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J Mater Inf 2023;3:21. 10.20517/jmi.2023.17 © The Author(s) 2023.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

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1Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China.

2Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA.

3Yunnan Precious Metal Laboratory Co. LTD, Kunming 650106, Yunnan, China.

4Laboratory of Computational Physics, Institute of Applied Physics and Computational Mathematics, Beijing 100088, China.

*Correspondence to: Prof. Shun-Li Shang, Department of Materials Science and Engineering, The Pennsylvania State University, 324 Steidle Building, University Park, PA 16802, USA. E-mail: sus26@psu.edu.cn; Prof. Jing Feng, Faculty of Materials Science and Engineering, Kunming University of Science and Technology, 326 Materials Science and Engineering Building, Xuefu Road, Kunming 650093, Yunnan, China. E-mail: jingfeng@kust.edu.cn

© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Abstract

Thermodynamic calculations of Ellingham diagrams and the forming oxides have been performed relevant to the Pt-based alloys Pt82Al12M6 (M = Cr, Hf, Pt, and Ta). The predicted Ellingham diagrams indicate that the elements Hf and Al are easy to oxidize, followed by Ta and Cr, while Pt is extremely difficult to oxidize. Oxidation experiments characterized by X-ray diffraction (XRD) and electron probe micro-analyzers verify the present thermodynamic predictions, showing that the best alloy with superior oxidation resistance is Pt82Al12Cr6, followed by Pt88Al12 due to the formation of the dense and continuous α-Al2O3 scale on the surface of alloys; while the worse alloy is Pt82Al12Hf6 followed by Pt82Al12Ta6 due to drastic internal oxidation and the formation of deleterious HfO2, AlTaO4, and Ta2O5. The present work, combining computations with experimental verifications, provides a fundamental understanding and knowledgebase to develop Pt-based superalloys with superior oxidation resistance that can be used in ultra-high temperatures.

Keywords

Ellingham diagrams, Pt-based alloys, oxidation resistance, XRD, EPMA

INTRODUCTION

Aiming to design ultra-high-temperature materials for use in harsh environments has led to the development of Pt-based superalloys. These superalloys are superior to Ni-based superalloys since Pt has a higher melting point (2,042 K for Pt vs. 1,728 K for Ni[1]) and is more chemically inert for outstanding oxidation resistance, c.f., the predicted Ellingham diagrams[2] in the present work. Currently, a higher application temperature is urgently needed in gas turbine sections to improve engine efficiency and reduce fuel consumption and carbon dioxide emissions for a greater thrust. For example, the current application temperature of Pt-based superalloys can be extended to 1,350 °C (1,623 K)[3]. Although Pt-based alloys are expensive, they are the most reliable materials under ultrahigh temperatures and are widely used as bonding coating alloys in the thermal barrier coating (TBC) system and in the aerospace engine nozzle. The excellent oxidation resistance of Pt-based superalloys is their great advantage. The oxidation behavior of pure Pt has been fully investigated[4]; however, the oxidation of Pt-Al-based alloys is far from fully understood.

The Pt-Al system is one of the widely used superalloys, fueling up considerable studies such as the strength and creep properties[5,6]. The mechanism of oxidation resistance for pure Pt and Pt-Al alloys is quite different. It shows that the Pt-Al alloy can be further stabilized at high temperatures in terms of the L12-based γ’ phase by adding alloying elements such as Cr, Hf, and Ta. Especially, Cr can form the dense and continue Cr2O3 coating to provide a protective scale on Cr-containing alloys; Hf is a reactive element to reduce the growth of a thermally grown Al2O3 scale in alloys such as the NiAlCr and NiAlPt[7]; additionally, Ta can hinder ion diffusion and improve oxidation resistance in, e.g., the Ni-10Al alloy[8]. One of the purposes of the present work is to develop a series of Pt-Al-based alloys with strong oxidation resistance used as bonding coating in the TBC system. Notably, oxidation resistance for an alloy of interest is an important indicator to determine whether it can be applied to high temperatures or not[9,10], with the best oxide being α-Al2O3 (represented by Al2O3 in the present work if no further explanation), which will be formed on the surface of Al-containing alloys at high temperatures (> 1,100 °C)[11-13]. Here, Al2O3 usually acts as the oxidation resistance barrier of the alloys at high temperatures, thus further preventing oxidation inside the alloys[14,15].

