2.1 Second Phases and Possible Reinforcement

Platinum can form Pt₃X and Pt₅X high melting point intermetallic compounds on the platinum-rich side of the phase diagram with transition metals and rare earth metals (44), such as Pt₃Al (1550°C), Pt₃Hf (2250°C), Pt₃Sc (1850°C), Pt₃Y (2020°C) and Pt₃Zr (2250°C). Their melting points are higher than Ni₃Al (1390°C). Most platinum compound precipitation phases with γ’ structure have a high melting point, high thermal conductivity, low thermal expansion coefficient, high strength and a large number of possible slip systems. It can be expected that γ/γ’ platinum alloys will have high thermal strength and precipitation strengthening effects, leading to the possible development of a new generation of precipitation strengthened platinum-based superalloys (45). Figure 1 represents the binary Pt-Al phase diagram (44). The diagram shows that the maximum solubility of aluminium in platinum is about 10 at% at relatively low temperatures, whereas at the eutectic temperature (1507°C) the value is slightly higher. The Pt₃Al phase forms at the eutectic temperature and the eutectic reaction can be found in the top right of the diagram. At high temperatures, the Pt₃Al intermetallic compound shows a L1₂ structure. It transforms into a tetragonal structure (D0’c) at lower temperatures. Pt₃Al is the most important intermetallic phase.

Table I lists the common precipitation phases and main performance evaluations in platinum-based alloys (46). The Pt₃X phase mainly appears in platinum-transition metals and platinum-simple metal alloy systems, while Pt₅X mainly appears in platinum-rare earth and platinum-alkaline earth metal alloy systems. The Pt₃X phase formed by chromium, vanadium and platinum decomposes at 1130°C and 1015°C respectively, and was excluded from the design study of platinum-based superalloys. Although tin, lead, gallium and other elements can form a L1₂ structure phase with platinum, they are also excluded due to the low melting point. Initial research on binary systems such as Pt-Zr, Pt-Hf and ternary system alloys such as Pt-Rh-Zr, Pt-Rh-Hf showed that γ’ phases Pt₃Zr and Pt₃Hf are formed in the alloy which are coherent with the matrix (47). The presence of zirconium and hafnium as solid solution strengthening elements in combination with γ’ precipitation strengthening give these alloys high-temperature mechanical properties, but they all have the problem of poor oxidation resistance. The formation of brittle zirconium and hafnium oxides leads to embrittlement of the material (48).

The refractory metals tantalum and niobium can increase the solidus temperature of platinum-based superalloys and are worthy of further study. Alloying with aluminium leads to the formation of Pt₃Al, a strengthening intermetallic of the Pt₃X type that has two different crystal structures: the cubic structure (L1₂) at high temperatures and allotrope tetragonal structure (D0₂₄) at low temperatures. The high-temperature allotrope can be stabilised at lower temperatures by adjusting the composition of the alloy matrix.

It is necessary to study the structure and properties of the Pt₃X second phase particles in order to develop precipitation strengthened platinum-based superalloys. However, there are few reports on the experimental measurement of the mechanical properties of Pt₃X intermetallic compounds. Adjal (49), Pan (50) and Li (51) investigated the electronic structure, thermodynamic properties, oxidation resistance and mechanical properties of Pt₃X intermetallic compounds by simulation calculation methods. The mechanical properties and anisotropy of Pt₃M alloy were determined from these studies. The anisotropy is mainly derived from the d electronic state of platinum and the d electronic state (or p electronic state) of M. Pt₃Hf has the highest modulus (bulk modulus, shear modulus, Young’s modulus) and hardness, while Pt₃Y has the lowest values (51). Liu (52) used first-principles calculations to study the effect of pressure on the structure and mechanical properties of Pt₃Al. The study found the elastic modulus, bulk modulus and shear modulus of Pt₃Al increase linearly with increasing pressure. Pt₃Al changes from tetragonal to cubic structure when the pressure reaches 60 MPa, indicating that the cubic structure of Pt₃Al has higher resistance to volume deformation.

