Protective aluminide coating by pack cementation for Beta 21-S titanium alloy

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Protective aluminide coating by pack cementation for Beta 21-S titanium alloy ⁎

N. Chaiaa, , C.M.F.A. Cossua, L.M. Ferreiraa, C.J. Parrischb, J.D. Cottonc, G.C. Coelhoa, C.A. Nunesa a

Escola de Engenharia de Lorena, Universidade de São Paulo, Lorena, SP, Brazil Boeing Research and Technology, São José dos Campos, SP, Brazil c The Boeing Company, Seattle, WA, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermodynamic analysis Pack cementation Beta-21S Aluminide coating High temperature oxidation

Pack cementation method was used to form high activity aluminide coating on titanium alloy Beta-21S using a cement activated by CrCl3 at 760 °C for a period of 9 h. Thermodynamic computations showed that growth mechanism of TiAl3 occurs in tree steps involving different chemical reactions. The effectiveness of the TiAl3 coating to protect Beta-21S alloy was demonstrated through oxidation tests performed at 750 and 850 °C during 300 h. The elevated resistance against oxidation of the TiAl3 layer is provided by a protective Al2O3 scale having low growth kinetics.

1. Introduction The oxidation/degradation phenomena of titanium and its alloys are strongly dependent on the composition of the oxidizing gases and the time of exposure at high temperatures. It is well established that during high temperature oxidation, dissolution of interstitials in titanium alloys are responsible for the degradation of their mechanical properties [1–3]. To improve their oxidation resistance in harsh environments, a modification of the alloy chemistry or surface treatments are generally adopted [4]. For titanium alloys that exhibit excellent resistance, the temperature limit of use is that identified with the beginning of an exacerbated oxidation of the surface along with significant formation of an alpha case which is detrimental to their mechanical properties [5,6]. For conventional titanium alloys, these limiting temperatures are close to 600 °C. Alloying elements present in Beta-21S, such as Mo, Al, Si and platinoids, provide a relatively enhanced mechanical properties up to temperatures close to 800 °C [7,8]. However, the dissolution of oxygen during long exposure durations leads to the stabilization of the α-Ti and the reduction of the β-Ti fraction, altering drastically the ductility and fatigue resistance, thus limiting the application of the alloy at temperatures near 550 °C [9]. This work aims at showing the effect of aluminization treatment applied by pack cementation method on the oxidation resistance of Beta-21S at temperatures of 750 and 850 °C. It is worth mentioning that

the use of coatings for titanium alloys may be discredited because of the possibility of crack formation or detachment of the coating due to thermal shock, which may lead to irreversible damage induced by an accelerated degradation of mechanical properties at elevated temperatures. For these reasons, to meet the specifications of a good coating, perfect physical continuity, excellent ability to accommodate thermo-mechanical stresses and excellent adhesion with the substrate are necessary. These requirements can be achieved by using the pack cementation technique that involves diffusion phenomena during formation of the protective layer. Coatings manufactured by this method have been the object of a large number of studies, demonstrating the effectiveness and capability to enhance the oxidation behavior of a large family of alloys intended for high temperature applications [10–19]. 2. Materials and experimental methods Beta-21S substrates used in the present investigation, with a nominal composition Ti-15Mo-3Nb-3Al-0.2Si in wt. %, was manufactured by Timetal® and supplied by BOEING Research & Technologies Brazil. Substrates with a parallelepipedic geometry having approximate dimensions of 10 × 10 x 1.5 mm were cut from a heat treated plate in STOA (solution treat and overage) condition. Prior to the pack cementation process, the substrates were ground with silicon carbide

⁎

Corresponding author. E-mail addresses: [email protected] (N. Chaia), [email protected] (C.M.F.A. Cossu), [email protected] (L.M. Ferreira), [email protected] (C.J. Parrisch), [email protected] (J.D. Cotton), [email protected] (G.C. Coelho), [email protected] (C.A. Nunes). https://doi.org/10.1016/j.corsci.2019.108165 Received 19 June 2019; Received in revised form 12 August 2019; Accepted 13 August 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: N. Chaia, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108165

