The ductility response of three high temperature materials tested under hydrostatic pressure
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Communications The Ductility Response of Three High Temperature Materials Tested Under Hydrostatic Pressure
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P E T E R V. DEMBOWSKI A N D J O S E P H P E P E
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When c o n s i d e r i n g alloys for applications which involve short t e r m dynamic mechanical p r o p e r t i e s at high t e m p e r a t u r e s , some thought must also be given to other f a c t o r s such as fatigue, r e s i s t a n c e to low and high t e m p e r a t u r e f r a c t u r e , and f a b r i c a b i l i t y . P r e s ently, m a t e r i a l s that look most p r o m i s i n g for these applications a r e high t e m p e r a t u r e alloys such as the nickel, cobalt, and c h r o m i u m b a s e alloys, and some of the r e f r a c t o r y m e t a l s such as columbium, tantalum, molybdenum, and tungsten. However, two of the b a r r i e r s limiting the use of these m a t e r i a l s a r e the high initial cost of these alloys and their l i m i t e d f a b r i c a b i l i t y . . , a factor which affects not only cost but also d e g r a d e s mechanical p r o p e r t i e s . The b e s t r u p t u r e strength at elevated t e m p e r a t u r e s is achieved in cast m a t e r i a l s with a c o a r s e grain s i z e and a s t a b i l i z e d c a r b i d e network at the grain boundar i e s . In c o n t r a s t , the b e s t combination of strength and toughness at ambient t e m p e r a t u r e s is achieved in a fine grained, homogeneous wrought m a t e r i a l with m e t a l l u r g i c a l l y clean g r a i n b o u n d a r i e s . As m a t e r i a l s used for s h o r t t e r m applications should provide good strength with adequate toughness for short exposure t i m e s at e l e vated t e m p e r a t u r e s , one would expect that a t r a d e - o f f between p r o p e r t i e s and s t r u c t u r e be c o n s i d e r e d . A review ~ of the p r o p e r t i e s of s e l e c t e d s u p e r a l l o y s concluded that cold forming r e s u l t s in an i n c r e a s e in the s h o r t - t i m e elevated t e m p e r a t u r e yield and t e n s i l e s t r e n g t h s . Unfortunately, the m a j o r i t y of high t e m p e r ature alloys that show p r o m i s e for these s h o r t - t i m e applications have poor f a b r i c a b i l i t y , p a r t i c u l a r l y at low or ambient t e m p e r a t u r e s . One possible solution to o v e r coming this drawback is the use of metalworking t e c h niques which involve the use of a high s u p e r i m p o s e d p r e s s u r e . The b a s i s of these techniques is the enhancement of ductility by means of a s u p e r i m p o s e d p r e s s u r e , and a c o m p r e h e n s i v e r e v i e w edited by laugh 2 yields theor e t i c a l , e x p e r i m e n t a l , and technological information on the effects of high p r e s s u r e on the deformation of m a terials. Of the v a r i o u s high p r e s s u r e metal forming t e c h niques, hydrostatic e x t r u s i o n has r e c e i v e d the most attention. In this technique, the uniaxial r a m force of the conventional extrusion techniqu
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