Standard molar enthalpies of formation of PdAl, PtAl, ScAl1.78, YAl2 and LaAl2

Journal of Alloys and Compounds, ] 76 ( 1991 ) 309-318 JAL 0026

309

S t a n d a r d molar enthalpies of formation of PdAI, PtA1, ScAll.78 , YA12 and L a i l 2 W.-G. Jung, O. J. K l e p p a and L. Topor* The James Franck Institute, and Departments of Chemistry and Geophysical Sciences, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 (U.S.A.)

(Received May 26, 1991)

Abstract The standard molar enthalpies of formation of PdAl, PtAl, ScAl~.Ts, YAle and LaAl2 have been determined using a high temperature calorimeter at 1473_+2 K: AH~(PdA1)= -182.5+_9.3 kJ tool -j, AH}(PtAl)=-195.1+_10.1 LI mol -~, z~/}(ScA1,.~s)= -132.6_+2.6 kJ m o l - ' , A//~(YAl2) = -151.2_+3.8 kJ m o l - ' and AH~(LaA12)= -149.6+_ 6.2 kJ tool- '. Comparisons are made with some available data and with predicted values.

1. I n t r o d u c t i o n During r e c e n t y e a r s t h e r e h a s b e e n a g r e a t l y i n c r e a s e d interest in the p h a s e stability and c h e m i c a l b o n d i n g of intermetallic c o m p o u n d s . E v i d e n c e for this interest c a n be f o u n d in the n u m b e r of t h e o r e t i c a l a n d s e m i e m p i r i c a l m o d e l s [ 1 - 8 ] w h i c h h a v e b e e n d e v e l o p e d in o r d e r to p r e d i c t the e n t h a l p i e s of f o r m a t i o n of binary alloy p h a s e s f o r m e d b y transition metals. H o w e v e r , owing to t h e s c a r c i t y o f e x p e r i m e n t a l i n f o r m a t i o n , it h a s b e e n v e r y difficult to t e s t m a n y of t h e s e predictions. W e h a v e for a long t i m e p u r s u e d a r e s e a r c h p r o g r a m w h i c h a t t e m p t s to p r o v i d e reliable e x p e r i m e n t a l i n f o r m a t i o n on t h e t h e r m o c h e m i s t r y of b i n a r y alloy s y s t e m s . In the c o u r s e of t h e s e i n v e s t i g a t i o n s w e h a v e d e t e r m i n e d the e n t h a l p i e s of f o r m a t i o n of a n u m b e r of borides, silicides, g e r m a n i d e s a n d intermetallic c o m p o u n d s of transition metals. In the p r e s e n t w o r k w e e x t e n d this s t u d y to transition m e t a l a l u m i n i d e s a n d h a v e d e t e r m i n e d t h e s t a n d a r d e n t h a l p i e s of f o r m a t i o n o f five c o m p o u n d s : PdA1, PtAl, ScAl,.Ts , YAI2 a n d LaA12. C o m p a r i s o n s will b e m a d e with earlier r e p o r t e d e x p e r i m e n t a l data a n d with p r e d i c t e d values.

2. E x p e r i m e n t a l details T h e e x p e r i m e n t s w e r e carried out at 1473 + 2 K in a modified S e t a r a m t y p e high t e m p e r a t u r e c a l o r i m e t e r . Details of this a p p a r a t u s a n d o f the *Present address: Department of Geology, Princeton University, Princeton, NJ 08544, U.S.A.

