Platinum Metals Rev., 2007, 51, (3), 104–115 104

Work has been ongoing in building a thermody-namic database for the prediction of phaseequilibria in Pt-based superalloys (1–4). The alloysare being developed for high-temperature applica-tions in aggressive environments. The database will

aid the design of alloys by enabling the calculationof the composition and proportions of phases pre-sent in alloys of different compositions. Currently,the database contains the elements platinum, alu-minium, chromium and ruthenium. This paper is a

Building a Thermodynamic Database forPlatinum-Based Superalloys: Part IINTRODUCTION, AND INITIAL RESULTS FROM THE COMPOUND ENERGY FORMALISM MODEL

By L. A. CornishAdvanced Materials Division, Mintek, Private Bag X3015, Randburg 2125, South Africa,

DST/NRF Centre of Excellence in Strong Materials, Johannesburg 2050, South Africa,

and School of Chemical and Materials Engineering, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South

Africa

R. Süss*Advanced Materials Division, Mintek, Private Bag X3015, Randburg 2125, South Africa,

DST/NRF Centre of Excellence in Strong Materials, Johannesburg 2050, South Africa,

and School of Chemical and Materials Engineering, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South

Africa; *E-mail: rainers@mintek.co.za

A. WatsonInstitute for Materials Research, University of Leeds, Leeds LS2 9JT, U.K.

and S. N. PrinsNational Metrology Institute of South Africa, Private Bag X34, Lynwood Ridge 0040, South Africa,

and Phases Research Lab, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA

16802, U.S.A.

Work is being done at Mintek, the University of Leeds and the University of Bayreuth tobuild up a platinum-aluminium-chromium-ruthenium (Pt-Al-Cr-Ru) database for the predictionof phase diagrams for further alloy development by obtaining good thermodynamic descriptionsof all of the possible phases in the system. The available databases do not cover all of thephases, and these had to be gleaned from literature, or modelled using experimental data.Similarly, not all of the experimental data were known, and where there were gaps orinconsistencies, experiments had to be undertaken. A preliminary version of the database wasconstructed from assessed thermodynamic data-sets for the binary systems only. The binarydescriptions were combined, allowing extrapolation into the ternary systems, and experimentalphase equilibrium data were compared with calculated results. Very good agreement wasobtained for the Pt-Al-Ru and Pt-Cr-Ru systems, which was encouraging and confirmedthat the higher-order systems could be calculated from the binary systems with confidence.Since some of the phase models in earlier databases were different, these phases had to beremodelled. However, more work is ongoing for information concerning the ternary phasespresent in the Al-Cr-Ru, Pt-Al-Ru (two ternary compounds in each) and Pt-Al-Cr (possiblymore than three ternary compounds) systems. Later in the work, problems with thethermodynamic descriptions of the Cr-Ru and Pt-Cr binary systems were found, and aprogramme of experimental work to overcome these has been devised, and is being undertaken.

DOI: 10.1595/147106707X212967

Platinum Metals Rev., 2007, 51, (3) 105

revised account of work presented at the confer-ence: Southern African Institute of Mining andMetallurgy ‘Platinum Surges Ahead’ at Sun City,South Africa, from 8th to 12th October 2006 (4).