2.2 Alloy Composition Design

Table II lists the candidate solid solution strengthening elements and main performance evaluations of platinum-based superalloys (46). Platinum group metals ruthenium, iridium, rhodium, palladium, nickel and rhenium can all be used as solid solution strengthening elements. The alloys Pt-Al-Ni, Pt-Al-Ru, Pt-Al-Cr, Pt-Al-Ti, Pt‐Al‐Re, Pt-Ti-Ru, Pt-Ti-Re, Pt-Ta-Ru, Pt-Ta-Re and Pt‐Nb‐Ru were selected through the comprehensive evaluation of precipitation strengthening, solid solution strengthening and alloy properties. These alloys were optimised and screened and are all characterised by a fcc (platinum) solid solution matrix and fcc (L1₂) Pt₃X precipitation phase, thus a two-phase microstructure. Investigations on structural composition, mechanical properties and oxidation resistance were performed. The results showed that the two-phase structure of Pt-Al-X and Pt-Ti-X alloys confers significant precipitation strengthening, with hardness exceeding 400 Vickers hardness (HV1) and strong resistance to crack initiation and propagation. Aluminium-containing alloys also have better oxidation resistance than the other alloys due to the formation of an aluminium oxide-based protective film on the surface. Internal oxidation was observed in titanium-containing alloys. Therefore, aluminium is considered to be a necessary alloying element for the development of oxidation-resistant platinum-based superalloys (46, 53). For these reasons, the subsequent research mainly focuses on the Pt-Al-X alloy system.

3.1 Structural Characteristics of Ternary Alloys

In order to obtain an effective strengthening effect on platinum-based ternary alloys it is necessary to determine the low-temperature structure of the Pt₃Al phase in the representative binary alloy. Wolff (22) used an electric arc furnace (EAF) to smelt the Pt-12Al (at%) alloy and the material was then subjected to 96 h solution annealing and subsequent ageing at 1350°C. Figure 2 shows some structural features of the Pt-12Al alloy. It can be seen that Pt₃Al precipitation phase exhibits strong directional cubic alignment and the dispersed sub-micron nature of the precipitation phase exhibits a typical bimodal size distribution. This special-orientation geometrical configuration is a highly coherent phase which has low mismatch strain. The Pt₃Al-γ’ phase additionally has a low-temperature variant and a lath or twin structure.

The maximum volume fraction of the Pt₃Al phase is limited to about 30% due to the maximum solubility of aluminium in platinum. The volume ratio of the precipitation phase in nickel-based superalloys can be as high as 70–80%. Additional information about the microstructural features of Pt₃Al can be inferred from Figure 3 where a transmission electron microscopy (TEM) bright-field image microstructure characteristic of the precipitate phase in an alloy with slightly higher aluminium content, namely Pt-14Al (at%) alloy (54). It can be seen from Figure 3 that the martensitic transformation has occurred in the platinum matrix leading to the formation of a clear stacked sheet or plate-like structure. Electron diffraction analysis shows that the stacked plates are twinned with each other, the [001] directions of adjacent tetragonal structure D0’c single packages are perpendicular to each other, and the twin planes are (112) planes. The [001] direction of D0’c is parallel to the <001> direction of the cubic matrix.

Adding transition metals such as nickel, titanium, chromium, ruthenium, iridium, rhenium and tantalum to Pt-Al alloys can improve the strengthening effect of γ’(Pt₃Al) precipitates, thermal stability, solid solution strengthening of the matrix and overall properties. Several typical ternary alloy microstructures are shown in Figure 4. The two-phase structure of Pt₃Al and platinum solid solution of all alloys has been confirmed by X-ray diffraction (XRD) experiments (55). The alloys with nominal compositions (at%) of Pt-14Al-3Re, Pt-14Al-4Ti, Pt-14Al-4Ta and Pt‐14Al‐4Cr were all characterised by a microstructure consisting of primary Pt₃Al surrounded by a fine two-phase eutectic-like mixture of a (platinum) matrix and fine particles of Pt₃Al. The proportion of primary Pt₃Al in Pt-14Al-4Ti, Pt-14Al-4Ta and Pt-14Al‐4Cr alloys is between 40% and 50% and in Pt‐14Al‐3Re alloy it is about 25% according to optical microstructure analysis. The fine martensite-like lamellar structure was observed in Pt-14Al-3Re (Figure 4(a)) and Pt-22Al-2Ru (Figure 4(e)) alloys. This means the Pt₃Al phase has transformed from the high-temperature cubic structure (L1₂) to the low-temperature tetragonal structure (D0’c). In the Pt-14Al-4Ti and Pt-14Al-4Ta alloys, it seems that the high-temperature Pt₃Al phase is formed and maintained. This demonstrates that the third metal elements titanium and tantalum can stabilise the L1₂ polymorph. It is important to observe that an alloying element must enter the Pt₃Al phase in order to prevent the low-temperature transformation of the Pt₃Al phase (55). Biggs (56) and Hill (57) have also shown the possibility for other alloy third components (nickel, titanium or chromium) in Pt-Al alloys to stabilise the cubic (L1₂) structure of Pt₃Al.