In the present work, four alloying elements, Al, Cr, Hf, and Ta, are selected to investigate oxidation resistance of the Pt-based alloys. The forming mechanism of Al2O3 and other oxides is understood by the predicted Ellingham diagrams[2] with experimental verifications. Here, the free energy (or partial pressure of gas species) versus temperature diagram, i.e., the Ellingham diagram, is a predictive tool to tailor thermodynamic stability of oxides that form in the present Pt-based alloys[16]. In addition, the oxidation kinetic curves are also measured to examine the formation rates of oxides. The present computations of Ellingham diagrams with experimental verifications provide fundamentals to develop Pt-based superalloys with superior oxidation resistance that can be used in ultra-high temperatures.

MATERIALS AND METHODS

Ellingham diagram

An Ellingham diagram[2] is a plot regarding the change of the standard Gibbs energy (ΔGo) as a function of temperature for a given reaction, evaluating the equilibrium phases obtained by chemical reactions. In the present work, the calculated Ellingham diagrams can judge whether the Pt-based alloys (i.e., metals M = Al, Cr, Hf, Pt, and/or Ta) can be oxidized or not in terms of the following two reactions (scenarios),

$$ \frac{2 \mathrm{x}}{\mathrm{y}} \mathrm{M}(s \text { or } l)+\mathrm{O}_{2}(g)=\frac{2}{\mathrm{y}} \mathrm{M}_{\mathrm{x}} \mathrm{O}_{\mathrm{y}}(s\text { or } l) \\ $$

$$ \frac{2 x}{a y-b x} \mathrm{M}_{\mathrm{a}} \mathrm{O}_{\mathrm{b}}(s\text { or } l)+\mathrm{O}_{2}(g)=\frac{2 a}{a y-b x} \mathrm{M}_{\mathrm{x}} \mathrm{O}_{\mathrm{y}}(s\text { or } l) \\ $$

where s, l, and g indicate the solid, liquid, and gas phases, respectively. Here, (i) ay > bx, indicating the oxidation is from MaOb to MxOy; (ii) the oxygen gas species O2 is selected due to its extreme dominance in comparison with the other oxygen species of O1 and O3[17]; and (iii) the reactions are under the total pressure of 1 atm with 1 mole of O2. Under the assumptions of (a) ideal gas with O2 being the dominant gas species and (b) the activities of liquids and solids as unity, the ΔGo of equation (1) or (2) at equilibrium can be determined by the activity of oxygen ($$ a_{\mathrm{O}_{2}} $$)[18], as described by the equation:

$$ \Delta \mathrm{G}^{\mathrm{o}}=R T \ln a_{\mathrm{O}_{2}} \\ $$

where R is the gas constant, and T is the absolute temperature. By setting the reference pressure and the total pressure Pref = Ptot = 1 atm (105 Pa), then, $$ a_{\mathrm{O}_{2}}=P_{\mathrm{O}_{2}} / P_{\mathrm{ref}} \\ $$ with $$ P_{\mathrm{O}_{2}} $$ being the partial pressure of O2. Note that (a) only the stable or metastable oxides at 1 atm are considered in the present work; and (b) the reactions in equations (1) and (2) only involve the phases that are able to be in equilibrium with each other, i.e., only the phases involving neighboring oxidation states are allowed in a reaction. For example, the oxidation in the Cr-O system is from Cr to Cr2O3 and then to Cr5O12 with increasing oxygen content, x(O). For the ternary oxide of AlTaO4 with x(O) = 0.667, only the reaction from the oxide with x(O) = 0.6 (i.e., Al2O3) is considered. Table 1 summarizes all the possible reactions to form oxides in the present Al-Cr-Hf-Pt-Ta-O system, including seven binary oxides (Al2O3, Cr2O3, Cr5O12, HfO2, PtO, PtO2, and Ta2O5) and 1 ternary oxide (AlTaO4). Note that the listed reaction to form NiO is for reference only, and four polymorphs of Al2O3 (α-Al2O3, δ-Al2O3, γ-Al2O3, and κ-Al2O3) were included in the SGTE substance database (i.e., the SSUB5)[19] but only α-Al2O3 is stable.

