THE DEVELOPMENT OF PLATINUM-BASED ALLOYS AND THEIR THERMODYNAMIC DATABASE

A series of quaternary platinum-based alloys have been demonstrated to exhibit the same two-phase structure as Ni-based superalloys and showed good mechanical properties. The properties of ternary alloys were a good indication that the quaternary alloys, with their better microstructure, will be even better. The quaternary alloy composition has been optimised at Pt84:Al11:Ru2:Cr3 for the best microstructure and hardness. Work has begun on establishing a thermodynamic database for Pt-Al-Ru-Cr alloys, and further work will be done to enhance the mechanical and oxidation properties of the alloys by adding small amounts of other elements to the base composition of Pt84:Al11:Ru2:Cr3.


Introduction
Nickel-based superalloys have excellent mechanical properties because they have a microstructure comprising many small, strained-coherent, particles in a softer matrix [1].The strengthening originates from dislocations being slowed down as they negotiate the small ordered particles.Additionally, there is solid solution strengthening in the (Ni) matrix.Although these alloys are used at relatively high temperatures, coarsening does not occur because the surface energy itself is very small.This is because the particle structure is very closely related to that of the matrix.Both are based on the face centred cubic structure: the matrix has a random fcc structure, and the particles have an L1 2 ordered structure.The lattice misfit between these structures is very small and renders the surface energy negligible [1].
The Ni-based superalloys have virtually reached their temperature limit for operation in turbine engines.However, there is a need to further increase the operational temperatures of these engines to achieve greater thrust, reduced fuel consumption and lower pollution.Thus, there is interest in developing a whole new suite of similar structured alloys based on a metal with higher melting point which can be used at temperatures of ~1300 o C.
Platinum has been selected as the base material for these alloys because of its similarity to Ni in fcc structure and similar chemistry.Thus, similar phases to Ni 3 Al could be used to give similar mechanisms as found in the Ni-based superalloys.The important differences are the higher melting point (1769 o C for platinum compared to 1455 o C for nickel) and improved corrosion resistance.Although platinum-based alloys are unlikely to replace all Ni-based superalloys on account of both higher price and higher density, it is likely that they can be used for the highest application temperature components.Pt 3 Al has two forms, and the more desirable high temperature L1 2 form needs to be stabilised.
Experimental Pt-based alloys have been studied.It was found that successful Nibased superalloy analogues could be manufactured with alloys of the approximate composition Pt 82 :Al 14 :X 4 where X was Cr, Ti and Ru [2,3].The best properties were exhibited by the Pt-Al-Cr and Pt-Al-Ru alloys, although the precipitate volume fraction was not as high as in the Ni-based superalloys.Although much heavier, the Pt-based alloys have the advantages of good mechanical properties and high temperature oxidation resistance [2,4].The ternary alloys have mechanical properties which are better than those of the Ni-and Co-based superalloys, higher than conventional solid-solution strengthened Pt-based alloys, and comparable with mechanically alloyed ferritic ODS alloys [5].

Experimental
Several alloys were made up in order to ascertain whether the two-phase structure could be achieved in the quaternary system.Compositions were selected based on the results of the ternary Pt-Al-Cr and Pt-Al-Ru systems.
The alloys were prepared by arc-melting the pure elements several times to achieve homogeneity.The samples were then heat treated at 1350ºC for 96 hours.The heattreated samples were then cut in half, mounted and polished.The microstructure was examined using scanning electron microscopy (SEM) and, where possible, the phases were analysed using electron dispersive X-ray spectroscopy (EDS).The hardness of the alloys was measured using a Vickers hardness tester with a 10 kg load.

