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Site occupation determination of Pd in Ni3Al by ALCHEMI
Acta metall, mater. Vol. 39, N o . 1, pp. 13-18, 1991 Printed in G r e a t Britain. All rights reserved
0956-7151/91 $3.00 + 0.00 Copyright © 1991 P e r g a m o n Press pie
SITE OCCUPATION DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI A. CHIBA,I" D. SHINDO and S. HANADA Institute for Materials Research, Tohoku University, Sendal 980, Japan (Received 27 September 1989; in revised form 4 July 1990) Abstract--Atom location, by channelling enhanced microanalysis (ALCHEMI), was performed to examine the substitution behavior of Pd atoms added to a Ni3AI intermetallic compound. From the quantitative analysis, it can be concluded that Pd has a strong tendency to locate in the Ni site of the compositions Ni75A123Pd 2, Ni74A124Pd2, and Ni73A125Pd2, which means that the anti-structure defects are formed in the compositions of Ni7s_xA125_yPdx+y(X~ y , y ~0). In addition, the above results are thermodynamically discussed by considering the formation of the anti-structure defects introduced by Pd addition. It is investigated as to whether the substitution behavior of ternary alloying elements, which are known to substitute for the Ni site in the composition of Ni75_xAI25Cx, depend on the composition of the host elements when they are added to the composition of Ni75AI25_xC~. Rrsumr--On effectue une localisation atomique par microanalyse renforcre par canalisation (ALCHEMI) pour examiner le comportement substitutionnel des atomes de Pd ajoutrs au compos6 intermrtallique Ni3A1. A partir de l'analyse quantitative, on peut conclure que le Pd a une forte tendance ~ se localiser sur les sites de Ni avec les compositions Ni75AI23Pd2,Ni74AI24Pd2 et Ni73A125Pd2, ce qui signifie que des drfauts sont cr66s avec les compositions Ni75- xA125- yPdx+. y , (x ~y, y # 0)- De plus, . . antistrueture . . , les resultats e~-dessus sont dtscutes du pont de rue thermodynamlque en cons~derant la formaUon de defauts antistructure introdults par l'addition de Pd. On 6tudie le problrme suivant: le comportement substitutionnel des 616ments d'aUiage ternaire, qui sont connus pour se substituer aux sites de Ni avec la composition Ni75- xA125Cx,drpend-il de la composition de 616ments-hrtes lorsqu'ils sont ajoutrs jusqu'~ la composition Ni75A125- xC~.
Zesammenfassung--Das Substitutionsverhalten von Pd-Atomen bie Zugabe zur der intermetallischen Verbindung Ni3A1 wird mittels ALCHEMI (Bestimmung yon Atomorten durch gitterfiihrungsverst/irkte Mikroanalyse) analysiert. Aus der quantitativen Analyse kann gefolgert werden, dab Pd eine grol3e Neigung aufweist, sich auf Nickelpl/itzen der Legierungen Ni75AI23Pd2, Ni74A124Pd2 und Ni73AI25Pd2 einzulagern, d.h. es werden Antistrukturdefekte in den Legierungen der Zusammensetzungen Ni75_xA125_yPdx+y (x ~
1. INTRODUCTION
Ni3A1 [2], has a great tendency to segregate to grain-boundaries [3]. It is generally accepted that the segregation of B to grain-boundaries results in improvement of ductility. On the other hand, ductilization by macroalloying with substitutional ternary elements is also a worthy consideration from the view point of practical use. Since grain-boundary cohesion depends on the type and strength of the interatomic bonds [4], it is important to determine the occupation site of ternary elements in order to examine the relation between substitution and ductilization in Ni3A1. According to the current concept of macroalloying, ternary elements, substituting for the Ni or the AI sites, are added to Ni3AI in replacement of Ni or AI atoms respectively. It has not been established,
A strongly ordered intermetallic c o m p o u n d Ni3A1 exhibits brittle fracture due to the intrinsic weakness of grain-boundaries. Its application as a hightemperature structural material has therefore been restricted in spite of its excellent high-temperature strength properties, i.e. the material becomes stronger when subjected to elevated temperatures. Considerable effort has been made in improving the ductility of NiaA1. It was discovered by Aoki and Izumi [1] that microalloying with B improves ductility of Ni3AI dramatically. B, an interstitial element in l'On leave from: Faculty of Engineering, Department of Metallurgy, Iwate University, Morioka 020, Japan. 13
14
CHIBA et al.:
D E T E R M I N A T I O N OF Pd IN Ni3A1 BY A L C H E M I
however, that the substitution behavior of ternary Table 1. Nominal composition and occupation fraction in the Ni site of the compound NiaAl~Pdc, together with experimental error due alloying elements is not affected by the composition to the counting statistics of the host elements. It is thus important to examine Counting experimentally such composition dependence of the Occupation statistical fraction error substitution behavior. Specimen Measurement k (%) _+Ak (%) The macroalloying behavior of ternary additions 1 117 20 (i.e. the substitution of ternary additions for Ni, for I Ni75A123Pd2 2 107 18 A1 or for both sites), has hitherto been discussed 3 97 22 1 99 13 from the knowledge of the direction of solubility lobe II Ni74A124Pd 2 2 82 17 of Ni3A1 in the ternary phase diagram [5]. However, 3 69 22 1 92 9 the site determination obtained from these consider- III Ni73A125Pd2 ations is deficient in the case of a small addition of a ternary element or in examining the effect of annealed at 1273K for 5 days for homogenization deviation from stoichiometry on the substitution and then dropped into iced water. After slicing, by a behavior. Quantitative investigations [6-8] concern- spark-cutting machine and polishing mechanically, ing the site occupation determination of both inter- the specimens for the ALCHEMI measurements were stitial and substitutional elements have been