Fracture and fractography of metastable austenites
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the V last d e c a d eE , it would aR p p e a r that a plateau O has been r e a c h e d with r e s p e c t to the s t r e n g t h - t o u g h n e s s c o m b i n a t i o n s in u l t r a h i g h - s t r e n g t h s t e e l s . F o r e x a m p l e , 18 N i - m a r a g i n g s t e e l s a r e l i m i t e d to plane s t r a i n f r a c t u r e toughness l e v e l s n e a r 120 ksi-in.1/2. ~'2 Although s o m e h i g h - s t r e n g t h , 12 N i - m a r a g i n g s t e e l s and 9 N i - 4 C o - 0 . 2 5 C s t e e l s have e x c e e d e d this v a l u e , t h e s e have g e n e r a l l y had yield s t r e n g t h s somewhat b e low 200,000 p s i . ~-s S i m i l a r l y , f o r thin s h e e t , it has b e e n difficult to obtain p la n e s t r e s s f r a c t u r e to u g h n ess l e v e l s o v e r 300,000 p s i - i n . I / 2 f o r m a t e r i a l s with y i e l d s t r e n g t h s g r e a t e r than 200,000 p s i . 3 One p o s s i b i l i t y of m a r k e d l y e x c e e d i n g t h e s e p l a t e a u s is uti l i zat i o n of the s t r a i n - i n d u c e d p h a s e t r a n s f o r m a t i o n as an e n e r g y d i s s i p a t i o n m e c h a n i s m at the tip of a c r a c k . 4,s S p e c i f i c a l l y , in m e t a s t a b l e a u s t e n i t e s that had p r e v i o u s l y been w a r m - w o r k e d above M D to p r o d u c e yie ld s t r e n g t h s in e x c e s s of 200,000 p s i , the a u s t e n i t e to m a r t e n s i t e p h as e t r a n s f o r m a t i o n p r o v i d e s an a r r a y of i n t e r e s t i n g m e c h a n i c a l p r o p e r t i e s , s-" P r e l i m i n a r y r e suit s s'" had i n d i cat ed that r e l a t i v e l y high toughness could be a c h i e v e d in both thin s h e e t and thick plate. The p r e s e n t study is an e x t e n s i o n of the e a r l i e r in v e s t i g a t i o n s and has the following o b j e c t i v e s : 1) A q u a n t i t a t i v e study of an u n u s u a l f r a c t u r e t r a n s i t i o n p h e n o m enon which was s t r a i n - r a t e dependent. 2) A m o r e d e t a i l e d e x p l o r a t i o n of the d e p e n d e n c e of the f r a c t u r e m o r p h o l o g y on c o m p o s i t i o n and t e s t i n g t e m p e r a t u r e . 3) F u r t h e r v e r i f i c a t i o n of a p r o p o s e d t h e o r e t i c a l f r a c t u r e m o d e l by v a r i a t i o n of a u s t e n i t e s ta b i li ty through W. W. GERBERICH, formerly with the Lawrence Radiation Laboratory, Berkeley, is now Associate Professor, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minn. P. L. HEMMINGS is Metallurgist, Kaiser Aluminum Research Center, Pleasanton, Calif. V. F. ZACKAY is Professor, Department of Materials Science and Engineering, University of California, Berkeley, Calif. Manuscript submitted November 24, 1970. METALLURGICALTRANSACTIONS
c o m p o s i t i o n v a r i a t i o n . By such m e a n s the amount of s t r a i n - i n d u c e d phase a p p e a r i n g at the c r a c k tip can be systematically varied. M A T E R I
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