0925-8388/91/$3.50

© 1991 -- Elsevier Sequoia, Lausanne

310 c a l o r i m e t e r liner a s s e m b l y h a v e a l r e a d y b e e n given in earlier p u b l i c a t i o n s [9, 10]. B e c a u s e of the possibility of r e d u c t i o n o f the b o r o n nitride (BN) crucible b y a l u m i n u m , a beryllium o x i d e (BeO) c r u c i b l e w a s u s e d in the direct c o m b i n a t i o n e x p e r i m e n t s . A s u r v e y of t h e p h a s e d i a g r a m s o f the c o n s i d e r e d binary s y s t e m s [ 1 1 - 1 3 ] s h o w e d t h a t the c o m p o u n d s of interest are all c o n g r u e n t m e l t i n g a n d h a v e quite high m e l t i n g t e m p e r a t u r e s ( a b o v e 1673 K). H o w e v e r , since v e r y s t r o n g c h e m i c a l i n t e r a c t i o n w a s e x p e c t e d in t h e s e alloy s y s t e m s , t h e direct c o m b i n a t i o n m e t h o d w a s applied as well as solute-solvent drop calorimetry. T a b l e 1 r e p o r t s the metallic purity a n d d e s c r i b e s the m a t e r i a l s u s e d in the direct c o m b i n a t i o n e x p e r i m e n t s . P a l l a d i u m a n d a l u m i n u m p o w d e r s w e r e p u r c h a s e d f r o m J o h n s o n - M a t t h e y AESAR: L o t # 1 9 9 0 1 a n d L o t # 1 8 9 1 8 respectively. P l a t i n u m p o w d e r w a s p u r c h a s e d f r o m B a k e r & Co. Inc. ( L o t # 145). S c a n d i u m , y t t r i u m a n d l a n t h a n u m in l u m p f o r m w e r e p u r c h a s e d f r o m J o h n s o n - M a t t h e y AESAR: L o t # 1 9 0 6 5 , B M 1 8 1 7 a n d B M 2 2 8 2 r e s p e c t i v e l y . S c a n d i u m a n d y t t r i u m p o w d e r s w e r e o b t a i n e d b y s h a v i n g o n a milling m a c h i n e , a n d t h e l a n t h a n u m p o w d e r by h a n d filing the l u m p j u s t b e f o r e m a k i n g the pellets. In this w a y c o n t a m i n a t i o n o f the s a m p l e b y o x i d a t i o n w a s a v o i d e d as far as p o s s i b l e T a b l e 2 d e s c r i b e s t h e m a t e r i a l s u s e d in the s o l u t e - s o l v e n t e x p e r i m e n t s . P a l l a d i u m m e t a l a n d p l a t i n u m m e t a l w e r e p u r c h a s e d f r o m E n g e l h a r d as 2 m m wire. W e also m a d e u s e of p a l l a d i u m foil 0 . 0 2 5 m m thick, p u r c h a s e d TABLE 1 Metallic purity and description of materials used in direct combination measurements Metal

Metallic purity (95)

Description

Pd Pt Sc Y La Al

99.95 99.9 99.9 99.9 99.9 99.5

200-mesh 325-mesh 80-mesh 80-mesh 80-mesh 325-mesh

powder powder shavings shavings shavings powder

TABLE 2 Metallic purity and description of materials used in solute-solvent drop calorimetry Metal

Metallic purity (95)

Description

Pd Pt A1 Ge

99.99 99.99 99.5 99.999

2 mm wire 0.025 mm foil 2 mm wire 2-3 mm pieces 2-3 mm pieces

311 from Engelhard, and s em i c onduc t or grade germanium. Prior to the calorimetric experiments the palladium foil was annealed overnight at 1100 K in an inert atmosphere. The PdAl and PtAl c o m p o u n d s used in the solute-solvent experiments were synthesized by arc melting on a water-cooled copper hearth in an inert atmosphere. The arc-melted buttons were turned over and remelted several times in order to obtain a h o m o g e n e o u s phase. The com pounds were checked by scanning electron m i cr os copy (SEM) and by energy-dispersive X-ray (EDX) analysis. The analysis indicated that both samples were h o m o g e n e o u s and of stoichiometric composition. For the calorimetric experiments samples were crushed into small pieces, ground in a mortar and sifted through 100mesh sieves. Calibration was achieved by dropping small pieces of high purity copper wire 2 mm in diameter from room temperature into the calorimeter at 1473 K. The enthalpy of pure c o p p e r was taken from ref. 1 4 : 4 6 465 J mol-~ at 1473 K. Within a single series of measurements the calibrations were reproducible to + 1.5%.

3. R e s u l t s a n d d i s c u s s i o n

The standard enthalpies of formation were determined by the direct combination method for all c o m p o u n d s investigated in the present study. For comparison, solute--solvent drop calorimetry was also applied to PdA1 and PtA1. In the direct combination experiments the well-mixed powders of transition metal and aluminum, which were weighed carefully in the desired ratio, were compressed into pellets 4 mm in diameter. These pellets were d r o p p ed from room t e m p e r a t u r e into the calorimeter at 1473+_2 K and the enthalpy effects determined. After the experiments the alloy products were removed from the crucible and broken into smaller pieces, which were used for the heat content measurements and also for analysis by SEM and EDX and by powder X-ray diffraction. The experimental procedures can be expressed through the following equations, here shown for MeAl, where Me represents the transition metal: Me(s, 298 K) +Al(s, 298 K)=MeAl(s, 1473 K)

(1)

MeAl(s, 298 K)=MeAl(s, 1473 K)

(2)

From reactions (1) and (2) we get Me(s, 298 K) +Al(s, 298 K) =MeAl(s, 298 K)

(3)

and the standard enthalpy of formation is obtained from AH~(MeAI) = AHm(1) - AHm(2)

(4)

where AHm(1) and AHm(2) r e pr es e nt the molar enthalpy changes for reactions (1) and (2) with respect to MeAl.