Ever since the possibility of basing a new seriesof alloys on platinum was seen (1), work has beenongoing at Mintek, Fachhochschule Jena andBayreuth University, Germany, with input fromthe National Institute for Materials Science(NIMS), Japan. Experimental work in this field istime-consuming and very expensive in terms ofequipment, materials and expertise. A number ofimportant commercial alloy systems, such as steels,nickel-based alloys, and aluminium alloys nowhave thermodynamic databases which have beenderived from copious experimental results pub-lished by experts. These databases can be usedwith appropriate software to calculate phase dia-grams, phase proportion diagrams, and Pourbaixdiagrams. They can be used instead of experimen-tation, saving both time and money. A similardatabase is being derived within this programme,so that it will facilitate further alloy development,and also be a tool to help designers to select alloycompositions and conditions in the future.However, steels, nickel-based and aluminiumalloys have been used extensively, and there aremore data and accepted phase diagrams for thesesystems than for Pt alloys. As inputs to the Pt-based database, there are fewer commercial alloys,experimental data, and very few accepted ternarysystems. There are problems even with some ofthe binary systems. Thus, part of this work includ-ed the study of phase diagrams to address the lackof data, and to use these data to compile a thermo-dynamic database. Since the basis of the alloys isthe Pt84:Al11:Cr3:Ru2 alloy, the thermodynamicdatabase will be built on the Pt-Al-Cr-Ru system.The Scientific Group Thermodata Europe(SGTE) database (5) includes all the elements andsome of the most commonly used phases, i.e.those that are industrially important, but containsfew of the required Pt phases. Additionally, theruthenium data have been updated to captureAnsara’s modification, to obtain a better estimateof the calculated melting temperature for (hypo-thetical) b.c.c.-Ru (6) than hitherto. Although there

is a database for precious metals (7) it is not suffi-ciently complete for the purposes of thisinvestigation, and does not contain all the elementsof interest to this study, or all the phases.Additionally, not all the phase descriptions neces-sary are present in Spencer’s database. The Al-Crsystem has also been independently assessed (8),although some of the phases might ultimately bemodelled a different way by this group.

The phase diagram programs (e.g. Thermo-CalcTM, MTDATA and Pandat) comprise thesoftware itself, accessed in modules through amain interface, and a series of databases where thestructural and thermodynamic data are stored. Inthese databases, each phase is described by a seriesof parameters. The SGTE database covers thephases of only the most common and well knownsystems, and all the stable elements (5). The inter-metallic phases in the Al-Ru and Pt-Al systems arenot included in the SGTE database. Providing thatthe elements are available in a database (and all ofthe stable elements are in the SGTE database, orother available databases), a phase diagram can becalculated and drawn. However, if there is nodescription for a particular phase, then the calcu-lated phase diagram cannot include it. A givendatabase can be modified to include new phases, orrun in conjunction with another database. The aimof this programme is to develop a database specif-ically for the Pt-rich alloys in this investigation.Prior to building a database, it must be knownwhich phases need descriptions. Information onthe elements, and on any phase that is alreadyincluded in the SGTE database (5), can beaccessed from that database. For phases that arenot represented by the SGTE database, a numberof factors must be taken into consideration. Firstly,the structure of the phase has to be decided,including the number of lattice sites for the atoms,and which particular atoms fit on the sites. Eachphase is modelled with sublattices, and each sublat-tice usually corresponds with a type of atomposition. Elements allowed in a particular sublat-tice are those actually found in those positions bycrystallographic measurement. This information isusually derived from X-ray diffraction (XRD)structural information and composition ranges,

Platinum Metals Rev., 2007, 51, (3) 106

and is usually made to be as simple as possible.Next, some values have to be obtained for theinteraction parameters. Initial values may beguessed, or the parameters set to zero, and the usercan decide which parameter can be varied duringoptimisation. In preparing for optimisation, exper-imental data are compared against thethermodynamic description, which is adjusted tobest fit the experimental data. A ‘pop’ file is creat-ed which contains the experimental data (these caninclude phase compositions in equilibrium witheach other at known temperatures, reaction infor-mation, enthalpies, etc.). Then optimisation can beconducted. Thermo-CalcTM uses the informationin the pop file and, through iteration, calculates theparameters required (those that were set to be var-ied) to best fit the data in the pop file. The resultof this process is the incorporation of new phases,which now have parameters that can be used tocalculate a phase diagram that agrees with theinput data.