jet-electropolished in a solution of 1 part sulfuric acid performed using Debye-Scherrer technique and by and 9 parts methanol. ion channeling and nuclear reaction analysis respectThe ALCHEMI measurements were performed ively. These methods did not deal with the effect of with a JEM 2000FX electron microscope equipped deviation from stoichiometry on the substitution with the EDX (Energy Dispersive X-ray specbehavior. Recently, Ochiai et al. [9], based on the trometer) system. The electron beam was focused to Bragg-Williams approximation of nearest neighbour about 2/~m in diameter with an accelerating voltage interactions, have investigated the substitution be- of 200 kV. The direction of the incident beam was havior of ternary additions in intermetallic com- chosen nearly parallel to the (110) planes instead of pounds with the L12-type ordered crystal structure. the (001) planes, of which the neighboring fundamenIn their investigation, there has been no discussion tal reflections were closely excited. Near the 2-beam relating the substitution behavior of the ternary diffraction, the X-ray spectra were taken at one non addition to the deviation from the stoichiometry of channelling and two diffraction conditions, s > 0, host elements. However, since it has been revealed s < 0, where s is the excitation error. Examples of that the ductility of B-doped Ni3Al is affected by the electron diffraction patterns, representing the deviation from stoichiometry [3, 10], studying on s < 0 and s > 0 diffraction conditions, are shown in substitution behavior accompanied with the devi- Fig. 1. ation from stoichiometry is required for the underALCHEMI formulation [11] for the Ll2-type standing of ductility by macroalloying. structure was set up as follows: The present work presents an investigation of the site determination of Pd in Ni3A1 by means of Let ~'Ni,J~/-(n)N~l and N~ I be the nine X-ray counts ALCHEMI [ll] under a planar channelling confrom the Ni, A1 and the additional element X dition. Pd is selected as a substitutional ternary for the diffraction conditions n = 1, 2 and 3. element, because Pd has been found to strongly Then we have the nine relations occupy the Ni site in Ni3A1 on the basis of thermodynamic analysis. Much attention is focused on the N~! = eNi" {A= "I=+ A~'I~ } (1) dependence of the substitution behavior of Pd as a function of the composition of the host elements. N ~ I = PA," {B,'I, + B#'I~} (2) Therefore, three specimens for A L C H E M I measurements, NiTaAI25Pd2, Ni74A124Pd2, and Ni75A123Pd2 U~ ) = Px" { C," I~ + C a"/~ } are examined. Furthermore the obtained results are = P x . { 2 . c . r . I , + l ' c ' t ¢ • + c ' ( 1 - t c ) ' I ~ } (3) discussed in terms of the thermodynamic analysis. 2. E X P E R I M E N T A L
Three types of A L C H E M I specimens were used in this investigation. Alloy buttons of 50 mm diameter were prepared by arc-melting, raw materials four times in argon gas atmosphere at a pressure of about 700 torr. The nominal compositions and the chemical analyses of the A L C H E M I specimens are listed in Table 1 and Table 2 respectively. Rod-shaped bulk materials obtained by cutting the alloy buttons were encapsuled into I x 10-Storr with Ti foil getter,
where I, and I~ are the thickness averaged electron intensities on the ~ and fl planes, respectively. Here, for the atomic arrangement of Ni3AI with L12-type structure, the ~t plane and the fl plane represent the Table 2. Results of chemical analysis
Specimen I II III
Nominal composition (at. %) Ni-23A1-2Pd Ni-24Al-2Pd Ni-25AI-2Pd
Analysed composition (at. %) Ni-22.7Al-1.92Pd Ni-23.7Al-1.89Pd Ni-24.TAI-1.91Pd
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
15
Experimental error due to the counting statistics, Ax, are estimated to be
Ax =A~LON~ !
N,
O N ~ A N ~ + ON~-----sANx].
(4) In the above equation, the sum of the three diffraction conditions n = 1, 2 and 3 is calculated and AN is given by AN = 2 x / ~
(5)
which shows a 96% confidence level. 3. RESULTS
Fig. 1. Diffraction patterns for the two diffraction conditions. (a) Negative excitation error for the T10 reflection. (b) Positive excitation error for the I10 reflection.
Figure 2 shows an example of spectra taken from the Ni75A123Pd2 at the two diffraction conditions (s < 0, s > 0), where the peak intensity of A1 is set equal for comparison. The X-ray intensity from Ni on the first diffraction condition (s < 0) is higher than that of the second one (s > 0). Also, the X-ray intensity from Pd changes in accordance with that of Ni, indicating that the electron channelling effect under a planar channeling condition, is reflected directly in the difference between the two diffraction conditions and that a Pd atom has a tendency to occupy the Ni site qualitatively. With these X-ray spectra, taken from three analysis points for specimen I and II and from a single analysis point for specimen III, quantitative evaluation of the site occupancy of Pd in Ni site was carried out by using ALCHEMI formulation as described above in detail. The results are listed in Table 1, together with experimental error due to the X-ray counting statistics, Ak. As can be seen in this table, site occupancies, k, of the specimen I and II are 107 and 99% respectively, when chosen at the minimal Ak value of individual measurement. For specimen III, 92% site occupancy is derived. These results show that Pd atoms always substitute for Ni sites independent of the composition of the host elements. Furthermore, the results of this investigation are plotted in Fig. 3, together with those of Fe and Co additions obtained by Shindo et al. [12]. As can be seen in this figure, one can find that the substitution behavior of Fe depends on the composition of the host elements, whereas Pd always substitutes for Ni site.
