Steady state creep in two Ni-Al alloys
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i. INTRODUCTION
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of the contemporary creep resistant alloys owe their creep strength to the presence of finely dispersed particles of the secondary phase. Thus the nickel-chromium base alloys are usually alloyed by relatively small amounts of aluminum and titaniumthe elements forming the stable Ni3(A1,Ti) phase coherent with the m a t r i x , if it p r e c i p i t a t e d u n d e r c e r t a i n a g i n g conditions. Though c o n s i d e r a b l e a t t e n t i o n has b e e n given to the c r e e p s t r e n g t h e n i n g due to Ni3(A1, Ti) p a r t i c l e s in c o m m e r c i a l n i c k e l - c h r o m i u m and n i c k e l - c h r o m i u m - c o b a l t b a s e c r e e p r e s i s t a n t a l l o y s , the c r e e p in one of the m o s t i m p o r t a n t subs y s t e m s of these alloys, n a m e l y in the b i n a r y Ni-A1 s y s t e m , has b e e n studied to a r e l a t i v e l y r e s t r i c t e d extent. 1-3 T h a t is why the p r e s e n t work was done. Its a i m is to obtain s o m e f u n d a m e n t a l c r e e p c h a r a c t e r i s t i c s in a) an alloy r e p r e s e n t i n g a solid s o l u t i o n of a l u m i n u m in n i c k e l in a t e m p e r a t u r e i n t e r v a l 873 to 1073 K, and b) a n alloy r e p r e s e n t i n g a two p h a s e s y s t e m in the t e m p e r a t u r e i n t e r v a l m e n t i o n e d . The c o r r e s p o n d i n g a l u m i n u m c o n c e n t r a t i o n s w e r e 4.8 and 7.0 wt pct (i.e. 9.85 and 14.07 at. pct), r e s p e c t i v e l y . F u r t h e r on the alloys a r e r e f e r r e d to as A5 and A7, where n u m b e r s indicate a p p r o x i m a t e a l u m i n u m c o n c e n t r a tions in wt pct.
2. E X P E R I M E N T A L PROCEDURES The a l l o y s i n v e s t i g a t e d , p r e p a r e d by v a c u u m i n d u c t i o n m e l t i n g f r o m component m e t a l s of p u r i t i e s not lower than 99.99 pct were r e c e i v e d in the f o r m of hot r o l l e d s t r i p s 3.2 m m in t h i c k n e s s . F r o m the s t r i p s s p e c i m e n s for c r e e p t e s t s w e r e m a c h i n e d in such a way that the t e n s i l e s t r e s s axis was p a r a l l e l to the r o l l i n g d i r e c t i o n . The s p e c i m e n s w e r e heat t r e a t e d as follows: the Alloy A5 was a n n e a l e d for 30 FERDINAND DOBES is Research Assistant, and JOSEF CADEK is Head, Department of High Temperature Processes, Institute of Physical Metallurgy,CzechoslovakAcademy of Sciences, 616 62 Brno, Czechoslovakia. Manuscript submitted October 27, 1976. METALLURGICALTRANSACTIONSA
m i n at 1273 K in v a c u u m and f u r n a c e cooled; the Alloy A7 was a n n e a l e d at the s a m e t e m p e r a t u r e in a i r and w a t e r quenched. A f t e r the q u e n c h i n g the l a t t e r alloy was aged for 24 h at 1123 K. T h i s t r e a t m e n t r e s u l t e d in u n i f o r m l y d i s t r i b u t e d p a r t i c l e s of Ni3A1 (~') phase; the d e n s i t y of p a r t i c l e s was 3 to 6 • 10 x9 m-3; the m e a n edge length of cube shaped p a r t i c l
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