Optimisation is the iterative process in whichselected expressions of the thermodynamicdescriptions are allowed to vary so that agreementwith the experimental results is improved. Theoptimisation was carried out with the PARROTmodule (for the assessment of experimental data,and establishment of thermodynamic coefficients)(9) of the Thermo-CalcTM software (10). With thismodule, the Gibbs energy functions can bederived by a least-squares fit to experimental data.Different types of experimental data can be used,with weightings assigned on the basis of the uncer-tainties associated with the original data. Oncecalculated phase diagrams that agree with theexperimental data are obtained, and the thermody-namic descriptions have been rationalised, thebase systems will be complete. Selected importantbinaries were optimised first, for example, Al-Ru(11) and Al-Pt (12). More work has to be carriedout on the Al-Pt system because there is nodescription for the two major Pt3Al phases. Sincethese phases are crucial to the programme, theyhave to be modelled satisfactorily, before incorpo-ration into the main database. Once each binarysystem has been modelled satisfactorily, it can beadded into the ternary systems, after which each

ternary system must be optimised individually.This is done using the experimental data, eitherdrawn from the literature, or, as was mostly thecase, derived experimentally within the pro-gramme at the University of the Witwatersrandand Mintek (for Al-Cr-Ru (13, 14), Pt-Cr-Ru (15,16), Pt-Al-Cr (17, 18) and Pt-Al-Ru (19)) or theCSIR and Mintek (for Pt-Al-Ru (20)). Only oncethe ternaries have been finalised can they be com-bined for the Pt-Al-Cr-Ru quaternary. TheThermo-CalcTM database will then be optimisedagainst some quaternary alloys that have alreadybeen made for the alloy development work (2–4).Once this stage is complete, then the other smalladditions, to improve the properties (as in nickel-based superalloys), can be included in theoptimisation. It is envisaged that the very finalstage will be focused on the optimisation of onlythe important phases: at least the cubic and tetrag-onal structures of ~ Pt3Al, (Pt), ~ Pt2Al and (Ru).Here, (Pt) and (Ru) denote combinations of fouratoms of the elements in the four-compound sub-lattice formalism (4CSF); arithmetically, Pt4 andRu4, respectively.

The Pt-Al-Cr-Ru system is optimised by study-ing the four-component ternary systems. Thereason for undertaking an optimisation of wholeternaries rather than portions of them is that thereare very few data available for the system, and anythermodynamic model needs to be valid over thecomplete range of compositions in the base sys-tem before the minor components can be added.If only a small region is to be optimised (e.g. theregion between the (Pt) and Pt3Al phases only),then it is likely that although the model would besufficiently good locally, the fit would either bevery erratic or the calculations would not be capa-ble of converging when new elements were added,or other elements added beyond their originalcompositions. (This phenomenon is well knownfor Thermo-CalcTM and has been experienced atMintek for copper additions in duplex stainlesssteels (21).) Thus, the ternary systems for the Pt-Al-Cr-Ru quaternary will be studied in full toprovide a sound basis for the computer database.The Pt-Al-Cr-Ru system is shown schematically inFigure 1.

Platinum Metals Rev., 2007, 51, (3) 107

Using the Compound EnergyFormalism Model

At the beginning of the programme, it wasassumed that the six binary phase diagrams report-ed by Massalski (22) were correct, but it wasrealised after subsequent ternary work that thisassumption was wrong. For the ternary systems,experimental work has already been completed forAl-Cr-Ru (13, 14), Pt-Cr-Ru (15, 16), Pt-Al-Ru (19,20), and is nearly complete for Pt-Al-Cr (17, 18).Some quaternary alloys have already beenaddressed (3, 4), but any new alloys chosen willprobably be located only within the Pt-rich corner.The aim is to input results from the phase diagramwork, together with enthalpies from the single-phase or near single-phase compositions from theUniversity of Leeds (23), for optimisation. Therewill also be inputs from ab initio work from theUniversity of Limpopo, South Africa, onenthalpies of formation for the Pt3Al (24) and Cr-Ru (25) phases. Additionally, the transmissionelectron microscopy (TEM) results will be utilisedin changes to modelling, especially of the ~ Pt3Alphase (26–28). The Pt-Al-Cr-Ru system needed tobe thoroughly researched through experimentalwork, so that the phases could be realisticallydescribed (to be as true to their crystallographicform as possible, so that any additional elementscould be correctly incorporated) and then opti-mised using the software. Only then can the otherelements be added to the database descriptions.These will be the additional elements, added insmaller proportions to ‘tweak’ the metallurgicalproperties of the systems. These will include at

