DOElNASAll6310-6 NASA TM-I 00848

Characterization of Precipitates in a Niobium-Zirconium-Carbon Alloy

INASA-Tn- 100848)

tHA3ACTEBIZATIOB OF 888-22981 PRECIPITATES IN A N I O E I U f f - 2 IBCONIUR-CARBON ALLOY Final Report (NASA) 18 p

C S C L 11F Unclas

G3/26 0142685

T.L. Grobstein and R.H. Titran National Aeronautics and Space Administration Lewis Research Center

Work per.formed for U.S. DEPARTMENT OF ENERGY Nuclear Energy Reactor Systems Development and Technology

Prepared for The Metallurgical Society Fall Meeting sponsored by the American Institute of Mechanical Engineers Orlando, Florida, October 5-9, 1986

https://ntrs.nasa.gov/search.jsp?R=19880013597 2018-07-01T03:48:02+00:00Z

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Characterization of Precipitates in a Niobium-Zirconium-Carbon Alloy

T.L. Grobstein and R.H. Titran National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 441 35

Work performed for U.S. DEPARTMENT OF ENERGY Nuclear Energy Reactor Systems Development and Technology Washington, D.C. 20545 Under Interagency Agreement DE-A103-86SF16310

DOE/NASA/I 631 0-6 NASA TM-100848

Prepared for The Metallurgical Society Fall Meeting sponsored by the American Institute of Mechanical Engineers Orlando, Florida, October 5-9, 1986

CHARACTERIZATION OF PRECIPATES I N A Nb-Zr-C ALLOY

T.L. G r o b s t e i n and R.H . T i t r a n N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n

Lewis Research Center C leve land , Oh io 44135

SUMMARY

A n i o b i u m a l l o y w i t h 1 p e r c e n t z i r c o n i u m and 0.063 p e r c e n t carbon by w e i g h t was i n v e s t i g a t e d i n t h e a s - r o l l e d and annealed c o n d i t i o n s , and a f t e r h igh - tempera tu re (1350 and 1400 K ) exposure w i t h and w i t h o u t an a p p l i e d s t r e s s . I n t h e a s - r o l l e d and i n t h e annea led c o n d i t i o n s , l a r g e m e t a s t a b l e c a r b i d e s were observed i n a d d i t i o n t o a r e g u l a r d i s t r i b u t i o n of smal l p a r t i c l e s . D u r i n g t h e h igh - tempera tu re exposure, t h e m a j o r i t y o f t h e l a r g e c a r b i d e s were d i s s o l v e d and a more s t a b l e c a r b i d e phase formed. T h i s f i n e l y d i s p e r s e d phase had a com-

ev idence o f an o r i e n t a t i o n r e l a t i o n s h i p w i t h t h e m a t r i x . The p r e c i p i t a t e s appeared t o coarsen s l i g h t l y a f t e r -5000 h r exposure a t t empera tu res between 1350 and 1400 K . T h i s same h igh - tempera tu re exposure i n t h e presence o f an a p p l i e d s t r e s s r e s u l t e d i n a decrease i n t h e s i z e and i n t h e i n t e r p a r t i c l e spac ing o f t h e s t a b l e p r e c i p i t a t e s . However, t h e c o m p o s i t i o n o f t h e D r e c i D i - t a t e phase and i t s a b i l i t y t o p i n d i s l o c a t i o n s were n o t a f f e c t e d by

? p o s i t i o n de termined t o be -70 p e r c e n t Z r C and -30 p e r c e n t NbC and showed some d 0 d I

w

a t u r e or s t r e s s c o n d i t i o n s

INTRODUCTION

Niobium-zirconium-carbon a l l o y s were f i rst developed f o r space c o n v e r s i o n a p p l i c a t i o n s due t o t h e i r e l e v a t e d tempera tu re c reep r e s r e s i s t a n c e t o c o r r o s i o n by l i q u i d a l k a l i me ta l s , and r e l a t i v e l y low These a l l o y s r e l y on complex c a r b i d e p r e c i p i t a t e s f o r s t r e n g t h e n i n g z i r c o n i u m o x i d e p r e c i p i t a t i o n as i n b i n a r y niobium-1 p e r c e n t z i r c o n been e s t a b l i s h e d t h a t c a r b i d e p r e c i p i t a t e s a r e more e f f e c t i v e s t r e n

t h e temper-

energy s tance , d e n s i t y . r a t h e r than um. I t has theners ,

b u t t h e a c t u a l compos i t i on , morphology, and a g i n g c h a r a c t e r i s t i c s o , t h e second phase i n a l l o y s c o n t a i n i n g carbon have not been f u l l y i n v e s t i g a t e d . Severa l workers have ana lyzed p r e c i p i t a t i o n i n t h e Nb-Zr-C system (Arzamozov and V a s i l ' e v a , 1978; Begley e t a l . , 1963; C u f f , 1962; K i s s i l e t a l . , 1976, 1978; Koro tayev e t a l . , 1980, 1981; L y u t y i e t a l . , 1978; Maksimovich e t a l . , 1978; Ostermann and B o l l e n r a t h , 1966, Ostermann, 1971; T s v i k i l e v i c h , 1980> , b u t b e t - t e r unders tand ing o f t h e k i n e t i c s o f a g i n g r e a c t i o n s w i t h r e s p e c t t o t h e i n t e n d e d s e r v i c e c o n d i t i o n s i s r e q u i r e d f o r d e t e r m i n a t i o n of d e s i g n pa ramete rs . The p r e s e n t work was under taken t o i n v e s t i g a t e t h e c a r b i d e morphology i n a Nb-Zr-C a l l o y h e a t t r e a t e d w i t h and w i t h o u t t he presence o f an a p p l i e d s t r e s s .

