- Email: [email protected]
Void growth and failure in tensile specimens under superimposed hydrostatic pressure
d e f o r m a t i o n of t h e Nb phase. The objective of t h e s t u d y w a s to improve fracture a n d fatigue properties.
D E F O R M A T I O N P R O P E R T I E S OF NICKEL ALUMININDE/ALUMINA COMPOSITES T.C. Chou, K.Forbes. ( S t a n l o r d University, Stanford, California, USA-) A description of t h e fabrication, by hot p r e s s i n g a n d r e a c t i v e s i n t e r i n g of Nb ahlminidc materials was given. The structures produce were a composite c o n t a i n i n g ductile Nb solid solution in a brittle Nb:~A1 m a t r i x . T h e r m o m e c h a n i c a l t r e a t m e n t p r o d u c e d a l a m e l l a r NbaA1/Nb s t r u c t u r e after aging. Electron microscopy of a g e d s p e c i m e n s a l s o s h o w e d t h e precipitation o f Nb by a m a s s i v e t r a n s f o r m a t i o n r e s u l t i n g in a u n i f o r m a n d fine d i s t r i b u t i o n of f i l a m e n t a r y Nb in t h e NbaAl matrix. It w a s s u g g e s t e d t h a t t h i s slructure can enhance crack growth resistance by c r a c k b r i d g i n g and deformation o f t h e Nb p h a s e . T h e objective of t h e s t u d y w a s to i m p r o v e fracture a n d fatigue properties.
D E F O R M A T I O N P R O P E R T I E S OF NICKEL ALUMININDE COMPOSITES T.C. Chou, K.Forbes. (Stanford University, Stanford, California, USA.) The use of PM t e c h n i q u e s to fabricate Ni:~A1/AI2Oa c o m p o s i t e s c o n t a i n i n g 50, 32, a n d 17wt% NiaAl was described. It w a s reported t h a t t h e a d d i t i o n of t h e NiaAI increased t h e room t e m p e r a t u r e t o u g h n e s s of t h e c o m p o s i t e while r e t a i n i n g t h e h i g h t e m p e r a t u r e s t r e n g t h a n d good o x i d a t i o n resistance. The d e f o r m a t i o n b e h a v i o u r at a m b i e n t a n d elevated t e m p e r a t u r e s w a s described and correlated with the microstructures. A model of the d e f o r m a t i o n c h a r a c t e r i s t i c s was presented.
F O R M A l ' I O N OF N I C K E L A L U M I N I D E I N T E R M E T A L L I C C O M P O S I T E S BY REACTIVE I N F I L T R A T I O N . tI.Chen el, al. ( Q u e e n ' s University, Kingston, Ontario, C a n a d a . ) A novel p r o c e s s for t h e p r o d u c t i o n of Ni aluminide matrix composites was d e s c r i b e d . Fully d e n s e p r o d u c t s w i t h aligned c o n t i n u o u s fibres were produced. The possibility of m a k i n g c o m p o n e n t s to n e a r n e t s h a p e w a s discussed.
DUCTILE INTERGRANULAR F R A C T U R E OF PM 3 0 4 STAINLESS STEEL E.Y. C h e n et al. ( N o r t h w e s t e r n University, Evanston, Illinois, USA-) A s t u d y of t h e i n t e r g r a n u l a r fracture of PM 304 s t a i n l e s s steel w a s d e s c r i b e d .
Localized plastic instability at grain b o u n d a r i e s a n d m i c r o s t r u c t u r a l features, 400 n m in size, were observed over 75% of t h e fracture surface. This p a t t e r n of fracture was e x a m i n e d from t h e p o i n t of view of a ductile f r a c t u r e m e c h a n i s m . The i n t e r a c t i o n between t h e crack tip s t r e s s field a n d grain 'corner' s i n t e r i n g voids w a s d i s c u s s e d with possible void n u c l e a t i o n m e c h a n i s m s . The r e d u c t i o n in f r a c t u r e energy a s s o c i a t e d w i t h c o n f i n e m e n t of plasticity w a s c o m p a r e d with t h e availability of n u c l e a t i o n sties.
