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 been made on twenty two m a n g a n e s e - s i l i c o n al loys in the liquid s ta te , at t e m p e r a t u r e s between 1400 to 1900 K, us ing a to r s ion-e f fus ion technique. The the rmodynamic p roper t i e s of the sys tem have been calculated f rom the observed vapor p r e s s u r e s of manganese over the al loys at 1700 K. The ac t iv i t ies of the components show very s t rong negat ive deviat ions f rom ideal i ty and the heats of fo rma- t ion a re markedly exo thermic . Excess f ree ene rg ies evaluated f rom 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 measu red heats of format ion , avai l - able in the l i t e ra tu re , to obtain the excess en t rop ies of mixing which a re found to be modera te ly negat ive . The p resen t r e su l t s a re a s s e s s e d with r e spec t to the exis t ing phase equ i l ib r i a and other the rmodynamic data for the solid and liquid s ta tes . Among the fac- to rs inf luencing the p rope r t i e s of these a l loys , a tendency to fo rm covalent l inkages in the liquid s tate appears mos t s ignif icant .

THIS study of liquid m a n g a n e s e - s i l i c o n al loys is a par t of the continuing p rog ram of r e s e a r c h at the au tho r s ' l abora tory a imed at co r re l a t ing 1 the the r - modynamic p roper t i e s with component c h a r a c t e r i s - t ics such as s ize, valency, e lec t ronegat iv i ty , magnet ic in te rac t ions and chemical bond effects . P r ev ious work on b ina ry liquid al loys of manganese with copper, 2 gold, 3 and t in 4 has shown that the fac tors r espons ib le for solid phase equ i l ib r i a may p e r s i s t in the liquid state and therefore inf luence its t he rmodynamic prop- e r t i e s . It has been suggested that such p rope r t i e s of liquid manganese -coppe r a l loys owe the i r behavior main ly to the magnet ic in te rac t ions while the e l ec t ro - negat ivi ty factor has been seen to be more s igni f icant in manganese -go ld a l loys . The the rmodynamic prop- e r t i e s of m a n g a n e s e - t i n l iquids have been a t t r ibuted to the d i f fe rences of s ize , valency, and poss ib ly some covalent c lus te r ing of t in . The p re sen t inves t iga t ion was under taken to examine the effect of a l loying man- ganese with a more s t rongly covalent e lement and to provide data for an improved a s s e s s m e n t of the the r - modynamic p rope r t i e s of the sys tem.

As with above examples , the re la t ive vola t i l i ty of manganese makes the m a n g a n e s e - s i l i c o n a l loys a m- menable to study by means of a vapor p r e s s u r e method and the to r s ion -e f fus ion technique has again been employed.

EXPERIMENTAL DETAILS

The p r inc ip les of the to r s ion -e f fus ion technique and i ts use in vapor p r e s s u r e m e a s u r e m e n t s of meta l s have been reviewed by Car te r .5 The spec imen is con- ta ined in an effusion cel l suspended f rom a fine wire, within a ve r t i c a l vacuum chamber . The cell is pro- vided with two or i f ices , so disposed, in i ts f ront and

NAZ1R AHMAD, formerly Research Student/Research Fellow, Department of Physical Metallurgy and Science and Materials, Univer- sity of Birmingham, is now Research Associate, Department of Mate- rials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 and JOHN N. PRATT is Reader in Metallur- gical Thermochemistry, Department of Physical Metallurgy and Science of Materials, University of Birmingham, Birmingham, England.

Manuscript submitted February 17, 1978.

METALLURGICAL TRANSACTIONS A

r e a r v e r t i c a l faces, that the effusing vapor cause a ro ta t ion of the cel l about the axis of suspens ion . In free suspens ion , the equ i l ib r ium deflect ion occurs when the torque due to the effusing vapor is ba lanced by that in the suspension, so that the vapor p r e s s u r e is given by:

p : 27ot / (alq~f l + a2q2f2)

when p is the p r e s s u r e , z the to r s ion constant of the suspens ion , ~ the deflection, al, a2, ql and q2, r e spec - t ively, the or i f ice a r e a s and the d i s tances f rom the axis of rotat ion, and f l and fz the F r e e m a n and Searcy co r rec t ion fac tors for or if ice geomet ry and molecu- lar d i s t r ibu t ion .