least cobalt and nickel. Experimental work hasincluded the phase investigations alluded to above.Studies of as-cast alloys were done to determinetheir solidification reactions (13–15, 20). The solid-ification reactions and the temperatures at whichthey occur (found by differential thermal analysis(DTA)) are important inputs to the phase diagramprograms. The as-cast alloy samples were alsoheat-treated at 600ºC and 1000ºC (16), thenanalysed so that the phase compositions at knowntemperatures could be input.

Ruthenium-AluminiumInitially, a simplified version of the four-com-

pound sublattice formalism (4CSF), a version ofthe compound energy formalism model (29),which models different combinations of theatoms, was used for the RuAl phase (11). Thisworked very well for the Al-Ru system, as is shownin Figures 2(a)–(c), where the calculated diagram iscompared both with that of Boniface and Cornish(30, 31) and with a phase diagram by Mücklich andIlic (32) which was published subsequently to thecalculated work. The RuAl (B2) phase was actuallydescribed by two different models: the compoundenergy formalism (CEF), which is a simplifiedform of the 4CSF model, and is designated SL (forsublattice model) in Figure 2, as well as the modi-fied sublattice formalism (MSL), which describesthe order-disorder transformation with one Gibbsenergy function. The MSL model allowed a widerRuAl phase field (by giving more flexibility in atompositions), which is closer to experimental find-ings. The RuAl6 phase was described as astoichiometric phase (i.e. ‘line compound’, with nocomposition range), and the other intermetallicphases (Ru4Al13, RuAl2 and Ru2Al3) were modelledwith the sublattice model. The solubility of Ru in(Al) was considered negligible. The coefficientswere also within a range comparable to those ofother phases in other systems.

It will be noticed that the two experimentalphase diagrams are very similar, except for the sta-bility of the Ru2Al3 phase, and the appearance ofthe Ru2Al5 phase. Boniface had observed a similarphase, but attributed it to being a ternary phasebecause it was found only with zirconium and

Fig. 1 Schematic diagram of the Pt-Al-Cr-Ru system,showing four ternary systems and six binary systems

Pt

Cr

Al

Ru

Platinum Metals Rev., 2007, 51, (3) 108

silicon impurities (33). Differences in the experi-mental phase diagrams are due to the use ofdifferent techniques. Boniface and Cornish (30,31) studied both as-cast and annealed samples, andthe as-cast specimens showed that Ru3Al2 solidi-fied at higher temperatures than RuAl2. Annealedsamples (32) are less likely to show this feature.Since data from the experimental diagram are usedto optimise the calculated phase diagram, the lattershould agree with the former. Where there are dif-ferences, this is usually due to the mathematicalmodel not allowing flexibility, or sometimes toomuch flexibility for complex models with limiteddata. In some cases, simpler models have to be

used because there are insufficient data for all theparameters required by a more complex (butpotentially more accurate) model.

Platinum-AluminiumAt the outset of the programme, there were

two conflicting phase diagrams: those of McAlisterand Kahan (34) and Oya et al. (35). The major dif-ference, which was very important to thedevelopment of the Pt-based alloys using the ~ Pt3Al precipitates in a (Pt) solid solution, was thephase transformations in the ~ Pt3Al (γ′) phase,and the number of types of the ~ Pt3Al (γ′) phase.McAlister and Kahan (34) reported one transfor-

Fig. 2 Comparison of the Al-Ru phasediagrams: (a) calculated (11) (Courtesy ofCALPHAD; MSL: modified sublattice model;SL: sublattice model)

