Thermodynamic Properties of Liquid Manganese-Silicon Alloys NAZIR

AHMAD

AND

JOHN

N. PRATT

Vapor p r e s s u r e m e a s u r e m e n t s have b e e n made on twenty two m a n g a n e s e - s i l i c o n alloys in the liquid state, at t e m p e r a t u r e s b e t w e e n 1400 to 1900 K, u s i n g a t o r s i o n - e f f u s i o n technique. The t h e r m o d y n a m i c p r o p e r t i e s of the s y s t e m have been calculated f r o m the o b s e r v e d v a p o r p r e s s u r e s of m a n g a n e s e over the alloys at 1700 K. The a c t i v i t i e s of the components show v e r y s t r o n g negative d e v i a t i o n s f r o m ideality and the heats of f o r m a tion a r e m a r k e d l y e x o t h e r m i c . E x c e s s free e n e r g i e s e v a l u a t e d f r o m the vapor p r e s s u r e s have been combined with the c a l o r i m e t r i c a l l y m e a s u r e d heats of f o r m a t i o n , a v a i l able in the l i t e r a t u r e , to obtain the e x c e s s e n t r o p i e s of m i x i n g which a r e found to be m o d e r a t e l y n e g a t i v e . The p r e s e n t r e s u l t s a r e a s s e s s e d with r e s p e c t to the e x i s t i n g phase e q u i l i b r i a and other t h e r m o d y n a m i c data for the solid and liquid s t a t e s . A m o n g the fact o r s i n f l u e n c i n g the p r o p e r t i e s of these a l l o y s , a tendency to f o r m covalent linkages in the liquid state a p p e a r s m o s t s i g n i f i c a n t .

THIS

study of liquid m a n g a n e s e - s i l i c o n alloys is a p a r t of the continuing p r o g r a m of r e s e a r c h at the a u t h o r s ' l a b o r a t o r y a i m e d at c o r r e l a t i n g 1 the t h e r m o d y n a m i c p r o p e r t i e s with component c h a r a c t e r i s t i c s such as size, valency, e l e c t r o n e g a t i v i t y , m a g n e t i c i n t e r a c t i o n s and c h e m i c a l bond effects. P r e v i o u s work on b i n a r y liquid alloys of m a n g a n e s e with copper, 2 gold, 3 and t i n 4 has shown that the f a c t o r s r e s p o n s i b l e for solid phase e q u i l i b r i a may p e r s i s t in the liquid state and t h e r e f o r e influence its t h e r m o d y n a m i c p r o p e r t i e s . It has b e e n suggested that such p r o p e r t i e s of liquid m a n g a n e s e - c o p p e r alloys owe t h e i r b e h a v i o r m a i n l y to the m a g n e t i c i n t e r a c t i o n s while the e l e c t r o negativity factor has b e e n s e e n to be m o r e s i g n i f i c a n t in m a n g a n e s e - g o l d a l l o y s . The t h e r m o d y n a m i c p r o p e r t i e s of m a n g a n e s e - t i n liquids have b e e n a t t r i b u t e d to the d i f f e r e n c e s of s i z e , valency, and p o s s i b l y some covalent c l u s t e r i n g of tin. The p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to e x a m i n e the effect of alloying m a n ganese with a m o r e s t r o n g l y covalent e l e m e n t and to provide data for an i m p r o v e d a s s e s s m e n t of the t h e r m o d y n a m i c p r o p e r t i e s of the s y s t e m . As with above e x a m p l e s , the r e l a t i v e v o l a t i l i t y of m a n g a n e s e m a k e s the m a n g a n e s e - s i l i c o n a l l o y s a m m e n a b l e to study by m e a n s of a vapor p r e s s u r e method and the t o r s i o n - e f f u s i o n technique has again b e e n employed. EXPERIMENTAL

DETAILS

The p r i n c i p l e s of the t o r s i o n - e f f u s i o n technique and its use in vapor p r e s s u r e m e a s u r e m e n t s of m e t a l s have b e e n r e v i e w e d by C a r t e r .5 The s p e c i m e n is contained i n an effusion cell s u s p e n d e d f r o m a fine wire, within a v e r t i c a l v a c u u m c h a m b e r . The cell is provided with two o r i f i c e s , so disposed, in its f r o n t and NAZ1R AHMAD, formerly Research Student/Research Fellow, Department of Physical Metallurgyand Science and Materials, University of Birmingham,is now Research Associate, Department of Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, MA 02139 and JOHN N. PRATT is Reader in Metallurgical Thermochemistry, Department of Physical Metallurgy and Science of Materials, University of Birmingham,Birmingham,England. Manuscript submitted February 17, 1978. METALLURGICALTRANSACTIONSA