The high t e mpe r a t u r e to r s ion -e f fus ion appara tus used in the p resen t inves t iga t ions i s e s s e n t i a l l y the same as desc r ibed previous ly , ~ but a n u m b e r of modi- f ica t ions have been made to improve its operat ion. A r ev i sed d iag ram of the p re sen t appara tus is shown in Fig . 1. The wate r -coo led vacuum envelope, evacuated through a side a r m of the lower (furnace) chamber is unchanged. The or ig ina l s l i t - tube heating e l emen t has now been rep laced by a un i form tan ta lum tube (C), 15.3 cm long, 3.8 cm d iam and 0.045 cm wall th ickness with tan ta lum leads connected to e i ther end. This provides a more un i form cu r r en t flow through the heat ing e lement and r e su l t s in a longer zone of uni- fo rm t e m p e r a t u r e . Power to the hea ter is now car - r ied through the vacuum chamber base plate (E) by means of two wate r -coo led (Edwards Type 9A High Current ) e lec t rodes (F), with consequent improvemen t of vacuum condit ions. As before , a 500 amp, 8 V t r a n s f o r m e r suppl ies power, but the input to this is now control led by a Stanton-Redcrof t (Model LVP-C) t e m p e r a t u r e con t ro l l e r and p r o g r a m m e r . The feed- back to the con t ro l l e r is f rom a P t /13 pct RhPt ther - mocouple (S) which has i ts hot junct ion contact ing, but e l ec t r i ca l ly insula ted from, the ins ide wall of the heat ing e lement . P r o g r a m m e d heat ing and cooling r a t e s (0 to 10~ and constant t e mpe r a t u r e con- t ro l (+0.5~ a re obtainable between 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 dur ing heating and cooling a re now achievable . Some modif icat ions have now been made to the rad ia t ion

ISSN 0360-2133/78/1211-1857500.75/0 �9 1978 AMERICAN SOCIETY FOR METALS AND VOLUME 9A, DECEMBER 1978-1857

THE METALLURGICAL SOCIETY OF AIME

shie ld ing (H and L) which now cons is t s of two inner shields of t an ta lum su r rounded by three of molybde- num; a lumina space r s are used to e l imina te contact between ve r t i c a l shields and the a s s e m b l y is made s table by enc los ing in an a lumina tube (I). A s e r i e s of c i r cu l a r shie lds (J) have been added beneath the heat- ing e l emen t . The design of the effusion cel l su s - pens ion sys t em (Q) has been s impl i f ied by the e l im ina - t ion of the magnet ic control sys t em and cel l me a s - u r e m e n t s a re now made by de t e rmin ing the free ro- ta t ion of the cel l f rom observa t ions of def lect ions of

0

R

0

.•

M-

L

F

-A

.C

5

U

E

~I~OXI~AT[ SCAt[

i 1 12 13 d~ C.ms

Fig. 1--The torsion-effusion apparatus.

1858-VOLUME 9A, DECEMBER 1978

the ga lvanometer mixer (X). Tungs ten to r s ion wire of 0.005 cm diam is now used in a l l exper imen t s , suspens ion lengths being va r i ed between 16.0 to 27.0 cm accord ing to the magnitude of the vapor p r e s s u r e involved and the sens i t iv i ty r equ i red . The effusion cel ls , as before , have been machined f rom boron ni - t r ide . Diff icul t ies were encountered due to the in- s tab i l i ty at high t e m p e r a t u r e s of some grades of this ma te r i a l , but t r i a l s have shown that Union Carbide grade HD 0092 is sa t i s fac to ry and this is therefore no rma l ly used. Dur ing exper imen ta l runs the nomina l cel l t e mpe r a t u r e is indicated by means of the in s i t u 20 pct R h - P t / 5 pct R h - P t thermocouple (U) located just beneath the effusion cell . This is ca l ib ra ted to give t rue cel l t e m p e r a t u r e s by making vapor p r e s - sure m e a s u r e m e n t runs with the pure meta l s s i l ve r and manganese , for which the vapor p r e s s u r e s a re accura te ly es tabl i shed, 7 over the t e mpe r a t u r e range of in t e re s t . The n e c e s s a r y t e mpe r a t u r e cor rec t ions a re found to be reproduc ib le for given heat ing e lement and shield a s s e m b l i e s .

