Introduction

Titanium aluminide (TiAl) based alloys are considered to be a new class of promising advanced high-temperature structural materials in the aerospace, gas turbine and automotive industries, because of their lightweighting with a low density (3.9–4.2 g/cm3), high specific yield strength and stiffness, and superior creep resistance at elevated temperatures1,2,3,4,5,6. This is highly inspired by the recent successful application of a TiAl-based alloy in General Electric’s high-thrust GEnex jet engines for powering Boeing 747-8 and 787 Dreamliner, to substitute Ni-based superalloys in the temperature range of 650–750 °C with the benefit of a weight reduction of ~50%1,2,3. However, their wide commercial applications are still limited due to low room-temperature ductility7, 8, lack of a cost-effective processing method, and unsatisfactory oxidation resistance at temperatures above 750 °C9, 10. Unlike Ni-Al alloys, no protective Al2O3 layer could occur on the titanium aluminide alloys because both titanium and aluminum form oxides of similar stability11. Therefore, many measures were taken to improve the oxidation resistance of titanium aluminide alloys such as surface treatment12, 13 and coating technologies14,15,16. In particular, both mechanical properties and oxidation resistance at high temperatures can be simultaneously improved via adding moderate ternary or quartic elements such as niobium17, tungsten, silicon, molybdenum18, chromium19 and yttrium20 in binary titanium aluminide alloys21, 22. Yttrium exhibits a strong grain refinement effect, thus improving the tensile strength of TiAl-based alloys23, 24. The addition of yttrium also significantly improves oxidation resistance due to its strong affinity to oxygen20, 23, 25, 26. Oxide pegs protruding into the substrate26 and convex-shaped nail-like oxides25 were observed at the scale-substrate interface, which play an important role in anchoring the oxide scale and improving anti-spalling ability of the scale on the surface of titanium aluminide alloys with an addition of yttrium. However, it is unclear how and in which form the oxidation occurs at high temperatures. In the present study, a new type of core-multishell globular oxide structure consisting of different oxide layers formed in a newly-developed Ti-44Al-4Nb-1.5Cr-0.5Mo-0.1B-0.1Y alloy induced by the presence of yttrium oxide (Y2O3) particles during oxidation at a high temperature of 900 °C is observed and discussed. Special attention is paid to the formation and growth mechanism of this unique type of globular oxides to give underlying insights about the effect of yttrium on the oxidation process of titanium aluminide alloys.

Results

Microstructures of TiAlNbCr alloy

Back-scattered electron (BSE) SEM micrograph, TEM bright field image along with the relevant selected area diffraction (SAD) patterns, and XRD pattern of as-cast TiAlNbCr alloy are shown in Fig. 1(a) through (d). The elemental compositions of points marked by “1” to “5” in Fig. 1(a) via electron probe microanalysis (EPMA) are summarized in Table 1. These analyses revealed that the microstructure consisted mainly of white B2-phase, dark γ-TiAl phase at colony (or grain) boundaries, which has an ordered L10 structure, gray α2-Ti3Al, which has an ordered hexagonal D019 structure, and α2/γ lamellar colonies as identified by the SAD and XRD patterns in Fig. 1. This is also corroborated via EPMA point chemical microanalyses, where points “3”, “4” and “5” corresponded to γ-TiAl, ordered titanium with B2 structure and α2/γ lamellar colony, respectively. The disordered β phase existing at high temperatures, which exhibits multiple slip systems due to its body-centered cubic crystal structure, will transform into the brittle B2-ordered phase in Ti-Al alloys at room temperature5, 22. Therefore, the volume fraction of B2 phase should be carefully controlled. Points “1” and “2” and other bright white particles imbedded in α2/γ lamellar colonies were identified to be Y2O3 particles. However, YAl2 compounds reported in other TiAl-based alloys with yttrium addition26 were not observed in the present TiAlNbCr alloy.

Figure 1
figure 1

Microstructures of as-cast TiAlNbCr alloy. (a) SEM back-scattered electron micrograph showing gray α2-Ti3Al and dark γ-TiAl lamellae, white B2-phase, and bright white Y2O3 particles, (b) TEM bright field image along with (c) the corresponding selected area diffraction (SAD) patterns of points A-D, where A represents α2-Ti3Al lamella, B corresponds to γ-TiAl lamella, C indicates γ-TiAl, and D stands for B2, and (d) XRD pattern further confirming the presence of α2-Ti3Al, γ-TiAl, B2-phase and Y2O3 particles.