The s t r e n g t h o f Nb-Zr-C a l l o y s has been shown t o be much h i g h e r than b i n a r y Nb-Zr or Nb-C a l l o y s (De lGrosso e t a l . , 1967; G r i g o r o v i c h and S h e f t e l , 1982). I t i s assumed t h a t t h e suppress ion o f t h e t r a n s i t i o n o f t h e c a r b i d e phase from coheren t t o i n c o h e r e n t ( a s i n b i n a r y Nb-C a l l o y s ) i s due t o t h e fo r - m a t i o n o f complexes o f carbon, n iob ium, and z i r c o n i u m atoms (Koro tayev e t a l . , 1981). The i n c r e a s e d s t r e n g t h o f t h e t e r n a r y a l l o y i s due, t h e r e f o r e , t o t h e p r e c i p i t a t i o n of s t a b l e , z i r c o n i u m - e n r i c h e d (Nb,Zr>C i n s t e a d o f NbC, wh ich breaks down a t lower tempera tures than t h e complex c a r b i d e . The i n c r e a s e d s t a - b i l i t y o f t h e (Nb,Zr)C c a r b i d e can be a t t r i b u t e d t o t h e r e l a t i v e i m m o b i l i t y o f

z i r c o n i u m i n n i o b i u m due t o i t s l a r g e r a tomic s i z e . I n a d d i t i o n , a l t h o u g h t h e p r e c i p i t a t e phase may c o n t a i n oxygen, i t must be p r i m a r i l y a c a r b i d e due t o t h e i n c r e a s e d s t r e n g t h e n i n g o f Nb-Zr-C a l l o y s o v e r Nb-1Zr w i t h Zr02 p r e c i p i t a t e s .

MAT E R I A L

The m a t e r i a l used i n t h i s s t u d y was r e c e i v e d from t h e Oak Ridge N a t i o n a l L a b o r a t o r y i n t h e form o f a s - r o l l e d , 1-mm t h i c k t e n s i l e specimens. r i a l was from a s i n g l e a r c - m e l t f o l l o w e d by a s i n g l e p r i m a r y breakdown e x t r u - s i o n i n t h e 1350 K reg ime. I t was subsequen t l y p rocessed i n t o tube m a t e r i a l , wh ich was s p l i t and r o l l e d t o produce sheet specimens. A l l m a t e r i a l was annea led for 1 h r a t 1755 K t h e n 2 h r a t 1475 K p r i o r to any t e s t i n g . Anneal- i n g t r e a t m e n t s were conducted i n t i t a n i u m s u b l i m a t i o n pumped systems a t a p res - sure below 10-5 Pa and i n c l u d e d wrapp ing each specimen i n c leaned t a n t a l u m f o i 1 .

T h i s mate-

Two o f t h e sheet samples were c u t i n t o s m a l l e r p i e c e s , wrapped i n Nb-1Zr f o i l , and aged a t 1350 K wi th no a p p l i e d s t r e s s . c a r r i e d o u t i n LN2 t r a p p e d o i l d i f f u s i o n pumped systems a t a p r e s s u r e o f -1 xlO-4 Pa. Severa l samples were aged i n t h e presence of an app l i e d s t r e s s a t 1350 and 1400 K as d e s c r i b e d by T i t r a n (1986) i n i n t e r n a l l y loaded chambers a t c o n s t a n t l o a d . The vacuum, m a i n t a i n e d by bakeab le t i t a n i u m s u b l i m a t i o n pumps, was i n i t i a l l y -10-5 and ranged t o 5x10-7 Pa a f t e r s e v e r a l hundred hours .

The h e a t t r e a t m e n t s were

EXPERIMENTAL PROCEDURE

T r a d i t i o n a l e t c h i n g f o r o p t i c a l and scann ing e l e c t r o n m e t a l l o g r a p h y was a d i f f i c u l t t echn ique f o r examin ing t h e c a r b i d e s t r u c t u r e due t o t h e tendency o f t h e p a r t i c l e s to f a l l o u t o f t h e s t r u c t u r e d u r i n g e t c h i n g . T h i s was due t o t h e f a c t t h a t t h e e t c h a n t removed m a t e r i a l underneath t h e p a r t i c l e s w i t h o u t a t t a c k - i n g t h e c a r b i d e s . An anod ic s t a i n i n g techn ique was t r i e d (Crouse, 1965), b u t was n o t e a s i l y i n t e r p r e t a b l e due to t h e sma l l s i z e o f t h e p r e c i p i t a t e s . L i m i t e d success was o b t a i n e d u s i n g a n i t r i c l h y d r o f l u o r i c a c i d e t c h a n t .

Phase e x t r a c t i o n was pe r fo rmed on specimens suppor ted w i t h p l a t i n u m w i r e s i n 100 m l of bromine i n 900 m l o f methanol w i t h 10 g o f t a r t a r i c a c i d i n so lu - t i o n . s i v e l a y e r on t h e s u r f a c e o f t h e samples. X-ray a n a l y s i s was used to i d e n t i f y and t o de te rm ine t h e c o m p o s i t i o n o f t h e p r e c i p i t a t e phase. was a l s o per fo rmed on t h e e x t r a c t e d m a t e r i a l by i n d u c t i v e l y coup led plasma a tomic emiss ion spec t romet ry .