VOID GROWTH A N D F A I L U R E I N TENSILE SPECIMENS UNDER S U P E R I M P O S E D HYDROSTATIC PRESSURE S.B. Biner, W.A Spitzig. (Iowa State University, Ames, Indiana, USA.) An experimental and numerical investigation of void growth a n d ductile f a i l u r e in p a r t i a l l y c o n s o l i d a t e d a n d s i n t e r e d Fe p o w d e r w i t h 1.5 a n d 3.7% porosities w a s described. The s p e c i m e n s w e r e t e s t e d in u n i a x i a l t e n s i o n w i t h s u p e r i m p o s e d h y d r o s t a t i c s t r e s s e s of 0.1, 138 or 276 MPa a n d void growth kinetics were d e t e r m i n e d . The m o d e l l i n g u s e d a n elastic/plastic constitutive relationship t h a t a c c o u n t s for t h e d e g r a d a t i o n o f s t r e n g t h w h i c h r e s u l t s from void growth. The lack of a correlation between experimental and numerical data was a t t r i b u t e d to t h e o m i s s i o n of a t e r m for the p r e s s u r e build u p in t h e pores d u r i n g initial c o n s o l i d a t i o n a n d densification.
E N V I R O N M E N T A L P R O T E C T I O N OF REFRACTORY METALS R.A. Perkins, (Lockheed Missiles a n d Space Co., Palo Alta, California, USA.) A t t e n t i o n w a s d r a w n to t h e fact t h a t whilst refractory m e t a l s h a v e h i g h s t r e n g t h to weight ratios at high t e m p e r a t u r e s t h e o x i d a t i o n r e s i s t a n c e is i n a d e q u a t e for a e r o s p a c e applications. It w a s c o n s i d e r e d unlikely t h a t t h e long t e r m b u l k o x i d a t i o n p r o p e r t i e s c o u l d be i m p r o v e d or t h a t c o a t i n g c o u l d be u s e d to give h i g h r e l i a b i l i t y f a i l - s a f e p r o t e c t i o n . It w a s s u g g e s t e d t h a t M o - W a l l o y s c a n be d e s i g n e d to p o s s e s s s h o r t t e r m o x i d a t i o n r e s i s t a n c e w i t h c o a t i n g s to e x t e n d service life w i t h fail safe p r o t e c t i o n . T h i s w a s d i s c u s s e d in t h e light of r e c e n t research.
P R O T E C T I O N OF R E F R A C T O R Y M E T A L AGAINST OXIDATION AT HIGH TEMPERATURES R.H.Tuffias et al. ( U l t r a m e t , P a c o i m a , California, USA.) Three m e t h o d s of p r o t e c t i n g refractory m e t a l s a g a i n s t oxidation, at 2225°C a n d above, sacrificial barriers, t h e r m a l barriers and diffusion barriers were discussed.
E x a m p l e s were given. Pt, Ir a n d Rh were said to be effective as sacrificial barriers a n d refractory oxide coating as t h e r m a l a n d diffusion barriers. It w a s r e p o r t e d t h a t thrust rocket chambers have been protected and have operated e x p e r i m e n t a l l y for over 15 h o u r s a t a mean temperature above 2275°C. C o m b i n a t i o n of t h e t h r e e types of barrier a n d c u r r e n t r e s e a r c h were discussed.
