RESISTIVITY

OF

LIQUID

E . S. L e v i n , P. B. a n d G. D. A y u s h i n a

IRON--ALUMINUM

ALLOYS UDC 669.715.1:537

Gel'd,

This study will examine the t e m p e r a t u r e and concentration dependence of e l e c t r i c a l r e s i s t a n c e in liquid alloys of iron with aluminum f r o m the melting point to 1700-1800~ and 0 - NA1 -< 1. It is established that the i s o t h e r m s (1535 and 1650~ of r e s i s t i v i t y have a complex extremal c h a r a c t e r with a diffuse m a x i m u m at the equiatomic alloy. The data obtained a r e evaluated from the viewpoint of peculiarities in i n t e r p a r t i c l e interaction and microinhomogeneous s t r u c t u r e of the alloys studied. It is known that the determination of the c h a r a c t e r of t e m p e r a t u r e and concentration dependence of s t r u c t u r e - s e n s i t i v e p r o p e r t i e s of alloys (density, v i s c o s i t y , s u r f a c e energy, e l e c t r i c a l conductivity, and others) p e r m i t s evaluation of s t r u c t u r e and electron p r o p e r t i e s . Study of the p h y s i c o c h e m i c a l p r o p e r t i e s of liquid alloys of iron with aluminum has been relatively neglected. In p a r t i c u l a r , t h e r e is no knowledge of e l e c t r i c a l conductivity, which, beside its own i m p o r t a n c e , is important for clarification of i n t e r p a r t i c l e interaction peculiarities and the c h a r a c t e r of c a r r i e r diffusion. We will offer below the r e s u l t s of r e s i s t i v i t y m e a s u r e m e n t s of liquid i r o n - - a l u m i n i u m alloys (compositions p r e s e n t e d in Table 1) as functions of t e m p e r a t u r e . The samples were p r e p a r e d from type AV-00 aluminium (99.99% A1) and specially pure class V-3 carbonyl iron, which was p r e l i m i n a r i l y reduced in a hydrogen flow and degassed in a vacuum (99.988% Fe). Alloying was done in corundum crucibles in a h e r m e t i c a l l y sealed high frequency furnace with an a t m o s p h e r e of carefully purified argon. Electrical conductivity was m e a s u r e d by the eontactless method in a rotating magnetic field in an app a r a t u s the construction of which and method of operation were d e s c r i b e d p r e v i o u s l y [1, 2]. The data on density of the liquid alloys n e c e s s a r y for calculation of resistivity, were taken from [3]. The e r r o r in r e s i s t i v i t y determination was 7%. P o l y t h e r m s of r e s i s t i v i t y from the melting point to 1700-1800~ a r e p r e s e n t e d in Fig. 1. It follows f r o m these graphs that p and its t e m p e r a t u r e coefficient a r e quite dependent on composition and t e m p e r a ture. Most noticeable is the fact that iron poor alloys display a s e m i c o n d u c t o r type conductivity in the liquid state. Their r e s i s t i v i t y t e m p e r a t u r e coefficient is quite high in absolute value and negative (in the f i r s t approximation for an alloy with 77.7 atomic % A1 for t > 1300~ it proves to be no/St = --8.6 910 -1~ 12 9m / d e g , and for an alloy with 75 atomic % A1, ~ p / S t = --8.0 910 -i~ ~2 "m/deg). On the other hand, the lower aluminides Fe3A1 and FeA1 a r e c h a r a c t e r i z e d by a metallic type conductivity at high t e m p e r a t u r e s (at least above 1650~ Alloys close in composition to Fe2A15 and FeA12 occupy an i n t e r m e d i a t e position.

TABLE

i. Compositions

of Iron--Aluminium

Alloys Studied

1

4 I

1

Alloy No.

0

At. %A1 Wt. %A1 Compound

100 100 AI

2

3

66,7 77,7 75,0 71,~ 63,0 [ 59,2 54,7 49,1 FeeAlT[ FeAta F%Ats FeAI2

5 50,0 32,6 FeAi

6 25,0 12,2 F%AI

7 0 0 Fe

S. M. Kirov Ural Polytechnic Institute, Sverdlovsk. Translated from Izvestiya Vysshikh Uehebnykh Zavedenii, Fizika, No. i0, pp. 135-18, October, 1972. Original article submitted June 17, 1971, 9 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, \'. Y. I0011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

1505

P~'01~ ~.

2:! t

~

- ~-- ~

i"

!