planes composed of only Ni atoms and of both Ni and A1 atoms respectively, when the incident electron beam is set nearly parallel to the (I10) planes. The superscript (n) indicates the three diffraction conditions, i.e. n = 1 for s < 0, n = 2 for s > 0 and n = 3 for non-channelling condition. PNi, PAl and Px are factors for considering the absorption and the difference of X-ray fluorescence yields of each element. A~,,, B~., and C~,~ indicate the atomic ratios of Ni, A1 and X in the ~, fl planes respectively. A~,, and B~., refer to the three compounds I, II and III in Table 3, where a, b and c indicate the atomic ratios of the constituents Ni, A1 and X (a + b + c = 100). Here, we take into account the anti-site occupancy of the host elements, i.e. A1 atoms in the Ni site in compound I, and Ni atoms in the A1 site in compound III. As for compound II, one of the coefficients A~,B and B~,, assigned to compounds I and III is used depending on the substitution behavior of the addition, i.e. if Pd favors the AI site, the coefficients for compound I should be used, otherwise those for compound III should be used. Eliminating the unknown parameters of PNi,AI,X and I,.,(n), from the equations (1), (2) and (3), k, the 4. DISCUSSION atomic fraction of X atom at the Ni site can be obtained in the same manner as described in the Ochiai et al. investigated the substitution behavior previous paper [10]. of transition metal elements in Ll2-type compounds
Specimens I(II) III ( I I )
Table 3. Atomicratios A~.a and B~.a in the planes a and fl Atomic ratio A~ Ap B~ Ba 2/3.a l/3"a 2/3-(75 - a -c'k) 1/3"(75- a -c'k) +[25 - c "(1 - k)l
2/3.(75-c'k)
1/3"(75-c.k) +a -75+c-k
0
b
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
16 20
ii
!i1i
AI...... ~...................
Q
15
i
i =
AI
i :
........i................ ~
i Nii
i
s<0i ....... i !
-
...............i .......
................. i ................ '-i
..................
0
~a
1
2
3
4
7
8 9 KeV
1
2
3
4
7
8 9 KeY
Fig. 2. Characteristic X-ray spectra of Ni75A123Pd2 on two diffraction conditions (s > 0, s < 0). Ni3AI, Ni3Ga, Ni3Si and Ni3Ge based on the Bragg-Williams approximation of "Nearest Neighbour Interaction" [9]. They considered the change in total bond energy in the A3B system by adding a small amount of ternary element "C" in two extreme substitution cases, namely in the case of exclusively substituting for Ni site in the composition of Ni75_ xAl2sCx and in the case of exclusively substituting for A1 site in the composition of NiTsA125_xC~. In both cases, the anti-structure defect was assumed not to be formed. According to the present investigation, however, it is shown that Pd atoms substitute for Ni sites even in the composition of Ni75Al:~Pd 2, suggesting that the anti-structure defects of Ni atoms in AI sites are formed by the Pd addition. Therefore, we consider the reason why Pd atoms have a strong preference to the Ni sites in spite of forming such anti-structure defects in Ni atoms. According to Ochiai et al. [9], in Ni75A12s_~Cx alloy, the total bond energy, H (A1), can be expressed as follows when C atoms exclusively substitute for the AI sites H (AI) = 6N[3HNiNi + (~1 -- x c ) H ~ J
+ x c H ~ a + xcHcc] + 12N[¼ - xc)VN~ + xcVN~] (6)
xc the mole fraction of C atom, HNiN~, etc. the-bond energy for NiNi pairs, etc. and V N i A I , etc. the interaction parameter VNiAI= HNiAI-- (HNiNi+ HAIAI)/2, etc. When C atoms completely substitute for Ni sites, the total bond energy, H(Ni), can be expressed as follows if the mole fraction, "z", of Ni atoms in the AI site is introduced as a parameter for the anti-structure defect. H(Ni) = ~ N ' HNiNi+
3(1
--
X¢) N ' H ~ , I
+ 6xcN" Hcc + N ~ - 8z + 3 6 4 z 2 - 5 - ( 1 6 z 4 2 4 ) VNiAI
+5-
+N" ~ f i ( 1 2 - 16z - 3 2 X c ) V~c 3 X~
+ N . 3 ( 1 6 z + 24)VNic
(7)
where the relationship, z = x¢, holds in this case. From equations (6) and (7), the following relation can be derived
H(A1)-/-/(Ni)
where N is the total number of atoms in the crystal.
= 4 VNi~a"N "z [ ( 1 - 4z )( 1 - VNiA IValc// 4 ( 1 - - ~ Z ) Vale ] .
VNiAIj
(8)
By differentiating equation (8), we obtain
1 _FaL dxc jI_~c=O
dxc Jxc=0
+' VNiC']_ ~-'4VNiAI'N[C|- VNild/V/ A1c
Ni;,sAI2s-zX=Nivs~tzAlzs-z~Z Xz Ni';,5.zA[2sXz Fig. 3. Occupation fraction in the Ni site of added elements X (X = Pd, Fe and Co).
(9)
Here, let C atom be Pd atom, the values of VAIpd/VNi~ and VNipd/VNi~d can be estimated at 1.8 and 0.04, respectively from the data evaluated by Ochiai et al.
CHIBA et al.: DETERMINATION OF Pd IN NisAl BY ALCHEMI [9]. When this numerical data is substituted in equation (9), we obtain
_ Fd.(Ni)]
j .....
dxpd
L
J..... = -- 3.04' N ' VNiAI> 0.
Because the VNiAI is expected to be negative, then
L dxed ~ 0 ..I-• =0'
dxea j . . . . .
