Phase equilibria and structural stability of intermetallics in the PtPdHf and PtPdZr systems

Journal ofthe Less-Common

Metals, 163 ( 1990) l-8

PHASE EQUILIBRIA AND STRUCTURAL STABILITY OF INTERMETALLICS IN THE Pt-Pd-Hf AND Pt-Pd-Zr SYSTEMS V. N. KUZNETSOV,

G. P. ZHMURKO

and E. M. SOKOLOVSKAYA

Moscow State University, Chemistry Department, Moscow 119899 (U.S.S.R.) (Received July 14, 1989)

Summary Isothermal sections (1000 “C) of the Pt-Pd-Hf and Pt-Pd-Zr systems were plotted experitientally and thermodynamic calculations were performed. Thermodynamic properties of stable compounds were evaluated by the Kaufman and Bernstein method. The unstable compounds were subdivided into two groups: (1) those reported in the literature (probably in metastable or admixture-stabilized form) with a “small” (850 J mol- ‘) stability difference with respect to stable states and (2) those with a “large” stability difference (2100 J mol- I). The estimates are confirmed by the good agreement of calculated and experimentally determined isothermal sections for both systems. No correlation was found between estimated stability differences and the position of points on the structure maps of Watson and Bennett.

1. Introduction Palladium and platinum alloys with zirconium and hafnium are quite promising compounds since they ensure the preparation of dispersive-strengthened materials by internal oxidation. Moreover, they also have a theoretical interest for the investigation of the structural stability of intermetallics since some fine differences in the metallochemical behaviour of 4d and 5d metals are observed in such systems. Present studies mainly deal with the problem of predicting the single stable structure of intermetallics of the composition studied [l-3]. However, the theory of phase diagrams has another paradigm-the “phase competence” principle. The latter assumes the possibility of the existence of different structures at the composition given with subsequent “selection” by the minimum Gibbs energy criterion. The existence of structures with somewhat lower stability than equilibrium structures may be detected when studying interactions in higher order systems [4]. The phase stability of different phases of metals has been analysed by Kaufman and Bernstein [5] within an approach based on binary phase diagrams. By analogy, one can suppose that the study of ternary systems can clarify the problem of structural stability of binary compounds to a certain degree. 0022-5088/90/$3.50

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2. Binary systems

There are no data in the literature on the phase diagrams of the Pt-Pd-Hf and Pt-Pd-Zr ternary systems; however, the binaries Pt-Hf, Pd-Hf, Pt-Zr and Pd-Zr were investigated in refs. 6-22, although the results are incomplete and contradictory. To resolve this problem we checked the existence and structure of some compounds at 1000 ’ C. In the platinum systems with zirconium and hafnium the existence of A,B, A,,B, and AB (A=Pt, B =Zr or Hf) structures were confirmed. They have the Ni,Ti, Ni, ,Zr, and CrB-type structures respectively. The unit cell parameters are similar in all systems. In the Pt-Hf system the AB, compound with the NiTi, type structure was found, whereas in the Pt-Zr binary Pt,Zr, was found with the hexagonal Mn,Si,type structure, in agreement with refs. 7-9; we were not able to confirm the existence of PtZr, with the NiTi,-type structure [lo]. In the palladium systems with zirconium and hafnium we have confirmed the presence of A,B compounds having the TiNi,-type structure, and A,B and AB, both having the MoSi,-type structure (Pd,Hf, stable only above 1500 “C according to ref. 15, was found both in the as-cast state and after annealing at 1000 “C). There several different structure types were reported for some of these, e.g. Zr,Cu for PdZr, and PdHfi [ 12, 131, TiNi, for PdHfi [ 141, but they were not observed by us. PdZr and PdHf also exist, although their structures have not been determined. There are indications [15-l 81 that both compounds have the CsCl-type structure at high temperature, but undergo a very complex transformation with decreasing temperature. We were not able to confirm the results [ 14, 18-221 concerning the existence of Pt,Zr, “Pt3+Zr” (Pt,Zr, Pt,Zr), “Pd,,Hf,,“, Pt,Hf, Pt,Hf and Pd,Hf,.