'

It should be noted that the r e s i s t i v i t y of the alloy containing 77.7 atomic % A1 from melting to 1325~ undergoes no change with t e m p e r a t u r e and is approximately 96 910 -8 ~2 . m . It is p o s sible that this is related to p r e s e r v a t i o n of a h e r e d i t a r y solid phase s t r u c t u r e and an unusual competition between two opposing factors -- an i n c r e a s e in s c a t t e r i n g of electron waves on phonons with t e m p e r a t u r e , and an i n c r e a s e in charge c a r r i e r c o n c e n t r a tion or mobility. With further i n c r e a s e in t e m p e r a t u r e a change in the nature of i n t e r p a r t i c l e interaction o c c u r s , and a m o r e significant role begins to be played by not only resonant but localized bonds, bearing a directed c h a r a c t e r . The p r o c e s s e s of t h e r m a l electron excitation of these bonds predominate over s c a t t e r i n g on phonons and p r o d u c e a d e c r e a s e in r e s i s t i v i t y . The s a m e fact o r s explain the polytherm of e of the 75.0 atomic % A1 alloy. As for the t e m p e r a t u r e dependence of the alloy corresponding to the compound Fe2A15, it m a y be said that after melting a d e c r e a s e in p up to 1430~ ( a p / D t = - - 9 . 0 . 1 0 -1~ ~ " m / d e g ) o c c u r s , and with f u r t h e r i n c r e a s e in t e m p e r a t u r e the r e s i s t i v i t y does not change (102.10 -8 ~2 -m).

;_ r>

' "--.,.t..i

i'i'j_

,~oI-

~

9

i

~. i.___,A

i

i

~

i

F

i

i

8

!

. . . .

The t e m p e r a t u r e dependence of e l e c t r i c a l r e s i s t a n c e in FeA12 has an extremal c h a r a c t e r with a diffuse m i n i m u m o c c u r f200 ing at approximately t = 1550~ F r o m the melting point to Fig. 1. Resistivity p o l y t h e r m s 1550~ directed homeopolar i n t e r a t o m i c interactions already disfor i r o n - - a l u m i n u m alloys (numbers c u s s e d above in connection with the polytherm of p for the higher on c u r v e s a r e those of alloys; O, aluminide Fe2A17 a r e r e s p o n s i b l e for the e l e c t r i c a l c h a r a c t e r i s heating; 9 cooling. t i c s . T e m p e r a t u r e growth above 1550~ leads to development of t h e r m a l atomic oscillations and i n c r e a s e d s c a t t e r i n g of electron waves, and destruction of h o m e o p o l a r bonds and t h e i r t r a n s f o r m a t i o n to metallic, which produces increased resistivity. o

i

o-

t400

160#

i;

t~

The r e s i s t i v i t y of the solid monaluminide FeA1 n e a r the melting point was 140 910 -8 ~2 9m, which a g r e e s well with the data of [4]. In that study the t e m p e r a t u r e dependence (from 20 to 1200~ of r e s i s t i vity of solid i r o n - - a l u m i n u m alloys with 0.16 -< NA1 - 0.50 was examined. The authors noted a m i n i m u m p at r o o m t e m p e r a t u r e , o c c u r r i n g at the equiatomie alloy. With i n c r e a s e in t e m p e r a t u r e to 1200~ this m i n i m u m t r a n s f o r m s to a maximum, which was explained by d i s o r d e r i n g , d i s a p p e a r a n c e of magnetic p r o p e r t i e s , and change in the energy s p e c t r u m of valent e l e c t r o n s . F r o m melting to 1450~ a significant i n c r e a s e o c c u r s in r e s i s t i v i t y from 1 4 0 . 1 0 -8 to 155.10 -8 ~2 -m, which then r e m a i n s p r a c t i c a l l y constant up to 1650~ With further i n c r e a s e in metal t e m p e r a t u r e , p inc r e a s e s . It is possible that this is due, among other causes, to the formation of t h e r m a l l y stable m i c r o groupings (consisting of FeA1 quasimolecules), which a r e supplementary c e n t e r s for electron wave JD Io, 8 ~ 9 m

lo

d.lO: s

2o

5o

4o

5o

~o

70

8o

90at. ~AI

Fig. 2. Resitivity (p) and conductivity (~) of iron - - a l u m i n u m liquid alloys as functions of concentration: 1) P1650oc, 2) P1535"C, 3) ff1650oc .