As a result, equation (10) clearly shows that the present A L C H E M I results, which indicate that Pd has a preference to locate in the Ni atom site independent of composition of the host elements, can be explained in terms of the thermodynamic basis. In addition, the relationship of equation (10) can be generally derived from equation (9) if the following condition is satisfied VAACx
VNiC
VNiC
VAIC
1 -- VNiA1)+V--~iAI<0 VNIA
(11)
Figure 4 indicates the correlation between the bond energy ratios of VNic/VNi~ and V~uc/VNiAafor various ternary transition elements with the site preferences in the Ni3A1 compounds [91. The solid line in this figure divides the ternary elements into two categories; one substituting for the A1 site and the other substituting for the Ni site. According to Ochiai et aL [9], the elements situated below the solid line substitute for the Ni site, whereas those above the line substitute for the AI site. Furthermore the dotted line, derived from equation (11), in this figure divides the ternary elements substituting for the Ni site into two parts. That is, the elements situated below the dotted line always substitute for the Ni site regardless of the composition of the host elements. On the other hand, the elements situated between the solid line and the dotted line substitute for the Ni site and the AI site 3
i
Ni3AI } OchiQiat al. / . . . . This work / . / rZ/r / , //// •
2 ~_
ri
z
>
..b . /
:~
~ /
M ~ OeCreMn 0
We °e / .~'~e Co /
/c'~U ~AU ": A'g:"
-10
~
"/.so / ~., ,"
I II/ /
•
Pt
• PO
i
2
3
VAIc / VNiAI Fig. 4. The substitution behavior of ternary addition in Ni3AI. • marks and solid line are referred to from the data evaluated by Ochiai et al. [9]. The dotted line is derived from the equation (I 1). AM 39/I--B
17
if ternary alloying elements are added in the compositions of Ni75_xAI25Cx and Ni75AI25_xCx respectively. It was actually found in previous investigation [12] shown in Fig. 3 that the substitution behavior of Fe atom, which is situated between the solid line and dotted line in Fig. 4, has a tendency to depend on the composition of the host elements. It can also be seen in Fig. 4 that among the transition elements, which substitute for the Ni site in the composition of Ni75_xAI25X x (i,e. situated below the solid line), Pt as well as Pd may also substitute for Ni site independent of the composition of the host elements. Wu et al. [13] studied the site preference of ternary additions in an A 3B intermetallic compound with L 12 structure using the tetrahedron approximation of the Cluster Variation Method and derived the similar results to those obtained from the present investigation. Thus we concluded that the substitution behavior of the ternary alloying element Pd in the compositions of Ni75 _ xA125_ yPdx +y (X <~y, y 5 0 ) , can be explained by taking the formation of the anti-structure defects into account. Finally it should be noted that it is interesting to examine the mechanical properties of Pd or Pt doped Ni3A1, Ni3Ga, Ni3Si or Ni3Ge because Pd or Pt shows a peculiar substitution behavior in comparison with other transition elements such as Co, Fe and Mn, etc. having the compositional dependence of the host elements on the substitution behavior. 5. CONCLUSIONS 1. From the quantitative ALCHEMI, it is found that Pd has a strong preference to locate in the Ni site of the compositions of Ni7sA123Pd2, Ni74A124Pd2 and Niv3AlzsPd2, which means that antistructure defects are formed in the composition of Ni75_ xA125_yPdx +y(X ~ y, y # 0). 2. By considering the formation of the anti-structure defects, the present A L C H E M I results can be explained in terms of the thermodynamic basis. 3. It is generally revealed from the thermodynamic analysis whether or not the substitution behavior of ternary alloying elements, which substitute for Ni site in the composition of Ni75 xA125Cx, depend on the composition of the host elements, when they are added in the composition of Ni75AI25_xC x. 4. It is suggested that Pt may also have a preference to locate in the Ni site regardless of the composition of the host element. Acknowledgements--The authors are grateful to Dr T. Sekiguchi for his kind help with our operating the electron microscope. One of us (A.C.) would like to thank Professor K. Tanosaki, Associate Professor M. Fujita and Mr K. Nonaka, of Iwate University, for their encouragement. REFERENCES
1. K. Aoki and O. Izumi, Nippon Kinzoku Gakkaishi 43, 1190 (1979).
18
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
2. I. Baker, B. Huang and E. M. Schulson, Acta metall. 36, 493 (1988). 3. C. T. Liu, C. L. White and J. A. Horton, Acta metall. 33, 213 (1985). 4. O. Izumi and T. Takasugi, J. Mater. Res. 3, 426 (1988). 5. R. W. Guard and J. H. Westbrook, Trans. metalL Soc. A.LM.E. 215, 807 (1959). 6. N. Masahashi, T. Takasugi and O. Izumi, Acta metall. 36, 1815 (1988). 7. H. Lin, L. E. Seiberling, P. F. Lyman and D. P. Pope, M R S Proc., Vol. 81, High Temperature Ordered Intermetallic Alloys II, pp. 165-172. MRS (1987).
8. A. I. Taub, C. L. Briant, S. C. Huang, K. M. Chang and M. R. Jackson, Scripta metall. 20, 129 (1986). 9. S. Ochiai, Y. Oya and T. Suzuki, Acta metall. 32, 289 (1984). 10. V. Vitek, J. J. Kruisman and J. Th. M. Dehosson, Proc. M R S Syrup., Reno, Nev. (edited by M. H. Yoo, W. A. T. Clark and C. L. Braiant), p. 139. MRS (1988). 11. J. C. H. Spence and J. Tafto, J. Microsc. 130, 147 (1983). 12. D. Shindo, M. Kikuchi, M. Hirabayashi, S. Hanada and O. Izumi, Trans. Japan Inst. Metals 29, 956 (1988). 13. Y. P. Wu, N. C. Tso, J. M. Sanchez and J. K. Tien, Acta metall. 37, 2835 (1989).