INTERFACE STRENGTHENED REACTIVELY S I N T E R E D T I T A N I U M A L U M I N I D E MATRIX C O M P O S I T E S J.E. Talia et al. (Rice University, Houston, Texas, USA.) The fabrication of Ti a l u m i n i d e s from elemental powders was described. The p o w d e r s were cold isostatically p r e s s e d at 267 MPa a n d reactively s i n t e r e d at 700°C in air, inert gas or v a c u u m . It w a s e s t a b l i s h e d t h a t A13Ti w a s formed. An intermetallic m a t r i x c o m p o s i t e w a s f o r m e d w h e n Al wires were incorporated. It w a s r e p o r t e d t h a t t h e control of p r o c e s s p a r a m e t e r s m a d e it possible to tailor t h e interface and hence control the mechanical p r o p e r t i e s of t h e c o m p o s i t e
O X I D A T I O N O F T U N G S T E N ALLOYS D.B. Lee et al, (S.K. Kwan University, Scewon, S o u t h Korea.) A c o m p a r i s o n of t h e o x i d a t i o n s w a s reported. Static t e s t s in O a n d cyclic t e s t s in air were described. It w a s s h o w n t h a t t h e scale c o n t a i n e d Cr oxide. S u g g e s t e d m e c h a n i s m s for s c a l e f o r m a t i o n a n d p r o t e c t i o n of t h e alloys were discussed.
A P P L I C A T I O N S OF R E F R A C T O R Y METALS IN FUSION REACTORS R.F Mattas, ( A r g o n n e National Laboratory, Argonne, Illinois, USA.) The use of refractory m e t a l s in fusion reactors w a s d i s c u s s e d with r e s p e c t to t h e p r o p e r t i e s required to w i t h s t a n d t h e service conditions. Nb, Mo a n d W alloys are being c o n s i d e r e d for s t r u c t u r a l c o m p o n e n t s w h i c h will be e x p o s e d to h i g h h e a t loads d u e to t h e fusion p l a s m a a n d m u s t resist erosion by p l a s m a particles. V-Cr-ti alloys were c o n s i d e r e d for u s e in s t r u c t u r a l p a r t s in t h e fuel b l a n k e t a n d were r e p o r t e d to have high strength at elevated temperatures.
P R O C E S S I N G OF N A N O P H A S E M A T E R I A L S BY H I G H E N E R G Y BALL M I L L I N G C.C. Koch, (North Carolina State University, Raleigh, North Carolina, USA.) T h e e v o l u t i o n of m i c r o s t r u c t u r e s in m e c h a n i c a l l y alloyed (MA) m a t e r i a l s w a s
MPR June 1992 55
D E F O R M A T I O N P R O P E R T I E S OF NICKEL ALUMININDE/ALUMINA COMPOSITES T.C. Chou, K.Forbes. ( S t a n l o r d University, Stanford, California, USA-) A description of t h e fabrication, by hot p r e s s i n g a n d r e a c t i v e s i n t e r i n g of Nb ahlminidc materials was given. The structures produce were a composite c o n t a i n i n g ductile Nb solid solution in a brittle Nb:~A1 m a t r i x . T h e r m o m e c h a n i c a l t r e a t m e n t p r o d u c e d a l a m e l l a r NbaA1/Nb s t r u c t u r e after aging. Electron microscopy of a g e d s p e c i m e n s a l s o s h o w e d t h e precipitation o f Nb by a m a s s i v e t r a n s f o r m a t i o n r e s u l t i n g in a u n i f o r m a n d fine d i s t r i b u t i o n of f i l a m e n t a r y Nb in t h e NbaAl matrix. It w a s s u g g e s t e d t h a t t h i s slructure can enhance crack growth resistance by c r a c k b r i d g i n g and deformation o f t h e Nb p h a s e . T h e objective of t h e s t u d y w a s to i m p r o v e fracture a n d fatigue properties.
D E F O R M A T I O N P R O P E R T I E S OF NICKEL ALUMININDE COMPOSITES T.C. Chou, K.Forbes. (Stanford University, Stanford, California, USA.) The use of PM t e c h n i q u e s to fabricate Ni:~A1/AI2Oa c o m p o s i t e s c o n t a i n i n g 50, 32, a n d 17wt% NiaAl was described. It w a s reported t h a t t h e a d d i t i o n of t h e NiaAI increased t h e room t e m p e r a t u r e t o u g h n e s s of t h e c o m p o s i t e while r e t a i n i n g t h e h i g h t e m p e r a t u r e s t r e n g t h a n d good o x i d a t i o n resistance. The d e f o r m a t i o n b e h a v i o u r at a m b i e n t a n d elevated t e m p e r a t u r e s w a s described and correlated with the microstructures. A model of the d e f o r m a t i o n c h a r a c t e r i s t i c s was presented.