3. Estimation of thermodynamic ternary systems

properties and thermodynamic

calculation

of

The differences in the phase diagrams of the systems which were generally analogous (Pt, Pd) with (Hf, Zr) reflect some fine metallochemical differences between 4d (Pd, Zr) and 5d (Pt, Hf) metals. We have tried to quantify the differences by thermodynamic calculations of ternary phase diagrams. Such calculations simplified the subsequent experimental work. It is well known that the interactions of “early” and “late” d transitional metals are extremely strong [23-251, whereas those of platinum and palladium, belonging to the same group of the periodic table, are much weaker. This enables us to assume that the main features of ternaries under consideration will mainly depend on the relative stability of compounds in the binaries. Moreover, we suggest that at least a qualitatively correct picture can be obtained by the simplest (ideal) approximation of the solutions between binary compounds. The estimation of the thermodynamics of the stable compounds has been performed by use of the Kaufman and Bernstein method [5], since the Miedema

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approach, usually considered as giving the best numerical values [23, 241, is “structureless”. We expect that some information on fine “structural” effects is effectively encoded in the experimental melting temperature upon which it is based. The properties of the liquid phases were taken from ref. 5; the melting points were taken from refs. 6, 12, 14,25 and 26. Graphical extrapolation was performed for the peritectically melting phases. The resulting A,H values are presented in Table 1. For the heats of formation of the unstable structures, the original approach is not suitable, also being structureless and giving stability values which are too low. So we therefore used a different procedure. For the structures which were reported in literature, but not confirmed in subsequent investigations and most probably observed in the metastable or the admixture-stabilized state, we ascribe a “small stability difference with the stable TABLE 1 Results of assessment of A,H” of stable compounds in Pt-Zr(Hf) Number

1 2 3 4 5 6 7 8 9 10 11 12 13 14

and Pd-Zr(Hf) systems

Compound

Structure

T,,, W)

A,H”(J molV’)

Ptzr Pt,Zr, Pt,Zr PdZr? PdZr Pd,_Zr Pd,Zr PtHf, PtHf Pt,Hf PdHf, PdHf Pd>Hf Pd,Hf

CSCI Mn& TiNi, TizNi CSCI MoSiz TiNi, TizNi CsCl TiNi, MoSi, CSCI MoSiz TiNi,

2370 2020 2270 1360 1860 1900 2050 1940 2370 2420 1700 1900 1300 2240

-21600 - 19200 - 17000 - 16800 - 20900 - 18800 - 16850 - 16800 -21400 - 16300 - 17400 - 20600 - 20000 - 15300

TABLE 2 Results of assessment of A,H” of metastable compounds in Pt-Zr(Hf)

and Pd-Zr(Hf) systems

Number

Compound

Structure

A (J mol- ‘)

A,H”(Jmol-‘)

1 2 3 4 5 6

PtZrz PtzZr Pd,Zr, PtHfi PdHf, Pt,Hfz

TizNi MoSiz Mn$i, MoS& T&Ni MoSiz

850 2100 2100 2100 850 850

-

16800 17600 15700 14700 16550 18500

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state (compound or two-phase mixture of the same composition) of 850 J mall ’ (200 cal mall ‘). If no such indication was found in the literature, the corresponding stability difference was taken to be “great”, ie. 2100 J mall’ (500 cal mol-‘). The results of such evaluation are given in Table 2. Since the published results [18, 201 enabled us to suggest that Pt,,Zr, and Pd,,Hf, are in some sense structural modifications of the equiatomic phases, we have not taken those into account in the calculations. We did not calculate the detailed equilibria with solid solutions of components. Most attention was focused on the interactions of compounds of the same composition, but different structures. The calculated isothermal sections ( 1000 “C) are given in Fig. 1.

4. Experimental

investigation

of the isothermal

sections of ternaries

A total of 270 samples were prepared to study the isothermal sections of Pt-Pd-Hf and Pt-Pd-Zr experimentally. The starting materials were platinum and

.

-1’

_____--

Pt

.?o a)

BI

40

60

Pd, 06

Pd,el

80

./ *

Pd

%

%

Fig. 1. Calculated isothermal section (1000 “C) of the Pt-Pd-Zr

(a) and Pt-Pd-Hf

(b) systems.

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palladium (purity, 99.99O~) and, iodide zirconium and hafnium (purity 99.9%). The samples were pressed and melted in an arc vacuum furnace in a purified argon atmosphere. Titanium and zirconium were used as getters. After melting, the samples were sealed in double quartz ampoules and annealed at 1000 “C for 1000 h. The phase equilibria were studied by X-ray, metallography, electron probe and differential thermal analysis (DTA) methods. Measurements of hardness and microhardness were also performed. The results obtained enabled us to construct the 1000 “C isothermal sections of the ternary systems (Fig. 2). As expected, the ternary phases are absent in both systems. The solid solubilities of zirconium and hafnium in group VIII metals are rather large. The isostructural AB, compounds (Pt,Hf and Pd,Hf, as well as Pt,Zr and Pd,Zr) form continuous solid solutions with a wide homogeneity range. The main specific feature of the Pt-Pd-Hf system is that Pd?Hf solid solution penetrates

Fig. 2. Isothermal sections (1000 “C) of the Pt-Pd-Zr

(a) and Pt-Pd-Hf

systems.