0956-7151/91 $3.00 + 0.00 Copyright © 1991 P e r g a m o n Press pie
SITE OCCUPATION DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI A. CHIBA,I" D. SHINDO and S. HANADA Institute for Materials Research, Tohoku University, Sendal 980, Japan (Received 27 September 1989; in revised form 4 July 1990) Abstract--Atom location, by channelling enhanced microanalysis (ALCHEMI), was performed to examine the substitution behavior of Pd atoms added to a Ni3AI intermetallic compound. From the quantitative analysis, it can be concluded that Pd has a strong tendency to locate in the Ni site of the compositions Ni75A123Pd 2, Ni74A124Pd2, and Ni73A125Pd2, which means that the anti-structure defects are formed in the compositions of Ni7s_xA125_yPdx+y(X~ y , y ~0). In addition, the above results are thermodynamically discussed by considering the formation of the anti-structure defects introduced by Pd addition. It is investigated as to whether the substitution behavior of ternary alloying elements, which are known to substitute for the Ni site in the composition of Ni75_xAI25Cx, depend on the composition of the host elements when they are added to the composition of Ni75AI25_xC~. Rrsumr--On effectue une localisation atomique par microanalyse renforcre par canalisation (ALCHEMI) pour examiner le comportement substitutionnel des atomes de Pd ajoutrs au compos6 intermrtallique Ni3A1. A partir de l'analyse quantitative, on peut conclure que le Pd a une forte tendance ~ se localiser sur les sites de Ni avec les compositions Ni75AI23Pd2,Ni74AI24Pd2 et Ni73A125Pd2, ce qui signifie que des drfauts sont cr66s avec les compositions Ni75- xA125- yPdx+. y , (x ~y, y # 0)- De plus, . . antistrueture . . , les resultats e~-dessus sont dtscutes du pont de rue thermodynamlque en cons~derant la formaUon de defauts antistructure introdults par l'addition de Pd. On 6tudie le problrme suivant: le comportement substitutionnel des 616ments d'aUiage ternaire, qui sont connus pour se substituer aux sites de Ni avec la composition Ni75- xA125Cx,drpend-il de la composition de 616ments-hrtes lorsqu'ils sont ajoutrs jusqu'~ la composition Ni75A125- xC~.
Zesammenfassung--Das Substitutionsverhalten von Pd-Atomen bie Zugabe zur der intermetallischen Verbindung Ni3A1 wird mittels ALCHEMI (Bestimmung yon Atomorten durch gitterfiihrungsverst/irkte Mikroanalyse) analysiert. Aus der quantitativen Analyse kann gefolgert werden, dab Pd eine grol3e Neigung aufweist, sich auf Nickelpl/itzen der Legierungen Ni75AI23Pd2, Ni74A124Pd2 und Ni73AI25Pd2 einzulagern, d.h. es werden Antistrukturdefekte in den Legierungen der Zusammensetzungen Ni75_xA125_yPdx+y (x ~
1. INTRODUCTION
Ni3A1 [2], has a great tendency to segregate to grain-boundaries [3]. It is generally accepted that the segregation of B to grain-boundaries results in improvement of ductility. On the other hand, ductilization by macroalloying with substitutional ternary elements is also a worthy consideration from the view point of practical use. Since grain-boundary cohesion depends on the type and strength of the interatomic bonds [4], it is important to determine the occupation site of ternary elements in order to examine the relation between substitution and ductilization in Ni3A1. According to the current concept of macroalloying, ternary elements, substituting for the Ni or the AI sites, are added to Ni3AI in replacement of Ni or AI atoms respectively. It has not been established,
A strongly ordered intermetallic c o m p o u n d Ni3A1 exhibits brittle fracture due to the intrinsic weakness of grain-boundaries. Its application as a hightemperature structural material has therefore been restricted in spite of its excellent high-temperature strength properties, i.e. the material becomes stronger when subjected to elevated temperatures. Considerable effort has been made in improving the ductility of NiaA1. It was discovered by Aoki and Izumi [1] that microalloying with B improves ductility of Ni3AI dramatically. B, an interstitial element in l'On leave from: Faculty of Engineering, Department of Metallurgy, Iwate University, Morioka 020, Japan. 13
14
CHIBA et al.:
D E T E R M I N A T I O N OF Pd IN Ni3A1 BY A L C H E M I
however, that the substitution behavior of ternary Table 1. Nominal composition and occupation fraction in the Ni site of the compound NiaAl~Pdc, together with experimental error due alloying elements is not affected by the composition to the counting statistics of the host elements. It is thus important to examine Counting experimentally such composition dependence of the Occupation statistical fraction error substitution behavior. Specimen Measurement k (%) _+Ak (%) The macroalloying behavior of ternary additions 1 117 20 (i.e. the substitution of ternary additions for Ni, for I Ni75A123Pd2 2 107 18 A1 or for both sites), has hitherto been discussed 3 97 22 1 99 13 from the knowledge of the direction of solubility lobe II Ni74A124Pd 2 2 82 17 of Ni3A1 in the ternary phase diagram [5]. However, 3 69 22 1 92 9 the site determination obtained from these consider- III Ni73A125Pd2 ations is deficient in the case of a small addition of a ternary element or in examining the effect of annealed at 1273K for 5 days for homogenization deviation from stoichiometry on the substitution and then dropped into iced water. After slicing, by a behavior. Quantitative investigations [6-8] concern- spark-cutting machine and polishing mechanically, ing the site occupation determination of both inter- the specimens for the ALCHEMI measurements were stitial and substitutional elements have been jet-electropolished in a solution of 1 part sulfuric acid performed using Debye-Scherrer technique and by and 9 