Results and discussion
Three alloys were single-phase ~Pt 3 Al, while three had two-phase microstructures.Two of these had large areas of ~Pt 3 Al, together with a mixture of (Pt) and ~Pt 3 Al (Figure 1a).The third (Pt 84 :Al 11 :Ru 2 :Cr 3 ) was composed entirely of a fine two-phase mixture, which is the desired microstructure (Figure 1b).The EDS analyses of the overall and phase compositions are given in Table 1.Very fine phases were not analysed..The hardness of the three two-phase heat treated alloys was measured and the results are given in Table 2.The alloys were reasonably ductile, although some of the hardness indentations exhibited small cracks on the edges.
More alloys were produced to ascertain if the volume fraction of the ~Pt 3 Al precipitates could be increased.Table 3 shows the measured compositions after heat treatment for 96 hours at 1350ºC in argon.Only the Pt 85 :Al 11 :Ru 2 :Cr 2 alloy had a clear fine two-phase mixture, but there were also small areas of primary ~Pt 3 Al.
The hardness of the alloys was measured after heat treatment and the results are given in Table 4.The hardness ranged from 417 to 430 HV 10 .The alloys showed good ductility, with no cracking around the indentations.
In an attempt to improve the microstructure of the second batch of alloys, a second heat treatment was conducted for 96 hours at 1350ºC in air.Some oxidation took place, and due to the small size of the samples, this caused loss of aluminium.In all but one of the alloys, there was no improvement.However, alloy Pt 81.5 :Al 11.5 :Ru 2.5 :Cr 4.5 showed a clear, fine two-phase microstructure after this heat treatment, possibly due to the change in its overall composition.There was no primary ~Pt 3 Al in evidence, so the overall composition is that of the two-phase mixture: 85.2±0.3 at.%Pt, 7.1±0.8at.%Al, 3.1±0.8at.%Ru and 4.6±0.1 at.%Cr.Since the overall composition changed, the sample was redesignated as Pt 85 :Al 7 :Ru 3 :Cr 5 .
Table 3. Compositions of the Pt-Al-Ru-Cr alloys after heat treatment at 1350ºC for 96 hours.
Figure 2 shows the microstructure of this alloy after the first and second heat treatments, and that of the Pt 84 :Al 11 :Ru 2 :Cr 3 alloy from the first batch.It can be seen that the precipitates in Pt 84 :Al 11 :Ru 2 :Cr 3 are approximately twice as large, but more well-defined than those of Pt 85 :Al 7 :Ru 3 :Cr 5 .The hardnesses were re-measured and are given in Table 5.They range from 396 to 415 HV 10 , and were less after the second anneal.

Discussion
As has been shown before [6], it is possible to obtain a (Pt) + ~Pt 3 Al microstructure in the quaternary alloys.The volume fraction of ~Pt 3 Al was estimated, using image analysis, to be approximately 25-30%.The highest hardness was found in the alloy without primary ~Pt 3 Al.In the second batch of quaternary alloys, there was no clear relationship between the hardness and the composition or microstructure.The decrease in hardness after the second heat treatment is likely to be due to the changes in composition due to oxidation.
The best alloy to date is Pt 84 :Al 11 :Ru 2 :Cr 3 it has the required structure, no primary ~Pt 3 Al and reasonable hardness.Other work has already shown that its oxidation resistance is better than the original ternary alloys [7].
The other part of the project is the development of a thermodynamic database to facilitate the further development of these Pt alloys, while simultaneously developing the alloys further.This work will build on the information already gleaned from prior work, and will also extend the work to Pt alloys of higher order (i.e.alloys with more components, such as Ni).
The Parrot module in Thermo-Calc TM [8] that is being used to optimise the database is based on that of SGTE [9]  Although a calculated phase diagram for Al-Pt has been published by Wu and Jin [11], this was re-calculated [12] as Wu and Jin's description did not exhibit any ordering in the Pt 3 Al phase.They also only described one form of the Pt 3 Al phase, and since both the cubic and tetragonal structures of this phase are important for this work, they must both be included.The Al-Ru system has also been optimised by the group [12].Next, each ternary system will be optimised individually (already started for Al-Cr-Ru), and then once finalised, they will be combined for the quaternary.Experimental work has already commenced on the ternary systems: Al-Cr-Ru [13][14][15], Pt-Al-Ru [16] and Pt-Cr-Ru [17].The Pt-Al-Cr system will also be studied.Additionally, more quaternary alloys will also be studied.Results from the phase diagram work, together with enthalpies from the single-phase or near single-phase compositions from Leeds, UK will be input to Thermo-Calc TM for optimisation.
Table 5. Vickers hardness of the second batch of quaternary alloys after the second heat treatment, using a 10 kg load.
Once the Pt-alloy database has been optimised against some quaternary alloys, other small additions, added to improve the properties (as in nickel-based superalloys), will be included in the optimisation.

Conclusion
It is possible to produce a fine two-phase γ/γ′ structure in the Pt-Al-Ru-Cr system, with precipitates of similar shape to those in the nickel-based superalloy systems.The composition of Pt 84 :Al 11 :Ru 2 :Cr 3 is the optimum composition because it has no primary phase.Development of the Pt-Al-Ru-Cr thermodynamic database has commenced.
The assistance of DACST and the PDI is gratefully acknowledged.

Figure 1 .
Figure 1.SEM micrographs, in back-scattered electron (BSE) mode, of the two types of twophase alloys.a) With primary ~Pt3Al (dark contrast) in a fine mixture of (Pt) and ~Pt3Al; b) Fine mixture of (Pt) and ~Pt3Al.

Table 1 .
Compositions of the Pt-Al-Ru-Cr alloys after heat treatment at 1350ºC for 96 hours.

Table 2 .
Vickers hardness of the two-phase quaternary alloys, using a 10 kg load.

Table 4 .
Vickers hardness of the second batch of quaternary alloys, using a 10 kg load.