Alloys were p repa red f rom spec t roscop ica l ly pure components . Approximate ly 1.0 g samples , careful ly weighed to 0.00002 g, were mel ted under a rgon by R. F. heat ing in boron n i t r ide c ruc ib les su r rounded by a graphite susceptor ; the r e su l t ing ingots were used without fu r the r t r ea tmen t for the liquid alloy vapor p r e s s u r e s tudies . Some al loys, pa r t i cu l a r ly those in the s i l i c o n - r i c h range, were p repared di- r ec t ly in the effusion cell immedia te ly p r io r to the vapor p r e s s u r e m e a s u r e m e n t s .

EXPERIMENTAL RESULTS

The equat ions for vapor p r e s s u r e s of manganese over the a l loys were calculated on the a s sumpt ion that they approximate to C laus ius -C lapey ron behavior over the t e m p e r a t u r e range of in t e re s t . L inea r equat ions of the fo rm:

log p = - A / T + B

were computed by least square ana lys i s of the raw data and a re p resen ted in Table I. Assoc ia ted unce r - ta in t ies in A and B were calculated f rom the s tandard deviat ions , s, of the p a r a m e t e r s accord ing to the fo rmula : +s . t~ where t~ is a probabi l i ty factor for a s s u m i n g n o r m a l d i s t r ibu t ion of the data and 95 pct confidence limits.S The uncer t a in t i e s in log p values due to ext rapola t ion to 1700 K were es t ima ted using the fo rmula :

1 (x K _ ~)2 ]1/2

where x is 1 / T , T being the m i d - t e m p e r a t u r e within the range 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 observa t ions .

For the a l loys , the ac t iv i t i es and par t i a l thermody- n a m i c p rope r t i e s of manganese at 1700 K (Table II) have been calculated d i rec t ly f rom the above vapor p r e s s u r e equat ions and the a s s e s s e d data for pure manganese . 7 Cor responding values for s i l icon were evaluated us ing the Gibbs-Duhem re la t ion , while in - t eg ra l va lues were obtained by the usual summat ion of the pa r t i a l s . Al l p rope r t i e s r e fe r to the pure liquid f o r ms of the components as the s tandard s ta tes .

METALLURGICAL TRANSACTIONSA

Table 1. The Vapor Pressures of Mn Over Liquid Mn-Si Alloys

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

Nmn A B • &log p, 1700 K Temp. Range, K

0.100 16044 • 496 4.380 • 0.269 0.0297 i9lM- 1775 0.167 15685 • 191 4,509 • 0.106 0.0141 1886 - 1693 0.290 15393 • 417 4,733 • 0.250 0.0196 1737 - 1592 0.303 15320 • 183 4.736 • 0.110 0.0118 1591 - 1802 0.386 14817 • 432 4.749 • 0,261 0.0283 1576 - 1664 0.395 15079 • 722 4,946 • 0.427 0.0405 t590 - 1774 0.438 14694 • 692 4.921 • 0.411 0.0370 1760 - 1590 0.481 14621 • 487 5,045 • 0.293 0.0206 1721 - 1593 0.530 15222 • 1097 5,589 • 0.655 0.0520 1744- 1590 0.567 15115 • 640 5,688 • 0.383 0.0235 1734 - 1608 0.590 15737 • 781 6,198 • 0.459 0.0313 1756 - 1648 0.650 14590 • 785 5,897 • 0.461 0.0437 1800 - 1615 0.662 15458 • 1041 6,436 • 0.619 0.0283 1696 - 1637 0.700 14500 • 692 6,014 • 0.412 0.0554 1800- 1573 0.725 12470 • 495 4,950 • 0.329 0.0541 1608 - 1403 0.750 12428 • 350 4,989 • 0.232 0.0348 1593 - 1425 0.750 12477 • 226 5,032 • 0,144 0.0296 1440 - 1720 0.773 12337 • 142 4,982 • 0.092 0.0192 1402 - 1641 0.830 12267 • 555 5,069 • 0.293 0.0642 1638 - 1403 0.867 12576 • 5.334 • 0.284 0.0581 1680 - 1402 0.868 12469 • 215 5,274 • 0.139 0.0301 1403 - 1708 0.895 12718 • 142 5,448 • 0.089 0.0087 1519 - 1644 1.000 12673 • 185 5,490 • 0.t15 0.0137 I527 - 1693

