Superplasticity in low-alloy steels
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where (r is the flow s t r e s s , K is a constant, and ~ is the s t r a i n r a t e . The s t r a i n r a t e s e n s i t i v i t y is a function of the s t r a i n r a t e , t e m p e r a t u r e , and grain s t r u c t u r e . F o r s u p e r p l a s t i c i t y (m > 0.3) the t e m p e r a t u r e is usually g r e a t e r than one half the absolute melting point, the s t r a i n r a t e is low (usually 10 -4 to 1 per rain), and the g r a i n s i z e is of the o r d e r of one micron. The flow s t r e s s is also v e r y low during s u p e r p l a s t i c flow. Because of the high s t r a i n r a t e s e n s i t i v i t y these m a t e r i a l s r e s i s t necking during t e n s i l e deformation, thus accounting for the v e r y l a r g e elongations that have been observed. The l a r g e elongations, coupled with the low flow s t r e s s e s , make these m a t e r i a l s d e s i r a b l e c o m m e r c i a l l y for f o r m i n g v e r y complex p a r t s with low applied f o r c e s . The use of s u p e r p l a s t i c i t y in n i c k e l - and i r o n - b a s e alloys gives the d e s i g n e r e x cellent f o r m a b i t i t y , coupled with a high r o o m t e m p e r a t u r e strength. The l a r g e elongations p o s s i b l e during s u p e r p l a s t i c flow o c c u r s only if the deformation m e c h a n i s m can allow this l a r g e s t r a i n . It is g e n e r a l l y accepted that a l a r g e p a r t of the flow is a c c o m m o d a t e d by grain boundary sliding, l~ F o r s u p e r p l a s t i c i t y to be u s e ful the grain s i z e must r e m a i n s t a b l e at the high f o r m ing t e m p e r a t u r e . This is usually a c c o m p l i s h e d by h a v ing a m i c r o d u p l e x s t r u c t u r e , whereby the two finely d i s p e r s e d phases tend to prevent grain growth. This technique is used in the Zn-A1, Pb-Sn and NS-Fe-Cr systems and also in carbon steels, by forming in the two-phase ferrite-austenite region.4 The fine-grained steel structures have been obtained in carbon and low-alloy steels by both thermal cycling3 and thermomechanical treatments.4 The thermal cycling process normally used is to cycle the steel MURRAY JOHN STEWARTis Head, Metal ProcessingSection, Physical MetallurgyResearch Laboratories,Canada Centre for Mineral and Energy Technology,Department of Energy, Mines and Resources, Ottawa, Canada. KI A OG 1 Manuscript submitted March 25, 1975. METALLURGICALTRANSACTIONSA
through the f e r r i t e ~ a u s t e n i t e t r a n s f o r m a t i o n a numbe of t i m e s . ~ However, if these m a t e r i a l s a r e to be c o m m e r c i a l l y viable, a continuous p r o c e s s i n g p r o c e dure (thermomechanical) is most d e s i r a b l e and, hence, was used in this study. The t h e r m o m e c h a n i c a l method can be adapted to conventional r o l l i n g m i l l equipment. In the p r e s e n t study, C-Mn s t e e l s w e r e used with a l loy additions of V, Nb and Ti to form fine p a r t i c l e s . In this way, it was hoped that g r a i n growth would be
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