r e a r v e r t i c a l faces, that the effusing vapor cause a r o t a t i o n of the cell about the axis of s u s p e n s i o n . In free s u s p e n s i o n , the e q u i l i b r i u m deflection o c c u r s when the t o r q u e due to the effusing v a p o r is b a l a n c e d by that in the s u s p e n s i o n , so that the v a p o r p r e s s u r e is given by: p : 27ot/(alq~fl + a2q2f2) when p is the p r e s s u r e , z the t o r s i o n constant of the s u s p e n s i o n , ~ the deflection, al, a2, ql and q2, r e s p e c tively, the o r i f i c e a r e a s and the d i s t a n c e s f r o m the axis of rotation, and f l and fz the F r e e m a n and S e a r c y c o r r e c t i o n f a c t o r s for orifice g e o m e t r y and m o l e c u lar distribution. The high t e m p e r a t u r e t o r s i o n - e f f u s i o n a p p a r a t u s used in the p r e s e n t i n v e s t i g a t i o n s i s e s s e n t i a l l y the s a m e as d e s c r i b e d p r e v i o u s l y , ~ but a n u m b e r of m o d i f i c a t i o n s have b e e n made to i m p r o v e its o p e r a t i o n . A r e v i s e d d i a g r a m of the p r e s e n t a p p a r a t u s is shown in F i g . 1. The w a t e r - c o o l e d v a c u u m envelope, evacuated through a side a r m of the lower (furnace) c h a m b e r is unchanged. The o r i g i n a l s l i t - t u b e heating e l e m e n t has now b e e n r e p l a c e d by a u n i f o r m t a n t a l u m tube (C), 15.3 cm long, 3.8 cm d i a m and 0.045 cm wall t h i c k n e s s with t a n t a l u m leads connected to e i t h e r end. T h i s p r o v i d e s a m o r e u n i f o r m c u r r e n t flow through the heating e l e m e n t and r e s u l t s in a longer zone of unif o r m t e m p e r a t u r e . P o w e r to the h e a t e r is now c a r r i e d through the v a c u u m c h a m b e r b a s e plate (E) by m e a n s of two w a t e r - c o o l e d (Edwards Type 9A High C u r r e n t ) e l e c t r o d e s (F), with consequent i m p r o v e m e n t of v a c u u m conditions. As b e f o r e , a 500 amp, 8 V t r a n s f o r m e r s u p p l i e s power, but the input to this is now c o n t r o l l e d by a S t a n t o n - R e d c r o f t (Model L V P - C ) t e m p e r a t u r e c o n t r o l l e r and p r o g r a m m e r . The feedback to the c o n t r o l l e r is f r o m a P t / 1 3 pct RhPt t h e r mocouple (S) which has its hot j u n c t i o n contacting, but e l e c t r i c a l l y i n s u l a t e d from, the i n s i d e wall of the heating e l e m e n t . P r o g r a m m e d heating and cooling r a t e s (0 to 10~ and constant t e m p e r a t u r e cont r o l (+0.5~ a r e obtainable b e t w e e n 800 to 1600~ and continuous m e a s u r e m e n t s of vapor p r e s s u r e s d u r i n g heating and cooling a r e now a c h i e v a b l e . Some m o d i f i c a t i o n s have now b e e n made to the r a d i a t i o n

ISSN 0360-2133/78/1211-1857500.75/0 9 1978AMERICANSOCIETYFOR METALSAND THE METALLURGICALSOCIETYOF AIME

VOLUME9A, DECEMBER 1978-1857

s h i e l d i n g (H and L) which now c o n s i s t s of two i n n e r s h i e l d s of t a n t a l u m s u r r o u n d e d by t h r e e of molybdenum; a l u m i n a s p a c e r s a r e used to e l i m i n a t e contact between v e r t i c a l s h i e l d s and the a s s e m b l y is made s t a b l e by e n c l o s i n g in an a l u m i n a tube (I). A s e r i e s of c i r c u l a r s h i e l d s (J) have b e e n added b e n e a t h the heating e l e m e n t . The d e s i g n of the effusion cell s u s p e n s i o n s y s t e m (Q) has b e e n s i m p l i f i e d by the e l i m i n a tion of the m a g n e t i c c o n t r o l s y s t e m and cell m e a s u r e m e n t s a r e now made by d e t e r m i n i n g the free r o tation of the cell f r o m o b s e r v a t i o n s of d e f l e c t i o n s of

0

0

E X P E R I M E N T A L RESULTS

.•

The equations for vapor p r e s s u r e s of m a n g a n e s e o v e r the a l l o y s w e r e calculated on the a s s u m p t i o n that they a p p r o x i m a t e to C l a u s i u s - C l a p e y r o n b e h a v i o r over the t e m p e r a t u r e r a n g e of i n t e r e s t . L i n e a r equations of the f o r m :

