The origin of the shear stress increment in aged ferritic Fe-Ni-Ti-Al alloys
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by G l e i t e r 9 would not have r e s u l t e d in the s a m e d e g r e e of c o r r e l a t i o n . I n v e s t i g a t i o n of the s h e a r s t r e s s i n c r e m e n t obtained in f e r r i t i c F e - S i - T i a l l o y s which p r e c i p i t a t e o r d e r e d Fe2SiTi, i n d i c a t e s that in the p r e p e a k and p e a k - a g e d conditions a n t i - p h a s e b o u n d a r y h a r d e n i n g is d o m i n a n t , while in the o v e r a g e d condition the m a t e r i a l d e f o r m s by the O r o w a n m e c h a n i s m . ~~ The c o n c l u s i o n s d r a w n f r o m q u a n t i t a t i v e u s e of the a p p r o p r i a t e s t r e n g t h e n i n g m o d e l s a r e f u r t h e r v a l i d a t e d by the o b s e r v a t i o n s of p a i r e d d i s l o c a t i o n s at e a r l y aging t i m e s , and s h e a r and p r i s m a t i c loops in the o v e r a g e d condition. In the p r e s e n t i n v e s t i g a t i o n a study h a s been m a d e of the o r i g i n of the s h e a r m o d u l u s i n c r e m e n t obtained on aging f e r r i t i c F e - N i - A 1 - T i a l l o y s which p r e c i p i t a t e the o r d e r e d b o d y - c e n t e r e d cubic i n t e r m e t a l l i c c o m pound, Ni(A1Ti)J 4 M A T E R I A L S AND METHODS T h e a l l o y s w e r e p r e p a r e d by v a c u u m induction m e l t ing and c a s t i n g into a f o r m s u i t a b l e f o r hot e x t r u s i o n to 15 m m d i a m e t e r rod. T h i s rod was r e d u c e d to 7 m m d i a m e t e r by cold s w a g i n g and t e n s i l e t e s t p i e c e s w e r e m a c h i n e d f r o m this stock. T h e t e n s i l e t e s t p i e c e s w e r e then s e a l e d in e v a c u a t e d s i l i c a c a p s u l e s f o r s o l u t i o n h e a t - t r e a t m e n t for 2 h at 1050~ followed by w a t e r quenching, b r e a k i n g the c a p s u l e s . Aging was c a r r i e d out under v a c u u m for v a r y i n g t i m e s at 620~ The c h e m i c a l c o m p o s i t i o n s of the two a l l o y s s t u d i e d a r e given in T a b l e I. T e n s i l e t e s t s w e r e conducted at 23~ using a h a r d b e a m t e n s i l e m a c h i n e at a n o m i n a l s t r a i n r a t e of 2 • 10-4 s -1. The load was r e c o r d e d f r o m a load c e l l and the s t r a i n o b t a i n e d f r o m the t i m e s c a l e and the m a c h i n e m o d u l u s . A 0.I p c t p r o o f s t r e s s was obtained for e a c h condition t o g e t h e r with w o r k - h a r d e n i n g r a t e s at 0.01, 0.1, 1, 3, and 4 p c t s t r a i n s . Thin foil e l e c t r o n m i c r o s c o p y e x a m i n a t i o n s w e r e m a d e to obtain the p a r t i c l e s i z e s and v o l u m e f r a c -
Table I. The Chemical Analysis of the Alloys (Wt Pct}
I. O. SMITH is Lecturer in Physical Metallurgy, University of Queensland, Brisbane, Queensland. M. G. WHITE, formerly Graduate Student at University of Queensland is now with the Australian Public Service Board, Canberra, Australia. Manuscript submitted April 14, 1975. METALLURGICAL TRANSACTIONS A
Alloy
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