The c e l l was p e r i o d i c a l l y s u b j e c t e d t o u l t r a s o u n d t o d i s p e r s e any pas-

Chemical a n a l y s i s

Transmiss ion e l e c t r o n m ic roscopy f o i l s were c u t by e l e c t r i c d i scha rge mach in ing . were j e t t h i n n e d i n an e l e c t r o l y t e c o n s i s t i n g o f 936 m l methano l , 62.8 m l H2SO4, 40 m l b u t y l a l c o h o l , and 1 . 5 m l HF. The f o i l s were prepared u s i n g a S t r u e r s Tenupol appara tus o p e r a t e d a t a c u r r e n t d e n s i t y o f -27 mA/mm2. A l though t h e l a r g e c a r b i d e p a r t i c l e s tended t o f a l l o u t o f t h e f o i l s d u r i n g t h i n n i n g , adequate t h i n areas were o b t a i n e d . I n a d d i t i o n , some f o i l s were d imp led then i o n - m i l l e d .

A f t e r t h e o x i d e l a y e r was removed by hand p o l i s h i n g , some samples 0

2

RESULTS AND DISCUSSION

Chemical A n a l y s i s

The oxygen c o n t e n t s i n c r e a s e d a f t e r a g i n g w i t h and w i t h o u t an a p p l i e d s t r e s s as ev idenced i n t a b l e I , which l i s t s t h e compos i t i ons i n b o t h we igh t p e r c e n t and atomic p e r c e n t . Th i s i n c r e a s e was expec ted and r e a l i s t i c s i n c e t h e vacuum l e v e l s a t which t h e m a t e r i a l was t e s t e d approx imated what t h e m a t e r i a l would see d u r i n g use i n space power systems. The oxygen c o n t e n t o f t h e a l l o y was 2.5 t imes g r e a t e r a f t e r a g i n g w i t h o u t a p p l i e d s t r e s s , and was f o u r t i m e s g r e a t e r a f t e r a g i n g w i t h an a p p l i e d s t r e s s o f 40 MPa. These two cases were for a p p r o x i m a t e l y t h e same t i m e and temperature, and t h e vacuum was one t o two o r d e r s o f magnitude b e t t e r for t h e sample under s t r e s s . The l a r g e r i n c r e a s e i n t h i s case i s a t t r i b u t e d t o t h e i n c r e a s e d oxygen m o b i l i t y i n t h e presence o f t h e a p p l i e d s t r e s s . The i n c r e a s e d oxygen c o n t e n t , however, was n o t d i s c e r n i b l e th rough m e t a l l o g r a p h y or phase e x t r a c t i o n , i n d i c a t i n g t h a t most o f t h e oxygen remained i n s o l u t i o n .

M e t a l l o g r a p h y

The m i c r o s t r u c t u r e s o f t h e a s - r o l l e d c o n d i t i o n and t h e annea led c o n d i t i o n for t h e Nb-Zr-C a l l o y a r e shown i n f i g u r e 1 . Va r ious shapes and s i z e s o f par- t i c l e s were n o t i c e a b l e i n b o t h m i c r o s t r u c t u r e s . The morphology ranged from massive 5- t o 10-pm p a r t i c l e s d e c o r a t i n g g r a i n boundar ies t o submicron needle- l i k e p a r t i c l e s . The l a r g e p a r t i c l e s observed a r e b e l i e v e d t o be p r i m a r y meta- s t a b l e c a r b i d e s o f Nb2C formed d u r i n g t h e i n i t i a l s o l i d i f i c a t i o n which were n e i t h e r broken up no r d i s s o l v e d d u r i n g e x t r u s i o n or d u r i n g t h e sheet r o l l i n g p rocess . The annealed m a t e r i a l had a m i x t u r e o f e l o n g a t e d and equ iaxed g r a i n s w i t h an average g r a i n s i z e o f -25 pm.

I t appears from t h e s i z e o f t h e c a r b i d e s i n t h e annea led c o n d i t i o n ( f i g . l ( b > ) t h a t t h e anneal was i n s u f f i c i e n t to s o l u t i o n t h e l a r g e c a r b i d e par- t i c l e s . Aged samples ( f i g . 2 ) , however, show t h a t these p a r t i c l e s a r e d i s - s o l v e d and t h a t a u n i f o r m d i s t r i b u t i o n o f a new p r e c i p i t a t e forms, w i t h a s i z e r a n g i n g from 500 to 1000 nm. for t i m e g r e a t e r t h a n 500 h r e x h i b i t e d t h i s same p a t t e r n o f p r e c i p i t a t i o n . Scanning e l e c t r o n mic roscopy c o u l d n o t d i f f e r e n t i a t e between short and l o n g exposures to tempera tu re and /o r s t r e s s .