2700

2400

2100

1800

1500

1200

900

600

3000 0.2 0.4 0.6 0.8 1.0

Ru, mole fraction

Tem

pera

ture

, K

- - - MSL— SL

RuA

l 6

RuA

l 2

Ru 4

Al 13

Ru 2

Al 3

RuA

l

(a)

0 10 20 30 40 50 60 70 80 90 100Ru, at.%

2400

1900

1400

900

400

Tem

pera

ture

, ºC Al 5R

u

Al 13

Ru 4

Al 2R

u Al 3R

u 2

AlR

u

Fig. 2(b) Comparison of the Al-Ruphase diagrams: Experimental fromBoniface and Cornish (31) (Courtesyof Elsevier Science)

(b)

Platinum Metals Rev., 2007, 51, (3) 109

mation of the high-temperature Pt3Al phase fromL12 (γ′) to a tetragonal low-temperature variant(designated D0′c) (γ′1) at ~ 1280ºC. However, Oyaet al. (35) observed the highest transformationγ′ → γ′1 at ~ 340ºC, and gave an additional trans-formation γ′1 → γ′2 at 127ºC. Previous attempts toresolve this conundrum by scanning electronmicroscopy (SEM), XRD and DTA had beenunsuccessful, although Biggs found peaks at 311 to337ºC and 132ºC for different composition sam-ples using DTA (36). Recent work (4) using in situheating in a TEM showed that the diagram of Oyaet al. (35) was more correct, although there is a pos-sibility that very minor impurities are responsible,since the different workers sourced their raw mate-rial differently. The Pt-Al system had beencalculated by Wu and Jin (37) using the CALPHAD method, but there was a need forreassessment (12) because they considered onlyone Pt3Al phase, i.e. they did not reflect the order-ing in the Pt3Al phases. This needs to be doneusing a model that allows ordering to be calculated(described below). Wu and Jin did not include thePtAl2 or β phases, owing to a lack of experimentaldata. A study of Pt-Al-X ternaries (where X = Ru,Ti, Cr, Ni) confirmed the presence of the Pt2Alphase (19, 36). Experimental work on the Pt-Al-Ru

ternary confirmed the presence of the β phase inthe Al-Pt binary (20).

Initially, the four-compound sublattice formal-ism (4CSF) was applied. This models differentcombinations of four atoms of two different ele-ments, for example: (A) (arithmetically A4), A3B,AB (arithmetically A2B2), AB3, and (B) (arithmeti-cally B4) where at least two of these appear in asystem. This method was used for the (Pt) andPt3Al phases (12), because this model had beenused in the development of the nickel-based super-alloy database (38, 39).

However, when the 4CSF model was applied tothe Pt-Al system (12), the results were less success-ful, mainly because there were very few data, andthe system was more complex. The intermetalliccompounds Pt5Al21, Pt8Al21, PtAl2, Pt2Al3, PtAl,Pt5Al3 and Pt2Al were treated as stoichiometriccompounds. The β phase was assumed to be stoi-chiometric, since very little experimentalinformation was available, and was treated asPt52Al48. The phase diagram shown in Figures3(a)–(c) appears to agree with that of Massalski(22), which is actually from McAlister and Kahan(34), but the 4CSF model did not produce the dif-ferent Pt3Al phases. The calculated compositionsand temperatures for the invariant reactions of the

Fig. 2(c) Comparisonof the Al-Ru phasediagrams: Experimentalby Mücklich and Ilic(32) (Courtesy ofElsevier Science; EDS:energy dispersivespectroscopy; WDS:wavelength dispersivespectroscopy; referencenumbers are as cited inReference (32))

Tem

pera

ture

, ºC

0 10 20 30 40 50 60 70 80 90 100Ru, at.%

Al(R

u)

Al 6R

u

Al 13

Ru 4

Al 2R

u

Al 3R

u 2

RuA

l

Ru(

Al)