*1 atm = 101325 Pa.

T h e p r e c i s i o n of t he 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 in T a b l e I . C o n s i d e r i n g a n a v e r a g e d e r r o r v a l u e f o r the e q u i - a t o m i c c o m p o s i t i o n a n d t h e s c a t t e r in t he m e a s u r e -

m e n t s on p u r e m a n g a n e s e , t he u s u a l m e t h o d of e r r o r p r o p a g a t i o n s w a s a p p l i e d . A s s u m i n g a p o s s i b l e e r r o r

of +5 K in t e m p e r a t u r e a n d +0.02 NMn in c o m p o s i t i o n , t he e r r o r l i m i t s f o r the p r o p e r t i e s of t he e q u i a t o m i c a l l o y a r e :

AGMn = -- 49 ,321 + 1452 J / g - a t o m .

AG = - 37 ,367 + 1377 J / g - a t o m ,

A/-/ = - 24 ,690 • 3008 J / g - a t o m ,

AS = 7 .4 6 0 • 1 .452 J / d e g . g - a t o m .

T h e v a p o r p r e s s u r e o b s e r v a t i o n s m a d e d u r i n g b o t h h e a t i n g a n d c o o l i n g 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 ng h e a t i n g w h e r e a s t he 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 - t i o n s a r e m a d e d u r i n g a s i n g l e r u n , t h e f o r m e r a r e t r e a t e d a s c h a r a c t e r i s t i c of the s t a r t i n g a l l o y co rn -

p o s i t i o n a nd 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 the f i n a l

c o m p o s i t i o n ; t h i s w a s e s t i m a t e d f r o m the w e i g h t l o s s of m a n g a n e s e d u r i n g the 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 by t he c o n s i s t e n c y of the 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 in F i g . 3, w h e r e the i d e n t i c a l l y f l a g g e d p o i n t s a r e r e s u l t s f r o m the s a m e

e x p e r i m e n t a l r u n . A n o m a l o u s e v a p o r a t i o n , r e s u l t i n g in a d e l a y e d e s -

t 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 o b - s 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 qu id 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 to 65 a t . pc t m a n g a n e s e , i . e .

in t he v i c i n i t y of the 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 on t he e x a c t m a n g a n e s e c o n t e n t (<>MnsSi3) of t he 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 a t c o n s t a n t

t e m p e r a t u r e f o r s h o r t p e r i o d s . S y s t e m a t i c c y c l i n g

e x p e r i m e n t s i n d i c a t e d t h a t t h i s b e h a v i o r i s a s s o c i a - t e d w i th t he 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 he l i qu id p h a s e . F o r a l l o y s on t h e m a n g a n e s e - r i c h s i d e of t he

c o m p o u n d , t he 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 the i n i t i a l l i q u i d s a nd 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 ; the s i m i l a r h o m o g e n i z a t i o n of the s i l i c o n - r i c h a l l o y s , on t he o t h e r h a n d , wi l l be a c c o m p a n i e d by a g r a d u a l m a n g a n e s e - e n r i c h m e n t of the l i qu id p h a s e a n d s o by i n c r e a s i n g v a p o r p r e s s u r e s . B e c a u s e of t h i s b e h a v i o r , on ly r e s u l t s o b t a i n e d d u r i n g c o o l i n g r u n s h a v e b e e n u s e d in d e t e r m i n i n g t he 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 plot- ted in Fig. 3 show very strong negative deviations from ideality, but these observations are in reason- able 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 devia- tions from ideal behavior have been reported by Petrushevskii, Kocherov, Geld, Zamyatin and Suchilni- key z~ 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 man- ganese activities at 1673 K from their vapor pres- sure equations and from those of Table I show good

