e n t a l data points a r e c o n n e c t e d by s t r a i g h t c u r v e s as shown in Fig. 4. The point of t h e i r i n t e r s e c t i o n d e t e r m i n e s the a p p r o x i m a t e i n t r i n s i c h a r d n e s s of the m e t a l . The thus d e t e r m i n e d V i c k e r s h a r d n e s s of 16.4 kg p e r sq m m is in good a g r e e m e n t with the d i r e c t l y * m e a s u r e d value of 16.6 kg p e r sq m m . An
continuous function shown in F i g . 4. If, t h e r e f o r e , e x t r a p o l a t i o n to z e r o d i s p e r s i o n p r o d u c e s an i n t r i n s
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