parts methanol. ion channeling and nuclear reaction analysis respectThe ALCHEMI measurements were performed ively. These methods did not deal with the effect of with a JEM 2000FX electron microscope equipped deviation from stoichiometry on the substitution with the EDX (Energy Dispersive X-ray specbehavior. Recently, Ochiai et al. [9], based on the trometer) system. The electron beam was focused to Bragg-Williams approximation of nearest neighbour about 2/~m in diameter with an accelerating voltage interactions, have investigated the substitution be- of 200 kV. The direction of the incident beam was havior of ternary additions in intermetallic com- chosen nearly parallel to the (110) planes instead of pounds with the L12-type ordered crystal structure. the (001) planes, of which the neighboring fundamenIn their investigation, there has been no discussion tal reflections were closely excited. Near the 2-beam relating the substitution behavior of the ternary diffraction, the X-ray spectra were taken at one non addition to the deviation from the stoichiometry of channelling and two diffraction conditions, s > 0, host elements. However, since it has been revealed s < 0, where s is the excitation error. Examples of that the ductility of B-doped Ni3Al is affected by the electron diffraction patterns, representing the deviation from stoichiometry [3, 10], studying on s < 0 and s > 0 diffraction conditions, are shown in substitution behavior accompanied with the devi- Fig. 1. ation from stoichiometry is required for the underALCHEMI formulation [11] for the Ll2-type standing of ductility by macroalloying. structure was set up as follows: The present work presents an investigation of the site determination of Pd in Ni3A1 by means of Let ~'Ni,J~/-(n)N~l and N~ I be the nine X-ray counts ALCHEMI [ll] under a planar channelling confrom the Ni, A1 and the additional element X dition. Pd is selected as a substitutional ternary for the diffraction conditions n = 1, 2 and 3. element, because Pd has been found to strongly Then we have the nine relations occupy the Ni site in Ni3A1 on the basis of thermodynamic analysis. Much attention is focused on the N~! = eNi" {A= "I=+ A~'I~ } (1) dependence of the substitution behavior of Pd as a function of the composition of the host elements. N ~ I = PA," {B,'I, + B#'I~} (2) Therefore, three specimens for A L C H E M I measurements, NiTaAI25Pd2, Ni74A124Pd2, and Ni75A123Pd2 U~ ) = Px" { C," I~ + C a"/~ } are examined. Furthermore the obtained results are = P x . { 2 . c . r . I , + l ' c ' t ¢ • + c ' ( 1 - t c ) ' I ~ } (3) discussed in terms of the thermodynamic analysis. 2. E X P E R I M E N T A L
Three types of A L C H E M I specimens were used in this investigation. Alloy buttons of 50 mm diameter were prepared by arc-melting, raw materials four times in argon gas atmosphere at a pressure of about 700 torr. The nominal compositions and the chemical analyses of the A L C H E M I specimens are listed in Table 1 and Table 2 respectively. Rod-shaped bulk materials obtained by cutting the alloy buttons were encapsuled into I x 10-Storr with Ti foil getter,
where I, and I~ are the thickness averaged electron intensities on the ~ and fl planes, respectively. Here, for the atomic arrangement of Ni3AI with L12-type structure, the ~t plane and the fl plane represent the Table 2. Results of chemical analysis
Specimen I II III
Nominal composition (at. %) Ni-23A1-2Pd Ni-24Al-2Pd Ni-25AI-2Pd
Analysed composition (at. %) Ni-22.7Al-1.92Pd Ni-23.7Al-1.89Pd Ni-24.TAI-1.91Pd
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
15
Experimental error due to the counting statistics, Ax, are estimated to be
Ax =A~LON~ !
N,
O N ~ A N ~ + ON~-----sANx].
(4) In the above equation, the sum of the three diffraction conditions n = 1, 2 and 3 is calculated and AN is given by AN = 2 x / ~
(5)
which shows a 96% confidence level. 3. RESULTS
Fig. 1. Diffraction patterns for the two diffraction conditions. (a) Negative excitation error for the T10 reflection. (b) Positive excitation error for the I10 reflection.
Figure 2 shows an example of spectra taken from the Ni75A123Pd2 at the two diffraction conditions (s < 0, s > 0), where the peak intensity of A1 is set equal for comparison. The X-ray intensity from Ni on the first diffraction condition (s < 0) is higher than that of the second one (s > 0). Also, the X-ray intensity from Pd changes in accordance with that of Ni, indicating that the electron channelling effect under a planar channeling condition, is reflected directly in the difference between the two diffraction conditions and that a Pd atom has a tendency to occupy the Ni site qualitatively. With these X-ray spectra, taken from three analysis points for specimen I and II and from a single analysis point for specimen III, quantitative evaluation of the site occupancy of Pd in Ni site was carried out by using ALCHEMI formulation as described above in detail. The results are listed in Table 1, together with experimental error due to the X-ray counting statistics, Ak. As can be seen in this table, site occupancies, k, of the specimen I and II are 107 and 99% respectively, when chosen at the minimal Ak value of individual measurement. For specimen III, 92% site occupancy is derived. These results show that Pd atoms always substitute for Ni sites independent of the composition of the host elements. Furthermore, the results of this investigation are plotted in Fig. 3, together with those of Fe and Co additions obtained by Shindo et al. [12]. As can be seen in this figure, one can find that the substitution behavior of Fe depends on the composition of the host elements, whereas Pd always substitutes for Ni site.