A l l o f t h e samples exposed t o h i g h temperatures

Phase E x t r a c t i o n

A n a l y s i s o f t h e phase-ex t rac ted m a t e r i a l i s l i s t e d i n t a b l e 11. M a t e r i a l i n t h e a s - r o l l e d c o n d i t i o n showed mostly Nb2C and a v e r y m ino r phase w i t h a, = 0.468 nm, and w i t h v i r t u a l l y no z i r c o n i u m i n t h e p r e c i p i t a t e phase. t h e annealed c o n d i t i o n , b o t h Nb2C and an f c c phase w i t h a. = 0.468 were iden- t i f i e d as ma jo r phases, though t h e z i r c o n i u m c o n t e n t o f t h e p r e c i p i t a t e d i d n o t i n c r e a s e a p p r e c i a b l y . t h e o n l y phase i d e n t i f i e d was f a c e c e n t e r e d c u b i c w i t h a l a t t i c e parameter between 0.461 and 0.467 nm. I t i s i n t e r e s t i n g t o n o t e t h a t Koro tayev e t a l . (1981) i d e n t i f i e d a phase w i t h a l a t t i c e parameter o f 0.462 nm u s i n g e l e c t r o n d i f f r a c t i o n a n a l y s i s o f e x t r a c t e d r e p l i c a s o f Nb-1Zr-O.1C a l l o y samples

I n

I n those samples exposed f o r t imes g r e a t e r t han 100 h r ,

3

quenched from 2075 K and aged for 100 h r a t 1275 K. They s t a t e t h a t t h e t r a n s - f o r m a t i o n t o t h e s t a b l e monocarbide i s l a r g e l y completed i n 100 h r a t tempera- t u r e s o v e r 1300 K . Khandarov e t a l . (19781, however, e s t i m a t e d t h a t f u l l decompos i t i on o f t h e m e t a s t a b l e c a r b i d e would r e q u i r e a g i n g o v e r 10 000 h r a t 1375 K .

A l though t h e r e i s c o n s i d e r a b l e s c a t t e r i n t h e da ta , t h e z i r c o n i u m c o n t e n t o f t h e p r e c i p i t a t e phase was v i r t u a l l y c o n s t a n t a f t e r -300 h r a t e i t h e r 1350 or 1400 K . A wide v a r i e t y o f p r e c i p i t a t e compos i t i ons a r e p o s s i b l e , some o f wh ich a r e l i s t e d i n t a b l e 111. I t i s p o s s i b l e t h a t t h e e x t r a c t e d m a t e r i a l i s a com- p l e x phase c o n s i s t i n g o f a l l o f these compounds, b u t i t i s more l i k e l y t h a t i t i s p r i m a r i l y a m i x t u r e o f Z r C and NbC, t h e s t a b l e c a r b i d e phases. I f we assume t h a t t hese a r e t h e components o f t h e p r e c i p i t a t e , i t i s p o s s i b l e t o de te rm ine t h e r e l a t i v e amounts o f each c a r b i d e p r e s e n t b y r e f e r r i n g to f i g u r e 3, which p l o t s t h e s o l u b i l i t y r e l a t i o n s h i p between NbC and Z r C ( N o r t o n and Mowry, 1949). I f t h e p r e c i p i t a t e phase i s i n f a c t (Zr,Nb>C w i t h a l a t t i c e parameter o f 0.462 nm, t h e c o m p o s i t i o n i s 70 p e r c e n t Z r C and 30 p e r c e n t NbC, a Zr/Nb r a t i o o f -2.3. The Zr/Nb r a t i o found from t h e chemica l a n a l y s i s and t h e l a t t i c e para,l ieter va lues from x- ray d i f f r a c t i o n l e a d t o t h e c o n c l u s i o n t h a t t h e compo- s i t i o n o f t h e p r e c i p i t a t e i s v e r y c l o s e t o t h e s t o i c h i o m e t r i c monocarbide.

C o n s i d e r i n g t h e i n c r e a s e o f t h e oxygen c o n t e n t d u r i n g a g i n g , i t i s l i k e l y t h a t oxygen i s p r e s e n t i n a t l e a s t some o f t h e p r e c i p i t a t e p a r t i c l e s . McCoy and Douglas (1961) p r e d i c t t h a t when oxygen, n i t r o g e n , and carbon a r e a l l p r e s e n t i n solution, Zr02 w i l l form p r e f e r e n t i a l l y s i n c e b o t h f r e e energy of f o r m a t i o n va lues and d i f f u s i o n r a t e s f a v o r t h e f o r m a t i o n o f t h e o x i d e o v e r t h e n i t r i d e or c a r b i d e . Some e x p e r i m e n t a l s u p p o r t o f t h i s was p r o v i d e d by Koro tayev e t a l . (1980) , who observed an i n t e r n a l o x i d a t i o n f r o n t i n a Nb-1Zr-O.1C a l l o y where t h e o r i g i n a l c a r b i d e phase t r a n s f o r m e d to Zr02 p l a t e - l e t s . No Zr02 was observed i n t h e x - ray d i f f r a c t i o n a n a l y s i s , b u t s i n c e t h e l a t t i c e parameter o f Z r O ( a m e t a s t a b l e phase wh ich does n o t r e a d i l y form on i t s own) i s so c l o s e t o t h a t o f Z r C and NbC, a n a l y s i s if t h e oxygen absorbed d u r i n g ag complex (Nb,Zr)(O,C). S ince v i r t u a l l y no do n o t expec t n i t r i d e f o r m a t i o n .