R

M-

-A

L

.C

5

U

F

E ~I~OXI~AT[ SCAt[

i1

12

13 d~ C.ms

Fig. 1--The torsion-effusion apparatus. 1858-VOLUME 9A, DECEMBER 1978

the g a l v a n o m e t e r m i x e r (X). T u n g s t e n t o r s i o n wire of 0.005 cm d i a m is now used in a l l e x p e r i m e n t s , s u s p e n s i o n lengths b e i n g v a r i e d between 16.0 to 27.0 cm a c c o r d i n g to the magnitude of the v a p o r p r e s s u r e involved and the s e n s i t i v i t y r e q u i r e d . The effusion cells, as b e f o r e , have b e e n m a c h i n e d f r o m b o r o n n i t r i d e . D i f f i c u l t i e s were e n c o u n t e r e d due to the ins t a b i l i t y at high t e m p e r a t u r e s of some g r a d e s of this m a t e r i a l , but t r i a l s have shown that Union Carbide grade HD 0092 is s a t i s f a c t o r y and this is t h e r e f o r e n o r m a l l y u s e d . D u r i n g e x p e r i m e n t a l r u n s the n o m i n a l cell t e m p e r a t u r e is indicated by m e a n s of the i n s i t u 20 pct R h - P t / 5 pct R h - P t t h e r m o c o u p l e (U) located just b e n e a t h the effusion cell. T h i s is c a l i b r a t e d to give t r u e cell t e m p e r a t u r e s by making vapor p r e s s u r e m e a s u r e m e n t r u n s with the pure m e t a l s s i l v e r and m a n g a n e s e , for which the vapor p r e s s u r e s a r e a c c u r a t e l y e s t a b l i s h e d , 7 over the t e m p e r a t u r e range of i n t e r e s t . The n e c e s s a r y t e m p e r a t u r e c o r r e c t i o n s a r e found to be r e p r o d u c i b l e for given heating e l e m e n t and shield a s s e m b l i e s . Alloys were p r e p a r e d f r o m s p e c t r o s c o p i c a l l y p u r e c o m p o n e n t s . A p p r o x i m a t e l y 1.0 g s a m p l e s , c a r e f u l l y weighed to 0.00002 g, were m e l t e d u n d e r a r g o n by R. F . h e a t i n g in b o r o n n i t r i d e c r u c i b l e s s u r r o u n d e d by a graphite s u s c e p t o r ; the r e s u l t i n g ingots were used without f u r t h e r t r e a t m e n t for the liquid alloy v a p o r p r e s s u r e s t u d i e s . Some alloys, p a r t i c u l a r l y those in the s i l i c o n - r i c h r a n g e , were p r e p a r e d dir e c t l y in the effusion cell i m m e d i a t e l y p r i o r to the vapor pressure measurements.

log p = - A / T + B were computed by least s q u a r e a n a l y s i s of the raw data and a r e p r e s e n t e d in T a b l e I. A s s o c i a t e d u n c e r t a i n t i e s in A and B were calculated f r o m the s t a n d a r d d e v i a t i o n s , s, of the p a r a m e t e r s a c c o r d i n g to the f o r m u l a : + s . t ~ where t~ is a p r o b a b i l i t y f a c t o r for a s s u m i n g n o r m a l d i s t r i b u t i o n of the data and 95 pct confidence limits.S The u n c e r t a i n t i e s in log p v a l u e s due to e x t r a p o l a t i o n to 1700 K were e s t i m a t e d u s i n g the f o r m u l a :

1

(xK _ ~)2 ]1/2

where x is 1 / T , T b e i n g the m i d - t e m p e r a t u r e within the r a n g e of m e a s u r e m e n t , xK is 1/1700 K and n is the n u m b e r of o b s e r v a t i o n s . F o r the a l l o y s , the a c t i v i t i e s and p a r t i a l t h e r m o d y n a m i c p r o p e r t i e s of m a n g a n e s e at 1700 K (Table II) have b e e n c a l c u l a t e d d i r e c t l y f r o m the above v a p o r p r e s s u r e equations and the a s s e s s e d data for p u r e m a n g a n e s e . 7 C o r r e s p o n d i n g v a l u e s for s i l i c o n were e v a l u a t e d u s i n g the G i b b s - D u h e m r e l a t i o n , while i n t e g r a l v a l u e s were obtained by the u s u a l s u m m a t i o n of the p a r t i a l s . A l l p r o p e r t i e s r e f e r to the pure liquid f o r m s of the components as the s t a n d a r d s t a t e s . METALLURGICALTRANSACTIONSA

Table 1. The Vapor Pressuresof Mn Over Liquid Mn-Si Alloys

logp (atm)* = - A / T + B

Nmn 0.100 0.167 0.290 0.303 0.386 0.395 0.438 0.481 0.530 0.567 0.590 0.650 0.662 0.700 0.725 0.750

B

• &logp, 1700 K

4.380• 0.269 4,509• 0.106 4,733• 0.250

0.0297 0.0141 0.0196

4.736 • 0.110

0.0118

4.749• 4,946• 4.921 • 5,045• 5,589 • 5,688• 6,198• 5,897• 6,436 • 6,014• 4,950• 4,989•

0.0283 0.0405 0.0370 0.0206 0.0520 0.0235 0.0313 0.0437 0.0283 0.0554 0.0541 0.0348 0.0296 0.0192 0.0642 0.0581 0.0301 0.0087 0.0137

A

16044• 15685• 15393• 15320• 14817• 15079• 14694• 14621• 15222• 15115• 15737• 14590• 15458• 14500• 12470• 12428•