F O R M A l ' I O N OF N I C K E L A L U M I N I D E I N T E R M E T A L L I C C O M P O S I T E S BY REACTIVE I N F I L T R A T I O N . tI.Chen el, al. ( Q u e e n ' s University, Kingston, Ontario, C a n a d a . ) A novel p r o c e s s for t h e p r o d u c t i o n of Ni aluminide matrix composites was d e s c r i b e d . Fully d e n s e p r o d u c t s w i t h aligned c o n t i n u o u s fibres were produced. The possibility of m a k i n g c o m p o n e n t s to n e a r n e t s h a p e w a s discussed.
DUCTILE INTERGRANULAR F R A C T U R E OF PM 3 0 4 STAINLESS STEEL E.Y. C h e n et al. ( N o r t h w e s t e r n University, Evanston, Illinois, USA-) A s t u d y of t h e i n t e r g r a n u l a r fracture of PM 304 s t a i n l e s s steel w a s d e s c r i b e d .
Localized plastic instability at grain b o u n d a r i e s a n d m i c r o s t r u c t u r a l features, 400 n m in size, were observed over 75% of t h e fracture surface. This p a t t e r n of fracture was e x a m i n e d from t h e p o i n t of view of a ductile f r a c t u r e m e c h a n i s m . The i n t e r a c t i o n between t h e crack tip s t r e s s field a n d grain 'corner' s i n t e r i n g voids w a s d i s c u s s e d with possible void n u c l e a t i o n m e c h a n i s m s . The r e d u c t i o n in f r a c t u r e energy a s s o c i a t e d w i t h c o n f i n e m e n t of plasticity w a s c o m p a r e d with t h e availability of n u c l e a t i o n sties.
VOID GROWTH A N D F A I L U R E I N TENSILE SPECIMENS UNDER S U P E R I M P O S E D HYDROSTATIC PRESSURE S.B. Biner, W.A Spitzig. (Iowa State University, Ames, Indiana, USA.) An experimental and numerical investigation of void growth a n d ductile f a i l u r e in p a r t i a l l y c o n s o l i d a t e d a n d s i n t e r e d Fe p o w d e r w i t h 1.5 a n d 3.7% porosities w a s described. The s p e c i m e n s w e r e t e s t e d in u n i a x i a l t e n s i o n w i t h s u p e r i m p o s e d h y d r o s t a t i c s t r e s s e s of 0.1, 138 or 276 MPa a n d void growth kinetics were d e t e r m i n e d . The m o d e l l i n g u s e d a n elastic/plastic constitutive relationship t h a t a c c o u n t s for t h e d e g r a d a t i o n o f s t r e n g t h w h i c h r e s u l t s from void growth. The lack of a correlation between experimental and numerical data was a t t r i b u t e d to t h e o m i s s i o n of a t e r m for the p r e s s u r e build u p in t h e pores d u r i n g initial c o n s o l i d a t i o n a n d densification.
E N V I R O N M E N T A L P R O T E C T I O N OF REFRACTORY METALS R.A. Perkins, (Lockheed Missiles a n d Space Co., Palo Alta, California, USA.) A t t e n t i o n w a s d r a w n to t h e fact t h a t whilst refractory m e t a l s h a v e h i g h s t r e n g t h to weight ratios at high t e m p e r a t u r e s t h e o x i d a t i o n r e s i s t a n c e is i n a d e q u a t e for a e r o s p a c e applications. It w a s c o n s i d e r e d unlikely t h a t t h e long t e r m b u l k o x i d a t i o n p r o p e r t i e s c o u l d be i m p r o v e d or t h a t c o a t i n g c o u l d be u s e d to give h i g h r e l i a b i l i t y f a i l - s a f e p r o t e c t i o n . It w a s s u g g e s t e d t h a t M o - W a l l o y s c a n be d e s i g n e d to p o s s e s s s h o r t t e r m o x i d a t i o n r e s i s t a n c e w i t h c o a t i n g s to e x t e n d service life w i t h fail safe p r o t e c t i o n . T h i s w a s d i s c u s s e d in t h e light of r e c e n t research.