Al5Ru2

657ºC734ºC

1340ºC1420ºC

1492ºC

1675ºC

1805ºC

2334ºC

1920 ± 20ºC

[64] Annealed, EDS[64] DTA heating, EDS[64] DTA cooling, EDS[78] Quenched, WDS[78] Annealed, WDS[36] Annealed, WDS[63] As-cast, EDS[58] Annealed, EDS

~ 54 74 ~ 86

Liquid2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

0

660

(c)

Platinum Metals Rev., 2007, 51, (3) 110

intermetallic phases are generally in good agree-ment with those reported from experiment.However, there are some areas in less goodagreement, in most cases for reasons inherent inthe models being used.

Calculated results for the congruent forma-tion of the Pt3Al phase and L → Pt3Al + (Pt)eutectic reactions are not in very good agree-

ment with the experimental diagram, as bothreactions are shifted to lower platinum compo-sitions in the calculated system. The 4CSFmodel is such that the formation composition ofPt3Al is fixed at 75 at.%, whereas the phase hasbeen reported in the literature to form congru-ently at 73.2 at.%. This non-stoichiometricformation cannot be described with the model,

Fig. 3 Comparison of Al-Pt phasediagrams: (a) calculated (12)

Fig. 3(b) Comparisonof Al-Pt phasediagrams: Experimentalfrom Massalski (22)(Courtesy of ASMInternational; LT: lowtemperature structure)(McAlister and Kahan(34).)

Tem

pera

ture

, K2100

1800

1500

1200

900

600

3000 0.2 0.4 0.6 0.8 1.0

Pt, mole fraction

Pt 5A

l 21

Pt 8A

l 21P

tAl 2

Pt 2A

l 3

PtA

l

Pt 5A

l 3

Pt 2A

l

β Pt 3A

l (Pt)

(Al)

Liquid

Pt, wt.%0 20 40 50 60 70 80 85 90 95 100

0 10 20 30 40 50 60 70 80 90 100Pt, at.%

Tem

perta

ure,

ºC

2000

1800

1600

1400

1200

1000

800

600

400

Pt 5A

l 21

Pt 8A

l 21P

tAl 2

Pt 2A

l 3

PtA

l

Pt 2A

l

Pt2Al(LT)

Pt3Alβ (Pt)

(Al)

Pt3Al(LT)

Liquid

660.452

657ºC

806ºC

1127ºC

1406

1527ºC 1554ºC 1556ºC

1468ºC13971260

1456ºC1507ºC

1769ºC

Pt5Al3

(a)

(b)

Platinum Metals Rev., 2007, 51, (3) 111

and had consequences for the temperature aswell as the enthalpy of formation for the Pt3Alphase. The symmetry and fixed compositionsassumed in the 4CSF model also made it diffi-cult to fix the eutectic reaction to lower Ptcontents in the calculation. Furthermore, thephase area of the (Pt) solid solution is too nar-row, especially at lower temperatures, althoughthe phase area for the Pt3Al phase is acceptable.However, the Pt3Al phase is ordered throughoutits phase area. The unstable PtAl3 (L12) andPt2Al2 (L10) phases, which are introducedthrough the 4CSF model, are not stable at anycomposition or temperature in the phase dia-gram, which is correct. Further work on thissystem was postponed until more data todescribe the (Pt) and Pt3Al phases had beenobtained. Currently, the Pt-Al binary is beinginvestigated at Mintek using the NovaNanoSEM, and good results are being obtained(40). The data from these alloys will be used tooptimise the Pt3Al phase in the Pt-Al binary.

