The effects of prior deformation and transformations on the microstructure of an iron-nickel alloy
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has been e s t a b l i s h e d that the m a r t e n s i t e ( a ' ) to austentte (V) t r a n s f o r m a t i o n in i r o n - n i c k e l a l l o y s can occur (under p r o p e r conditions) by a r e v e r s e s h e a r m e c h a n i s m and that the r e v e r t e d austenite is significantly s t r o n g e r than austenite f o r m e d from a nucleation and growth p r o c e s s . 1-s Koppenaal and Gold 6 r e c e n t l y d e t e r m i n e d the effects of p r i o r d e f o r m a t i o n of m a r t e n s i t e on the strength of r e v e r t e d austenite in an i r o n 31 pct nickel alloy. Their r e s u l t s indicated that r e v e r t e d austenite can be strengthened by ~30 pct with the use of 80 pct p r i o r deformation of the m a r t e n s t t e . In the p r e s e n t study, the m i c r o s t r u c t u r e s of the s a m e alloy following v a r i o u s t h e r m o m e c h a n i c a l t r e a t m e n t s have been examined to elucidate the o r i g i n s of the mechanical b e h a v i o r of this alloy.
quenching. A bath t e m p e r a t u r e of 538~ (1000~ was used for complete a ' - - * V t r a n s f o r m a t i o n and lower t e m p e r a t u r e s for p a r t i a l t r a n s f o r m a t i o n . Thin foil s p e c i m e n s for 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 were p r e p a r e d from disk s a m p l e s 3 mm (0.12 in.) diam. The thickness of the s t a r t i n g sheet stock was reduced to 0.15 mm (0.006 in.) by chemical polishing. In some c a s e s , disks were m e c h a n i c a l l y ground using a jig. These disks were e l e c t r o p o l i s h e d by a jet dimpling method using 25 pct HC1 at 50V and finally thinned to produce a p e r f o r a t e d a r e a by c h e m i cal polishing in HzO2 containing 2 pct HF at r o o m t e m p e r a t u r e . The thin foils were examined in a Jeolco E l e c t r o n M i c r o s c o p e (JEM-120) o p e r a t i n g at 120 KV. RESULTS AND DISCUSSIONS
EXPERIMENTAL PROCEDURE The m a t e r i a l used in this work was the s a m e v a c u u m - m e l t e d Fe-31.4 pct Ni-0.03 pct C alloy used in the previous investigationfl All the s p e c i m e n s were a u s t e n i t i z e d in an i n e r t a t m o s p h e r e at 982~ (1800~ and water quenched. These were t r a n s f o r m e d to m a r t e n s i t e (approximately 88 pct a ' ) by i m m e r s i o n in liquid nitrogen and holding for two minutes after bubbling subsided. Some of the m a r t e n s i t i c s p e c i m e n s w e r e cold r o l l e d to 20, 50 and 80 pct reduction in t h i c k n e s s at r o o m t e m p e r a t u r e (the final thickness was held constant at 0.5 mm). M a r t e n s i t e s p e c i m e n s with or without cold work were r e v e r t e d by up-quenching in a lead bath, holding for 1 minute, and water KANJI ONO is AssociateProfessor, MaterialsDepartment, University of California,Los Angeles,California90024. T. J. KOPPENAALis Supervisor,PhysicalMetallurgy,AdvancedDevelopment Operation, Aeronutronic Division,Philco-FordCorporation, Newport Beach, California 92663, Manuscript submit
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