Aging and brittleness in an Fe-Ni-Mn alloy
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A study of the aging behavior of an Fe-12 Ni-6 Mn alloy has been made. Rapid hardening was found to occur over a wide temperature range. Extraction of the overaged precipitate has been carried out and the precipitate identified as fct 0 NiMnby electron diffraction. Studies of the effect of aging on the toughness and tensile ductility have shown that severe embrittlement occurs over a wide temperature range. The relationship between the hardening and the embrittlement was studied by comparison of "activation energies" determined from Arrhenius plots. It is suggested that embrittlement in this alloy is the result of segregation of manganese to prior austenite grain boundaries resulting in intergranular failure. To avoid embrittlement, aging was performed at temperatures above the embrittlement range which resulted in good toughness with moderate hardness.
THE remarkable properties of the 18 pct Ni maraging steels may be largely attributed to two factors:i) The lath or massive martensite obtained under most cooling conditions.I This retains substitutional elements in solid solution and, on aging the high dislocation density of the martensite laths provides heterogeneous sites for precipitation, z This results in a fine d i s p e r s i o n of p r e c i p i t a t e . ii) T h e high t o u g h n e s s of the low c a r b o n 18 pct Ni b a s e . T h i s is the r e s u l t of the l o w e r i n g of the duct i l e / b r i t t l e t r a n s i t i o n of p u r e i r o n by the a d d i t i o n of n i c k e l . ~'4 A p a r t f r o m the g r a i n r e f i n i n g a c t i o n of n i c k e l , r e c e n t work S s u g g e s t s that the d u c t i l e / b r i t t l e t r a n s i t i o n is a l s o l o w e r e d b e c a u s e n i c k e l p e r m i t s c r o s s - s l i p to o c c u r at v e r y low t e m p e r a t u r e s . L a t h o r m a s s i v e rrrartensite a l s o f o r m s u n d e r m o s t cooling conditions in Fe-Mn alloys,6'7 and thus Fe-Mn alloys would appear to provide an attractive base for maraging. However, recent work8 suggests that such alloys are brittle and exhibit similarities to alloys s u f f e r i n g t e m p e r e m b r i t t l e m e n t . It m a y w e l l be t h e r e f o r e , that only a p a r t i a l r e p l a c e m e n t of m a n g a n e s e f o r n i c k e l m ay be p o s s i b l e . P a t t e r s o n et al.9 s t u d i e d m a r a g i n g s t e e l s with r e duced n i c k e l and containing m a n g a n e s e up to 3 pct. T h e y r e p o r t e d p r o p e r t i e s c o m p a r a b l e with 18 pet Ni maraging steels. O t h e r w o r k e r s ~~ have u s e d m a n g a n e s e a s an a d d i tion to m a r a g i n g s t e e l s but the m e c h a n i c a l p r o p e r t i e s of s u c h a l l o y s have not b e e n d e t e r m i n e d . R u s s i a n n and J a p a n e s e ~z w o r k e r s f i r s t r e p o r t e d aging in F e - N i Mn a l l o y s . I n t en s e a g i n g o c c u r r e d o v e r the t e m p e r a t u r e r a n g e 350 ~ to 550~ T h e m a x i m u m i n c r e a s e in h a r d n e s s was found b
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