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abrasive papers down to 2400 grits, the corners were rounded, ultrasonically cleaned in acetone then in ethanol and dried in hot air. The deposition of aluminum was performed by the halide activated pack cementation technique using a powder mixture of Al (99.99%) as masteralloy, Al2O3 as inert filler and CrCl3 as activator. Samples to be coated and the aluminizing pack were placed under primary vacuum in sealed silica tubes. The deposition process was performed isothermally at 760 °C during 9 h based on results of growth kinetics determined by [20]. For the oxidation tests, coated samples were placed in alumina crucibles in preheated furnaces under static lab air at 750 and 850 °C for durations of 100, 200 and 300 h. Samples were hand-weighted after each period in order to determine mass variation and oxidation kinetics. Prior to the metallographic preparation, the oxidized samples were electroplated with Ni using a Watts bath to protect the oxide scale. Oxidized as well as as-coated samples were cleaned ultrasonically in ethanol, sectioned, cold mounted in epoxy resin and prepared with conventional methods for metallographic examination by optic/electron microscopy and XRD analysis. Chemical composition was measured by EDS.

Fig. 2. Gibbs energy of formation for fluorides and chlorides used as activator in the pack cementation technique.

thickness after 9 h of deposition at 760 °C is approximately 150 μm. The micrograph shows a uniform layer, completely covering the surface of the substrate with no cracks/porosities formation. Some irregularities in the thickness can appear in the regions near the borders, due to the high concentration of defects and mechanical stress in these regions. Results from EDS microanalysis (Fig. 1.b) performed on the cross section along with XRD performed on the surface, indicate that the interdiffusion region is composed only by TiAl3. The concentration profile shows that only Mo is dissolved in the coating with considerable amount (about 3 at. %) while the content of Si and Nb do not exceed 0.4 at. %, approximately.

3. Results 3.1. Morphology and composition of the coatings Typical microstructure of the coatings formed on Beta-21S alloy at 760 °C using CrCl3 as activator is shown in Fig. 1.a. The coating

3.2. Thermodynamics of the aluminization process In the present work, the use of chlorides as transport agent rather than fluorides is justified by their low thermodynamic stability, as illustrated by Gibbs energies of formation presented in Fig. 2. The decomposition/volatilization of the chlorides leads to the formation of larger amounts of gaseous species even for small amount introduced in the powder mixture. The pack cementation technique used in this work may be performed in two different configurations: (a) using a sealed silica reactor operating under primary or secondary vacuum, or (b) using an alumina crucible introduced into furnace operating under controlled atmosphere of inert gas like argon, or a reducing gas like H2 (g) [21,22]. Both types of assembly offer the possibilities of operating with the part to be coated inside or outside the powder mixture with configurations called "in pack" or "out of pack", respectively. In the present work, in pack configuration using silica tubes sealed under primary vacuum is used to avoid kinetic limitations that can be associated to diffusion in the gas phase. The formation of coatings using pack cementation is effective only if the following criteria are satisfied: (a) Quasi-steady state and near thermodynamic equilibrium conditions are established at the cement/gas and at the substrate/gas interfaces; (b) Between the cement and the substrate, a negative gradient of chemical activity (driving force) exist through the reactive gas so that the species carrying the element(s) to be deposited can migrate by diffusion. Taking into account these criteria, the pack cementation conditions can be described by adopting a classical thermodynamic approach which minimizes the total Gibbs energy of the system [23]. The overall composition of the gas phase can be calculated using the routines of ChemSage, Solgasmix or Gibbs modules [24–28] of the HSC Chemistry software [29]. The minimization of the Gibbs energy function of the

Fig. 1. a) SEM/BSE micrograph of the cross section of aluminized Beta 21S at T = 760 °C for during 9 h using using a cement activated by CrCl3 ; b) the corresponding concentration profile determined by EDS on the cross section. 2

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phases shows that TiAl is the most stable compound, in contrast to what it is observed experimentally, i.e. coating composed only of TiAl3. Thus, in this first step of the cementation process, aluminum deposition is expected to occur involving displacement reactions: AlCl3(g) + 2Ti → TiCl3(g) + TiAl(ΔGr° = -23,6 kJ/mol) 2AlCl3(g) + 4Ti → Ti2Cl6(g) + 2TiAl(ΔGr° = -74,7 kJ/mol) Al2Cl6(g) + 4Ti → 2TiCl3(g) + 2TiAl(ΔGr° = -62,5 kJ/mol) Al2Cl6(g) + 4Ti → Ti2Cl6(g) + 2TiAl(ΔGr° = -89,9 kJ/mol) It is worth noting that the most favorable reactions are those involving the gaseous compound Ti2Cl6. According to the second computation, TiAl considered as the new substrate, is consumed by chemical reactions of the same nature of those involved in the first step, leading to the formation of the TiAl3 phase:

Fig. 3. Evolution of the gas phase composition for the aluminizing cement (15 mg of CrCl3 +100 mg of Al) as a function of temperature.