496 191 417 183 432 722 692 487 1097 640 781 785 1041 692 495 350

0.750

12477 • 226

0.773 0.830 0.867 0.868 0.895 1.000

12337• 12267• 12576• 12469• 12718• 12673•

142 555 215 142 185

0,261 0.427 0.411 0.293 0.655 0.383 0.459 0.461 0.619 0.412 0.329 0.232 5,032 • 0,144 4,982• 0.092 5,069• 0.293 5.334• 0.284 5,274• 0.139 5,448• 0.089 5,490• 0.t15

Temp.Range,K i9lM1886 1737 1591 1576 t590 1760 1721 17441734 1756 1800 1696 18001608 1593 1440 1402 1638 1680 1403 1519 I527 -

1775 1693 1592 1802 1664 1774 1590 1593 1590 1608 1648 1615 1637 1573 1403 1425 1720

1641 1403 1402 1708 1644 1693

*1 atm = 101325 Pa.

T h e p r e c i s i o n of t h e e v a l u a t e d p r o p e r t i e s w a s e s t i m a t e d f r o m t h e u n c e r t a i n t i e s r e c o r d e d i n T a b l e I. Considering an averaged error value for the equia t o m i c c o m p o s i t i o n and the s c a t t e r in the m e a s u r e m e n t s o n p u r e m a n g a n e s e , t h e u s u a l m e t h o d of e r r o r propagation s was applied. Assuming a possible error of +5 K i n t e m p e r a t u r e a n d + 0 . 0 2 NMn i n c o m p o s i t i o n , t h e e r r o r l i m i t s f o r t h e p r o p e r t i e s of t h e e q u i a t o m i c alloy are : AGMn = -- 4 9 , 3 2 1 + 1452 J / g - a t o m . AG = -

3 7 , 3 6 7 + 1377 J / g - a t o m ,

A/-/ = - 2 4 , 6 9 0 • 3 0 0 8 J / g - a t o m , AS = 7 . 4 6 0 • 1.452 J / d e g .

g-atom.

The vapor pressure observations made during both h e a t i n g and cooling a r e p r e s e n t e d d i a g r a m a t i c a l l y in F i g . 2. T h e b r o k e n l i n e s i n d i c a t e v a l u e s o b t a i n e d d u r i n g h e a t i n g w h e r e a s t h e s o l i d l i n e s c o r r e s p o n d to c o o l i n g e x p e r i m e n t s . W h e r e b o t h t y p e s of o b s e r v a tions are made during a single run, the former are t r e a t e d a s c h a r a c t e r i s t i c of t h e s t a r t i n g a l l o y c o r n -

p o s i t i o n a n d t h e l a t t e r a s r e p r e s e n t a t i v e of t h e f i n a l composition; this was estimated from the weight loss of m a n g a n e s e d u r i n g t h e e x p e r i m e n t . T h e v a l i d i t y of t h i s t r e a t m e n t i s s u p p o r t e d b y t h e c o n s i s t e n c y of t h e r e s u l t i n g a c t i v i t y d a t a p l o t t e d i n F i g . 3, w h e r e t h e identically flagged points are results from the same experimental run. Anomalous evaporation, resulting in a delayed est a b l i s h m e n t of e q u i l i b r i u m p r e s s u r e s , w a s a l w a y s obs e r v e d d u r i n g i n i t i a l h e a t i n g of l i q u i d a l l o y s c o n t a i n i n g a p p r o x i m a t e l y 55 t o 65 a t . p c t m a n g a n e s e , i . e . i n t h e v i c i n i t y of t h e c o m p o u n d MnsSi3 ( s e e F i g . 5). D e p e n d i n g o n t h e e x a c t m a n g a n e s e c o n t e n t (<>MnsSi3) of t h e a l l o y , c o n t i n u o u s l y d e c r e a s i n g o r i n c r e a s i n g v a p o r p r e s s u r e s w e r e n o t e d on h o l d i n g at c o n s t a n t temperature for short periods. Systematic cycling experiments indicated that this behavior is associat e d w i t h t h e s l u g g i s h d i s s o l u t i o n of MnsSi3 a n d s l o w d i s p e r s i o n of i t s c o m p o n e n t s t h r o u g h o u t t h e l i q u i d p h a s e . F o r a l l o y s o n t h e m a n g a n e s e - r i c h s i d e of t h e c o m p o u n d , t h e a p p r o a c h to h o m o g e n e i t y a f t e r m e l t i n g w i l l r e s u l t in a g r a d u a l m a n g a n e s e i m p o v e r i s h m e n t of t h e i n i t i a l l i q u i d s a n d h e n c e to f a l l i n g v a p o r p r e s s u r e s ; t h e s i m i l a r h o m o g e n i z a t i o n of t h e s i l i c o n - r i c h a l l o y s , on t h e o t h e r h a n d , w i l l b e a c c o m p a n i e d b y a gradual manganese-enrichment of t h e l i q u i d p h a s e a n d so by increasing vapor pressures. B e c a u s e of t h i s behavior, only results obtained during cooling runs h a v e b e e n u s e d in d e t e r m i n i n g the v a p o r p r e s s u r e of a l l o y s in t h i s c o m p o s i t i o n r a n g e .