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Table 1 Elemental composition (at.%) determined via EPMA at points 1–5 in Fig. 1(a).
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Isothermal oxidation kinetics

Figure 2 shows a curve of isothermal oxidation kinetics of TiAlNbCr alloy at 900 °C. The obtained weight gain of this alloy after 100 h at 900 °C was about 2.3777 mg/cm2. To identify which law of oxidation kinetics is followed, the obtained experimental data could be fitted using the following equation,

$${\rm{\Delta }}{M}^{n}={k}_{n}t,$$
(1)

where ∆M represents weight gain per unit area (mg/cm2), n is an oxidation exponent (n = 1, linear relationship; n = 2, parabolic relationship), k n is a rate constant (mgn/cm2n h) and t is oxidation time (h). The obtained oxidation exponent was close to 2, suggesting that the oxidation kinetics of TiAlNbCr alloy at 900 °C obeyed a parabolic relationship.

Figure 2
figure 2

Oxidation kinetics of TiAlNbCr alloy at 900 °C up to 100 h. Weight gain per unit area as a function of time.

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Core-multishell globular oxidation

Figure 3(a) and (b) shows the cross-sectional morphology of a typical oxide with globular structures after oxidation at 900 °C for 100 h. The globular oxides were normally located inside or underneath the oxide scale and they were also observed in all other specimens after oxidation at 900 °C for 12, 24 and 50 h. The bright white core of the globular oxide marked by “1” in Fig. 3(b) was identified to be Y2O3 according to EPMA point chemical microanalysis results shown in Table 2 and EPMA elemental distribution maps in Fig. 3(c). The adjacent gray loop marked by “2” and “3” and outer dark loop marked by “4” and “5” in Fig. 3(b) were confirmed as TiO2 and Al2O3-rich shells, respectively. The outermost layer of TiO2-rich shell in this globular oxide contacted with the TiO2+Al2O3 mixture layer of the oxide scale. Therefore, the globular oxide structure consisted of a core Y2O3 particle and multi-layered TiO2/Al2O3-rich/TiO2-rich shells from the inside to outside. This kind of oxides protruding into the substrate could improve the adhesion of scale25, 26. It should be noted that transient or incomplete core-shell globular oxidation could also be observed, as shown in Fig. 4. This was mainly dependent on the oxidation time or the location of Y2O3 particles (i.e., the distance of Y2O3 particles to the scale-substrate interface). Additionally, while some Y2O3 particles were embedded or buried beneath the polished surface, as indicated by arrows in Fig. 4(a) and (b), they still induced the core-shell like globular oxidation. Such embedded Y2O3 particles were expected to be shallowly positioned just under the surface skin.

Figure 3
figure 3

A typical core-multishell globular oxide of TiAlNbCr alloy after oxidation at 900 °C for 100 h. (a) and (b) Morphologies of oxides with a globular structure, which consist of a core Y2O3 particle and multi-layered TiO2/Al2O3-rich/TiO2-rich shells from the inside to outside, and (c) elemental distributions via EPMA mapping for the globular oxide, where the detailed chemical microanalysis results are shown in Table 2.

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Table 2 Elemental composition (at.%) determined via EPMA at 1–5 points in Fig. 3(b).
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Figure 4
figure 4

Typical micrographs showing transient core-shell globular oxides in TiAlNbCr alloy after oxidation at 900 °C for 100 h. (a) and (b) Morphologies of two oxides with a transient or incomplete core-shell globular structure at an intermediate distance to the scale-substrate interface.

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“Daisy” flower-like inner oxidation

It is also of special interest to observe a unique type of “daisy” flower-like inner oxides present in the alloy substrate close to the oxide scale (but without linking to the scale-substrate interface), as shown in Fig. 5(a) and (b). This was unlike the situation of inner oxides in other alloys, such as intergranular internal oxides and internal-oxide bands27. In the present study, all the inner oxide structures exclusively centered on a bright white particle. EPMA point chemical microanalysis results of points marked by “1”–“7” in Fig. 5(b) were summarized in Table 3 and the bright white core marked by “1” was again determined to be a Y2O3 particle. At points “2” and “3”, the concentration of aluminum was higher than that of titanium, which was in contrast to points “6” and “7” in the lamellar colony of substrate. This means that aluminum with a higher chemical activity at high temperatures was preferentially oxidized to form the divergent inner Al2O3 oxides around Y2O3 particles in the substrate. Points “4” and “5” close to the inner oxides were speculated as α2-Ti3Al rich in oxygen. This was likely due to the fact that (1) the formation of inner Al2O3 consumed aluminum which led to an increase of the relative concentration of titanium; (2) the saturation concentration of oxygen in the α2-Ti3Al and γ-TiAl alloys was calculated to be 16% and 2%, respectively28, and the oxygen concentration at points “4” and “5” were about 23.6% and 18.9% which were closer to 16%. However, the inner oxides embedded in the substrate may be detrimental to the mechanical properties29, 30.