NMn aMn aSi

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

(k J/g-atom) (J/deg �9 g-atom) (k J/g-atom) AG, AS, AH,

AGMn AGsi ASMn ASsi A/tMn A/~Si k J /g -a tom J/deg �9 g-atom kJ/g-atom

0.1 0.0008 0.8934 0.2 0.0023 0.752 0.3 0.0049 0.5830 0.4 0.0117 0.3613 0.5 0.0305 0.1656 0.6 0.0929 0.0408 0.7 0.2852 0.0052 0.8 0.5829 0.0006 0.9 0.8645 6.7 X 10 "s

-100.776 -1.594 21.338 0.795 -64.501 -0.234 -11.5t4 2.849 -6.669 -85.851 -4.029 17.573 1.456 -55.978 -1.552 -20.393 4.678 -12.439 -75A66 -7.627 15.062 2.301 -49.560 -3.715 -27.891 6.130 -17.468 -62.865 -14.389 11.715 4.176 -42.949 -7.289 -33.777 7.192 -21.552 -49.321 -25.414 5.021 9.895 -40.786 -8.594 -37.367 7.460 -24.690 -33.581 -45.212 -8.368 2 5 , 5 6 8 -47.806 -1.745 -38.233 5.205 -29.380 -17.732 -74.325 -9.205 2 6 . 9 5 3 -33.380 -28.506 -34.710 1.644 -31,920 -7.627 -104.847 9.623 -31.698 +8.732 -158.733 -27.070 1.360 -24.761 -2.059 -I35.846 2.092 14.3t3 +1.498 -111.512 -15.439 3.314 -9.803

METALLURGICAL TRANSACTIONS A VOLUME 9A, DECEMBER 1978-1859

3-0

2-5

~2"0 x

0

e~

o 1-5

1"0

0"5

"x, "~.

' ~ " x , , "=,

\ x ' , "=,

,x,,

I I i I

5-5 6.0 11T~10 4 (OK)

NMn

o 0100 �9 0167 o 0.290 m 0303 �9 0386 �9 0395

* 0"438 �9 0"481 �9 0"530 " 0"567 �9 0"590

NMn

0-650 0662

x {)700 0-725

�9 0750 �9 0-750 e 0-773 T 0"830 + 0"867 * 0"868 �9 0"895

1 I

65 7-0

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

a g r e e m e n t be tween these two mos t r e c e n t i nves t iga - t ions ove r mos t of the compos i t ion range , but dev ia te f rom each o the r a t the h ighes t manganese contents . A s i s d e m o n s t r a t e d below, however , in th is r ange the r e s u l t s of the p r e s e n t s tudy appea r mos t cons i s t en t with va lues ind ica ted by the phase d i a g r a m . A com- p a r i s o n of the v a r i o u s l iquid s ta te s tud ies i s shown in F ig . 4.

The va l id i ty 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 cons i s t ency of the t h e r m o d y n a m i c da ta for the s y s t e m is a l so con f i rmed by c o m p a r i s o n with va lues com- puted f rom the phase 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 tud ies of so l id a l l o y s . The phase d i a g r a m shown in F ig . 5 is l a r g e l y a s compi led by Shunk 13 but modif ied, in the s i l i c o n - r i c h r eg ions , 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 Wach te l . 14 T h e r e have been two s ign i f i can t s tud ies of f r ee e n e r g i e s of f o r m a - t ion in the so l id s t a t e . Us ing a Hz/HC1 equ i l i b ra t i on technique, R o s s e m y r and Rosenqv i s t 15 have d e t e r m i n e d f r ee e n e r g i e s of f o rma t ion of MnSi2, MnSi and MnsSi3