Hill et al. (58) studied the microstructure and lattice mismatch of Pt-Al-X alloy systems with stabilising elements titanium, chromium, tantalum, ruthenium and iridium. Figure 5 shows TEM images of the typical two-phase microstructure. All the precipitated phases show a bimodal or even trimodal size distribution. Titanium, chromium and tantalum elements enter into the Pt₃Al phase to stabilise the cubic L1₂ structure, the precipitation phase has a cubic appearance with no clearly discernible internal structure (Figure 5(a)). On the contrary, when ruthenium and iridium enter the platinum matrix, the Pt₃Al precipitation phase transforms into a D0’c structure, and presents a band-like structure with alternating light and dark distributions (Figure 5(b)). Under higher magnification, it was confirmed that the lamellar structure in the precipitation phase of the D0’c tetragonal structure should belong to the twin structure (54).

The lattice misfits between the matrix and precipitate phase in the Pt-Al-X alloy system at room temperature and 800°C were measured respectively by XRD (58). The (220), (211) and (112) diffraction peaks are used to obtain the lattice constants of the platinum solid solution matrix, L1₂-Pt₃Al and D0’c-Pt₃Al precipitate phases, respectively (a_matrix and a_ppt represent the lattice constants of the platinum solid solution matrix and Pt₃Al precipitation phase, respectively). Then, the Lattice misfits δ between the precipitation phase and the matrix are calculated by Equation (i), and the results are listed in Table III. It can be seen that the degrees of mismatch for all alloys are negative and the difference in mismatch degrees at different temperatures is very small. Besides alloys containing ruthenium, the lattice misfit increases at high temperature and the cubic L1₂ structure (alloys containing chromium, tantalum or titanium) has a lower degree of mismatch than the D0’c structure (alloys containing iridium, ruthenium).

(i)

3.2 Mechanical Properties of Ternary Alloys

Figure 6 shows the high-temperature compression strength of Pt-10Al-4Ru compared to Mar-M247 (a nickel-based superalloy) and the tensile strength of PM2000 (an iron-based superalloy) (22). It can be seen that the Pt-10Al-4Ru alloy based on γ/γ〉 precipitation strengthening has higher compressive strength and the ability of withstand higher temperatures than the traditional nickel-based and iron-based superalloys at 1200°C.

Süss (59) studied the stress-rupture strength and high-temperature creep properties of the Pt-Al-X (X = chromium, ruthenium, iridium) ternary alloy system. Figure 7 shows the stress-rupture curves of Pt-10Al-4Ru and Pt-10Al-4Cr alloys at 1300°C. The alloys’ interpolated strength levels for a rupture time of 10 h are summarised in Table IV. PM2000 shows the highest high temperature rupture strength among all the tested alloys, but the lower slope of the stress-rupture curve indicates that the alloy has high stress sensitivity and brittle creep behaviour. This means PM2000 alloy is likely to be damaged by stress concentration or short-term overload in practical use. In contrast, the stress-rupture curve of platinum-based alloys has a steeper slope. Pt-10Al-4Cr alloy has the highest strength at 1300°C. The high-temperature durability of this precipitation strengthened alloy at 1300°C is higher than the ODS and DPH alloys (53) and the solid solution-strengthened Pt‐20wt%Rh alloys, and is close to the strength of Pt-30wt%Rh alloys (10). However, due to wide price fluctuations of rhodium as well as processing difficulties, the practical application of Pt-30wt%Rh alloy is restricted (10).