DISCUSSION Activities of manganese and silicon at 1700 K plotted in Fig. 3 show very strong negative deviations from ideality, but these observations are in reasonable agreement with liquid alloy activity data at 1673 K recently obtained by Batalin and Sudavtsova~ in molten chloride electrolyte galvanic cell studies (1520 to 1700 K). Somewhat smaller negative deviations from ideal behavior have been reported by Petrushevskii, Kocherov, Geld, Zamyatin and Suchilnikeyz~ following Knudsen vapor pressure measurements at 1623 K. However, coincident with the completion of the present work,z~ Gee and Rosenqvist 12 reported a further vapor pressure study of the same system, using a transport method over various temperature ranges between 1517 and 1977 K. Computation of manganese activities at 1673 K from their vapor pressure equations and from those of Table I show good

Table II. Thermodynamic Properties of Liquid Manganese-Silicon Alloys at 1700 K (Reference States: Mn(I) and Si(I))

(kJ/g-atom) NMn 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

aMn 0.0008 0.0023 0.0049 0.0117 0.0305 0.0929 0.2852 0.5829 0.8645

(J/deg 9 g-atom)

aSi

AGMn

AGsi

ASMn

0.8934 0.752 0.5830 0.3613 0.1656 0.0408 0.0052 0.0006 6.7 X 10"s

-100.776 -85.851 -75A66 -62.865 -49.321 -33.581 -17.732 -7.627 -2.059

-1.594 -4.029 -7.627 -14.389 -25.414 -45.212 -74.325 -104.847 -I35.846

21.338 17.573 15.062 11.715 5.021 -8.368 -9.205 9.623 2.092

METALLURGICAL TRANSACTIONS A

ASsi 0.795 1.456 2.301 4.176 9.895 25,568 26.953 -31.698 14.3t3

(kJ/g-atom) A/tMn

A/~Si

-64.501 -55.978 -49.560 -42.949 -40.786 -47.806 -33.380 +8.732 +1.498

-0.234 -1.552 -3.715 -7.289 -8.594 -1.745 -28.506 -158.733 -111.512

AG,

AS,

AH,

k J / g - a t o m J/deg 9 g-atom

kJ/g-atom

-11.5t4 -20.393 -27.891 -33.777 -37.367 -38.233 -34.710 -27.070 -15.439

-6.669 -12.439 -17.468 -21.552 -24.690 -29.380 -31,920 -24.761 -9.803

2.849 4.678 6.130 7.192 7.460 5.205 1.644 1.360 3.314

VOLUME 9A, DECEMBER 1978-1859

o

3-0

"x, '~"x,, \

9 o m

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9

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Fig. 2--Vapor pressures of manganese over liquid manganese-silicon alloys, 1 atm = 1.01325 • l0 s Pa.

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11T~10 4 (OK) a g r e e m e n t b e t w e e n t h e s e two m o s t r e c e n t i n v e s t i g a tions o v e r m o s t of the c o m p o s i t i o n r a n g e , but d e v i a t e f r o m e a c h o t h e r at the h i g h e s t m a n g a n e s e c o n t e n t s . A s i s d e m o n s t r a t e d below, h o w e v e r , in t h i s r a n g e the r e s u l t s of the p r e s e n t study a p p e a r m o s t c o n s i s t e n t with v a l u e s i n d i c a t e d by the phase d i a g r a m . A c o m p a r i s o n of the v a r i o u s liquid s t a t e s t u d i e s i s shown in F i g . 4. The v a l i d i t y of the p r e s e n t r e s u l t s and the g e n e r a l c o n s i s t e n c y of the t h e r m o d y n a m i c d a t a f o r the s y s t e m is a l s o c o n f i r m e d by c o m p a r i s o n with v a l u e s c o m puted f r o m the p h a s e d i a g r a m and f r o m t h e r m o d y n a m i c s t u d i e s of s o l i d a l l o y s . The p h a s e d i a g r a m shown in F i g . 5 is l a r g e l y a s c o m p i l e d by Shunk 13 but modified, in the s i l i c o n - r i c h r e g i o n s , in a c c o r d a n c e with the r e i n v e s t i g a t i o n s by M a g e r and W a c h t e l . 14 T h e r e have b e e n two s i g n i f i c a n t s t u d i e s of f r e e e n e r g i e s of f o r m a tion in the s o l i d s t a t e . U s i n g a Hz/HC1 e q u i l i b r a t i o n technique, R o s s e m y r and R o s e n q v i s t 15 have d e t e r m i n e d f r e e e n e r g i e s of f o r m a t i o n of MnSi2, MnSi and MnsSi3 1860-VOLUME9A, DECEMBER 1978