312 Since the PdA1 and PtAl alloy products could not be removed completely from the crucible, compounds synthesized by arc melting were used in the heat content measurements (reaction (2)). These arc-melted samples were also checked by SEM and EDX along with reaction products. The analyses indicated that for PdAl only one phase was present in both reaction p r o d u c t and arc-melted sample, while for PtAl a very small amount of a second phase (Pt~kl2) was detected in both samples. We believe that this small amount of an undesired phase does not significantly influence the measured enthalpy of formation. This is confirmed by the agreement with the results obtained by solute-solvent drop calorimetry for the same compounds. Liquid (Pd ÷ Ge) alloy was used as solvent in the solute-solvent drop experiments. The standard enthalpy of formation was obtained from the enthalpy effects associated with forming a liquid alloy of the same composition from the elements and from the compound. For PdA1 the reactions in the calorimeter were 0.35Pd(s, 298 K ) + 0.60Ge(s, 298 K ) + 0.05Al(s, 298 K) = Pdo.~sGeo.8oAlo.os(l, 1473 K)

(5)

0.30Pd(s, 298 K ) + 0 . 6 0 G e ( s , 298 K ) + 0.05PdAl(s, 298 K) = Pd0.3sGeo.6oAlo.os(l, 1473 K)

(6)

where (s) and (l) denote solid and liquid respectively. From reactions (5) and (6) we get 0.05Pd(s, 298 K ) + 0.05Al(s, 298 K ) = 0.05PdAl(s, 298 K)

(7)

Hence the standard enthalpy of formation can be obtained from AH~(PdAI) = AHm(5) - AHm(6)

(8)

where AHm(5) and AHm(6) r e pr es e nt the molar enthalpy changes for reactions (5) and (6) with respect to PdAl. Similar equations can be written for PtAl forming the liquid alloy Pdo.3oGeo.~oPto.o~Alo.os. After the experiments the solidified ingots were examined by SEM and EDX analyses; these examinations confirmed the complete dissolution of the elements and the c o m p o u n d s in the melts. The experimental results for PdAl and PtA1 by the direct combination method are summarized in Tables 3 and 4 and by the solute-solvent drop method in Tables 5 and 6. The enthalpies of formation of PdA1 and PtA1 obtained by the two different methods are in good agreement with each other: - 181.2 _ 4.4 and - 1 8 3 . 8 _ 8.2 kJ m o l - ' for PdA1 and - 1 8 9 . 8 _ 4.9 and -200.4___8.8 kJ m o l - ' for PtA1. Furthermore, these values agree well with the calorimetric values of Ferro and coworkers [15, 16]: - 1 8 7 . 2 kJ m o l - ' for PdAl and - 2 0 0 . 8 kJ m o l - ' for PtAl. In Tables 3 - 6 the average values of AHm for each reaction are given with their standard deviations 6, and ~ . The uncertainties in AH~ were calculated from 6 = (6~z + ~22)~/z. From the standard enthalpies of formation

313 TABLE 3 S t a n d a r d e n t h a l p y o f f o r m a t i o n of PdAl by direct c o m b i n a t i o n m e t h o d at 1473_-+2 K Exp. no.

n(Pd) (mmol)

n(Al) (retool)

1-1 1-2 1-3 1-4 1-5

1.2878 1.7069 1.0607 1.2466 1.7384

1.2878 1.7069 1.0607 1.2466 1.7384

n(PdAl) (retool)

2-1 2-2 2-3 2-4 2-5

AHo~ (J)

AH,,( 1 ) (kJ t o o l - J)

-

-

1.'1274 2.0967 1.6297 2.3635 2.5704

141.9 202.3 121.5 151.2 206.3

AH,,(2) (kJ t o o l - i)