planes composed of only Ni atoms and of both Ni and A1 atoms respectively, when the incident electron beam is set nearly parallel to the (I10) planes. The superscript (n) indicates the three diffraction conditions, i.e. n = 1 for s < 0, n = 2 for s > 0 and n = 3 for non-channelling condition. PNi, PAl and Px are factors for considering the absorption and the difference of X-ray fluorescence yields of each element. A~,,, B~., and C~,~ indicate the atomic ratios of Ni, A1 and X in the ~, fl planes respectively. A~,, and B~., refer to the three compounds I, II and III in Table 3, where a, b and c indicate the atomic ratios of the constituents Ni, A1 and X (a + b + c = 100). Here, we take into account the anti-site occupancy of the host elements, i.e. A1 atoms in the Ni site in compound I, and Ni atoms in the A1 site in compound III. As for compound II, one of the coefficients A~,B and B~,, assigned to compounds I and III is used depending on the substitution behavior of the addition, i.e. if Pd favors the AI site, the coefficients for compound I should be used, otherwise those for compound III should be used. Eliminating the unknown parameters of PNi,AI,X and I,.,(n), from the equations (1), (2) and (3), k, the 4. DISCUSSION atomic fraction of X atom at the Ni site can be obtained in the same manner as described in the Ochiai et al. investigated the substitution behavior previous paper [10]. of transition metal elements in Ll2-type compounds
Specimens I(II) III ( I I )
Table 3. Atomicratios A~.a and B~.a in the planes a and fl Atomic ratio A~ Ap B~ Ba 2/3.a l/3"a 2/3-(75 - a -c'k) 1/3"(75- a -c'k) +[25 - c "(1 - k)l
2/3.(75-c'k)
1/3"(75-c.k) +a -75+c-k
0
b
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
16 20
ii
!i1i
AI...... ~...................
Q
15
i
i =
AI
i :
........i................ ~
i Nii
i
s<0i ....... i !
-
...............i .......
................. i ................ '-i
..................
0
~a
1
2
3
4
7
8 9 KeV
1
2
3
4
7
8 9 KeY
Fig. 2. Characteristic X-ray spectra of Ni75A123Pd2 on two diffraction conditions (s > 0, s < 0). Ni3AI, Ni3Ga, Ni3Si and Ni3Ge based on the Bragg-Williams approximation of "Nearest Neighbour Interaction" [9]. They considered the change in total bond energy in the A3B system by adding a small amount of ternary element "C" in two extreme substitution cases, namely in the case of exclusively substituting for Ni site in the composition of Ni75_ xAl2sCx and in the case of exclusively substituting for A1 site in the composition of NiTsA125_xC~. In both cases, the anti-structure defect was assumed not to be formed. According to the present investigation, however, it is shown that Pd atoms substitute for Ni sites even in the composition of Ni75Al:~Pd 2, suggesting that the anti-structure defects of Ni atoms in AI sites are formed by the Pd addition. Therefore, we consider the reason why Pd atoms have a strong preference to the Ni sites in spite of forming such anti-structure defects in Ni atoms. According to Ochiai et al. [9], in Ni75A12s_~Cx alloy, the total bond energy, H (A1), can be expressed as follows when C atoms exclusively substitute for the AI sites H (AI) = 6N[3HNiNi + (~1 -- x c ) H ~ J
+ x c H ~ a + xcHcc] + 12N[¼ - xc)VN~ + xcVN~] (6)
xc the mole fraction of C atom, HNiN~, etc. the-bond energy for NiNi pairs, etc. and V N i A I , etc. the interaction parameter VNiAI= HNiAI-- (HNiNi+ HAIAI)/2, etc. When C atoms completely substitute for Ni sites, the total bond energy, H(Ni), can be expressed as follows if the mole fraction, "z", of Ni atoms in the AI site is introduced as a parameter for the anti-structure defect. H(Ni) = ~ N ' HNiNi+
3(1
--
X¢) N ' H ~ , I
+ 6xcN" Hcc + N ~ - 8z + 3 6 4 z 2 - 5 - ( 1 6 z 4 2 4 ) VNiAI
+5-
+N" ~ f i ( 1 2 - 16z - 3 2 X c ) V~c 3 X~
+ N . 3 ( 1 6 z + 24)VNic
(7)
where the relationship, z = x¢, holds in this case. From equations (6) and (7), the following relation can be derived
H(A1)-/-/(Ni)
where N is the total number of atoms in the crystal.
= 4 VNi~a"N "z [ ( 1 - 4z )( 1 - VNiA IValc// 4 ( 1 - - ~ Z ) Vale ] .
VNiAIj
(8)
By differentiating equation (8), we obtain
1 _FaL dxc jI_~c=O
dxc Jxc=0
+' VNiC']_ ~-'4VNiAI'N[C|- VNild/V/ A1c
Ni;,sAI2s-zX=Nivs~tzAlzs-z~Z Xz Ni';,5.zA[2sXz Fig. 3. Occupation fraction in the Ni site of added elements X (X = Pd, Fe and Co).
(9)
Here, let C atom be Pd atom, the values of VAIpd/VNi~ and VNipd/VNi~d can be estimated at 1.8 and 0.04, respectively from the data evaluated by Ochiai et al.
CHIBA et al.: DETERMINATION OF Pd IN NisAl BY ALCHEMI [9]. When this numerical data is substituted in equation (9), we obtain
_ Fd.(Ni)]
j .....
dxpd
L
J..... = -- 3.04' N ' VNiAI> 0.
Because the VNiAI is expected to be negative, then
L dxed ~ 0 ..I-• =0'
dxea j . . . . .