Figure 5
figure 5

“Daisy” flower-like morphologies of an inner oxide after oxidation at 900 °C for 50 h. (a) A low-magnification image showing the location of inner oxide with respect to the scale-substrate interface, and (b) a magnified image of Fig. 3(a) showing the oxide details consisting of a core Y2O3 particle surrounded by divergent Al2O3 and oxygen-rich \({\alpha }\) 2-Ti3Al, where the EPMA microanalysis locations are marked by “1”–“7” with the obtained results summarized in Table 3.

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Table 3 Elemental composition (at.%) determined via EPMA at points 1–7 in Fig. 5(b).
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Discussion

Yttrium with a low concentration of 0.1% was added in our TiAlNbCr alloy aiming to remove the oxygen which was present in the alloy in the process of smelting. This goal was successfully achieved due to the reaction of yttrium with oxygen to form Y2O3 particles, as shown in Fig. 1(a). After high-temperature oxidation at 900 °C, Y2O3 particles were still stable and remained unchanged. However, they were observed to induce high-temperature oxidation and occupy the cores of both globular oxide structures and “daisy” flower-like inner oxide structures. Globular oxides were positioned inside or underneath the oxide scale on the surface, which were directly linked to the scale (Fig. 3), while the characteristic flower-like inner oxidation occurred within the substrate in the vicinity of scale-substrate interface (Fig. 5). This suggests that the area surrounding Y2O3 particles was susceptible to oxidation at a high temperature of 900 °C. As the oxidation continued, the scale-substrate interface moved inwards and flower-like inner oxide structures were increasingly oxidized and eventually become the globular oxide structures (Fig. 4). It was as if the “daisy” flower-like inner oxide structures were the precursor of the globular oxide structures and the Y2O3 particles were the seeds of oxidation at high temperatures.

Figure 6 illustrates the formation and growth mechanism of the core-multishell globular oxide structures in the TiAlNbCr alloy during high-temperature oxidation. First, a thin oxide scale formed on the surface in conjunction with the inward diffusion of oxygen and nitrogen as seen in “Stage 1”. Then aluminum, which was more active than titanium at a high temperature of 900 °C as seen from their standard free energy of oxidation as a function of temperature31, reacted with the penetrated oxygen preferentially near the interface between Y2O3 particles and substrate to form flower-like inner oxides as shown in Fig. 5. Al2O3 firstly formed at the Y2O3-substrate interface and the reasons could be considered as follows:

  1. (a)

    The Y2O3-substrate interfacial energy could be lowered when the nucleation of Al2O3 occurred there.

  2. (b)

    The local tensile stresses existent in the nearby substrate arising from the nucleation and growth of Y2O3 could drive the formation of inner oxides32.

  3. (c)

    The tensile stresses present at the Y2O3-substrate interface could result in the formation of microvoids or other defects at the interface which provided a growing space of Al2O3 or fast diffusion channel of oxygen27, 32.

Figure 6
figure 6

Formation and growth process of a distinctive core-multishell globular oxide in the TiAlNbCr alloy during high-temperature oxidation. Stage 1, inward diffusion of oxygen and nitrogen; Stage 2, aluminum reacted with the penetrated oxygen preferentially near the interface between Y2O3 particle and substrate to form a flower-like inner oxide; Stage 3, formation of a partial TiO2-rich shell mingled with a small amout of Al2O3; Stage 4, formation of a full TiO2-rich shell mingled with Al2O3, along with an inner aluminum-rich shell; Stage 5, formation of an Al2O3 shell; and Stage 6, formation of the innermost TiO2 shell.