110.2 118.5 114.5 121.3 118.7

72.8 134.5 104.1 151.6 169.3

64.6 64.2 63.9 64.1 65.9 64.5±0.8

-116.6±4.3 AH~=(-

116.6±4.3)-

( 6 4 . 5 ± 0 . 8 ) = - 1 8 1 . 2 ± 4 . 4 kJ tool -~

TABLE 4 S t a n d a r d e n t h a l p y o f f o r m a t i o n of PtA1 by direct c o m b i n a t i o n m e t h o d at 1 4 7 3 ± 2 Exp. no.

n(Pt) (mmol)

n(Al) (mmol)

1-1 1-2 1-3 1-4 1-5

0.9011 0.9583 0.8826 1.0439 0.7399

0.9011 0.9583 0.8826 1.0439 0.7399

n(PtA1) (mmol)

2-1 2-2 2-3 2-4

~J~o~ (J)

~/m(1) (kJ t o o l - ~)

-121.3 - 118.1 - 112.2 - 135.3 - 92.4

-

1.0929 0.8682 0.9493 1.0439

AH,,(2) (kJ t o o l - J)

134.7 123.2 127.1 129.6 124.8

65.7 56.1 59.4 63.1

60.1 64.6 62.6 60.5 -127.9±4.5

AH})= ( - 1 2 7 . 9 ± 4 . 5 ) - - ( 6 1 . 9 ± 2 . 1 ) =

for

PdAl

average molar

or

PtA1 obtained

of the enthalpy

two

AHKPtAI) For we

= - 195.1 the

adopted

direct the

by

results.

the

different

way

the

of PdA1 and

methods

mean

we

values

PtAl were

found

calculated of

the

the

standard

to be

i

kJ mo1-1

combination

MeAl2

two

In this

± 9.3 kJ mol± 10.1

61.9±2.1

- 1 8 9 . 8 ± 4.9 kJ m o l -~

of formation

AH~'(PdAI) = - 182.5

K

experiments

composition

(Me-Sc,

on

group

Y, L a ) .

III metal Examination

aluminides by

SEM

314 TABLE 5 Standard enthalpy of formation of PdAl obtained by generating the liquid alloy Pd0.3~Ge0.6oAlo.o~ at 1473_+2 K Exp. no.

n(Pd) (mmol)

n(Ge) (retool)

1-1 1-2 1-3

1.5545 1.5633 1.5859

3.1718 3.1266 3.1090

2-1 2-2 2-3

2.8552 2.9187 2.8505

4.8946 5.0034 4.8865

n(Al) (retool)

n(PdAl) (retool)

AHobs (J)

0.2643 0.2605 0.2590

162.7 156.6 157.9

0.4078 0.4169 0.4072

173.6 175.4 174.5

AH~,(5) (kJ mol- ~)

AHm(6 ) (kJ mol-~) 615.6 601.2 609.7

425.7 420.7 428.5 425.0 2=4.0

608.8 _ 7.2

AH~' = (425.0 -+ 4.0) - (608.8 -+ 7.2) = - 183.8 -+ 8.2 kJ tool-

TABLE 6 Standard enthalpy of formation of Pdo.a0Geo.6oPto.o,~,A10.o5 at 1473 2=2 K Exp. no.

n(Pd) (mmol)

n(Ge) (mmol)

1-1 1-2 1-3 1-4 1-5

1.1499 1.1217 1.1654 1.1273 1.2114

2.2998 2.2434 2.3308 2.2546 2.4228

2-1 2-2 2-3 2-4 2-5

1.7443 1.7344 1.7377 1.7321 1.7481

3.4886 3.4688 3.4754 3.4642 3.4962

n(Pt +AI) (retool)

PtAl

obtained

by

n(PtAl) (retool)

AHo~ (J)

0.1916 0.1869 0.1942 0.1878 0.2019

116.4 114.6 119.8 114.1 121.9

2(0.2907) 2(0.2890) 2(0.2896) 2(0.2896) 2(0.2913)