There i s some ev idence t h a t i n t e r n a l s u r f a c e phenomenon when t h e sample i s und

t i s i m p o s s i b l e t o t e l l from t h i s ng s t a y s i n s o l u t i o n or forms t h e n i t r o g e n a b s o r p t i o n was d e t e c t e d , we

o x i d a t i o n i s a b u l k r a t h e r t h a n a r an a m l i e d s t r e s s . a t t r i b u t e d to

i n c r e a s e d oxygen mob1 1 i t y (Koro tayev e t a1 . , 1980’; L y u t y i e t a1 . , 1984). For t h e c a r b i d e s , however, T s v i k i l e v i c h (1980) observed t h a t t h e a p p l i c a t i o n o f a l o a d d u r i n g a g i n g a t 1166 and 1366 K d i d n o t change t h e phase c o m p o s i t i o n and form o f t h e c a r b i d e p r e c i p i t a t e s . Ostermann (1971) p o i n t s o u t t h a t t h e l a r g e range o f l a t t i c e d imens ions when t h e MC1-x c a r b i d e d e v i a t e s from s t o i c h i o m e t r y o f f e r s t h e l i k e l y p o s s i b i l i t y o f p e r f e c t a t o m i c ma tch ing ac ross t h e h a b i t p l a n e . I n a d d i t i o n , L y u t y i e t a l . (1978) f o u n d v i r t u a l l y no d i f f e r e n c e i n t h e s i z e or i n t e r p a r t i c l e spac ing o f t h e p r e c i p i t a t e s w i t h and w i t h o u t an a p p l i e d s t r e s s o f -15 MPa i n samples exposed fo r up t o 10 000 h r a t 1375 K, a l t h o u g h t h e y do observe i n c r e a s e d s t r e n g t h i n those samples aged i n t h e presence of t h e a p p l i e d s t r e s s . Wi lcox and A l l e n (1967) a l s o f i n d s some ev idence o f dynamic s t r e n g t h e n i n g d u r i n g c reep o f a Nb-W-Zr a l l o y .

4

Transmiss ion E l e c t r o n M ic roscopy

”

E l e c t r o n mic roscopy of a sample aged for 25 h r a t 1350 K t y p i c a l l y r e v e a l e d f l a t p r e c i p i t a t e s i n t h r e e p e r p e n d i c u l a r v a r i a n t s , a p p r o x i m a t e l y 100 to 150 nm i n s i z e , spaced from 100 t o 200 nm a p a r t . They were imaged u s i n g t h e spo ts p o i n t e d o u t by t h e a r row i n f i g u r e 4. phase c o u l d n o t be i d e n t i f i e d by t h e d i f f r a c t e d spo t spac ing , b u t t h e spo ts i n d i c a t e an o r i e n t a t i o n r e l a t i o n s h i p w i t h t h e m a t r i x .

The l a t t i c e parameter o f t h i s

I n samples aged i n t h e presence of a s t r e s s , s i m i l a r p r e c i p i t a t e s , approx- i m a t e l y 50 t o 100 nm i n s i z e , were found which were g e n e r a l l y c l o s e l y and regu- l a r l y spaced a t -50 t o 100 nm. p r e c i p i t a t e s w i t h a l a t t i c e parameter of 0.462 nm by t h e d i f f r a c t i o n s p o t spac- i n g . The images i n f i g u r e s 5 and 6 a r e of a sample wh ich was under an a p p l i e d s t r e s s for 650 h r a t 1400 K . The p r e c i p i t a t e s were imaged i n d a r k f i e l d e l e c - t r o n mic roscopy Us ing t h e (200)Carb ide and (111)Carb ide s p o t s , r e s p e c t i v e l y . The square shape o f t h e p r e c i p i t a t e s i n d i c a t e s t h a t l i t t l e or no c o a r s e n i n g has taken p l a c e i n t h i s t i m e .

These were t e n t a t i v e l y i d e n t i f i e d as (Zr,Nb>C

I n samples aged i n t h e presence o f an a p p l i e d s t r e s s f o r l o n g e r t imes , t h e p r e c i p i t a t e s grew i n s i z e from 100 t o 150 nm ac ross and took on a more rounded shape, i n d i c a t i n g some c o a r s e n i n g had o c c u r r e d . The sample shown i n f i g u r e 7 was under an a p p l i e d s t r e s s for 4770 h r s a t 1350 K. The M o i r e f r i n g e s apparen t i n t h e p r e c i p i t a t e i n d i c a t e a l a t t i c e mismatch. A l l o f these p a r t i c l e s appeared capab le o f p i n n i n g d i s l o c a t i o n s t o a t l e a s t some e x t e n t , and v e r y l a r g e d i s l o c a t i o n t a n g l e s were common. s t r u c t u r e were observed, b u t t h e l a r g e amount o f smal l p a r t i c l e s appeared to hamper any d i s l o c a t i o n a l i g n m e n t .

Suggest ions of a d i s l o c a t i o n c e l l

L y u t y i e t a l . (1978) found an i n c r e a s e i n t h e average s i z e o f t h e p a r t i - c l e s from 86 t o 133 nm upon a g i n g f o r 5000 h r a t 1375 K. spac ing a l s o i n c r e a s e d from 390 to 450 nm. 10 000 h r r e s u l t e d i n a p a r t i c l e s i z e o f 150 nm and an i n t e r p a r t i c l e spac ing o f 450 nm. L y u t y i e t a l . r e p o r t e d v i r t u a l l y no d i f f e r e n c e when a s t r e s s o f 15 MPa was a p p l i e d . T h i s p a r t i c l e g rowth appears t o be i n a c c o r d w i t h t h a t f ound i n t h i s Nb-Zr-C a1 loy w i t h i n e x p e r i m e n t a l s c a t t e r .