P R O T E C T I O N OF R E F R A C T O R Y M E T A L AGAINST OXIDATION AT HIGH TEMPERATURES R.H.Tuffias et al. ( U l t r a m e t , P a c o i m a , California, USA.) Three m e t h o d s of p r o t e c t i n g refractory m e t a l s a g a i n s t oxidation, at 2225°C a n d above, sacrificial barriers, t h e r m a l barriers and diffusion barriers were discussed.
E x a m p l e s were given. Pt, Ir a n d Rh were said to be effective as sacrificial barriers a n d refractory oxide coating as t h e r m a l a n d diffusion barriers. It w a s r e p o r t e d t h a t thrust rocket chambers have been protected and have operated e x p e r i m e n t a l l y for over 15 h o u r s a t a mean temperature above 2275°C. C o m b i n a t i o n of t h e t h r e e types of barrier a n d c u r r e n t r e s e a r c h were discussed.
INTERFACE STRENGTHENED REACTIVELY S I N T E R E D T I T A N I U M A L U M I N I D E MATRIX C O M P O S I T E S J.E. Talia et al. (Rice University, Houston, Texas, USA.) The fabrication of Ti a l u m i n i d e s from elemental powders was described. The p o w d e r s were cold isostatically p r e s s e d at 267 MPa a n d reactively s i n t e r e d at 700°C in air, inert gas or v a c u u m . It w a s e s t a b l i s h e d t h a t A13Ti w a s formed. An intermetallic m a t r i x c o m p o s i t e w a s f o r m e d w h e n Al wires were incorporated. It w a s r e p o r t e d t h a t t h e control of p r o c e s s p a r a m e t e r s m a d e it possible to tailor t h e interface and hence control the mechanical p r o p e r t i e s of t h e c o m p o s i t e
O X I D A T I O N O F T U N G S T E N ALLOYS D.B. Lee et al, (S.K. Kwan University, Scewon, S o u t h Korea.) A c o m p a r i s o n of t h e o x i d a t i o n s w a s reported. Static t e s t s in O a n d cyclic t e s t s in air were described. It w a s s h o w n t h a t t h e scale c o n t a i n e d Cr oxide. S u g g e s t e d m e c h a n i s m s for s c a l e f o r m a t i o n a n d p r o t e c t i o n of t h e alloys were discussed.
A P P L I C A T I O N S OF R E F R A C T O R Y METALS IN FUSION REACTORS R.F Mattas, ( A r g o n n e National Laboratory, Argonne, Illinois, USA.) The use of refractory m e t a l s in fusion reactors w a s d i s c u s s e d with r e s p e c t to t h e p r o p e r t i e s required to w i t h s t a n d t h e service conditions. Nb, Mo a n d W alloys are being c o n s i d e r e d for s t r u c t u r a l c o m p o n e n t s w h i c h will be e x p o s e d to h i g h h e a t loads d u e to t h e fusion p l a s m a a n d m u s t resist erosion by p l a s m a particles. V-Cr-ti alloys were c o n s i d e r e d for u s e in s t r u c t u r a l p a r t s in t h e fuel b l a n k e t a n d were r e p o r t e d to have high strength at elevated temperatures.
P R O C E S S I N G OF N A N O P H A S E M A T E R I A L S BY H I G H E N E R G Y BALL M I L L I N G C.C. Koch, (North Carolina State University, Raleigh, North Carolina, USA.) T h e e v o l u t i o n of m i c r o s t r u c t u r e s in m e c h a n i c a l l y alloyed (MA) m a t e r i a l s w a s
MPR June 1992 55