Beta decomposition processes in Hf-Rich Hf-Nb alloys
- PDF / 1,853,041 Bytes
- 7 Pages / 613 x 788.28 pts Page_size
- 52 Downloads / 160 Views
TRANSACTIONS
A
cluded that t h i s high s t a b i l i t y of the a - p h a s e in Hf a c counts for the d i f f e r e n c e b e t w e e n the Z r - N b and HfNb p h a s e d i a g r a m s . K a u f m a n and B e r n s t e i n 3 d e t e r m i n e d f r o m a r e g u l a r s o l u t i o n a p p r o x i m a t i o n that both a l l o y s y s t e m s should show /3-phase i m m i s c i b i l i t y . In the Hf-Nb s y s t e m the t e n d e n c y of the a - p h a s e to be s t a b l e o v e r a wide r a n g e of c o m p o s i t i o n s r e s u l t s in a b e t a i s o m o r p h o u s type of p h a s e d i a g r a m r a t h e r than the m o n o t e c t o i d type e x h i b i t e d by the Z r - N b s y s t e m . T h e m e t a s t a b l e f o r m a t i o n of H f - r i c h c l u s t e r s in a Hfo.~eNbo.68 a l l o y i n d i c a t e s that t h e r e is a m e t a s t a b l e m i s c i b i l i t y gap within the (a +/3) p h a s e f i e l d in the Hf-Nb s y s t e m . 2 The p r e s e n t w o r k was u n d e r t a k e n to i n t e r p r e t the m i c r o s t r u c t u r a l p r o c e s s e s that o c c u r d u r i n g the d e c o m p o s i t i o n of the b e t a p h a s e in H f - r i c h Hf-Nb a l l o y s . S e v e r a l c o m p o s i t i o n s w e r e s t u d i e d in the c h i l l - c a s t -
i
f 2468 •
I0 ~
2400 "~ "~ ~" - ~
L
22)8+10~
2200
2000
58
r
2065_'15 ~
o
tJ
1770-+30 1800
1600 i.o I1400
/?§
1200
,
10000
L
,
20
Nb
I
40 ~
6'0
ATOMIC % HAFNIUM
8'0 P
IO0 Hf
Fig. 1--Equilibrium phase diagram of the Hf-Nb system. 19
lSSN 0360-2133/78/0510-0723500.75/0 9 1978 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
VOLUME 9A, MAY 1978-723
Table I. Selected Characteristics of Ti, Zr, Hf, and Nb
Element
Melting Point ~
Ti Zr Hf Nb
1668 1876 2218 2468
Th (a -->~) 0.60 0.53 0.82
Atomic Radius (A) 1.42 1.56 1~53 1.43
condition to e x a m i n e the p r o d u c t s of the a t h e r m a l t r a n s f o r m a t i o n p r o c e s s e s . The i s o t h e r m a l d e c o m p o s i t i o n p r o c e s s e s of the ~ - p h a s e in a Hfo.6sNbo.3s a l loy w e r e s t u d i e d by aging at 400 and 600~ for v a r i ous p e r i o d s of t i m e . EXPERIMENTAL
RESULTS
TECHNIQUES
Hafnium-niobium alloys containing 50, 58, 65, 70, and 75 at. pet Hf were prepared in a Ti-gettered argon atmosphere using an arc melting technique. After several meltings to insure homogeneity, the molten buttons were cast directly into 2 mm diam rods. The materials used in alloy preparation were 99.8 pct pure Nb and Hf iodide bar containing 2.5 at. pct Zr. Optical microscopy revealed that the samples prepared in this m a n n e r c o n s i s t e d of e q u i a x e d g r a i n s a p p r o x i m a t e l y 0.03 to 0.05 m m in d i a m . The e q u i l i b r i u m p h a s e d i a g r a m in F i g . 1 shows cong r u e n t m e l t i n g b e h a v i o r at a c o m p o s i t i o n of 58 at. pct Hf. In the v i c i n i t y of the c o n g r u e n t m e l t i n g point, s o l i d i f i c a t i o n o c c u r s throughout the s a m p l
Data Loading...