at 1363 K, while a m o r e ex tens ive s tudy of the t h e r - modynamic p r o p e r t i e s of so l id a l loys (0 to 100 pct Si, 950 to 1100 K) has been made by E r e m e n k o , Lukashenko mad Sidorko TM by means of mol ten ch lo r ide e l e c t r o l y t e ga lvanic ce 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 manganese along the l iquidus , obta ined f rom liquid and so l id s t a t e s tud ies and r e f e r r e d to a un i fo rm liquid manganese s t anda rd , a r e c o m p a r e d with each o ther in F ig . 6. A l so included a r e manganese r i ch data, d e r i v e d f rom the phase d i a g r a m a s s u m i n g Raoul t ian behav io r in the /3-Mn so l id so lu t ions , and s i l i con ac t i v i t i e s at 1700 K computed f rom the fo rm of the l iquidus r e l a t - ing to pure so l id s i l i con . The gene ra l l y good a g r e e - ment of the v a r i o u s da ta d e m o n s t r a t e s the mutual con- s i s t e n c y of the phase d i a g r a m and of the t h e r m o d y - namic p r o p e r t i e s of the so l id and l iquid a l l oys . The anomalous compos i t ion dependence of the computed points (open c i r c l e s ) in the r eg ion NMn = 0.6 to 0.7 is thought to be due to the s ens i t i v i t y of these va lues to the t rue m a n g a n e s e - r i c h so l idus of the e s s e n t i a l l y

1860-VOLUME9A, DECEMBER 1978 METALLURGICAL TRANSACTIONS A

10 1"0

-9

-8

7

6

a ' 5

"4

.3

.2

.1

0 0 1 .2 .3 4 .5 .6 .7 .8 .9

NNn Fig. 3--Activities of manganese and silicon in liquid man- ganese-silicon alloys at 1700 K.

1,0

S tO 15 1 5 0 0 " ~

.8

- 6 c'-

.4

.2

o

o o o �9

0 i 1 i i i

0 .2 .4 .~, ' - 8 ' ~ .0 NM n

Fig. 4--Comparison of experimental data for manganese activity coeff ic ients in liquid m a n g a n e s e - s i l i c o n al loys: o P e t r u s h e v s k i i et al (1623 K), �9 Batal in and Sudavtsova (1673 K), ~ Gee and Rosenqv i s t (1673 K), and -- P r e s e n t work (1673 K).

WEIGHT PER CENT S I L I C O N z o z~, 3o 4 0 5 0 6 0 7 0 o o 9 o

1401

130q

120r

Fig. 5- -The m a n g a n e s e - s i l i - con equ i l ib r ium d i a g r a m .

u I 1 0 0

E

I O 0 0 9[ m.

tll i - ~ n n I

8 0 0

7 0 0

600

5O0 L

0 Ma

METALLURGICAL TRANSACTIONS A

w g

11

( | - I d . )

Y~.,,;5. ( r - M . ) - \

IG.1

8tO*,f

15.3'

IO 2 0

'r iol 1 1 l ~85 o - ~

~ 4 l J I I

1 0 7 5 "

1 0 4 0 *

I

3O

OSO i I

! ' 1- , I I I

I , I

I

I I

4 0 5 0 6 0 7 0 eO 9 0 I 0 0

ATOMIC PER CENT S I L I C O N S'u

VOLUME9A, DECEMBER i978-1861

0 0

asi

M.5si 2 at

o 1 ; aMn *

0

1 . MnSi

2 Mnll$ 19 1

/

1 . . . . �9 NON-ISOTHE RMA

0 ?~ I I r I [ i 1 I

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 phase , MnsSi3, in e q u i l i b r i u m with the l iquidus in th is r ange .