The i n t e r p a r t i c l e Exposure a t t h i s tempera tu re f o r

CONCLUDING REMARKS

The i n i t i a l a n n e a l i n g t r e a t m e n t ( 1 h r a t 1755 K + 2 h r a t 1475 K ) , i n t e n d e d t o d i s s o l v e t h e l a r g e m e t a s t a b l e c a r b i d e s , was n o t s u f f i c i e n t . How- e v e r , these p a r t i c l e s d i d d i s s o l v e d u r i n g a g i n g a t 1350 and a t 1400 K and a f i n e , more s t a b l e p r e c i p i t a t e formed. I t i s b e l i e v e d t h a t some of these f i n e l y d i s p e r s e d p a r t i c l e s a r e (Zr,Nb>C w i t h a Zr/Nb r a t i o o f -2.3. I t i s a l s o p o s s i - b l e t h a t these p r e c i p i t a t e s c o n t a i n some oxygen. The c o m p o s i t i o n o f t h e p a r t i - c l e s d i d n o t change w i t h t i m e a t tempera ture , and a l l o f t h e p a r t i c l e s observed appeared t o be e f f e c t i v e a t p i n n i n g d i s l o c a t i o n s .

Ag ing w i t h and w i t h o u t t h e presence o f an a p p l i e d s t r e s s appears t o cause p r e c i p i t a t e s t o form w i t h some ev idence of an o r i e n t a t i o n r e l a t i o n s h i p w i t h t h e m a t r i x . coa rsen ing a t 1400 K f o r 650 h r , b u t appear t o grow to a p p r o x i m a t e l y t w i c e t h e i r o r i g i n a l s i z e a f t e r 5000 h r a t 1350 K. P r e c i p i t a t e s aged i n t h e presence

Square p r e c i p i t a t e s o f (Zr,Nb)C form which show l i t t l e ev idence o f

5

o f a s t r e s s e x h i b i t e d a somewhat s m a l l e r s i z e and i n t e r p a r t i c l e spac ing than those aged w i t h o u t an a p p l i e d s t r e s s . The i n c r e a s e i n t h e d i f f u s i o n r a t e of carbon, oxygen, and z i r c o n i u m i n t h e presence o f a s t r e s s may be t h e reason for t h i s . However, t h e p a r t i c l e s were v e r y s i m i l a r to those observed i n a sample aged a t 1350 K f o r o n l y 25 h r w i t h no a p p l i e d s t r e s s , i n d i c a t i n g t h e l ong - te rm s t a b i l i t y o f these p r e c i p i t a t e s . I n c o n c l u s i o n , t h e f i n e (Zr,Nb)C p r e c i p i t a t e i s a v e r y s t a b l e p a r t i c l e and, as such, i s an e f f e c t i v e c reep s t r e n g t h e n e r i n t h e 1350 K regime.

~

REFERENCES

V.B. Arzamosov, and E . V . V a s i l ' e v a : Phase Composition of Nb-M-C Alloys. Met. S c i . Heat T r e a t . ( E n g l . T r a n s l . ) , 1978, v o l . 20, pp. 291-293.

R .T . Beg ley , R.L. Ammon, and R . S t i c k e r : WADC-TR-57-344, P a r t V I , W r i g h t P a t t e r s o n A i r Force Base, OH, 1963.

Development of Niobium Base Al loys.

R.S. Crouse: I d e n t i f i c a t i o n of Carbides, N i t r i des , and Oxides o f Niobium and Niobium Al loys by Anodic Sta in ing. ORNL-3821, Oak Ridge N a t i o n a l Lab, TN, 1965.

F.B. C u f f Jr . : Research t o Determine the Composition o f Dispersed Phases i n Refractory Metal A l l o y s . ASD-TDR-62-7, P a r t I, W r i g h t P a t t e r s o n A i r Force Base, OH, 1962.

E.J. DelGrosso, C . E . C a r l s o n , and 3 .3 . Kaminsky: Development o f Niobium- Zirconium-Carbon Alloys. J . Less Common Met . , 1967, vo l . 12 , pp. 173-201.

V.G. G r i g o r o v i c h , and E .N . S h e f t e l : Physicochemical Fundamentals of the Development of Heat-Resistant Niobium A l l oys . Met. S c i . Heat T r e a t . (Eng l . T r a n s l . ) , 1982, v o l . 24, pp. 472-478.

P.A. Khandarov, A . N . Luk ' yanov , A .G. A r a k e l o v , O.S. T s v i k i l e v i c h , E.M. L y u t y i , and G.G. Maksimovich: K i n e t i c s of S t ruc tu ra l Changes i n an Nb-Zr-C A l l o y w i th Prolonged Holding under Load a t High Temperatures. Sov. Ma te r . S c i . (Eng l . T r a n s l . ) , 1978, vol . 14, pp. 431-435.

A . E . K i s s i l , P . A . Khandarov, A.D. Rapp, L.P. Onisenko, L.Z. Po lyak , A.G. A r a k e l o v , and 1.1. Maksimov: Study o f the Structure and Morphology of the Strengthening Phase i n an Nb-Zr-C Alloy During the Prolonged Act ion of High Temperature and Pressure i n Vacuo, Sov. Ma te r . S c i . ( E n g l . T r a n s l . ) ,

A . E . K i s s i l , E . M . L y u t y i , A . G . A r a k e l o v , G.G. Maksimovich, L.P. Onisenko, and O.S. T s v i k i l e v i c h : Ef fect o f Long-time High-Temperature Aging on Creep of an Nb-Zr-C A l l o y i n a Vacuum. Sov. M a t e r . S c i . ( E n g l . T r a n s l . ) , 1978, v o l . 14,

A.D. Koro tayev , A . N . Tyumentsev, Yu P . P i n s h i n , Yu R . Ko lobov , M . G . Glazunov, K.A. Markov, and N . G . Minayev: In f luence o f the S t ruc tu ra l State on the Mechanism by which a Non-Metallic Phase i s Formed During D i f f u s i o n Saturat ion of Niobium Al loys w i t h Oxygen. Phys. Met. M e t a l l o q r . ( E n g l . T r a n s l . ) , 1980,

1976, V O ~ . 12, pp. 640-643.

pp. 49-52.