121.0 115.7 118.8 116.6 121.2

generating

AHm(5 ) (kJ mol -J)

the

liquid

alloy

AHm(6) (kJ tool - j ) 607.5 613.2 616.9 607.6 603.8

416.2 400.3 410.2 404.0 416.1 409.4 :i=7.1

609.8___5.2

5H~ = (409.4 + 7. l) - (609.82= 5.2) = - 200.4 + 8.8 kJ mol- '

a n d E D X a n a l y s i s i n d i c a t e d t h a t 9 9 % YAle o r 9 6 % LaAl2 w a s p r e s e n t in t h e s e two reaction products. However, the ScAle sample showed a large deviation from this stoichiometry. On the basis of this analysis we carried out our measurements o n Sco.3e~,1o.84 (ScA1,.Ts). T h e c a l o r i m e t r i c r e s u l t s f o r Sco.3e~Alo.64, YAle a n d L a A l 2 o b t a i n e d b y t h e d i r e c t c o m b i n a t i o n m e t h o d a r e g i v e n in T a b l e s 7 - 9 . T h e s t a n d a r d m o l a r enthalpies of formation are AH~'(ScA1,.Ts) = - 1 3 2 . 6 :i= 2 . 6 k J m o l - '

315 TABLE 7 Standard enthalpy of formation of Sc0.3~A1o~4 by direct combination method at 1473 ± 2 K Exp. no.

n(Sc) (mmol)

n(Al) (retool)

1-1 1-2 1-3 1-4 1-5

1.6928 1.6683 1.6372 1.9419 1.9041

3.0094 2.9659 2.9105 3.4523 3.3850

2-1 2-2 2-3 2-4 2-5

n(Sc0=)6A10.64 ) (retool)

3.7725 4.3882 4.3075 5.4644 5.5451

AHo~ (J)

A//m( 1) (kJ (g a t o m ) - ' )

- 53.7 -55.7 - 52.3 - 69.0 - 63.4

-

AHm(2) (LI (g a t o m , - J )

11.7 12.3 11.8 13.1 12.2

132.3 157.1 152.6 199.1 191.4

35.1 35.8 35.4 36.4 34.5 - 12.2_+0.6

35.5 ±0.7

A H ~ = ( - 1 2 . 2 _ + 0 . 6 ) - ( 3 5 . 5 = [ = 0 . 7 ) = - 4 7 . 7 ± 0 . 9 kJ (g a t o m ) - '

TABLE 8 Standard enthalpy of formation of YAIa by direct combination method at 1473_+ 2 K Exp. no.

n(Y) (mmol)

n(Al) (mmol)

1-1 1-2 I-3 1-4 1-5 1-6

1.2214 1.7911 1.5847 1.7953 1.5140 1.4657

2.4428 3.5822 3.1694 3.5906 3.0280 2.9314

2-1 2-2 2-3 2-4 2-5

n(YA12) (mmol)

AHob~ (J)

AHm( 1) (kJ mol- ')

- 71.9 - 104.4 -94.9 - 102.9 -98.9 - 91.5

- 58.9 - 58.3 -59.9 - 57.3 - 65.3 - 62.4

1.1073 1.1948 1.7659 2. 3602 1.4734

97.7 108.4 164.5 219.9 131.0

AHm(2 ) (kJ tool- ')

88.2 90.7 93.1 93.2 88.9 90.8±2.3

-60.4±3.0 AH~' = ( - 6 0 . 4 ± 3 . 0 ) - ( 9 0 . 8 ± 2 . 3 ) + - 1 5 1 . 2 ± 3 . 8 kJ t o o l -

AHof(YAI2)= - 151.2+_3.8

L I m o l -~

AH~'(LaAI2) = - 1 4 9 . 6 _ + 6 . 2

k J m o l -~

w h e r e t h e v a l u e f o r ScA1LT8 w a s c a l c u l a t e d

by multiplying

(-

4 7 . 7 +- 0 . 9 ) b y

(1 + 1 . 7 8 ) . T h e e x p e r i m e n t a l r e s u l t s o b t a i n e d i n t h e p r e s e n t w o r k a r e s h o w n i n F i g . 1, w h e r e t h e y a r e c o m p a r e d w i t h a v a i l a b l e v a l u e s . B e c a u s e t w o d i f f e r e n t compositions

are compared

i n t h i s f i g u r e , AH~' i s p r e s e n t e d

in kilojoules per

316 TABLE 9 Standard enthalpy of formation of Exp. no.

n(La) (mmol)

n(Al) (mmol)

1-1 1-2 1-3 1-4

1.2983 1.5321 1.5005 1.6197

2.5966 3.0642 3.0010 3.2394

2-I 2-2 2-3 2-4 2-5

LaA] 2

by direct combination method at 1 4 7 3 ± 2 K

n(LaA12) (retool)

1.0312 1.0027 1.6005 1.2210 1.7291

AHo~ (J)

AH~,(1) (kJ tool- 3)