Table 1

Reactions of one mole of O2 (g) from metal and/or metal oxide to metal oxide. The mole fractions of oxygen content, x(O), in oxides, and the $$ P_{\mathrm{O}_{2}} $$ value (in Torr) for each reaction at 1,300 °C and 1 atm are also shown. The reaction numbers (No.) are given according to the descending order of these $$ P_{\mathrm{O}_{2}} $$ values, and the reaction No. 4 to form NiO is for reference only

No. Metals x(O) $$ P_{\mathrm{O}_{2}} $$ at 1,300 °C Reactions
1 Pt 0.50 to 0.67 1.279e + 08 2PtO + O2 = 2PtO2
2 Cr 0.60 to 0.71 1.151e + 08 $$ \frac{10}{9} $$Cr2O3 + O2 = $$ \frac{4}{9} $$Cr5O12
3 Pt 0.50 2.036e + 07 2Pt + O2 = 2PtO
4 Ni 0.50 1.306e - 04 2Ni + O2 = 2NiO
5 Cr 0.60 4.660e - 14 $$ \frac{4}{3} $$Cr + O2 = $$ \frac{2}{3} $$Cr2O3
6 Ta 0.71 4.426e - 16 $$ \frac{4}{5} $$Ta + O2 = $$ \frac{2}{5} $$Ta2O5
7 Al, Ta 0.60 to 0.67 1.939e - 16 $$ \frac{2}{5} $$Al2O3 + $$ \frac{4}{5} $$Ta + O2 = $$ \frac{4}{5} $$AlTaO4
8 Al 0.60 8.256e - 24 $$ \frac{4}{3} $$Al + O2 = $$ \frac{2}{3} $$Al2O3
9 Hf 0.67 1.553e - 25 Hf + O2 = HfO2

Thermodynamic database and thermodynamic calculations

Calculating Ellingham diagrams requires a thermodynamic database (TDB) for the present Al-Cr-Hf-Pt-Ta-O system, which was created by a combination of the metal system Al-Cr-Hf-Pt modeled by Kim[20] with a thermodynamic description of the binary oxides from the SSUB5 database[19]. The missing thermodynamic properties of the Pt-O compounds (PtO, PtO2, and Pt3O4) were added to SSUB5 using the reported enthalpies and entropies of formation at room temperature[4]. The missing thermodynamic properties of AlTaO4 were estimated with the reference states being the binary oxides in SSUB5,

$$ \mathrm{Al}_{2} \mathrm{O}_{3}+\mathrm{Ta}_{2} \mathrm{O}_{5}=\mathrm{AlTaO}_{4} \\ $$

The enthalpy of formation (ΔH) of equation (4) was predicted by the density functional theory (DFT)-based first-principles calculations using the Vienna Ab initio Simulation Package (VASP)[21]. The ion-electron interaction was described by the projector augmented wave (PAW) method[22], and the exchange-correlation (X-C) functionals were described by the local density approximation (LDA)[23] and the generalized gradient approximation (GGA) improved for densely packed solids and their surfaces (PBEsol)[24]. Other details of first-principles calculations are given in Table 2, as well as the default settings used by the software DFT Tool Kits (DFTTK)[25], including crystal structures of the three oxides, the used k-point meshes, the employed plane-wave cutoff energy, and the selected potentials for elements Al, Ta, and O. In addition, Table 2 also lists the DFT results reported by the Materials Project (MP)[26] and the Open Quantum Materials Database (OQMD)[27] using the X-C functional of GGA-PBE[28]. It shows that the (ΔH) value by PBEsol (-13490 J per 6-mole-atoms) is roughly in the middle of these (ΔH) values and hence was selected for equation (4) and implemented into SSUB5 together with the heat capacity of AlTaO4 estimated by the Neumann-Kopp rule[29,30].

Table 2

Enthalpy of formation (H) for the reaction of “Al2O3 + Ta2O5 = AlTaO4” based on DFT-based first-principles calculations at 0 K according to MP[26], OQMD[27], and the present work, where the IDs of MP and OQMD are also listed. The final cutoff energy for accurate energy is 520 eV, and the other details are the default ones adopted by DFTTK[25]