at 1363 K, while a m o r e e x t e n s i v e study of the t h e r m o d y n a m i c p r o p e r t i e s of s o l i d a l l o y s (0 to 100 pct Si, 950 to 1100 K) h a s b e e n m a d e by E r e m e n k o , L u k a s h e n k o mad S i d o r k o TM by m e a n s of m o l t e n c h l o r i d e e l e c t r o l y t e g a l v a n i c c e l l m e a s u r e m e n t s . A c t i v i t i e s of m a n g a n e s e along the liquidus, obtained f r o m liquid and s o l i d s t a t e s t u d i e s and r e f e r r e d to a u n i f o r m liquid m a n g a n e s e s t a n d a r d , a r e c o m p a r e d with e a c h o t h e r in F i g . 6. A l s o included a r e m a n g a n e s e r i c h data, d e r i v e d f r o m the p h a s e d i a g r a m a s s u m i n g R a o u l t i a n b e h a v i o r in the /3-Mn s o l i d s o l u t i o n s , and s i l i c o n a c t i v i t i e s at 1700 K c o m p u t e d f r o m the f o r m of the liquidus r e l a t ing to p u r e s o l i d s i l i c o n . The g e n e r a l l y good a g r e e ment of the v a r i o u s d a t a d e m o n s t r a t e s the m u t u a l cons i s t e n c y of the p h a s e d i a g r a m and of the t h e r m o d y n a m i c p r o p e r t i e s of the s o l i d and liquid a l l o y s . The a n o m a l o u s c o m p o s i t i o n d e p e n d e n c e of the c o m p u t e d points (open c i r c l e s ) in the r e g i o n NMn = 0.6 to 0.7 is thought to be due to the s e n s i t i v i t y of t h e s e v a l u e s to the t r u e m a n g a n e s e - r i c h s o l i d u s of the e s s e n t i a l l y METALLURGICAL TRANSACTIONS A

10

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-6

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.4 .3 .2

.2 .1 o o

0 0

1

.2

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.5 .6 .7 .8 .9 1,0 NNn Fig. 3--Activities of manganese and silicon in liquid manganese-silicon alloys at 1700 K.

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NM n Fig. 4--Comparison of experimental data for manganese activity c o e f f i c i e n t s in liquid m a n g a n e s e - s i l i c o n alloys: P e t r u s h e v s k i i e t al (1623 K), 9 B a t a l i n and S u d a v t s o v a (1673 K), ~ Gee and R o s e n q v i s t (1673 K), and -- P r e s e n t w o r k (1673 K). o

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METALLURGICAL TRANSACTIONS A

IO

20

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40

ATOMIC

50

60

PER C E N T

SILICON

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VOLUME9A, DECEMBER i978-1861

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0 -2 4 .6 .8 10 si Nlqn Nn Fig. 6--Comparison of liquid, solid and computed activity data for manganese-silicon alloys. 9 Eremenko e t a l , • Rossemyr and Rosenqvist, 9 Batalin and Sudavtsova, u Gee and Rosenqvist, [] Calculated from phase diagram, and - Present work. s t o i c h i o m e t r i c p h a s e , MnsSi3, in e q u i l i b r i u m with the liquidus in this r a n g e . I n t e g r a l h e a t s and e n t r o p i e s of f o r m a t i o n of the liquid a l l o y s y i e l d e d by the p r e s e n t v a p o r p r e s s u r e s t u d i e s alone a r e given in T a b l e II. The h e a t s a r e c o n s i d e r a b l y l e s s e x o t h e r m i c than the e q u i v a l e n t q u a n t i t i e s s u g g e s t e d by the e m f s t u d i e s by B a l a t i n and Sudavstova, 9 but the p r e s e n t v a l u e s a r e s i m i l a r in m a g n i t u d e to t h o s e obtained, by G e r t m a n and Geld, t~ b y d i r e c t c a l o r i m e t r y at 1743 K and r e a s s e s s e d by Chart; ts t h e s e c a l o r i m e t r i c h e a t s of f o r m a t i o n a r e , h o w e v e r , s i g n i f i c a n t l y l e s s a s y m m e t r i c with r e s p e c t to c o m p o s i t i o n . Since the c a t o r i m e t r i c a l l y m e a s u r e d h e a t s of f o r m a t i o n should be m o r e a c c u r a t e than t h o s e obtained f r o m the v a p o r p r e s s u r e t e m p e r a t u r e coeff i c i e n t s , the e n t r o p i e s of f o r m a t i o n have finally been c a l c u l a t e d by c o m b i n i n g the c a l o r i m e t r i c h e a t s with the p r e s e n t i n t e g r a l f r e e e n e r g i e s at 1700 K; the v a r i a t i o n of &H b e t w e e n 1700 and 1743 K is cons i d e r e d to be n e g l i g i b l e . The r e s u l t i n g v a l u e s of the t h r e e i n t e g r a l e x c e s s p r o p e r t i e s of the s y s t e m a r e p l o t t e d in F i g . 7. T h e s e show that the liquid m a n g a n e s e - s i l i c o n a l l o y s a r e c h a r a c t e r i z e d by v e r y m a r k e d e x o t h e r m i c b e h a v i o r ; of the liquid m a n g a n e s e a l l o y s s o f a r i n v e s t i g a t e d only the m a n g a n e s e - g o l d s y s t e m , 3 with its e x c e p t i o n a l l y l a r g e e l e c t r o c h e m i c a l f a c t o r , h a s shown m o r e n e g a t i v e h e a t s . It is obvious that s i m i l a r h e t e r o p o l a r i t y of bonding w i l l not e x i s t b e t w e e n m a n g a n e s e and s i l i c o n , s i n c e the e l e c tronegativity difference between these elements is much s m a t t e r a a d indeed i s c l o s e l y c o m p a r a b l e with t h o s e e x i s t i n g in the b i n a r i e s of m a n g a n e s e with c o p p e r , n i c k e l and tin. C o n s i d e r a t i o n of the t y p e s of i n t e r m e d i a t e p h a s e s o c c u r r i n g in the s o l i d s t a t e , h o w e v e r , shows that i n t e r c o m p o n e n t covalent bonding is a p r e d o m i n a n t f e a t u r e of the m a n g a n e s e - s i l i c o n 1862-VOLUME 9A, DECEMBER 1978