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With the consumption of aluminum, α2-Ti3Al rich in oxygen formed in the Al-depleted areas beside divergent inner Al2O3 products as shown in “Stage 2”. In “Stage 3”, the scale-substrate interface continued to move inward and contacted with the flower-like inner oxide. Then TiO2 formed preferentially in α2-Ti3Al(O)33 perpendicular to divergent Al2O3 along with two aluminum-rich layers. The previously formed Al2O3 was thus broken up and dissolved into TiO2 lattice, because of the volume expansion caused by the formation of TiO2 34. The so-called fast-grown TiO2 35, rapidly moved around Y2O3 particle to form a full TiO2-rich shell mingled with a small amount of Al2O3, along with an inner aluminum-rich shell as shown in “Stage 4”. In the subsequent oxidation in “Stage 5”, Al2O3 preferentially grew in the inner aluminum-rich shell because of the higher relative activity of aluminum, leading to an Al2O3-rich shell as seen at points “4” and “5” in Fig. 3(b), and the formation of an aluminum-depleted shell directly adjoining to the central Y2O3 particle. Finally, in “Stage 6” this innermost aluminum-depleted shell was replaced by TiO2 as oxygen diffused inward via a certain extent of break-up and dissolution of the previously-formed neighboring Al2O3 oxides in the form36,

$${{\rm{Al}}}_{2}{{\rm{O}}}_{3}=2{{\rm{Al}}}^{3+}+\tfrac{3}{2}{{\rm{O}}}_{2}+6{e}^{-}.$$
(2)

As a result, little aluminum was detected in the innermost TiO2 shell as shown at points “2” and “3” in Fig. 3(b). The generated Al3+ interstitial ions via Equ. (2) may escape via the outer mingled shell of TiO2 and Al2O3 to compensate the outermost aluminum-depleted shell in Stage 5, leading to its disappearance and a wider mingled shell (“Stage 6” in Fig. 6).

In conclusion, a minor addition of 0.1% yttrium in the TiAlNbCr alloy led to the formation of Y2O3 particles, together with B2-phase, γ-TiAl and α2-Ti3Al/γ-TiAl lamellar structures. The presence of Y2O3 particles was responsible for the occurrence of two distinctive characteristics of oxidation at high temperatures: core-multishell globular oxidation and “daisy” flower-like oxidation, which were observed for the first time, to the best of our knowledge. The globular oxide structures consisted of a core Y2O3 particle and multi-layered TiO2/Al2O3-rich/TiO2-rich shells from the inside to outside, while the flower-like inner oxide structures exhibited a core Y2O3 particle surrounded by the divergent Al2O3 and oxygen-rich α2-Ti3Al within the substrate in the neighborhood of oxide scale. As the scale-substrate interface moved inward, the inner flower-like oxide structures underwent increasingly more severe oxidation and eventually transformed into the globular oxide structures. We anticipate that the findings of unique oxidation characteristics and the understanding of relevant formation mechanisms pave the way for the exploration and development of advanced oxidation-resistant TiAl-based materials for the high-temperature applications.

Materials and Methods

The titanium aluminide alloy selected in this study with a nominal chemical composition of Ti-44Al-4Nb-1.5Cr-0.5Mo-0.1B-0.1Y (in at.%, and hereafter referred to as TiAlNbCr alloy) was prepared by double vacuum consumable arc melting technique. The test specimens with a size of 10 × 10 ×3 mm3 were cut from the cast ingot by electro-discharge machining. The surfaces of the specimens were polished with 1200-grit SiC papers, and then cleaned ultrasonically in acetone for 15 min before isothermal oxidation in air at 900 °C. To avoid that one of the specimen surfaces came into contact with a flat crucible and ensure that all of the specimen surfaces were fully exposed to the air during isothermal oxidation tests, the specimens were first placed into the hollow ceramic cylinders positioned horizontally in a larger flat crucible. Then the crucible was transferred into the furnace. The specimens after oxidation were weighed for determining the weight gain, and subsequently mounted with resin and polished to a mirror-like surface. To increase electrical conductivity and image resolution, the polished specimens were carbon-coated before scanning electron microscope (SEM) and electron probe microanalysis (EPMA) examinations. Elemental distributions of titanium, aluminum, oxygen, nitrogen, niobium, chromium, molybdenum, boron and yttrium were detected through EPMA and the corresponding standard samples used for calibration were pure titanium, Al2O3, BN, pure niobium, pure chromium, pure molybdenum, pure boron and YP4O12, respectively. X-ray diffraction (XRD) was used to identify phases in the alloy with a diffraction angle (2θ) from 10° to 100° with a step size of 0.02° and 1 s in each step.