Oxides Space group ID and notes X-C functionals Total energy (eV/atom) H
(J/6-mole-atoms)
Al2O3
(4 × 4 × 4)a
$$ R \overline{3} c $$ MP-1143 PBE -7.48
OQMD-1234774 PBE -7.477
This work PBEsol -7.388
This work LDA -8.313
Ta2O5
(6 × 5 × 1)b
Pmmn MP-1539317 PBE -9.77
OQMD-1739439 PBE -9.759
This work PBEsol -9.621
This work LDA -10.605
AlTaO4
(4 × 4 × 5)c
C12/m1 MP-14333 PBE -8.84 -13990
OQMD-6370 PBE -8.837 -16692
This work PBEsol -8.714 -13490d
This work LDA -9.669 -11091

In the present work, thermodynamic calculations were performed by using the Thermo-Calc software[31] and the presently built TDB (i.e., the TDB file, as shown in the Supplementary Material). Rather than calculating ΔGo directly for each reaction in Table 1, equilibrium calculations were performed in the present work using the following conditions for each reaction: (i) the total pressure fixed at 1 atm; (ii) 1 mole of O2; and (iii) the dormant gas and other phases, which were excluded in the reactions of interest; see more details and a macro tcm file in our recent work[18].

Experiments

The ingots were prepared by arc-melting. Then, the ingots were subjected to vacuum heat treatments. It was determined that the alloys include the L12-type γ’-Pt3Al phase. The square samples with the dimensions of 3 mm (length) × 3 mm (width) × 1 mm (thickness) were extracted from the as-cast ingots. All the samples were ground with the SiC abrasive papers (3,000 mesh) and then polished in 1 μm diamond suspension, and cleaned repeatedly in the ethanol solution. The oxidation experiments were carried out in the air at 1,300 °C using the high-temperature tube furnace. The samples were taken out after different oxidation times (1 to 100 h) at 1,300 °C and cooled down to room temperature within 15 min. After weighing by an electronic balance, the sample and crucible were preheated and put into the furnace. The sample was heated up to 1,300 °C within 3 min. An X-ray diffractometer was used to identify the phases of the samples before and after the high-temperature oxidation. The elemental mappings were conducted by the electron probe micro-analyzer (EPMA).

Table 3 shows the chemical composition of these alloys by the photoelectric direct reading spectrograph. Figure 1 shows the overall oxidation kinetic curves of the four different samples at 1,300 °C. The type of oxidation kinetics of these samples is parabolic. As it can be observed, the oxidation rate is initially high and gradually lowers with increasing oxidation time. NiCoCrAlY is the current most used bonding coating alloy in the TBC system. The melting point of the NiCoCrAlY alloy is only ~1,400 °C. The oxidation layer of NiCoCrAlY alloys is easily peeled off and disabled quickly at 1,300 °C.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 1. The oxidation kinetics of four Pt-based alloys and NiCoCrAlY after oxidation at 1,300 °C for 100 h in an air atmosphere.

Table 3

Measured chemical compositions of the as-prepared alloys

Alloys Al (at. %) M (at. %) Pt (at. %)
Pt88Al12 11.78 0 Balance
Pt82Al12Cr6 11.82 6.21 Balance
Pt82Al12Hf6 11.64 6.17 Balance
Pt82Al12Ta6 11.72 6.06 Balance

In addition, the Pt82Al12Cr6 and Pt88Al12 alloys show a relatively stable stage after exposure for 20 h. Note that in the diffusion-controlled thickening process of alumina (Al2O3), the mass gain of the Al2O3 scale should follow a parabolic law at the test temperatures according to the classical oxidation theory[32]. The results of oxidation kinetics regarding mass gain with respect to oxidation time can be approximately expressed as[33]:

$$ (\Delta m / A)^{2}=\mathrm{k}_{\mathrm{p}} t \\ $$

where Δm is mass gain of the sample in mg, A is the surface area of the sample in cm2, kp is the parabolic rate constant in mg2/(cm4·h), and t is the oxidation time.

