Core-multishell globular oxidation in a new TiAlNbCr alloy at high temperatures

Oxidation resistance is one of key properties of titanium aluminide (TiAl) based alloys for high-temperature applications such as in advanced aero-engines and gas turbines. A new TiAlNbCr alloy with micro-addition of yttrium has been developed, but its oxidation behavior is unknown. To provide some fundamental insights, high-temperature oxidation characteristics of this alloy are examined via scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. We show that distinctive core-multishell globular oxidation and “daisy” flower-like oxidation occur exclusively around Y2O3 particles. Globular oxides exhibit multi-layered Y2O3/TiO2/Al2O3-rich/TiO2-rich shell structures from the inside to outside. Flower-like inner oxides consist of core Y2O3 particles surrounded by divergent Al2O3 and oxygen-rich α2-Ti3Al in the near-scale substrate. As the scale-substrate interface moves inward, the inner oxide structures suffer deeper oxidation and transform into the globular oxide structures. Our results demonstrate that the unique oxidation characteristics and the understanding of formation mechanisms pave the way for the exploration and development of advanced oxidation-resistant TiAl-based materials.


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 L1 0 structure, gray α 2 -Ti 3 Al, which has an ordered hexagonal D0 19 structure, and α 2 /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 α 2 /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 temperature 5,22 . Therefore, the volume fraction of B2 phase should be carefully controlled. Points "1" and "2" and other bright white particles imbedded in α 2 /γ lamellar colonies were identified to be Y 2 O 3 particles. However, YAl 2 compounds reported in other TiAl-based alloys with yttrium addition 26 were not observed in the present TiAlNbCr alloy. 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/cm 2 . To identify which law of oxidation kinetics is followed, the obtained experimental data could be fitted using the following equation,   where ∆M represents weight gain per unit area (mg/cm 2 ), n is an oxidation exponent (n = 1, linear relationship; n = 2, parabolic relationship), k n is a rate constant (mg n /cm 2n 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.

Isothermal oxidation kinetics.
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 Y 2 O 3 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"   Table 2. in Fig. 3(b) were confirmed as TiO 2 and Al 2 O 3 -rich shells, respectively. The outermost layer of TiO 2 -rich shell in this globular oxide contacted with the TiO 2 +Al 2 O 3 mixture layer of the oxide scale. Therefore, the globular oxide structure consisted of a core Y 2 O 3 particle and multi-layered TiO 2 /Al 2 O 3 -rich/TiO 2 -rich shells from the inside to outside. This kind of oxides protruding into the substrate could improve the adhesion of scale 25,26 . 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 Y 2 O 3 particles (i.e., the distance of Y 2 O 3 particles to the scale-substrate interface). Additionally, while some Y 2 O 3 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 Y 2 O 3 particles were expected to be shallowly positioned just under the surface skin.
"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  Table 2. Elemental composition (at.%) determined via EPMA at 1-5 points in Fig. 3(b).   Fig. 3(a) showing the oxide details consisting of a core Y 2 O 3 particle surrounded by divergent Al 2 O 3 and oxygen-rich α 2 -Ti 3 Al, where the EPMA microanalysis locations are marked by "1"-"7" with the obtained results summarized in Table 3.
alloys, such as intergranular internal oxides and internal-oxide bands 27 . 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 Y 2 O 3 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 Al 2 O 3 oxides around Y 2 O 3 particles in the substrate. Points "4" and "5" close to the inner oxides were speculated as α 2 -Ti 3 Al rich in oxygen. This was likely due to the fact that (1) the formation of inner Al 2 O 3 consumed aluminum which led to an increase of the relative concentration of titanium; (2) the saturation concentration of oxygen in the α 2 -Ti 3 Al and γ-TiAl alloys was calculated to be 16% and 2%, respectively 28 , 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 properties 29,30 .

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 Y 2 O 3 particles, as shown in Fig. 1(a). After high-temperature oxidation at 900 °C, Y 2 O 3 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 Y 2 O 3 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 Y 2 O 3 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 temperature 31  With the consumption of aluminum, α 2 -Ti 3 Al rich in oxygen formed in the Al-depleted areas beside divergent inner Al 2 O 3 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 TiO 2 formed preferentially in α 2 -Ti 3 Al(O) 33 perpendicular to divergent Al 2 O 3 along with two aluminum-rich layers. The previously formed Al 2 O 3 was thus broken up and dissolved into TiO 2 lattice, because of the volume expansion caused by the formation of TiO 2 34 . The so-called fast-grown TiO 2 35 , rapidly moved around Y 2 O 3 particle to form a full TiO 2 -rich shell mingled with a small amount of Al 2 O 3 , along with an inner aluminum-rich shell as shown in "Stage 4". In the subsequent oxidation in "Stage 5", Al 2 O 3 preferentially grew in the inner aluminum-rich shell because of the higher relative activity of aluminum, leading to an Al 2 O 3 -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 Y 2 O 3 particle. Finally, in "Stage 6" this innermost aluminum-depleted shell was replaced by TiO 2 as oxygen diffused inward via a certain extent of break-up and dissolution of the previously-formed neighboring Al 2 O 3 oxides in the form 36 Table 3. Elemental composition (at.%) determined via EPMA at points 1-7 in Fig. 5(b).
Scientific RepoRts | 7: 3483 | DOI:10.1038/s41598-017-03690-0 As a result, little aluminum was detected in the innermost TiO 2 shell as shown at points "2" and "3" in Fig. 3(b). The generated Al 3+ interstitial ions via Equ. (2) may escape via the outer mingled shell of TiO 2 and Al 2 O 3 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 Y 2 O 3 particles, together with B2-phase, γ-TiAl and α 2 -Ti 3 Al/γ-TiAl lamellar structures. The presence of Y 2 O 3 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 Y 2 O 3 particle and multi-layered TiO 2 /Al 2 O 3 -rich/ TiO 2 -rich shells from the inside to outside, while the flower-like inner oxide structures exhibited a core Y 2 O 3 particle surrounded by the divergent Al 2 O 3 and oxygen-rich α 2 -Ti 3 Al 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 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 Y 2 O 3 particle and substrate to form a flower-like inner oxide; Stage 3, formation of a partial TiO 2 -rich shell mingled with a small amout of Al 2 O 3 ; Stage 4, formation of a full TiO 2 -rich shell mingled with Al 2 O 3 , along with an inner aluminum-rich shell; Stage 5, formation of an Al 2 O 3 shell; and Stage 6, formation of the innermost TiO 2 shell.
consumable arc melting technique. The test specimens with a size of 10 × 10 ×3 mm 3 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, Al 2 O 3 , BN, pure niobium, pure chromium, pure molybdenum, pure boron and YP 4 O 12 , 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.