s y s t e m , p a r t i c u l a r l y in the c e n t r a l c o m p o s i t i o n r e g i o n s . The l a r g e n e g a t i v e h e a t s of f o r m a t i o n of the liquids a r e thus m o s t p r o b a b l y a t t r i b u t a b l e to the e x i s t e n c e of s i m i l a r covalent i n t e r a c t i o n s in the m o l t e n a l l o y s . The magnitude of the h e a t s f u r t h e r s u g g e s t s that a t e n d e n c y to f o r m c o v a l e n t l y - l i n k e d c l u s t e r s m a y w e l l o c c u r and that this, through a r e duction of c o n f i g u r a t i o n a l e n t r o p y , m a y be r e s p o n s i b l e f o r a s i g n i f i c a n t p a r t of the o b s e r v e d n e g a t i v e e x c e s s e n t r o p i e s . H o w e v e r , the m a j o r p a r t of the l a t t e r m u s t be a t t r i b u t e d to t h e r m a l s o u r c e s , s i n c e the covalent bonding p r e s e n t o v e r much of the s y s t e m is a c c o m p a n i e d by n e g a t i v e d e v i a t i o n s f r o m N e u m a n n - K o p p b e h a v i o r in the s o l i d s 19'2~ and p r o b a b l y a t s o in the liquid s t a t e . It w i l l be o b s e r v e d that the e x c e s s e n t r o p i e s of the liquids a r e l e s s n e g a t i v e in the m a n g a n e s e - r i c h r e g i o n s and this is thought to be a t t r i b u t a b l e to the m o r e m e t a l l i c bonding l i k e l y to e x i s t at t h e s e c o m p o s i t i o n s . T h e s u g g e s t e d v a r i a t i o n of bond c h a r a c t e r a c r o s s the liquids is s u p p o r t e d by r e p o r t e d s t u d i e s of the s o l u t i o n of h y d r o g e n in m a n g a n e s e s i l i c o n m e l t s . 21 S o l u b i l i t i e s a r e l e a s t in c o n c e n t r a t e d a l l o y s (30 to 60 a t . pct m a n g a n e s e ) and r e l a t i v e l y g r e a t e r at the m a n g a n e s e r i c h c o m p o s i t i o n s ; the h e a t s of solution in t h e s e two r e g i o n s a r e r e s p e c t i v e l y e n d o - a n d e x o t h e r m i c . T h i s s u g g e s t s that the m a n g a n e s e - r i c h a l l o y s , b e i n g m o r e m e t a l l i c allow the h y d r o g e n r e a d i l y to f o r m m e t a l l i c l i n k a g e s on solution, while in the c o n c e n t r a t e d a l l o y s the p r e f e r ence for c o v a l e n t i n t e r a c t i o n b e t w e e n m a n g a n e s e and s i l i c o n r e d u c e s the f a c i l i t y f o r l i n k a g e s with h y d r o g e n and hence inhibits its s o l u b i l i t y . P e r s i s t e n c e of m i c r o i n h o m o g e n e i t i e s in at l e a s t s o m e of the liquids is a l s o i n d i c a t e d by the o b s e r v a t i o n of a b n o r m a l l y low h e a t s of fusion f o r MnsSi3.19 T h e s e have b e e n i n t e r p r e t e d a s due to the e n h a n c e m e n t of M n - S i covalent bonding on m e l t i n g so that q u a s i - m o l e c u l a r g r o u p s of MnSi type a r e f o r m e d l e a v i n g o t h e r p a r t s of m i c r o i n h o m o g e n e o u s r e g i o n s r i c h e r in m a n g a n e s e . T h i s s u g g e s t i o n is c o n s i s t e n t with the f o r m s of a n o m a l o u s v a p o r p r e s s u r e b e h a v i o r o b s e r v e d in the p r e s e n t w o r k d u r i n g the i n i t i a l heating of a l l o y s in the v i c i n i t y of the compound MnsSi3. That a t e n d e n c y to f o r m m o l e c u NMn 0 0

92

.4

.6

>/oo -8

1.0

E

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S &

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I

20

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40 Fig. 7 - - E x c e s s integral thermodynamic p r o p e r t i e s of liquid m a n g a n e s e - s i l i c o n alloys. METALLURGICAL TRANSACTIONS A

ganese

and silicon atoms

in these

particular

regions.