Platinum-Aluminium-RutheniumThe resulting database files from the Ru-Al,

Pt-Al and Pt-Ru systems were added and the phasediagram was plotted, as an extrapolation from thebinary systems, without any ternary interactionparameters or optimisation with ternary data (41).There were problems in the calculation of isother-mal sections; these arose because the currentmodels were not sufficiently robust to allow forextension into the ternary. However, the liquidusprojection showed very good agreement with theexperimental projection (Figures 4(a)–(b)).Obviously, the two ~ Pt18Al18Ru64 and ~ Pt12Al15Ru73 ternary phases (20) were not calcu-lated, because data for these were not input. Thestability of the Pt2Al phase was calculated as toohigh in the ternary, because it solidified from themelt as a primary phase, which rendered the liq-uidus inaccurate for that region. This was probablybecause of the inadequacies of modelling the phases, which resulted in Pt2Al being shown astoo stable.

1800

1400

1000

600

200

Tem

pera

ture

, K

0 10 20 30 40Al, at.%

γ

γ ′

γ1′

γ2′

Pt 3A

l

Pt 2A

l

Pt 5A

l 3

613 K

400 K

1780 K

1829 K

1738 K1670 K

1533 K

Fig. 3(c) Comparison of Al-Pt phasediagrams: Experimental from Oya et al. (35)(Courtesy of Carl Hanser Verlag)

(c)

Chromium-PlatinumAn assessment by Oikawa et al. (42) showed

that the values for the two eutectic temperatures inthe Cr-Pt binary should be reversed when com-pared with those results (22), as shown in Figure 5.Oikawa et al.’s conclusion was initially thought to

be wrong, even considering that the original tem-perature data were very close (within 30 ± 10ºC).Thus, when work began on the Cr-Pt system, thework of Oikawa et al. (42) was ignored and the4CSF model was used on the (Pt), Pt3Cr and PtCrordered phases (43). However, more data were

Platinum Metals Rev., 2007, 51, (3) 112

X, (L

iq., R

u)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00 0.40.2 0.6 0.8 1.0

X, (Liq., Pt)

Fig. 4 Comparison of Pt-Al-Ruliquidus surface projections:(a) calculated by Prins et al.(41) (Courtesy of CALPHAD;X = mole fraction)

Fig. 4(b) Experimental by Prinset al. (20) (Courtesy of ElsevierScience; X: ~ Ru12Pt15Al73;T: ~ Ru18Pt18Al64)

Ru

PtAl

Al, a

t.%

Ru, at.%

(Ru)

RuAl(Pt)

Pt8Al21

Pt5Al21

RuAl3

(Al)

Ru4Al13 RuAl2

Ru2Al3

Ru4Al13

PtAl2 Pt2Al3 PtAl

Pt3Al

Pt5Al21

β

Pt5Al21 Pt8Al21 PtAl2 Pt2Al3 PtAl Pt5Al3 Pt2Al Pt3Al

Pt5Al3Pt, at.%

X

X T

(a)

(b)

Platinum Metals Rev., 2007, 51, (3) 113

required for the 4CSF model than were available,and consequently the fit was very poor (22).Subsequently, results from experimental work onthe Cr-Pt-Ru, Al-Cr-Pt and Cr-Ni-Pt ternary sys-tems also agreed with the findings of Oikawa et al.(42), and those parameters are being used untilsubsequent experimental work indicates that arevision is necessary.

Chromium-RutheniumFor the Cr-Ru system, there was no previous

assessment, and the first calculation once againused the 4CSF model. Reproducibility was poorbecause the model was too complex for the data

available. Further work and an extrapolation ofthe Pt-Cr, Cr-Ru and Pt-Ru binaries (the latterfrom Spencer’s database (7)) demonstrated ratherthat the calculations were poor (44) although thefit with the ternary Pt-Cr-Ru liquidus was reason-able. It was evident that another form ofmodelling was required. Subsequently, it wasrevealed (45) that there were problems withMassalski’s (22) phase diagram, and two alloyswere manufactured to study the sequence of reac-tions in the Cr-Ru binary. The Cr-Ru system isvery difficult to study experimentally because thediffusion rates are very low (large atoms and highmelting points), and Cr oxidised easily on

0 20 40 60 80 100Pt, at.%

2000

1800

1600

1400

1200

1000

800Te

mpe

ratu

re, º

CCr3Pt

Müller [51]Waterstrat [52]Waterstrat [52]

f.c.c.

b.c.c.