1506

s c a t t e r i n g , with decay of t h e s e groupings at high t e m p e r a t u r e leading to " d i s o r d e r i n g " of the alloy and inc r e a s e in r e s i s t i v i t y . The p o l y t h e r m of p for the alloy with NA1 = 0.25 r e c a l l s that of the monaluminide in m a n y f e a t u r e s , but with a wider r a n g e of t e m p e r a t u r e (including both solid and liquid a g g r e g a t e states) changes in conductivity a r e e x p r e s s e d m o r e s h a r p l y . In the p r e m e l t and m e l t r a n g e up to 1500~ the r e s i s t i v i t y is p r a c t i c a l l y constant and equal to 113 -10 -s ~ 9m . With f u r t h e r i n c r e a s e in t e m p e r a t u r e to 1550~ p i n c r e a s e s to 118 9 10 -8 ~ - m . In the i n t e r v a l 1550-1650~ the r e s i s t i v i t y does not change, and for f u r t h e r t e m p e r a t u r e inc r e a s e , i n c r e a s e s s h a r p l y . The data obtained on e l e c t r i c a l p r o p e r t i e s of solid alloys with composition c l o s e to Fe3A1 c o r r e l a t e well with the m e a s u r e m e n t s m a d e in [4], a c c o r d i n g to which o v e r the r a n g e 5001200~ the r e s i s t i v i t y changes weakly and is about 120 910 -8 ~ 9m . Unfortunately t h e r e is no i n f o r m a t i o n in the l i t e r a t u r e on r e s i s t i v i t y of liquid i r o n - - a l u m i n u m alloys, and so c o m p a r i s o n of our figures with o t h e r s is obviously i m p o s s i b l e . It can only be a s s e r t e d that the nonparity of i n t e r p a r t i c l e i n t e r a c t i o n s in the alloys studied leads to t h e i r m i c r o i n h o m o g e n e i t y and the f o r m a t i o n of q u a s i m o l e c u l a r groupings (in which d i r e c t e d bonds a r e s i g nificant} which a r e r e s p o n s i b l e for the t e m p e r a t u r e dependence of v a r i o u s p h y s i c o c h e m i c a l p r o p e r t i e s , a m o n g t h e m r e s i s t i v i t y [3, 5]. I s o t h e r m s (1650 and 1535~ of p a r e p r e s e n t e d in Fig. 2, f r o m which it follows that they differ only insignificantly and h a v e an e x t r e m a l c h a r a c t e r with m a x i m u m at the equiatomic alloy. Addition of iron to p u r e aluminum leads to the e s t a b l i s h m e n t between different type a t o m s of a Significant n u m b e r of h o m e o p o l a r bonds and f o r m a t i o n of m i c r o g r o u p i n g s , i . e . , f o r m a t i o n within the alloys of s t r o n g e r and localized Fe--A1 bonds and i n c r e a s e in r e s i s t i v i t y . The g r e a t e s t development of t h e s e d i r e c t e d i n t e r a t o m i c i n t e r actions is acheived in the equiatomic alloy, c h a r a c t e r i z e d by a m o r e homogeneous s t r u c t u r e and l o w e r conductivity. It should be noted that the r e s i s t i v i t y of the alloy c o r r e s p o n d i n g to the compound Fe3A1 at t = 1535-1650~ does not change and p r o v e s to be l o w e r than that p e c u l i a r to p u r e i r o n at t h e s e t e m p e r a t u r e s . This is evidently r e l a t e d to a change in a t o m coordination and t e s t i f i e s to the quite complex c h a r a c t e r of the c h e m i c a l bond in liquid i r o n aluminides. In conclusion we point out that the r e s u l t s of the liquid i r o n - - a l u m i n u m alloy r e s i s t i v i t y m e a s u r e m e n t s and t h e i r evaluation should be r e g a r d e d only as a f i r s t a p p r o x i m a t i o n . Without doubt, f u r t h e r s y s t e m a t i c investigations of e l e c t r i c a l , m a g n e t i c , rheological, d e n s i t o m e t r i c , t h e r m a l , and other p h y s i c o c h e m i c a l p r o p e r t i e s of t h e s e alloys is n e c e s s a r y . Only a thorough knowledge of the t e m p e r a t u r e and concentration dependences of t h e s e and the s t r u c t u r a l c h a r a c t e r i s t i c s of the alloys will p e r m i t r e l i a b l e evaluation of the p e c u l i a r i t i e s of i n t e r p a r t i c l e i n t e r a c t i o n s and i n t e r n a l s t r u c t u r e of the liquid phase. CONCLUSIONS i. The temperature-concentration dependence of resistivity of liquid iron--aluminum alloys was studied. It was established that aluminum rich alloys are characterized by a negative resistivity temperature coefficient while aluminum poor alloys display metallic properties. It has been shown that the isotherms (1535 and 1650~ of p are distinguished by a complex extremal character with a maximum corresponding to the equiatomic alloy. 2. The peculiarities of electrical conductivity in the alloys studied are to be explained by their microinhomogeneity and formation of quasimolecular groupings in which directed interatomic interactions are significant. LITERATURE 1. 2. 3. 4. 5.

CITED

E. S. Levin and G. D. Ayushina, Izv. Akado Nauk SSSR, Metally, No. 6, 52 (1970). B. A. Baum, P. V. Gel'd, and S. I. Suchil'nikov, Izv. Akad. Nauk SSSR, OTN, Metallurgiya i g o r n o e delo, No. 2, 149 (1964}. G. D. Avushina, E. S. Levin, and P. V. Gel'd, Zh. Fiz. I(him., 42, No. 11, 2799 (1968). P. V. P e t r e n k o , and P. P. Kuz'menko, Ukr. Fiz. Zh., 3, No. 6, 820 (1958). E. S. Levin, G. D. Ayushina, L. V. Shchipanova, and P. V. Gel'd, in: Surface P h e n o m e n a in Alloys [in Russian], Naukova Dumka, Kiev (1968), p. 191.

1507