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into the ternary very nearly to the Pt-Hf side. This solid solution is spread not along the line of constant hafnium concentration, but is directed to Pt,Hf, which is the real composition of the compound with the MoSi,-type structure which exists in the Pt-Hf binary above 1000 “C. PtHf and PdHf, as well as PtZr and PdZr, also form continuous solid solutions. With decreasing temperature they undergo complex transformations without composition changes. Pt, ,Zr, and Pt, lHfY, whose composition is close to equiatomic, penetrate into the ternaries up to 15 at.% Pd and 23 at.% Pd respectively. The mutual solubilities in PtHf,, PdHf,, Pt,Zr, and PdZr, are 22 at.% Pd, 7 at.% Pt, 15 at.% Pd and 4 at% of Pt respectively. The solubilities of the noble metals in hafnium and zirconium are very small. The comparison of calculated and experimentally determined phase equilibria shows rather good agreement, especially taking into account some general approximations made in estimating the thermodynamic properties.

5. Comparison with the structural maps of Watson and Bennett (2,3] The obvious disadvantage of the method used to evaluate structure stability differences is that it is essentially based on “erroneous” experimental data for the corresponding binaries, i.e. on information about the observation of the structures of interest in the metastable or admixture-stabilized state which is naturally restricted. Therefore it seems to be of interest to compare the data obtained with some more general information of the systems studied. The main attention is naturally focused on those properties which are known to be connected with structural stability. We have tried to compare the evaluated structural stability differences with the positions of points of the systems considered on the structural maps. We expected that the loss of stability should be the greater, the further (in some sense) the depicting point of the system from the region of preferential stability for the structure under consideration. As the main body of structural maps [2, 3, 27-291 are topologically equivalent [3], we used the most developed approach of Watson and Bennett [2, 31. They use an average number of the holes in the d zone and the difference in effective electronegativities as coordinates, defined via volume contraction upon alloy formation taking into account the stoichiometry of the compound. We have found that such correlation probably does not exist. For example, for the PtZr, phase (Ti,Ni-type structure) the figurative point (Fig. 3) is closer to the region of stability of this structure than to any other structure. It agrees with the “small” energy difference found in this structure with the stable one. The points corresponding to PtHf, and PdHf, fall practically on the border separating the Ti,Ni region from MoSi,. We may expect that one of these structures should be stable whereas stability of the other structure should be somewhat lower. This is true for PdHf,, but not for PtHf, (see Table 2). The figurative point of Pt,Hf falls into its stability region when the stoichiometry is ideal (AB,); however, for its real

Fig. 3. Positions of points of some compounds on the structural map of Watson and Bennett [2,3]. See text for details.

composition (Pt,Hf,) it shifts to the region of Laves phases. This phase is actually only stable at high temperatures [26], probably because of entropy effects. Finally, the point missing in the stable diagram, i.e. the Pt,Zr phase (MoSi,type structure), falls in the region of stability of this structure. However, a comparison of the calculated and experimental diagrams of the Pt-Pd-Zr system suggests that the stability difference accepted as being 2100 J mall ’ is not high enough (the calculated solubility of platinum in PdZr, is considerably higher than the experimental value). The decrease in this difference would have an adverse effect on the final results.

6. Discussion The degree of agreement of calculated and experimentally determined phase diagrams leads us to conclude that the estimated difference of stabilities of modifications of compounds listed in Table 2 are at least of the right order of magnitude. Attempts to attain higher accuracy seem hardly justified without rejection of the ideal approximation for solutions between binary compounds. This would require the development of some new methods to estimate interaction parameters. The values obtained seem to support the similar stability of several different structures for the compounds of the group IV and VIII metals and suggest that the

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causes responsible for the final choice of the stable structures are rather subtle. It is also corroborated by difficulties in correlating the parameters of Watson-Bennett structural maps, giving rather good separation of stable structures of different compounds, with actual differences of thermodynamical stability of different structures of the fixed compound. This also seems to suppose that the factors which determine crystalline structure and numerical values of thermodynamical functions are of a different nature.

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