3AlCl(g) + 3TiAl → TiCl3(g) + 2TiAl3(ΔGr° = -137,0 kJ/mol) 6AlCl2(g) + 9TiAl → 4TiCl3(g) + 5TiAl3(ΔGr° = -69,6 kJ/mol)

system is performed taking into account the mass balance of the components present in the cement. For all the compounds involved in this study, which are associated to the Ti-Al-Cl-Cr quaternary system, the thermodynamic data are those compiled by Barin and Knacke [30] and by Pankratz [31] and implemented in the database of the HSC Chemistry software. The intensive variables considered for thermodynamic calculations are pressure (set at 1 atm) and temperature. The analysis of the thermodynamic equilibria involved during the deposition of aluminum on Beta-21S was particularly similar to approaches previously used for describing aluminizing or siliconizing processes for superalloys [32,33] and Nb-based alloys [34]. Computation of chemical equilibria is performed separately at the gas/cement interface and then at the gas/ substrate interface assuming that the cement and the substrate are physically separated in the reactor during the aluminization process. For the equilibrium at the gas/cement interface, the reaction of aluminum with CrCl3 (activator) determines the composition of the gas phase. The input amount of Al and CrCl3 were 0.10 g and 0.015 g, respectively. Results of this equilibrium computation are given in Fig. 3, which shows the evolution of the gas phase composition, expressed as partial pressure, with temperature. The gas phase is composed mainly of aluminum chlorides (AlCl3, Al2Cl6, Al2Cl4, AlCl2 and AlCl) that fix the total internal pressure in the reactor. The other species present in the gas phase Al (g), Cl (g), Cl2 (g) and CrClx (g) are formed with partial pressures not exceeding 10−7 atm up to 1000 °C. Taking into consideration their low activities, these species should not have a significant effect on the aluminization process. For the equilibrium between gas and substrate, the calculations were performed in different steps in order to evaluate the nature of the compounds that form at this interface. In the first step, the substrate was considered to be pure Ti (input amount arbitrarily fixed at 10−4 mol), since the amount of alloying elements Mo, Nb, Al and Si and by extension their activities are very low. It is plausible to assume that these elements have no significant effect on the coating formation mechanisms. The second step was performed considering the most stable condensed phases from the first computation as the new substrate and assuming that the composition of the reactive gas phase remains unchanged. Input (nin) and equilibrium (neq) amounts, given in mol, along with the gradients determined between the gas and the substrate interfaces (Δn = neq-nin), are given in Tables 1 and 2, for the gaseous and the condensed phases, respectively. The first step of thermodynamic calculations clearly showed the effectiveness of the aluminization process of titanium substrate using a high activity cement. Results of the first equilibrium show that the amount of all aluminum chlorides decreases when the reactive gas is placed in contact with the titanium substrate leading to the formation of titanium chlorides species. However the analysis of the condensed