350 Z

E

Z

A CKNOWLEDGMENTS

~

300

Financial support for the research program has been provided by the United States Government t h r o u g h t h e E u r o p e a n R e s e a r c h O f f i c e of t h e U n i t e d States Army. The authors wish to acknowledge cons t r u c t i v e c o m m e n t b y D r . J . W . J o h n s o n of A r m y M a terials and Mechanics Research Center, Watertown, Massachusetts, and Professor John F. Elliott of Mass a c h u s e t t s I n s t i t u t e of T e c h n o l o g y .

Mn~i

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I

i

1

i

t

REFERENCES

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I

> "r<~ 3 0 O Mn-Ni

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200

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q

1

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Fig. 8--Heats of vaporization of manganese from manganesesilicon and manganese-nickel liquid alloys. iar associations is not merely transient but is probably characteristic of the steady equilibrium state of some liquid manganese-silicon alloys is most clearly demonstrated b y c o m p a r i n g t h e h e a t s of v a p o r i z a t i o n of manganese from these alloys with those from manganese-nickel a l l o y s i n t h e l i q u i d s t a t e . ~ l V a l u e s of these heats for the two systems, calculated directly from the equilibrium vapor pressure equations, are c o m p a r e d i n F i g . 8. A s w o u l d b e e x p e c t e d , s i m i l a r values are observed for the manganese-rich regions of both systems, but in contrast to the essentially constant or monotonic variation across the manganesenickel alloys, the intermediate manganese-silicon compositions exhibit significantly increased heats of vaporization. This thus provides very direct evidence of the existence of enhanced interactions between man-

METALLURGICAL TRANSACTIONS A

1. J. N. Pratt: Rev. Int. Hautes Temper. etRefract., 1957, vol. 4, p. 97. 2. P. J. Spencer and ]. N. Pratt: Tran~ Faraday Soc., 1968, vol. 64, p. 1470. 3. P. J. Spencer and J. N. Pratt: Rev. Int. Hautes Tempe~ etRefract., 1968, vol. 5, p. 155. 4. P. J. Spencer and J. N. Pratt: Tran~ TMS-AIME, 1968, vol. 242, p. 1709. 5. E. D. Carter: Physicochemical Measurements in Metals Research, part 1, R. A. Rapp, ed, pp. 21-94, Wiley-lnterscience, New York, 1970. 6. P. J. Spencer and J. N. Pratt: Brit. J. AppL Phy~, 1967, vol. 18, p. 1473. 7. R. Hultgren, P. D. Desai, D. T. Hawkins, M. Gleiser, K. K. Kelley, and D. D. Wagman: Selected Values of the Thermodynamic Properties of the Elements, ASM, Metals Park, Ohio, 1973. 8. O. L Davies: Statistical Methods in Research and Production, Oliver and Boyd, London, 1967. 9. G. 1. Batalin and V. S. Sudavtsova: Ukrain. Khim Zhur., 1974, vol. 40, no. 5, p. 542. 10. M. S. Petrushevskii, P. V. Kocherov, P. V. Geld, V. M. Zamyatin, and S. I. Suchilnikov: Rus~ J. Phy~ Chen~, 1973, vol. 47, p. 158. 11. J. N. Pratt and N. Ahmad: Final Tech. Report, U.S.D.A. Grant No. DA-ERO124-74-G0060, June 1975. 12. R. Gee and T. Rosenqvist: Scand ,L Metall., 1976, vol. 5, p. 57. 13. F. A. Shunk: Constitution of Binary Alloys, 2rid Supplement, McGraw-Hill, New York, 1969. 14. T. Mager and E. Wachtel: Z. Metalk., 1970, vol. 61, p. 853. 15. L. Rossemyr and T. Rosenqvist: Tran~ TMS-AIME, 1962, vol. 224, p. 140. 16. V. N. Eremenko, G; M. Lukashenko, and V. P. Sidorko: Soy. PowderMet. Met. Ceram., 1964, vol. 5, p. 393; 1965, vol. 9, p. 765. 17. Yu. M. Gertman and P. V. Geld: [sv. Vys. Uchebn. Zaved. Chem. Met,, 1959, vol. 9, p. 15. 18. T. G. Chart: A Critical Assessment of Thermochemical Data for Transition Metal-Silicon Systems, N.P.L. Report Chem. 18, National Physical Laboratory, Teddington, England, 1972. 19. S. M. Letun and P. V. Geld: High Temp., 1965, vol. 3, p. 39. 20. S. M. Letun, P. V. Geld, and N. N. Serebrennikov: Russ. MetalL, 1965, vol. 6, p. 97; Izv. Vys. Uchebn. Zaved. Chem. Met., 1965, vol. 4, p. 5; 1966, vol. 12, p. 5. 21. T. K. Kostina, B. A. Baum, P. V. Geld, and K. T. Kuroschkin: Russ. Metall., 1971, vol. 4, p- 8t.

VOLUME 9A, DECEMBER 1 9 7 8 - 1 8 6 3