Figure 8 shows the creep curves of the above alloys under the test conditions of 1300°C and 30 MPa (59). The initial stage of creep was not observed in the three platinum-based alloys. After the second-stage creep, the platinum-based alloys undergo the third stage of creep and subsequent rupture. The creep rupture strain was as high as approximately 10–30%. For PM2000 alloy, it is impossible to divide the creep curve into different stages because of the very low creep rate and fracture strain of less than 1%.

3.3 Oxidation Behaviour of Ternary Alloy

The service environment for components such as aeroplane engines, industrial gas turbine blades and aerospace engine thrusters is very harsh: high temperature, strong oxidation and corrosion. Although platinum-based superalloys with potential for high-temperature applications have been confirmed from the perspective of microstructure features and high-temperature mechanical properties, further attention must be given to assessing their oxidation and corrosion behaviour (10). Hill et al. (46) conducted oxidation tests on Pt-Al-Ni, Pt-Al-Ru, Pt-Al-Re, Pt-Nb-Ru and Pt-Ti-Ru in flowing air at 900°C, 1100°C, 1300°C and 1400°C, respectively. The study found that all aluminium-containing alloys show negligible weight loss on oxidation, while the Pt-Ti-Ru and Pt-Nb‐Ru alloy systems have significant mass increases, indicating lower resistance to oxidation. Figure 9 shows optical micrographs of the transverse-sectional morphology of several platinum-based alloys (46). It can be seen that the Pt-Ti-Ru alloy has grain boundary oxidation, while severe internal oxidation has occurred in the Pt-Nb-Ru and Pt‐Ta‐Re alloys. The aluminium-containing alloy has formed a protective aluminium oxide film on its outer surface. This layer prevents oxidation of the underlying metal, conferring better oxidation resistance on this alloy.

Figure 10 shows the isothermal oxidation curves of aluminium-containing platinum-based ternary alloys Pt-10Al-4X (X = chromium, iridium, ruthenium, titanium) and PM2000 iron-based superalloys at 1350°C (10). It can be seen that Pt-10Al-4Ti and Pt-10Al-4Ru exhibit a parabolic oxidation law similar to the PM2000 alloy. The Pt‐10Al-4Ir and Pt-10Al-4Cr alloys exhibit a parabolic change in the initial stage of oxidation, and the oxidation rate is relatively high. After that, the oxide film is grown at a logarithmic rate. After 800 h of oxidation a continuous oxide layer is obtained, which has better oxidation resistance than the PM2000 alloy. Experimental research on the microstructure, mechanical properties and oxidation resistance of the Pt-Al-X series of superalloys and comparison with the PM2000 benchmark alloy leads to the conclusion that the highest performing platinum-based ternary superalloys are Pt-10Al-4Cr and Pt-10Al-4Ru (10).

4.1 Pt-Al-Cr-Ni Quaternary Alloy

Nickel has a good solid solution strengthening effect on the platinum matrix (64) and its addition can stabilise the L1₂ structure of the Pt₃Al phase. Researchers have added nickel to the Pt-Al-Cr alloy to form a Pt-Al-Cr-Ni quaternary alloy (36). In order to reflect the results of previous research on Pt-10Al-4Cr, the nominal composition ratio of Pt:Al:Cr in the quaternary alloy is designed to be approximately 86:11:3, and the maximum content of nickel is 10 at%. The Pt-11Al-3Cr-(0-10)Ni alloy system has a single-phase structure after solution treatment at 1450°C. The microstructure obtained is similar to that of nickel-based superalloys after ageing treatment at 1000°C, but alloys with nickel content below 6 at% seem to have a lower coherence of precipitation. After ageing treatment, the Pt-11Al-3Cr-6Ni alloy has a maximum γ’ phase volume fraction of about 23%. The cubic precipitate phase is arranged in a straight line with a side length of 200–500 nm, and the degree of mismatch between the precipitate phase and the matrix is about –0.1% (similar to nickel-based superalloys) (Figure 11). Spherical particles are observed in alloys with a nickel content of more than 6 at% and it is believed that the change in the γ’ phase morphology is due to the increase in the nickel concentration and a decrease in the degree of mismatch. Ageing at 1100°C will cause coarsening of γ’ phase and reduce the volume fraction of the γ’ phase. However, as ageing temperature increases, the volume fraction of the γ’ phase of the nickel-containing alloy decreases less than that of the nickel-free alloy.