- 74.6 - 79.5 -88.4 - 90.6

-57.5 - 51.9 -58.9 - 56.0

95.8 85.9 153.3 I13.1 173.6

92.9 85.7 95.8 92.7 100.4 -56.1 ±3.0

AH~=(-56.1 ±3.0)-(93.5±5.4)=

ve tooi MeAl I

I

AHm(2 ) (kJ m o l - 3)

93.5 ~: 5.4

-149.6±6.2 kJ mol -~

,~ i 1

°::I 6O 5

7-. 8O

Eo

i

o

I

I

Ni

Pd

I MeAl2

70

60

x~ ~

o~

.~I 5o

I

Pt

I

+

~x....... o

ox

I/

40 I

I

I

Sc°

Y

L0

Fig. 1. Standard enthalpies of formation for MeAl and MeAl2 from present investigation compared with available values:~, present work (direct combination method); @, present work (solu t e - s o l v e n t drop method); × , de Boer et al. [ 8 ] (estimated); V, Hultgren e t al . [ 17 ] (assessment); A, Ferro and coworkers [15, 16]; e , Ettenberg et a l . [18] (vapour pressure); ®, Pratt and Bryant [19]; + , Snyder [21]; O, Kober et al. [20, 24] (e.m.f.); E], Colinet e t al. [22]; A, C a n n e d and Rossi [23]. *, ScAI3.vs.

317 g r a m a t o m . The value for NiAl given in the figure is t h a t s e l e c t e d by H u l t g r e n et al. [17], t h o u g h t h e r e p o r t e d e x p e r i m e n t a l v a l u e s w e r e - 5 9 to - 7 1 kJ (g a t o m ) -1. T h e p r e d i c t e d v a l u e s of de B o e r et al. [8] s h o w c o n s i d e r a b l e d i s c r e p a n c y f r o m o u r e x p e r i m e n t a l values; h o w e v e r , the general t r e n d s a r e in fair a g r e e m e n t with e x p e r i m e n t . Our results for PdAl a g r e e well with t h o s e of E t t e n b e r g et al. [18] ( - 9 5 kJ (g a t o m ) -~) a n d P r a t t and B r y a n t [19] ( - 92.5 kJ (g a t o m ) - 1). To the b e s t of o u r k n o w l e d g e an e x p e r i m e n t a l value for ScAll.Ts is not f o u n d in the literature. Our n e w value for YAlz is s m a l l e r in m a g n i t u d e t h a n t h o s e of K o b e r et al. [20] ( - 5 7 . 7 kJ (g a t o m ) -1) and S n y d e r [21] ( - 81 kJ (g a t o m ) - 1 ) , w h i c h w e r e o b t a i n e d by the e.m.f, m e t h o d a n d c o m b u s t i o n c a l o r i m e t r y respectively. Our v a l u e f o r LaAl e a g r e e s well with t h o s e of Colinet et al. [22] ( - 5 4 . 2 kJ (g a t o m ) - I ) and Canneri a n d Rossi [23] ( - 50 kJ (g a t o m ) - 1 ) , b o t h o b t a i n e d b y c a l o r i m e t r y , but n o t with that of K o b e r et al. [24] ( - 67 k J (g a t o m ) - 1 ) o b t a i n e d b y the e.m.f, m e t h o d . Finally, it m a y b e m e n t i o n e d t h a t o u r values f o r ScAll.78 ( - 4 7 . 7 kJ (g a t o m ) - 1 ) a n d YAle ( - 5 0 . 4 kJ (g a t o m ) - 1 ) m a y be c o m p a r e d in m a g n i t u d e with the c a l o r i m e t r i c results o f K u b a s c h e w s k i a n d H e y m e r [25] f o r TiAla ( - 36 kJ (g a t o m ) - 1) a n d with the third-law v a p o r p r e s s u r e dat~: of K e m a t i c k a n d F r a n z e n [26] for ZrAl2 ( - 4 6 kJ (g a t o m ) - 1 ) .

Acknowledgments This investigation w a s s u p p o r t e d b y the DOE u n d e r g r a n t D e - F G 0 2 8 8 E R 4 5 3 6 3 a n d b y t h e Alcoa F o u n d a t i o n I n t e r m e t a l l i c C o m p o u n d s Grant; it also b e n e f i t e d f r o m the central facilities o f the University of C h i c a g o MRL. W e are i n d e b t e d to Dr. A. M. Davis w h o carried out the SEM a n d EDX analyses.

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