RESULTS AND DISCUSSION

Ellingham diagrams to tailor oxidation resistance

Figure 2 shows the predicted Ellingham diagrams for the nine reactions, as shown in Table 1. The lower the $$ P_{\mathrm{O}_{2}} $$ value [i.e., the lower ΔGo, as shown in equation (3)] indicates that the reaction is more likely to occur. It is seen that Hf is highly susceptible to oxidation (resulting in the formation of HfO2), even at extremely low oxygen partial pressures (e.g., $$ P_{\mathrm{O}_{2}} $$ = 1.553e-25 Torr at 1,573 K, i.e., 1,300 °C), followed by Al to form Al2O3. Besides the Hf- and Al-containing oxides, the oxides AlTaO4, Ta2O5, and Cr2O3 are also easy to form at relatively low $$ P_{\mathrm{O}_{2}} $$ values. However, the formation of PtO and PtO2 (and Cr5O12) is extremely difficult, implying the excellent oxidation resistance of the Pt-based alloys, which is better than the Ni-based alloys, c.f., the formation of NiO in Figure 2. At the present experimental temperature of 1,300 °C, Table 1 also lists the predicted $$ P_{\mathrm{O}_{2}} $$ values, and these reactions are ranked according to the descending order of these $$ P_{\mathrm{O}_{2}} $$ values.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 2. Predicted Ellingham diagrams for 9 reactions (see also Table 1) with one mole of oxygen gas (mainly the O2) and the total pressure fixed at 1 atm. Here, the reaction No. 4 to form NiO is for reference only.

As two examples, Figure 3 shows the formed phases as a function of $$ P_{\mathrm{O}_{2}} $$ at 1,300 °C for two representative alloys, Pt82Al12Hf6 and Pt82Al12Cr6, indicating the dominant fcc phase, together with Al2O3 formed for both alloys. In addition, HfO2 forms for Pt82Al12Hf6 at very low $$ P_{\mathrm{O}_{2}} $$ values and Cr2O3 forms for Pt82Al12Cr6 at higher $$ P_{\mathrm{O}_{2}} $$ values (such as > 10-7 Torr). In summary, Table 4 shows the oxides predicted for the Pt-rich alloys Pt82Al12M6 (M = Cr, Hf, Pt, and Ta), indicating that Al2O3 is easy to form and appears in all Pt82Al12M6 alloys due to the low $$ P_{\mathrm{O}_{2}} $$ value required for the reaction No. 8; c.f., Table 1. In addition, Cr2O3 forms for Pt82Al12Cr6, HfO2 forms for Pt82Al12Hf6, and AlTaO4 and Ta2O5 form for Pt82Al12Ta6. Experimental verifications are also listed in Table 4, with details shown in Section “Verifications by oxidation experiments”.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 3. Formed phases for two representative alloys of Pt82Al12Cr6 and Pt82Al12Hf6 at 1,300 °C as a function of the partial pressure of O2 gas ($$ P_{\mathrm{O}_{2}} $$), where the input of each alloy is 1 mole and the gas phase is fixed but with zero amount during thermodynamic calculations. Here, we do not distinguish the fcc phase and the L12 phase, which were modeled by the same sublattice model[20].

Table 4

Summary of the oxides formed in the Pt-rich alloys at 1,300 °C according to the present thermodynamic calculations and the present oxidation experiments

Alloys Predicted oxides Measured oxides XRD figures
Pt88Al12 Al2O3a Al2O3 (α-Al2O3 + γ-Al2O3) Figure 4
Pt82Al12Cr6 Al2O3, Cr2O3b Al2O3 Figure 7
Pt82Al12Hf6 Al2O3, HfO2 (Al, Hf)2O3, Al2O3, HfO2, PtO2c Figure 10
Pt82Al12Ta6 AlTaO4, Al2O3, Ta2O5 Al2O3, AlTaO4, Ta2O5, PtO2c Figure 13

Verifications by oxidation experiments

Pt88Al12

Figure 4 shows the patterns of X-ray diffraction (XRD) of the Pt88Al12 alloy, which was oxidized at 1,300 °C. Both Al2O3 and fcc-based Pt are identified by XRD. Moreover, a few metastable scales of θ-Al2O3 are also detected, mainly due to the incomplete phase transition to the stable α-Al2O3 during the cooling process of the sample[34]. Note that the present thermodynamic calculations predict only the stable α-Al2O3, albeit four kinds of Al2O3 (α-, δ-, γ-, and κ-Al2O3) are included in the SSUB5 database; see also Table 4.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 4. Surface XRD results of the alloy Pt88Al12 after oxidation at 1,300 °C for 100 h. XRD: X-ray diffraction.