Liquid

Fig. 5 Comparison of Cr-Pt phasediagrams: (a) calculated by Oikawaet al. (42) (Courtesy of ElsevierScience; reference numbers are ascited in Reference (42))

Pt, wt.%0 20 40 50 60 70 80 90 100

2000

1500

1000

500

Tem

pera

ture

, ºC

0 10 20 30 40 50 60 70 80 90 100Pt, at.%

(Pt)

Liquid

Cr 3P

t

CrP

t

Tc

CrPt3

~ 1130ºC

1785 ± 5ºC~ 80

1500ºC1530ºC

970ºC~ 23 ~ 34

(Cr)

~ 24~ 29

~ 20

~ 13~ 10

~ 1600ºC

1769ºC

1863ºC

~ 85

Fig 5(b) Experimental (22)(Courtesy of ASMInternational; Tc is thecritical temperature)

(b)

(a)

Platinum Metals Rev., 2007, 51, (3) 114

protracted annealing, despite precautions.However, this work is ongoing.

ConclusionAlthough good results were obtained for Ru-Al

using the 4CSF representation for RuAl, there wereinsufficient data to obtain good results for theother systems since more phases were representedin each system. A different approach was neededwith simpler representation to allow for sparsedata. The use of simpler thermodynamic models totreat binary and ternary alloys will be reported inPart II of this series of papers, to be published in afuture issue of Platinum Metals Review.

1 I. M. Wolff and P. J. Hill, Platinum Metals Rev., 2000,44, (4), 158

2 L. A. Cornish, J. Hohls, P. J. Hill, S. Prins, R. Süssand D. N. Compton, J. Min. Metall. Sect. B: Metall.,2002, 38, (3–4), 197

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13 D. Compton, “The Constitution of Al-Rich Alloys ofthe Al-Cr-Ni System”, Ph.D. Thesis, University ofthe Witwatersrand, Johannesburg, South Africa, 2002

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References

AcknowledgementsFinancial assistance from the South African

Department of Science and Technology (DST); thePlatinum Development Initiative (PDI: AngloPlatinum, Impala Platinum and Lonmin); DST/NRFCentre of Excellence in Strong Materials andEngineering; and Physical Sciences Research Council(EPSRC) Platform Grant GR/R95798 is gratefullyacknowledged. The authors would like to thankCompuTherm LLC, Wisconsin, U.S.A., and theNational Physical Laboratory (NPL), Teddington,U.K., for the provision of the WinPhaD, Pandat andMTDATA software. This paper is published with thepermission of Mintek and the Southern AfricanInstitute of Mining and Metallurgy.

Platinum Metals Rev., 2007, 51, (3) 115

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45 X. Gu (NIMS, Japan), private communication to L.A. Cornish, June 2005

Lesley Cornish is an Honorary Professor atthe University of the Witwatersrand, SouthAfrica, and is associated with the DST/NRFCentre of Excellence for Strong Materialsand Mintek. Her research interests includephase diagrams, platinum alloys andintermetallic compounds.

Dr Andy Watson is a Senior Research Fellowin the Institute for Materials Research at theUniversity of Leeds. He has worked inexperimental and computationalthermodynamics for many years and hasinterests in lead-free solders andintermetallic phases as well as pgm alloys.

Rainer Süss is Section Head of theAdvanced Metals Group in the AdvancedMetals Division at Mintek, as well as the co-ordinator of the Strong Metallic AlloysFocus Area in the DST/NRF Centre ofExcellence for Strong Materials. Hisresearch interests include phase diagrams,platinum alloys and jewellery alloys.

Sara Prins is a research metrologist at theNational Metrology Institute of South Africa.She is undertaking Ph.D. research at thePennsylvania State University, U.S.A., whereshe is working on phase diagram and first-principles calculations.

The Authors