A Pt-(12-15)Al-3Cr-(4-8)Ni alloy with aluminium content near the solubility limit (15 at%) was selected to obtain a platinum-based alloy with high volume fraction of the γ’ phase (36, 37). For alloys with aluminium content less than 13 at% it is possible to obtain a homogenised single-phase structure after heat treatment at 1500°C. Alloys with a higher aluminium content will form a eutectic two-phase dendritic structure even after heat treatment at 1530°C. For alloys with aluminium content less than 13 at%, ageing treatment at 120 h at 1000°C ageing produces a uniformly distributed precipitation of Pt₃Al. Figure 12 shows cubic Pt₃Al particles with an average side length of 520 nm in the Pt-14Al-3Cr-6Ni alloy. The absolute mismatch between γ and γ’ phases decreases with the increase of nickel content. For alloys with nickel content higher than 5 at%, the slightly negative mismatch value (less than –0.5%) at room temperature and the cubic or spherical particle morphology of the γ’ phase indicate that the γ and γ’ phases are in a coherent state. Increasing the main γ’ phase forming element (aluminium) to 13 at% can increase the volume fraction of γ’ up to 30% (37).

The alloy system Pt-Al-Cr-Ni was studied with an aluminium content limited to 12.5 at% and a chromium content up to 6 at%. The effect of chromium content and heat treatment on the volume fraction of the γ’ reinforcing phase is reported (34). Analysis of the microstructure showed that the dendritic cast structure of Pt‐12.5Al‐3Cr‐6Ni, Pt‐12Al-6Ni and Pt-12Al‐6Cr‐6Ni can be homogenised by heat treatment at 1500– 1510°C. After homogenisation treatment (12 h at 1500°C), the Pt₃Al precipitation in the Pt-12.5Al-3Cr-6Ni alloy was almost completely suppressed after water quenching (Figure 13(a)). Air cooling causes the Pt₃Al particles (average size 200 nm) to be uniformly distributed with a volume fraction of about 30% (Figure 13(b)). Furnace cooling from 1500°C resulted in cubic and coarse particles distributed in the alloy with a volume fraction of 34% (Figure 13(c)). Increasing the chromium content to 6 at% resulted in Pt₃Al with an average particle size of 500 nm and a volume fraction that reached 50% after solution heat treatment for 6 h at 1500–1510°C and air cooling.

The γ’ phase dissolves at very high temperatures, therefore the designed platinum-based superalloys have a maximum operating temperature of 1300°C. Figure 14 shows the stress-rupture strength curves of Pt-10wt%Rh, Pt-10wt%Rh DPH and Pt-12Al-6Cr-5Ni alloys at 1300°C (65). The latter has the highest fracture strength. The minimum creep rate of Pt-12Al‐6Cr‐5Ni alloy is almost three orders of magnitude lower than that of the initial Pt‐10Al‐4Cr alloy at 1300°C. Under the stress of 30 MPa, the creep performance of Pt-12Al-6Cr‐5Ni alloy is better than the PM2000 benchmark alloy (Figure 15). Adding a small amount of boron (0.3 at% or 0.7 at%) can significantly improve the creep strength and ductility of the Pt-12Al‐6Cr‐5Ni alloy (66). To investigate the oxidation resistance of Pt-12Al-6Cr-5Ni, Wenderoth et al. (67) studied its isothermal oxidation behaviour of after 400 h exposure to a temperature range of 1100–1300°C in air. A layer of Al₂O₃ was observed on the surface below which a free γ’-free layer was detected. It was also observed that the size of the γ’-free layer continuously increases with time and temperature. Furthermore, the local concentration of aluminium in the γ’-free layer increases with elevated ageing temperatures. After 400 h exposure at 1300°C a thick polycrystalline Al₂O₃ scale with large oxide grains developed on the surface. This is in good agreement with the typical behaviour of alloy systems forming protective Al₂O₃ scales.

