The CsCl-type order-disorder transition temperature in VMn alloys
- PDF / 175,338 Bytes
- 2 Pages / 612 x 792 pts (letter) Page_size
- 8 Downloads / 240 Views
wt~cu
The CsCl-type Order-Disorder Transition Temperature in VMn Alloys
6.~G 0"5"C/CM -
I-
i
i
I
TOMOO SUZUKI AND MASUO HAGIWARA
~2TEMPERATURE
of the g r e a t e r r a t e of heat e x t r a c t i o n f r o m the bulk liquid (position E 2 o n Fig. 2). This in t u r n will s t i l l f u r t h e r d e c r e a s e the growth r a t e of the c o l u m n a r i n t e r f a c e and i n c r e a s e the t e m p e r a t u r e g r a d i e n t and hence d e c r e a s e the t e m p e r a t u r e of the c o l u m n a r f r o n t (position C4, Fig. 2). The r a t e s of growth and t e m p e r a t u r e s of the c o l u m n a r and equiaxed c r y s t a l s will t h e r e fore follow the a r r o w s shown on Fig. 2. The growth r a t e of the equiaxed c r y s t a l s r a p i d l y i n c r e a s e s and the growth r a t e of the c o l u m n a r i n t e r f a c e d e c r e a s e s . E v e n t u a l l y a condition is r e a c h e d where the c o l u m n a r f r o n t has a l m o s t stopped but w h e r e the equiaxed c r y s t a l s a r e growing r a p i d l y . This p h e n o m e n a leads to the v e r y abrupt change in s t r u c t u r e o b s e r v e d in m a n y c a s t i n g s and ingots. It is much m o r e difficult to e n v i s a g e such an a b r u p t change if the effect of g r a d i e n t ahead of the c o l u m n a r f r o n t is n e g l e c t e d .
A C s C l - t y p e o r d e r e d s t r u c t u r e in an e q u i a t o m i c VMn alloy has b e e n o b s e r v e d b y W a t e r s t r a t I and Darby, J r . , 2 on the b a s i s of the X - r a y d i f f r a c t i o n u s i n g the CrK~ r a d i a t i o n whose f r e q u e n c y is close to the a b s o r p t i o n edge of one of the e l e m e n t s . This s t r u c t u r e has also b e e n c o n f i r m e d by Doroshenko e t a l 3 and Kuchin e t a l 4 u s i n g the n e u t r o n diffraction technique. The p u r p o s e of the p r e s e n t study is to d e t e r m i n e the C s C l - t y p e o r d e r - d i s o r d e r t r a n s i t i o n t e m p e r a t u r e in VMn alloys c o n t a i n i n g 30 ~ 55 at. pct Mn by m e a n s of the d i f f e r e n t i a l t h e r m a l a n a l y s i s and specific heat m e a s u r e m e n t . The alloys w e r e p r e p a r e d by a r c - m e l t i n g v a n a d i u m flakes 99.8 pet in p u r i t y and e l e c t r o l y t i c m a n g a n e s e 99.9 pet in p u r i t y u n d e r an a r g o n a t m o s p h e r e . The c o m p o s i t i o n of each alloy was c a l c u l a t e d b y a s s u m i n g that the total weight l o s s d u r i n g the m e l t i n g was due to the v a p o r i z a t i o n of m a n g a n e s e . T h e s e c a l c u l a t e d v a l u e s a g r e e d r e a s o n a b l y well with the r e s u l t s of c h e m i c a l a n a l y s e s which w e r e done on s o m e of the a l l o y s . All the Ingots were wrapped with m o l y b d e n u m foil and e n c a p s u l a t e d in a q u a r t z tube in a v a c u u m and then h o m o g e n i z e d at 1300~ for 2 h. S p e c i m e n s for the DTA and specific heat m e a s u r e m e n t w e
Data Loading...
