Jerky flow in Co-Ni-Cr-C FCC alloys
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Jerky Flow in Co-Ni-Cr-C FCC Alloys M. C H A T U R V E D I
JERKY flow,also known
as the Portevin-Le Chatelier effect or "serrated yielding" has been observed in m a n y alloy systems x and has been attributed to the locking of dislocations by either substitutional or interstitial solute atoms resulting in dynamic strain aging. The effect due to the interstitial solute atoms in bcc structures has been the subject of extensive investigation as compared to fcc systems where this phenomenon has been reported only in hydrogencharged nickel 2'3 and Ni-Cu alloys,4 Ni-C,S'6 and nickel austenitic steels containing carbon. 7-9 In this communication the occurrence of serrations during tensile testing of Co-Ni-Cr-C fcc alloys is reported. The following two a l l o y s were p r e p a r e d in a n a r g o n a r c f u r n a c e u s i n g 99.999 pct Ni, 99.999 pct Co, 99.99 pct C r and s p e c t r o g r a p h i c g r a p h i t e ,
~oN-
Fig. 1--Load-elongation curves of Alloy A.
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EI_ONGATION ~
METALLURGICAL
TRANSACTIONS
600 *C
~O05cm--~
Fig. 2--Load-elongation curves of Alloy B.
Alloy B: 40 wt pct C o - 2 0 wt pct Ni-20 wt pct C r 0.01 to 0.005 wt pct C.
M. CHATURVEDIis Associate Professor of Metallurgy, Metallurgical Sciences Laboratory, Faculty of Engineering,University of Manitoba, Winnipeg,Manitoba, Canada. Manuscript submitted May 10, 1971.
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7OOoC
Alloy A: 40 wt pct Co-40 wt pct Ni-19.8 wt pct C r 0.19 wt pct C ,
T h e s e a l l o y s were cold swaged, with an i n t e r m e d i a t e a n n e a l at 1200~ to 0.2 in. diam r o d s . A l l the h e a t t r e a t m e n t s were c a r r i e d out in an a r g o n a t m o s p h e r e in s e a l e d vycor t u b e s . The t e n s i l e s p e c i m e n s , 0.458 cm in d i a m e t e r , and 1.60 cm gage length, w e r e s o l u tion t r e a t e d at 1200~ and w a t e r - q u e n c h e d . The a v e r age g r a i n size of the s p e c i m e n s of alloy A was 105/~ and those of alloy B was 150p. The t e n s i l e s p e c i m e n s w e r e pulled in an I n s t r o n t e s t i n g m a c h i n e at a c r o s s head speed of 0.05 cm p e r rain. F i g s . 1 and 2 show the l o a d - e l o n g a t i o n d i a g r a m s of alloy A and B, r e s p e c t i v e l y , tested between r o o m t e m p e r a t u r e and 700~ r a n g e . It is seen that at r o o m t e m p e r a t u r e n e i t h e r of the two a l l o y s show a n y s e r r a tion and a s the t e s t i n g t e m p e r a t u r e is r a i s e d both a l l o y s develop s e r r a t i o n s . At i n t e r m e d i a t e t e m p e r a t u r e s (viz. 400 ~ and 500~ in alloy B and 300~ in alloy A) only " p e r i o d i c " s e r r a t i o n s a r e o b s e r v e d ; a s the t e m p e r a t u r e of t e s t i n g is r a i s e d f u r t h e r both a l l o y s develop " f i n e s e r r a t i o n s " . W h e r e a s t h e s e " f i n e " s e r r a t i o n s develop in alloy A at 400~ in alloy B they a r e o b s e r v e d only at 600~ A l s o , the m a g n i t u d e of the s t r e s s drop in alloy A is l a r g e r than tha
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