6AlCl(g) + 6TiAl → Ti2Cl6(g) + 4TiAl3(ΔGr° = -302,4 kJ/mol) 6AlCl2(g) + 9TiAl → 2Ti2Cl6(g) + 5TiAl3(ΔGr° = -151,7 kJ/mol) Thus, the thermodynamic analysis suggest that the deposition process takes place on the substrate of titanium involving a double-reaction mechanism. 3.3. Oxidation behavior Fig. 4 shows the specific mass change Δm/S (with “S” stands for the surface area) versus time for air-oxidized coated samples at 750 and 850 °C respectively. Since during the oxidation tests only 3 points were recorded, data obtained were not processed to determine reliable oxidation rate constants. The recorded mass variations are extremely low indicating a high oxidation resistance of the coatings. After 300 h of exposure, these variations were approximately of 0.15 and 0.60 mg.cm−2 at 750 and 850 °C respectively. Fig. 5 shows the XRD patterns obtained from the surface of the samples after the oxidation tests. The analyses of oxidized samples at both temperatures indicate the presence of α-Al2O3 (corundum) and peaks corresponding to the coating TiAl3. The low intensity of the peaks associated to Al2O3 and the identification of TiAl3 for all the oxidation conditions indicate low thicknesses of the protective Al2O3 scale which is in good agreement with the low mass gains. Fig. 6a and b present the cross-sections of the coated samples after the oxidation tests. Both samples were covered by a dense and perfectly adherent Al2O3 layer, as shown by the micrographs in the inset with a higher magnification. The micrographs evidenced that this layer has a thickness inferior to 1 μm at 750 °C and of 1–2 μm at 850 °C in average confirming the XRD results. Cracks perpendicular to the substrate surface are observed for samples oxidized at 850 °C, but their propagation was systematically stopped at the interdiffusion region as indicated by the SEM images. 4. Discussion Displacement reactions, when involved in the mechanism of deposition during the process of pack cementation lead to the production of coatings with low metallurgical quality, presenting porosities, lack of adhesion at the substrate/coating interface and, in some cases, formation of blisters. Chaia et al., [17,35] have shown that deposition of silicon on vanadium and its alloys is hindered by the formation of volatile vanadium chlorides resulting from displacement reactions with important partial pressure, leading to formation of blisters in coating structure and a subsequent loss of adhesion. In order to control the 3

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Table 1 Composition of the main gaseous species in (mol) from the thermodynamic equilibrium between the aluminizing gas and the titanium substrate calculated at 760 °C. For the first and the second steps the composition of the aluminizing gas generated from the cement activated by CrCl3 is 1/2nin while for the third step 3nin is considered as the new aluminizing gas. Species

Reactive gas composition 1/2 nin

1st Equilibrium gas/ substrate 1 neq

2nd Equilibrium gas/ substrate 2 neq

Δn1

Δn2

Reactive gas composition 3 nin

3rd Equilibrium gas/ substrate neq

Δn3

AlCl(g) AlCl2(g) AlCl3(g) TiCl3(g) TiCl4(g) Ti2Cl6(g)

