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

Ellingham diagram

An Ellingham diagram^[2] is a plot regarding the change of the standard Gibbs energy (ΔG^o) 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),

where s, l, and g indicate the solid, liquid, and gas phases, respectively. Here, (i) ay > bx, indicating the oxidation is from M_aO_b to M_xO_y; (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 ΔG^o 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:

where R is the gas constant, and T is the absolute temperature. By setting the reference pressure and the total pressure P_ref = P_tot = 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.

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,

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].

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 ΔG^o 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.

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.

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]:

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

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 ΔG^o, 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.

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”.

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].

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.

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].

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.

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.

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

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].

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).

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

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.

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.

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.

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.

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.