Figure 5 shows the EPMA elemental maps of Pt88Al12 oxidized at 1,300 °C for 100 h. Obviously, the light areas are fcc phase (mainly Pt), and the dark areas are Al2O3 according to the EPMA analysis; see Figure 5A. The Al2O3 layer is not fully dense. Figure 6 shows the cross-sectional EPMA of Pt88Al12 after oxidation at 1,300 °C for 100 h. The oxide scale on the Pt88Al12 surface is continuous and intact, which is from 10.1 to 13.3 μm in thickness, as shown in Figure 6A. However, the discontinuous Pt-rich layer is also observed outside the oxide layer, which is probably due to the deposition of the Pt vapor above 1,000 °C[10].

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 5. Surface morphologies and distribution of elements from EPMA of the alloy Pt88Al12 after 100 h oxidation in air at 1,300 °C. (A) Surface morphology; (B1)-(B3) Pt, Al, and O map distribution, respectively; (C) Linear distribution of Pt, Al, and O.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 6. Cross-sectional morphologies and distribution of elements from EPMA EPMA mapping images Pt88Al12 after 100 h oxidation in air at 1,300 °C. (A) Cross-sectional morphology; (B1)-(B3) Pt, Al, and O map distribution, respectively; (C) Linear distribution of Pt, Al, and O. EPMA: Electron probe micro-analyzer.

Pt82Al12Cr6

Figure 7 shows the XRD patterns of the Pt82Al12Cr6 alloy after oxidation at 1,300 °C for 100 h. It reveals that the α-Al2O3 is the major phase in the oxide layer with small peaks identified as Pt, indicating that Cr promotes the formation of α-Al2O3. Different from the case of Pt88Al12, after oxidation at 1,300 °C for 100 h, the Pt82Al12Cr6 alloy exhibits a smooth and dense surface. No spallation or crack is observed, and the Al2O3 scale adheres well with the base alloy [Figure 8]. Comparing and analyzing the XRD and EPMA results on the surface of the sample after oxidation in Figures 7 and 8, respectively, indicates that a dense Al2O3 layer forms on the surface of Pt82Al12Cr6, and the surface of the alloy does not have the element Cr.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 7. Surface XRD results of the alloy Pt82Al12Cr6 after oxidation at 1,300 °C for 100 h. XRD: X-ray diffraction.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 8. Surface morphologies at EPMA of the alloy Pt82Al12Cr6 after 100 h oxidation in air at 1,300 °C. (A) Surface morphology; (B1)-(B4) Pt, Al, O, and Cr map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Cr. EPMA: Electron probe micro-analyzer.

Figure 9A shows that the thickness of the Al2O3 scale is about 7.3 μm, which is less than the scale of the Pt88Al12 alloy. The chemical compositions from the surface to the base alloy Pt82Al12Cr6 are shown in Figure 9B and C. The spikes shown in Figure 9 indicate that Pt is diffused to the interior of the oxidation layer. Elements Al and O are distributed on the oxidation layer, and the elements Pt and Cr are in the substrate. The concertation of Cr in the substrate is much higher than that in the oxidation layer, suggesting that Cr in Pt82Al12Cr6 does not participate in the oxidation process. It is hence concluded that no Cr2O3 exists between Al2O3 and the substrate or on the surface of Pt82Al12Cr6. These results are also consistent with the XRD measurements in Figure 7. As seen from the oxidation kinetics of Pt82Al12Cr6 in Figure 1, the oxidation rate is initially high and gradually lowers with increasing oxidation time. During the oxidation process, it forms a continuous and protective Al2O3 scale on the surface, and then the oxidation rate reduces, leading to a slower scale growth rate. These results indicate that the dense oxide layer on the sample surface could effectively prevent the external oxygen into the substrate[35] and hence protect the internal stability of the sample[36,37].

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 9. Cross-sectional morphologies and distribution of elements from EPMA EPMA mapping images Pt82Al12Cr6 after 100 h oxidation in air at 1,300 °C. (A) Cross-sectional morphology; (B1)-(B4) Pt, Al, O, and Cr map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Cr. EPMA: Electron probe micro-analyzer.

Thermodynamic calculations [Figure 3] indicate that Cr2O3 will be formed in Pt82Al12Cr6 but at higher $$ P_{\mathrm{O}_{2}} $$ values (e.g., > 10-7 Torr), indicating that the $$ P_{\mathrm{O}_{2}} $$ values are not high enough to promote the formation of Cr2O3 in the present oxidation process of Pt82Al12Cr6.

