Increasing the fracture toughness of a maraging steel type alloy
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TR-73-83, 1973. 4. M. V. Hyatt:Program to Improve the Fracture Toughness and Fatigue Resistance of Aluminum Sheet and Plate for Airframe Applications, 198 pp., Air ForceMaterialsLab. Tech.ReportAFML-TR-73-224,I973. 5. P. J. Blau: Effect o f lron and Silicon Content on Stress Corrosion Cracking in a Thermomechanieally Processed Aluminum Alloy, 34 pp., Air Force Materials Lab. Tech. Report AFML-TR-75-215,1975. 6. P. J. Blau:Influence of Iron and Silicon Content on the Tensile Properties of 7 X 75 and Zr-Modified 7 • 75 Aluminum Plate, 35 pp, Air Force Materials Lab. Tech. Report AFML-TR-75-140,1975.
increasing the Fracture Toughness of a Maraging Steel Type Alloy
o b s e r v e d that the a u s t e n i t e f o r m s i n i t i a l l y at the p r i o r a u s t e n i t e and m a r t e n s i t e tath b o u n d a r i e s , I-3 a s it ha s s i m i l a r l y been o b s e r v e d to do in m o r e c o n v e n t i o n a l maraging steels, s T h e p r e c i p i t a t e in this alloy, h e x a g o n a l NisTi (DO~4), d o e s not a p p e a r to p r e c i p i t a t e in a u s t e n i t e , F o r e x a m p l e , in s p e c i m e n s aged at 650~ (a t e m p e r a t u r e at which a c o n s i d e r a b l e a m o u n t of a u s t e n i t e is f o r m e d v e r y r a p i d l y ) no p r e c i p i t a t i o n is o b s e r v e d in the a u s tenite; p r e c i p i t a t i o n o c c u r s onty in the m a r t e n s i t i c |00
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Bo FRANCIS R e c e n t unpublished s t u d i e s ~ h a v e shown that the f r a c t u r e t o u g h n es s of a F e - 1 6 N i - I . 5 T i m a r a g i n g s t e e l type alloy m a y be g r e a t l y i m p r o v e d by age h a r d e n i n g at h i g h e r t e m p e r a t u r e s (550~ to 600~ than t h o se c o n v e n t i o n a l l y used (~450~ for maraging steels. The p u r p o s e of this note is to d e s c r i b e the m e c h a n i s m s b e l i e v e d to be r e s p o n s i b l e f o r the i m p r o v e d f r a c t u r e t o u g h n es s in s p e c i m e n s aged at the h i g h e r temperature. In this alloy, a u s t e n i t e can f o r m at a l l the aging t e m p e r a t u r e s i n v e s t i g a t e d (450~ to 600~ whether or not it d o e s f o r m is s t r o n g l y dependent on the t i m e at t e m p e r a t u r e , as shown in F i g . 1, which shows the v o l u m e p e r c e n t ol a u s t e n i t e f o r m e d a s a function of t i m e of aging at 450, 500, 550 and 600~ T h e d a t a in F i g . 1 is for the i s o t h e r m a l r a t e of f o r m a t i o n of a u s t e n i t e at the aging t e m p e r a t u r e . T h i s i s not n e c e s s a r i l y the s a m e as the r a t e of f o r m a t i o n of r e t a i n e d a u s t e n i t e m e a s u r e d at r o o m t e m p e r a t u r e s u b s e q u e n t to aging. Thus t h e r e is n o m e a s u r a b l e retained a u s t e n i t e in the F e - 1 6 N i - l . 5 T i a l lo y f o r any of the aging t e m p e r a t u r e s and t i m e s r e
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