VOI. 50, pp . 92-99.

6

A.D. Korotayev, A.N. Tyumentsev, M.G. Glazunov, L.M. Dizhur, A . I . Yakushina, S.P. Semkin, and T . I . Vitkovskaya. Nature of Secondary Phases and Mechanism of Rupture i n a Niobium-Zirconium-Carbon Alloy. Phys. M e t . Meta l logr . (Engl. T r a n s l . ) , 1981, vo l . 52, pp. 128-135.

E.M. Lyuty i , G.G. Maksimovich, and O.S. Tsv ik i lev ich : Quan t i t a t i ve Analysis on the D i s t r i b u t i o n of Hardening-Phase Par t i c l es i n an Nb-Zr-C Alloy. Mater. S c i . (Engl. T r a n s l . ) , 1978, vo l . 14, pp. 284-288.

Sov.

E.M. Lyu ty i , O.D. Smiyan, and 0.1. El iseeva: Role of Stresses i n the High-Temperature Oxidat ion o f Niobium i n a Raref ied Oxygen Stream. Meta l l . (Engl. T rans l . ) , 1984, no. 4, pp. 211-215.

Russ.

G.G. Maksimovich. E.M. Lvu tv i , O.S. Tsv ik i lev ich , A . E . K i s s i l , and A . E . Demichev: o f Alloy 5VMTs. Sov. Mater. S c i . (Engl. Trans l . ) , 1978, vo l . 14,

E f f e c t of Piolonged High-Temperature Soaking on the Strength

pp. 585-589.

H.E . McCoy, and D . A . Douglas: E f f e c t of Various Gaseous Contaminants on the Strength and Formab i l i t y of Columbium, i n Columbium Metal lurgy, D.L. Douglass and F.W. Kunz, eds., I n t e r s c i e n c e Publishers, New York, 1961, pp. 85-118.

J.T. Norton, and A.L. Mowry: S o l u b i l i t y Relat ionships o f the Refractory Monocarbides. J . M e t . , 1949, vo l . 1 , no. 2 , pp. 133-136.

F . Ostermann: Con t ro l l i ng Carbide Dispersions i n Niobium-Base Al loys, J. Less Common M e t . , 1971 , v o l . 25, pp. 243-256.

F . Ostermann, and F. Bol lenrath: I nves t i ga t i on of Prec ip i ta tes i n Two Carbon-Containing Columbium-Base A l loys . AFML-TR-66-259, Wright Patterson A i r Force Base, OH, 1966. ( A v a i l . N T I S , AD-649968).

E.K. Storms: The Refractory C a r b i d e s , Academic P r e s s , NY, 1967.

R.H. T i t ran : Long-Time Creep Behavior of Nb-1Zr A l l o y Containing Carbon. NASA TM-100142, National Aeronautics and Space Administrat ion, Washington, D.C., 1986.

O . S . Tsv i k i l ev i ch : S t ruc tu ra l Transformations i n NTsU and 5VMTs A l loys a f t e r Long-Term T e s t s . Sov. M a t e r . S c i . (Eng l . T rans l . ) , 1980, v o l . 16 , pp. 73-77.

B.A. Wilcox. and B.C. A l l e n : The In f luence of Zirconium on Dynamic

J

Strengthening o f Niobium Alloys. J . Less Common M e t . , 1967, i o l . 13, pp. 186-192.

7

As- ro l l ed

A t % W t %

.040 N i obi urn balance balance

Aged a t 1350 K f o r -1500 h r

No s t ress Stress = 40 MPa

A t % W t % A t % W t %

TABLE 11. - ANALYSIS OF PHASE-EXTRACTED M A T E R I A L

Condi t ion I

-1 ance

I

As-rolled I 0 Annealed

I I

Annealed + aged

3 100 315

1000 1000 1246 1000 1531

1296

1489

---- ---- -_--

Aging cond i t ions

Temperature, K

---- 1350 1350 1350 1400 1400 ( a ) 1350 1350

1350

1350

-___ ---- -___

Stress Temperature, K Total

s t r a i n , percent

alOOO h r a t 1400 K + 246 h r a t 1350 K .

TABLE 111. - L A T T I C E PARAMETER AND STRUC- TURE OF POSSIBLE PRECIP ITATE PHASES

[Storms, 1967.1

I Phase I ao, nm I Nb C hcp

NbO cubic NbC f c c (61) Z r N f c c (B1) Z r O f c c (61) Z r C fcc (61 )

2 0.313 ; c = 0.497

.421

.447t

.456

.462

.470c

.509

Total h r a t

temperature

0 3

100 315

1329 1246 1246 1531 1531 1296 1296 1489 1489 2929

Zr/Nb r a t i o

0.01 .04 .27

1.62 2.30 2.41 1.87 2.11 2.39 1.11 1.87 1.58 1.31 2.42

a0 3

nm

NbZC+ .468 NbZC+ .458

.461

.462

.462

.462

.463

.461

.463

.465

.462

.467

.462

.465

8

ORIGINAL PAGE IS OF POOR QUALITY

( a ) AS-ROLLED SHEET. c ROLLING DIRECTION .-c

( b ) ANNEALED CONDITION.

FIGURE 1. - MICROSTRUCTURE OF Nb-Zr-C ALLOY.