4.2 Pt-Al-Cr-Ru Quaternary Alloy

The strong solid solution strengthening element ruthenium can be added to the Pt-Al-Cr ternary alloy to form a Pt-Al-Cr-Ru quaternary alloy. This increases the volume fraction of the γ’ phase and further improves the mechanical performance and oxidation resistance of the alloy (68). Figure 16 shows the typical TEM two-phase microstructure of Pt-12Al-4Cr-2Ru (at%) alloy with the Pt₃Al precipitation phase and the (platinum) matrix (69). The volume fraction of the Pt₃Al precipitate has increased significantly, with the highest volume fraction of the precipitation phase in platinum-based alloys found so far. The morphology of the precipitation phase is mainly cubic with a side length of about 200 nm and a small amount being irregular. XRD analysis confirmed that the Pt₃Al precipitation phase is cubic.

The quaternary alloy was prepared by arc melting and then aged in air for 100 h at 1250°C in a muffle furnace. It was then water quenched. Test samples for investigation of mechanical properties were obtained from the bulk material by machining. Table V lists the mechanical properties of several Pt-Al alloys, iron-based (PM2000) and nickel-based (CMSX-4) superalloys (68). From the performance comparison of the three platinum-based alloys, it can be found that the quaternary alloy has the highest hardness value, but its room temperature tensile strength and elongation are the lowest. This is inconsistent with the expected result of ruthenium as a strong solid solution strengthening element. The tensile fracture morphology of the alloy was analysed. It was found that only the Pt‐11Al‐3Cr‐2Ru alloy showed intergranular fracture, while all the ternary alloys had cleavage fracture with some localised signs of dimpling. It is likely that the lower ultimate tensile strength value of the quaternary alloy is related to the intergranular failure mode, which also correlates to the lower plasticity expressed by the lower elongation value. Compared with iron-based and nickel-based superalloys, the tensile strength of platinum-based superalloys is within the ultimate tensile strength range of high-temperature alloys at room temperature.

Odusote (70, 71) studied the isothermal oxidation behaviour of Pt-11Al-3Cr-2Ru (at%) at 1350°C in air. It is found that the oxide layer of the alloy is mainly composed of α-Al₂O₃ and the thickness of the oxide layer increases with the oxidation time according to a parabolic law (similar to Pt-Al-Cr or Pt-Al-Ru alloys). The growth mechanism of the oxide layer is mainly the diffusion of oxygen atoms into the inner layer along the oxide grain boundary, accompanied by the outward diffusion of a small amount of aluminium atoms. The oxide protective layer is dense, has good adhesion to the substrate and no local discontinuities or detrimental internal oxidation phenomena have been found (Figure 17) (71). These characteristics of the oxide layer indicate that the Pt-11Al-3Cr-2Ru alloy has good oxidation resistance and has the potential for high-temperature applications.

Alloys belonging to the Pt-Al-Cr-Ru series are currently being developed since they show the best overall performance. However platinum-based alloys have the disadvantages of high price and high density (10). The addition of a cheap and low-density alloy element to replace part of the platinum while maintaining the high-temperature properties and desired microstructure is a subject of current research. Wenderoth et al. (72) added the refractory metal niobium to the platinum-based alloy to improve the high-temperature strength through precipitation hardening. Vanadium belongs to the same group of the Periodic Table as niobium but has a smaller atomic radius and higher solid solubility in the platinum matrix. As well as its effect on precipitation hardening, vanadium may also have a solid solution hardening effect. Odera et al. (73) prepared and analysed four Pt-Al-Cr-Ru-V five-component alloys and two Pt-Al-Cr-Ru-V-Nb six-component alloys by adding vanadium and niobium on the basis of a Pt-12Al-4Cr-2Ru quaternary alloy with excellent properties. Vanadium was added in the range 5.2–19.0 at%, niobium was added in smaller amounts. The content of platinum was reduced to the range 63.2–78.7 at%. The study found that the expected Pt₃Al precipitate appeared and that a two-phase structure (matrix with precipitates) was obtained in the four cast alloys, while two alloys had a single-phase structure with vanadium preferentially distributed in the platinum matrix. The hardness of multi-element platinum-based alloys is higher than the quaternary alloys. The optimal vanadium content is about 15 at% to obtain the desired microstructure. A higher vanadium content leads to the formation of a brittle Pt-V mesophase. Niobium loss was too high for its alloying effect to be determined. According to the preliminary experimental results on microstructure and hardness of the multi-element systems based on platinum alloys, these materials have promise for high-temperature applications although further optimisation of their composition is necessary.