4.14 × 10−7 5.96 × 10−7 8.38 × 10−5 0 0 0

7.35 × 10−9 6.08 × 10−8 4.92 × 10−5 1.07 × 10−5 2.45 × 10−6 1.37 × 10−5

4.28 × 10−8 1.91 × 10−7 8.33 × 10−5 6.57 × 10−7 8.09 × 10−8 4.49 × 10−8

−4.07 × 10-7 −5.35 × 10-7 −3.46 × 10-5 1.07 × 10−5 2.45 × 10−6 1.37 × 10−5

−3.71 × 10-7 −4.05 × 10-7 −5.00 × 10-7 6.57 × 10−7 8.09 × 10−8 4.49 × 10−8

4.14 × 10−7 5.96 × 10−7 8.38 × 10−5 1.07 × 10−5 2.45 × 10−6 1.37 × 10−5

1.98 × 10−8 1.33 × 10−7 8.77 × 10−5 1.24 × 10−5 2.31 × 10−6 1.21 × 10−5

−1.55 × 10-7 −1.22 × 10-7 4.38 × 10−6 1.70 × 10−6 −1.40 × 10-7 −1.60 × 10-6

formation of these halides, the authors introduced moderators (disilicides compounds CrSi2, VSi2 or CrSi2) to reduce the fugacity of VCl2 (g) and to improve consequently the coatings quality. Another optimization of the process made by Chaia et al. [35,36] consisted in the use fluorides instead of chlorides, avoiding the formation of volatile vanadium halides with significant amounts. Nonetheless, the use of fluorides does not allow for simultaneous deposition of elements in a single step, reducing the possibilities of modification of coatings chemistry. Cockeram et al. [37,38] also mentioned that siliconizing of titanium and its alloys using chlorides leads to the formation of porous coatings. According to these authors, the porosity observed in the coatings is a consequence of displacement reactions that are associated with the high thermodynamic stability of titanium chlorides. Similarly to these observations [17, 36–38], the thermodynamic calculations performed in the present work clearly indicate a significant formation of titanium chlorides by displacement reactions. Since TiAl3 coating on Beta-21S did not present any metallurgical defects, a third thermodynamic computation was performed to clarify the observed discrepancy. The departure of titanium from the substrate in the form of chlorides through displacement reactions is questioned. Step 3 of the thermodynamic analysis was performed with a modification of the gas phase composition, taking into account the titanium chlorides formed in contact with the titanium substrate. The nature and amounts of the condensed phases considered are the same as those used for the second computation. The following observations were made considering the results obtained from the thermodynamic equilibrium: (a) All aluminum gaseous chlorides are consumed, except AlCl3(g) whose partial pressure increases considerably; (b) The amount of the superior halide Ti2Al6(g) decreases in contact with titanium substrate, indicating its consumption; (c) The amount of TiAl decreases to the detriment of the TiAl3 whose amount increases significantly. Thus, four types of reactions are suggested to be involved for the consumption of the gaseous halide Ti2Cl6(g) along with the aluminum carrier species in favor of TiAl3 formation:

Fig. 4. The mass change with time during the isothermal oxidation of the coated samples at 750 and 850 °C.

3AlCl(g) + TiAl → TiAl3 + AlCl3(g)(ΔGr° = -192,3 kJ/mol) 6AlCl2(g) + TiAl → TiAl3 + 4AlCl3(g)(ΔGr° = -316,1 kJ/mol) The oxidation tests performed on the aluminized samples showed the efficiency of TiAl3 coating to protect the alloy even at elevated temperatures. The mass gains of the oxidized coated specimen are by far lower than those for the bare alloy. The oxidation of Beta-21S was studied by Wallace et al. [39] in the temperature range 600–800 °C. Under these conditions, the alloy is generally characterized by a poor oxidation resistance. For the sake of example, the mass change during oxidation for the bare alloy is around 3 mg/cm2 at 800 °C for a duration of 100 h, whereas this variation for the aluminized alloy is only of 0.60 mg.cm−2 at 850 °C after the same duration. The formation of Al2O3 by selective oxidation is in good agreement with other observations from the literature [40–46]. Data from the literature indicate that the behavior of TiAl3 compound is in the same order of magnitude compared to the Al2O3-forming alloys, such as NiAl and other Ni-based superalloys [1,47]. However, it was mentioned that it is particularly difficult to propose a kinetic law to predict oxidation behavior of TiAl3 for long-term exposure. Extrapolations of mass gains from short periods usually do not allow predicting mass gain over very long periods.

12AlCl(g) + Ti2Cl6(g) → 2TiAl3 + 6AlCl3(g)(ΔGr° = -851,3 kJ/mol) 24AlCl2(g) + Ti2Cl6(g) → 2TiAl3 + 18AlCl3(g)(ΔGr° = -1346,5 kJ/ mol)

Table 2 Composition of the main condensed phases in (mol) from the thermodynamic equilibrium between the aluminizing gas and the titanium substrate calculated at 760 °C. Species