9

( a ) ANNEALED, AGED 1000 HR AT 1 3 5 0 K . THEN CREPT 770 HR AT 1350 K TO 1.2% STRAIN.

( b ) ANNEALED, THEN CREPT 3 8 4 0 HR AT 1 3 5 0 K TO 3 . 3 % STRAIN.

FIGURE 2. - MICROSTRUCTURE OF Nb-Zr-C ALLOY AFTER CREEP TESTING.

ORIGINAL PAGE IS OE POOR QUALITY

10

.

-47

I oz 2 .46

Z rC

1 c c 4

80 * 43

0 20 40 60 MOLECULAR PERCENT Z r C

FIGURE 3. - LATTICE PARAMETER-COMPOSITION CURVE FOR NbC-ZrC (FROM NORTON AND MOWRY. 1949).

FIGURE 4. - DARK F I E L D MICROSTRUCTURE OF Nb-Zr-C ALLOY AFTER 25 HR EXPOSURE AT 1350 K SHOWING TWO PERPENDICULAR VARIANTS OF A PRECIPITATE PHASE.

i

12

,

FIGURE 5. - DARK F I E L D MICROSTRUCTURt OF N b - l r - C ALLOY A F l t K III INb A N N t A L i D , [HEN CREEP TESTED AT 1400 K AND 40 MPA FOR 648 HR TO 3.6% STRAIN. TWO PERPENDICULAR VARIANTS ARE SHOWN.

F IGURE 6. - BRIGHT ANI) I N R K t I t L I / F l lLKUSTRUCI l l l l l 5 01 N t II L ALLOY. AS I N F IGURE 5 ,

FIGURE 7. - BRIGHT AND DARK F I E L D hICROSTRUCTURES OF Nb-Zr-C ALLOY AFTER BEING ANNEALED. THEN CREEP TESTED AT 1350 K AND 40 MPA FOR 4772 HR TO 1.6% STRAIN.

13

Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No.

NASA TM-100848 I I 4. Title and Subtitle 5. Report Date

C h a r a c t e r i z a t i o n o f P r e c i p i t a t e s i,n a Niobium- Zirconium-Carbon Al loy

7. Author@)

T.L. G r o b s t e i n and R.H . T i t r a n

I 6. Performing Organization Code

8. Performing Organization Report No.

E-4041

17. Key Words (Suggested by Author(@)

PWC-11; Creep; Niobium a l l o y s ; R e f r a c t o r y me ta l a l l o y s ; Transmiss ion e l e c t r o n m ic roscopy ; Phase e x t r a c t i o n ; Carb ide p r e c i p i t a t i o n

10. Work Unit No.

18. Distribution Statement

U n c l a s s i f i e d - U n l i m i t e d S u b j e c t Category 26 DOE Category UC-25

506-41 -38

9. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages

U n c l a s s i f i e d U n c l a s s i f i e d 14

9. Performing Organization Name and Address

N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n Lewis Research Center C leve land , Ohio 44135-3191

22. Price’

A02

11. Contract or Grant No. r--- I 1 3 Type of Report and Period Covered

12. Sponsoring Agency Name and Address

U.S . Department o f Energy Reactor Systems Development and Technology Washington, D.C. 20545

Techn ica l Memorandum

DOE/NASA/16310-6

15. Supplementary Notes

F i n a l Repor t . Prepared under I n t e r a g e n c y Agreement DE-AI03-86SF16310. Prepared for The M e t a l l u r g i c a l S o c i e t y F a l l Meet ing , sponsored by t h e American I n s t i t u t e o f Mechan ica l Eng ineers ( A I M E ) , Or lando, F l o r i d a , October 5-9, 1986.

16. Abstract

A n i o b i u m a l l o y w i t h 1 p e r c e n t z i r c o n i u m and 0.063 p e r c e n t carbon by w e i g h t was i n v e s t i g a t e d i n t h e a s - r o l l e d and annealed c o n d i t i o n s , and a f t e r h igh - tempera tu re (1350 and 1400 K ) exposure w i t h and w i t h o u t an a p p l i e d s t r e s s . I n t h e a s - r o l l e d and i n t h e annealed c o n d i t i o n s , l a r g e m e t a s t a b l e c a r b i d e s were observed i n add i - t i o n t o a r e g u l a r d i s t r i b u t i o n of sma l l p a r t i c l e s . exposure, t h e m a j o r i t y o f t h e l a r g e c a r b i d e s were d i s s o l v e d and a more s t a b l e c a r b i d e phase formed. t o be -70 p e r c e n t Z r C and -30 p e r c e n t NbC and showed some ev idence o f an o r i e n t a - t i o n r e l a t i o n s h i p w i t h t h e m a t r i x . The p r e c i p i t a t e s appeared t o coarsen s l i g h t l y a f t e r -5000 h r exposure a t tempera tures between 1350 and 1400 K. T h i s same h i g h - tempera tu re exposure i n t h e presence o f an a p p l i e d s t r e s s r e s u l t e d i n a decrease i n t h e s i z e and i n t h e i n t e r p a r t i c l e spac ing o f t h e s tab1 p r e c i p i t a t e s . How- eve r , t h e c o m p o s i t i o n o f t h e p r e c i p i t a t e phase and i t s ab l i t y t o p i n d i s l o c a - t i o n s were n o t a f f e c t e d by t h e tempera tu re or s t r e s s cond t i o n s .

D u r i n g t h e h igh - tempera tu re

T h i s f i n e l y d i s p e r s e d phase had a c o m p o s i t i o n de termined