On the steady state flow stress of iron at low temperature and large strains
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Fig. 1--Comparison of the true equivalent stress-true equivalent strain curve for polycrystalline bcc iron (gettered with 0.17 pct Ti) in torsion and in tension (after wire drawing). 1 A steady state resistance to plastic flow drawing leads to continued strain hardening. The dashed line represents the r e suits from wire drawing on Fe-0.007 C alloy 2 which shows similar strength and strain hardening rate when compared to wire drawn Fe-0.17 Ti material.
t u r e . S,4 A high p u r i t y i r o n s a m p l e g e t t e r e d with 0.17 wt pct Ti ( F e - 0 . 0 0 5 C - 0 . 1 7 T i - 0 . 0 0 5 A1-0.001 P-0.005 S-0.008 Si-0.0039 0 - 0 . 0 0 2 2 N) was d e f o r m e d in t o r s i o n at r o o m t e m p e r a t u r e at a c o n s t a n t s t r a i n r a t e (~) of 6 • 10-3/s to f r a c t u r e which o c c u r r e d at a t r u e s t r a i n (e) of 4.8. The a n n e a l e d m a t e r i a l had an e q u i a x e d g r a i n s i z e of 0.05 m m (ASTM 6). The s t r e s s - s t r a i n c u r v e , plotted a s e q u i v a l e n t t r u e s t r e s s - t r u e s t r a i n , 5 is shown in Fig. 1. The r e s u l t i n g c u r v e obtained by t o r s i o n a l def o r m a t i o n s u g g e s t s that at l a r g e s t r a i n s the flow s t r e s s b e c o m e s independent of s t r a i n . C o m p a r i n g our r e s u l t s with t h o s e of L a n g f o r d and Cohen one notes a fundam e n t a l d i f f e r e n c e b e t w e e n the r e s u l t s obtained in t o r sion and t h o s e obtained by w i r e d r a w i n g . A s t e a d y s t a t e flow s t r e s s was a c h i e v e d in t o r s i o n p r i o r to f r a c t u r e while in w i r e d r a w i n g t h e r e is no hint of a s t e a d y state flow s t r e s s . The p r e s e n c e of a s t e a d y s t a t e flow s t r e s s i m p l i e s that the s t r u c t u r e is no l o n g e r changing s i g n i f i c a n t l y with s t r a i n . This s u g g e s t s that the h a r d e n i n g r a t e f r o m d i s l o c a t i o n g e n e r a t i o n and i n t e r a c t i o n is j u s t b a l a n c e d by the softening r a t e due to d i s l o c a t i o n annihilation and r e a r r a n g e m e n t . E v i d e n c e of e x t e n s i v e r e c o v e r y o c c u r r i n g d u r i n g t o r s i o n a l d e f o r m a t i o n is s u p p o r t e d by the e l e c t r o n t r a n s m i s s i o n m i c r o g r a p h shown in Fig. 2. Well defined s u b g r a i n s a r e p r e s e n t which a r e s i m i l a r in a p p e a r a n c e to t h o s e f o r m e d at e l e v a t e d t e m p e r a t u r e d u r i n g s t e a d y s t a t e c r e e p . Subgrains f o r m e d in i r o n d u r i n g low t e m p e r a t u r e t o r s i o n a r e m o r e e q u i a x e d than O. D. SHERBY is Professor, Departme
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