Substrate composition nin

1st Equilibrium gas/ substrate 1 neq

Δn1

Substrate composition nin

2nd Equilibrium gas/ substrate 2 neq

3rd Equilibrium gas/ substrate 3 neq

Δn2

Δn3

Ti TiAl TiAl3

1.00 × 10−4 0 0

6.15 × 10−6 1.75 × 10−5 1.49 × 10−8

−9.39 × 10-5 1.75 × 10−5 1.49 × 10−8

0 1.75 × 10−5 1.49 × 10−8

2.36 × 10−7 8.51 × 10−6 6.24 × 10−7

1.09 × 10−6 9.09 × 10−6 3.54 × 10−8

2.36 × 10−07 −8.98 × 10-06 6.09 × 10−07

1.09 × 10−6 −8.41 × 10-6 2.05 × 10−8

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good capacity to accommodate thermo-mechanical stresses induced in the system due to CTE (coefficient of thermal expansion) mismatch between the coating and the substrate. Indeed, the values of the CTE reported in the literature for Beta-21S which is approximately 10 × 10−6 K-1 at 800 °C [49] is close to the CTE of the phases suspected to form in the interdiffusion zone, TiAl and Ti3Al, that are estimated to be 11 and 9 × 10−6 K-1, respectively, as reported in [50]. The depletion rate of the coating can be a detrimental factor in determining the coating durability. Therefore, the coating stability does not only depend on the aluminum consumption to form the Al2O3 layer, but it is also strongly affected by the interdiffusion phenomena observed at the substrate/coating interface. Taking into consideration the semi-infinite couple Beta-21S/TiAl3 as a pseudo-binary couple, equivalent to Ti/TiAl3, other intermetallics are predicted to form in the interdiffusion region according to the Ti-Al phase diagram [51,52]. Although local thermodynamic equilibria are established at each interface, the change in chemical activity of elements at the interfaces make the system coating/substrate thermodynamically unstable. The gradients of the chemical potential of the elements at the interfaces are responsible for establishing the driving force for diffusion and consequently the consumption of TiAl3. Further systematic investigations to determine the diffusion coefficients for the different intermetallic phases present in this system are needed to estimate the thermal stability of this alulminde coating. 5. Conclusions Protective aluminide coating were successfully processed on Beta21S using the halide activated pack cementation method using CrCl3 as activator. The obtained coating was composed by a unique layer of TiAl3, presenting good adherence to the substrate and free of porosities. Thermodynamic analysis was used to predict the deposition conditions and to propose chemical reactions and mechanism involved during the aluminization process. Two principal displacement reactions are involved during the formation of TiAl3 with the production of considerable amounts of titanium halides that are reduced in contact with the growing intermetallic layers. The oxidation resistance of the coatings was demonstrated through isothermal tests carried out at 750 and 850 °C for duration up to 300 h. The oxidation rate constants determined for TiAl3 were three orders of magnitude lower than those determined for the bare Beta-21S alloy demonstrating the capability of the coating to protect the alloy. The elevated resistance to oxidation at high temperatures of these coating is provided by a dense Al2O3 scale, having low growth rate and resulting from a selective oxidation of the TiAl3. The cracks formed during the oxidation tests were systematically deflected by the interdiffusion layer that forms due to the inward diffusion of aluminum. This behavior is a clear demonstration of the good ability to accommodate thermomechanical stresses that can be generated from severe temperature changes during service.

Fig. 5. X-ray diffraction patterns on the surface of the oxidized coated samples during 100, 200 and 300 h at (a) 750 °C and (b) 850 °C.

Kinetic transitions are often observed during oxidation at high temperatures for prolonged periods as has been observed already during the oxidation of aluminide-based coatings for γ-TiAl alloys [46]. The authors observed a transition from a parabolic regime (n = 2.0) established up to 820 h of oxidation to 800 °C, to a sub-parabolic regime (n = 1.4) up to 6200 h. This regime, which contradicts the growth of alumina at high temperatures, may be a reflection of an oxidation controlled by electron diffusion or a recrystallization of the oxide layer as proposed by Cabrera and Mott [48]. The oxidation of TiAl3 formed in the present work by aluminization of Beta-21S, at 750 and 850 °C, systematically showed a selective oxidation of aluminum. TiAl2 precipitates were observed at 850 °C in the sub-surface region in agreement to the observation of Xiang et al. [45,46]. They have observed the formation of a TiAl2 layer after 2788 h of oxidation at 800 °C due to the depletion of aluminum in TiAl3 coatings to form Al2O3 according to the following reaction:

Data availability All data used in this work, raw or processed, required to reproduce these results can be provided upon request by the corresponding author.

4TiAl3 + 3O2 → 4TiAl2 + 2Al2O3 However, this transformation was not evidenced in this work at 750 °C. Considering the low exposure durations (300 h), it is likely that the size of the TiAl2 particles is so small that the resolution of characterization techniques (DRX and SEM) used in this work is unable to detect their presence. The formation of an interdiffusion layer by inward diffusion of aluminum due to the natural decrease of its chemical potential was observed at the substrate/coating interfacial region. SEM images revealed formation of transversal cracks in the structure of the coatings which were deflected by this interdiffusion layer. This behavior indicates that the growing aluminides in the interdiffusion region have a

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) 5

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Fig. 6. SEM-BSE images on cross sections of the isothermally oxidized coated samples at (a) 750 °C and (b) 850 °C, after 300 h of exposure in air.

References

Finance Code 001. BOEING R&T Brazil is greatly appreciated for providing Beta-21S plates. M.Sc. C.M. Cossu acknowledges CAPES for scholarship.

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