Pt82Al12Hf6

Figure 10 shows the XRD results of Pt82Al12Hf6 after oxidation at 1,300 °C for 100 h, confirming the presence of Al2O3, HfO2, and a small amount of Pt on the surface after oxidation. By analyzing the EPMA results, as shown in Figure 11, we observe that the dark area mainly contains the element Hf, which is coincident with the elements O and Al. It is possible that the oxide (Al, Hf)2O3 has formed due to the solid solution of Hf in Al2O3. Therefore, we presume the presence of Al2O3 and HfO2 in the dark region, along with Pt, and possibly trace amounts of PtO2 in the bright region. In addition, it is possible that trace amounts of PtO2 are present on the surface of Pt82Al12Hf6 after oxidation at very high $$ P_{\mathrm{O}_{2}} $$. It is worth mentioning that the formation of PtO2 needs further verification using techniques such as the high-resolution transmission electron microscopy (HRTEM) and/or the electron backscattering diffraction (EBSD).

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 10. Surface XRD results of the alloy Pt82Al12Hf6 after oxidation at 1,300 °C for 100 h. XRD: X-ray diffraction.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 11. Surface morphologies at EPMA of the alloy Pt82Al12Hf6 after 100 h oxidation in air at 1,300 °C. (A) Surface morphology; (B1)-(B4) Pt, Al, O, and Hf map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Hf. EPMA: Electron probe micro-analyzer.

Thermodynamic calculations show that only Al2O3 and HfO2 can form [Figure 3] due to insolubility of Hf in Al2O3 in the present database and the very high $$ P_{\mathrm{O}_{2}} $$ to form PtO2; c.f., Figure 2.

From the cross-sectional EPMA mapping images of Pt82Al12Hf6, Figure 12A shows that no dense oxide layer is observed, but it forms internal oxides. Figure 12B shows the composition map distribution of these oxides from the surface to the alloy interior. Figure 12B and C reveals that Al and Hf are combined with O. In addition, the dispersive dark appears on the oxide regions with rich Al and O, along with some Hf, observed at a distance of about 4 μm and beyond from the surface. Due to the segregation of alloying elements in oxides, the matrix is almost pure Pt.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 12. Cross-sectional morphologies and distribution of elements from EPMA EPMA mapping images Pt82Al12Hf6 after 100 h oxidation in air at 1,300 °C. (A) Cross-sectional morphology; (B1)-(B4) Pt, Al, O, and Hf map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Hf. EPMA: Electron probe micro-analyzer.

In comparison with the Pt88Al12 and Pt82Al12Cr6 alloys, Pt82Al12Hf6 shows a worse oxidation resistance due to the extremely easy formation of HfO2, as indicated by the Ellingham diagrams [Figure 2], resulting in oxidation inside the alloy and the element Pt inside the scale, probably due to a decomposition of PtO2 at the scale surface after oxidation at 1,300 °C[10].

Pt82Al12Ta6

Figure 13 shows the XRD patterns of Pt82Al12Ta6, which is oxidized at 1,300 °C for 100 h. The oxide layer consists of Al2O3, AlTaO4, Ta2O5, and PtO2, agreeing with thermodynamic calculations (except for the formation of a trace amount of PtO2), c.f., Table 4.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 13. Surface XRD results of the alloy Pt82Al12Ta6 after oxidation at 1,300 °C for 100 h. XRD: X-ray diffraction.

These results are also consistent with the EPMA mapping analysis [Figure 14], where the dark region is Al2O3, and the bright region mainly contains Pt, Ta, and a small amount of O. So, we presume that the bright area mainly consists of Pt, PtO2, and Ta2O5; and Al2O3 and AlTaO4 in the dark region; and Ta2O5 mainly in the dark area.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 14. Surface morphologies at EPMA of the alloy Pt82Al12Ta6 after 100 h oxidation in air at 1,300 °C. (A) Surface morphology; (B1)-(B4) Pt, Al, O, and Ta map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Ta. EPMA: Electron probe micro-analyzer.

Figure 15 shows the cross-section EPMA mapping of the oxidized samples, indicating that no dense oxides on the sample surface are observed. Figure 15B shows that at the interior, both Al and Ta are found in combination with O. According to Figure 15C, the dispersive gray areas are enriched with O and Ta, but almost no Al, which could be identified as Ta2O5. The dark oxidized regions with enriched Al and O, along with some Ta, are observed in the interior of the alloy, indicating the formation of Al2O3, AlTaO4, and Ta2O5. The oxidation kinetics of Pt82Al12Ta6 is similar to that of Pt82Al12Hf6, as shown in Figure 1; even the weight gain is more pronounced, attributed to more intense internal oxides formed than those in the Pt82Al12Hf6 alloy.

Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications

Figure 15. Cross-sectional morphologies and distribution of elements from EPMA EPMA mapping images Pt82Al12Ta6 after 100 h oxidation in air at 1,300 °C. (A) Cross-sectional morphology; (B1)-(B4) Pt, Al, O, and Ta map distribution, respectively; (C) Linear distribution of Pt, Al, O, and Ta. EPMA: Electron probe micro-analyzer.

CONCLUSIONS

The present work investigates the forming mechanism of oxides in the Pt-based alloys Pt82Al12M6 (M = Cr, Hf, Pt, and Ta) in terms of thermodynamic calculations with experimental verifications. The presently predicted Ellingham diagrams indicate the metals Hf and Al are easy to oxidize, followed by the metals Ta and Cr, while Pt is extremely difficult to oxidize in terms of thermodynamics. Oxidation experiments are characterized by XRD and EPMA, verifying the present thermodynamic predictions and showing that the best alloy with superior oxidation resistance is Pt82Al12Cr6 followed by Pt88Al12 by forming the dense and continuous α-Al2O3 scale on the surface of alloys after oxidation; while the worse alloy is Pt82Al12Hf6 followed by Pt82Al12Ta6 due to drastic internal oxidation and the formation of deleterious oxides of HfO2, AlTaO4, and Ta2O5.

DECLARATIONS

Authors’ contributions

Investigation, Writing - original draft: Chong X

Writing - review & editing: Yu W

Methodology, Visualization, Writing - review and editing: Liang Y

Supervision, Data curation, Methodology, Writing - review and editing: Shang SL

Data curation, Methodology: Li C

Validation, Funding acquisition: Zhang A, Wei Y

Discussion, Funding acquisition: Gao X

Supervision, Methodology: Wang Y

Validation, Supervision: Feng J, Song H

Validation, Funding acquisition: Chen L

Conceptualization, Supervision: Liu ZK

Availability of data and materials

The Supplementary Materials are available including the TDB file for Ellingham Diagrams calculations.

Financial support and sponsorship

The authors would like to thank the financial support from the Rare and Precious Metals Material Genetic Engineering Project of Yunnan Province [202102AB080019-1, 202302AB080021]; Key R&D Program of Yunnan Province [202203ZA080001]; Open Project of Yunnan Precious Metals Laboratory Co., Ltd [YPML-2023050281]; Major R&D Project of Yunnan Precious Metals Laboratory Co., Ltd [YPML-2023050205] and Yunnan Fundamental Research Projects [202101AW070011, 202101BE070001-015].

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2023.

Supplementary Materials

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Cite This Article

OAE Style

Chong X, Yu W, Liang Y, Shang SL, Li C, Zhang A, Wei Y, Gao X, Wang Y, Feng J, Chen L, Song H, Liu ZK. Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications. J Mater Inf 2023;3:21. http://dx.doi.org/10.20517/jmi.2023.17

AMA Style

Chong X, Yu W, Liang Y, Shang SL, Li C, Zhang A, Wei Y, Gao X, Wang Y, Feng J, Chen L, Song H, Liu ZK. Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications. Journal of Materials Informatics. 2023; 3(4): 21. http://dx.doi.org/10.20517/jmi.2023.17

Chicago/Turabian Style

Chong, Xiaoyu, Wei Yu, Yingxue Liang, Shun-Li Shang, Chao Li, Aimin Zhang, Yan Wei, Xingyu Gao, Yi Wang, Jing Feng, Li Chen, Haifeng Song, Zi-Kui Liu. 2023. "Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications" Journal of Materials Informatics. 3, no.4: 21. http://dx.doi.org/10.20517/jmi.2023.17

ACS Style

Chong, X.; Yu W.; Liang Y.; Shang S.L.; Li C.; Zhang A.; Wei Y.; Gao X.; Wang Y.; Feng J.; Chen L.; Song H.; Liu Z.K. Understanding oxidation resistance of Pt-based alloys through computations of Ellingham diagrams with experimental verifications. J. Mater. Inf. 2023, 3, 21. http://dx.doi.org/10.20517/jmi.2023.17

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