I n t e g r a l hea t s and e n t r o p i e s of f o rma t ion of the l iquid a l loys y ie lded by the p r e s e n t vapor p r e s s u r e s tud ies alone a r e given in Tab le II . The hea 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 equiva lent quant i t i es sugges t ed by the emf s tud ies by Ba la t in and Sudavstova , 9 but the p r e s e n t va lues a r e s i m i l a r in magni tude to those obta ined, by Ge r tman and Geld, t~ by 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 these c a l o r i m e t r i c hea t s of f o r m a t i o n a r e , however , s ign i f i can t ly l e s s a s y m m e t r i c with r e s p e c t to compos i t ion . 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 hea 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 those 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 coef- f i c ien t s , the e n t r o p i e s of fo rma t ion have f inal ly been ca l cu la t ed by combining the c a l o r i m e t r i c hea t s with the p r e s e n t i n t e g r a l f r ee e n e r g i e s at 1700 K; the v a r i a t i o n of &H be tween 1700 and 1743 K is con- s i d e r e d to be neg l ig ib l e . The r e s u l t i n g va lues 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 lo t ted in F ig . 7. T h e s e show that the l iquid man- g a n e s e - s i l i c o n a l l oys 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 behav ior ; of the l iquid manganese a l l o y s so f a r i nves t iga t ed only the m a n g a n e s e - g o l d s y s t e m , 3 with i ts excep t iona l ly l a rge e l e c t r o c h e m i c a l f a c t o r , has shown m o r e nega t ive hea t s . I t is obvi- ous that s i m i l a r h e t e r o p o l a r i t y of bonding wi l l not ex i s t be tween manganese and s i l i con , s ince the e l e c - t r onega t i v i t y d i f f e rence be tween these e l e m e n t s i s much s m a t t e r aad indeed i s c lo se ly c o m p a r a b l e with those ex i s t i ng in the b i n a r i e s of manganese with copper , n i cke l and t in . Cons ide ra t ion of the types of i n t e r m e d i a t e phases o c c u r r i n g in the so l id s ta te , however , shows that i n t e rcomponen t covalent bonding is a p r edominan t f e a tu r e of the m a n g a n e s e - s i l i c o n

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 compos i t ion r e - g ions . The l a rge negat ive hea t s of f o r m a t i o n of the l iquids a r e thus mos t p robab ly a t t r i bu 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 ac t i ons in the mol ten a l l oys . The magni tude of the hea t s fu r the r sugges t s that a t endency to fo rm cova l en t ly - l i nked c l u s t e r s may wel l occur and that th is , through a r e - duct ion of conf igura t iona l en t ropy , may be r e s p o n s i b l e for a s ign i f ican t pa r t of the o b s e r v e d nega t ive e x c e s s e n t r o p i e s . However , the m a j o r p a r t of the l a t t e r mus t be a t t r i bu t ed to t h e r m a l s o u r c e s , s ince the covalent bonding p r e s e n t ove r much of the s y s t e m is a c c o m - pan ied by nega t ive dev ia t ions f rom Neumann-Kopp behav io r in the so l i d s 19'2~ and p robab ly a t so in the l iquid s t a t e . It wi l l be o b s e r v e d that the e x c e s s en- t r o p i e s of the l iquids a r e l e s s nega t ive in the man- g a n e s e - r i c h r e g ions and this is thought to be a t t r i bu - t ab le to the m o r e m e t a l l i c bonding l ike ly to ex i s t at these compos i t i ons . The sugges t ed 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 l iquids is suppor t ed by r e p o r t e d s tud ies of the so lu t ion of hydrogen in m a n g a n e s e - s i l i c on m e l t s . 21 So lub i l i t i e s a r e l ea s t in concen t r a t ed a l l oys (30 to 60 a t . pct manganese ) and r e l a t i v e l y g r e a t e r at the manganese r i c h compos i t ions ; the hea t s of solut ion in these two r eg ions a r e r e s p e c - t ive ly endo-and exo the rmic . Th is sugges t s that the m a n g a n e s e - r i c h a l l oys , be ing more m e t a l l i c al low the hydrogen r e a d i l y to fo rm m e t a l l i c l inkages on solut ion, while in the concen t ra t ed a l loys the p r e f e r - ence for covalent i n t e r ac t ion be tween manganese and s i l i con r e d u c e s the f ac i l i t y for l inkages with hydrogen and hence inhibi ts i t s so lub i l i ty . P e r s i s t e n c e of mi - c ro inhomogene i t i e s in at l e a s t some of the l iquids is a l so ind ica ted 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 hea t s of fusion for MnsSi3.19 T h e s e have been i n t e r - p r e t e d as due to the enhancement of Mn-Si covalent bonding on me l t ing so that q u a s i - m o l e c u l a r groups of MnSi type a r e f o r m e d leaving o the r p a r t s of m i c r o - inhomogeneous r eg ions r i c h e r in manganese . Th is s u g ges t ion is cons i s t en t with the f o r m s of anomalous vapor p r e s s u r e behav io r o b s e r v e d in the p r e s e n t work dur ing the in i t i a l heat ing of a l l oys in the v ic in i ty of the compound MnsSi3. That a t endency to fo rm molecu -