As a result, equation (10) clearly shows that the present A L C H E M I results, which indicate that Pd has a preference to locate in the Ni atom site independent of composition of the host elements, can be explained in terms of the thermodynamic basis. In addition, the relationship of equation (10) can be generally derived from equation (9) if the following condition is satisfied VAACx
VNiC
VNiC
VAIC
1 -- VNiA1)+V--~iAI<0 VNIA
(11)
Figure 4 indicates the correlation between the bond energy ratios of VNic/VNi~ and V~uc/VNiAafor various ternary transition elements with the site preferences in the Ni3A1 compounds [91. The solid line in this figure divides the ternary elements into two categories; one substituting for the A1 site and the other substituting for the Ni site. According to Ochiai et aL [9], the elements situated below the solid line substitute for the Ni site, whereas those above the line substitute for the AI site. Furthermore the dotted line, derived from equation (11), in this figure divides the ternary elements substituting for the Ni site into two parts. That is, the elements situated below the dotted line always substitute for the Ni site regardless of the composition of the host elements. On the other hand, the elements situated between the solid line and the dotted line substitute for the Ni site and the AI site 3
i
Ni3AI } OchiQiat al. / . . . . This work / . / rZ/r / , //// •
2 ~_
ri
z
>
..b . /
:~
~ /
M ~ OeCreMn 0
We °e / .~'~e Co /
/c'~U ~AU ": A'g:"
-10
~
"/.so / ~., ,"
I II/ /
•
Pt
• PO
i
2
3
VAIc / VNiAI Fig. 4. The substitution behavior of ternary addition in Ni3AI. • marks and solid line are referred to from the data evaluated by Ochiai et al. [9]. The dotted line is derived from the equation (I 1). AM 39/I--B
17
if ternary alloying elements are added in the compositions of Ni75_xAI25Cx and Ni75AI25_xCx respectively. It was actually found in previous investigation [12] shown in Fig. 3 that the substitution behavior of Fe atom, which is situated between the solid line and dotted line in Fig. 4, has a tendency to depend on the composition of the host elements. It can also be seen in Fig. 4 that among the transition elements, which substitute for the Ni site in the composition of Ni75_xAI25X x (i,e. situated below the solid line), Pt as well as Pd may also substitute for Ni site independent of the composition of the host elements. Wu et al. [13] studied the site preference of ternary additions in an A 3B intermetallic compound with L 12 structure using the tetrahedron approximation of the Cluster Variation Method and derived the similar results to those obtained from the present investigation. Thus we concluded that the substitution behavior of the ternary alloying element Pd in the compositions of Ni75 _ xA125_ yPdx +y (X <~y, y 5 0 ) , can be explained by taking the formation of the anti-structure defects into account. Finally it should be noted that it is interesting to examine the mechanical properties of Pd or Pt doped Ni3A1, Ni3Ga, Ni3Si or Ni3Ge because Pd or Pt shows a peculiar substitution behavior in comparison with other transition elements such as Co, Fe and Mn, etc. having the compositional dependence of the host elements on the substitution behavior. 5. CONCLUSIONS 1. From the quantitative ALCHEMI, it is found that Pd has a strong preference to locate in the Ni site of the compositions of Ni7sA123Pd2, Ni74A124Pd2 and Niv3AlzsPd2, which means that antistructure defects are formed in the composition of Ni75_ xA125_yPdx +y(X ~ y, y # 0). 2. By considering the formation of the anti-structure defects, the present A L C H E M I results can be explained in terms of the thermodynamic basis. 3. It is generally revealed from the thermodynamic analysis whether or not the substitution behavior of ternary alloying elements, which substitute for Ni site in the composition of Ni75 xA125Cx, depend on the composition of the host elements, when they are added in the composition of Ni75AI25_xC x. 4. It is suggested that Pt may also have a preference to locate in the Ni site regardless of the composition of the host element. Acknowledgements--The authors are grateful to Dr T. Sekiguchi for his kind help with our operating the electron microscope. One of us (A.C.) would like to thank Professor K. Tanosaki, Associate Professor M. Fujita and Mr K. Nonaka, of Iwate University, for their encouragement. REFERENCES
1. K. Aoki and O. Izumi, Nippon Kinzoku Gakkaishi 43, 1190 (1979).
18
CHIBA et al.: DETERMINATION OF Pd IN Ni3A1 BY ALCHEMI
2. I. Baker, B. Huang and E. M. Schulson, Acta metall. 36, 493 (1988). 3. C. T. Liu, C. L. White and J. A. Horton, Acta metall. 33, 213 (1985). 4. O. Izumi and T. Takasugi, J. Mater. Res. 3, 426 (1988). 5. R. W. Guard and J. H. Westbrook, Trans. metalL Soc. A.LM.E. 215, 807 (1959). 6. N. Masahashi, T. Takasugi and O. Izumi, Acta metall. 36, 1815 (1988). 7. H. Lin, L. E. Seiberling, P. F. Lyman and D. P. Pope, M R S Proc., Vol. 81, High Temperature Ordered Intermetallic Alloys II, pp. 165-172. MRS (1987).
8. A. I. Taub, C. L. Briant, S. C. Huang, K. M. Chang and M. R. Jackson, Scripta metall. 20, 129 (1986). 9. S. Ochiai, Y. Oya and T. Suzuki, Acta metall. 32, 289 (1984). 10. V. Vitek, J. J. Kruisman and J. Th. M. Dehosson, Proc. M R S Syrup., Reno, Nev. (edited by M. H. Yoo, W. A. T. Clark and C. L. Braiant), p. 139. MRS (1988). 11. J. C. H. Spence and J. Tafto, J. Microsc. 130, 147 (1983). 12. D. Shindo, M. Kikuchi, M. Hirabayashi, S. Hanada and O. Izumi, Trans. Japan Inst. Metals 29, 956 (1988). 13. Y. P. Wu, N. C. Tso, J. M. Sanchez and J. K. Tien, Acta metall. 37, 2835 (1989).