�9 2 .4 .6

E o 10

S &

20 I

x

~ 3o

40

NMn

1.0

-8 1.0 E >/oo 2 6~

6% <~

I

Fig. 7- -Excess integral thermodynamic proper t ies of liquid manganese-s i l icon alloys.

1862-VOLUME 9A, DECEMBER 1978 METALLURGICAL TRANSACTIONS A

350

300

E 2 5 O o S

~ "

8 - 2 0 0 >

"r- <~ 3 0 O

250

Z E Z

~ M n ~ i

i I i 1 i t I I

gi M n - N i

i + q , , . ,

2 0 0 0 I I I I 1 I t I t -2 .4 "6 ' 8 1"0

N M n Fig. 8--Heats of vaporization of manganese from manganese- silicon and manganese-nickel liquid alloys.

i a r a s s o c i a t i o n s i s n o t m e r e l y t r a n s i e n t b u t i s p r o b a -

b l y c h a r a c t e r i s t i c o f t h e s t e a d y e q u i l i b r i u m s t a t e o f

s o m e l i q u i d m a n g a n e s e - s i l i c o n a l l o y s i s m o s t c l e a r l y d e m o n s t r a t e d 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 o f m a n g a n e s e f r o m t h e s e a l l o y s w i t h t h o s e f r o m m a n - g a n e s e - n i c k e l 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 t h e s e h e a t s f o r t h e t wo s y s t e m s , c a l c u l a t e d d i r e c t l y

f r o m t h e e q u i l i b r i u m v a p o r p r e s s u r e e q u a t i o n s , a r e c o m p a r e d in 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 v a l u e s a r e o b s e r v e d f o r t h e m a n g a n e s e - r i c h r e g i o n s

o f b o t h s y s t e m s , b u t i n c o n t r a s t t o t h e e s s e n t i a l l y c o n -

s t a n t o r m o n o t o n i c v a r i a t i o n a c r o s s t h e m a n g a n e s e -

n i c k e l a l l o y s , t h e i n t e r m e d i a t e m a n g a n e s e - s i l i c o n c o m p o s i t i o n s e x h i b i t s i g n i f i c a n t l y i n c r e a s e d h e a t s o f

v a p o r i z a t i o n . T h i s t h u s p r o v i d e s v e r y d i r e c t e v i d e n c e o f t h e e x i s t e n c e o f e n h a n c e d i n t e r a c t i o n s b e t w e e n m a n -

g a n e s e a n d s i l i c o n a t o m s in t h e s e p a r t i c u l a r r e g i o n s .

A C K N O W L E D G M E N T S

F i n a n c i a l s u p p o r t f o r t h e r e s e a r c h p r o g r a m h a s

b e e n p r o v i d e d b y t h e U n i t e d S t a t e s G o v e r n m e n t 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 S t a t e s A r m y . T h e a u t h o r s w i s h to a c k n o w l e d g e c o n - s 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 - t e r i a l s a n d M e c h a n i c s R e s e a r c h C e n t e r , W a t e r t o w n ,

M a s s a c h u s e t t s , a n d P r o f e s s o r J o h n F . E l l i o t t o f M a s -

s 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 .

R E F E R E N C E S

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-ERO- 124-74-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.

METALLURGICAL TRANSACTIONS A VOLUME 9A, DECEMBER 1978-1863