Ternary carbides such as Ti₃AlC₂ and Ti₃SiC₂ consist of edge-sharing Ti₃C₂ layers, which are separated by hexagonal nets of Al and Si atoms, respectively [1]. Like metals, they have good thermal and electrical conductivity, high toughness, and good machinability. Like ceramics, they display excellent chemical resistance, high Young’s moduli, high temperature strength, and high melting points. Ti₃AlC₂ and Ti₃SiC₂ form Ti₃Al_xSi_1-xC₂ solid solutions, which have attracted enormous interest for a number of applications because of their high strength, hardness, and good oxidation resistance [2-4]. To utilize Ti₃Al_xSi_1-xC₂ quaternary carbides as structural components, a comprehensive understanding of their oxidation properties over a wide range of temperatures is essential. Previously, Ti₃Al_xSi_1-xC₂ carbides with various compositions were synthesized via powder metallurgical routes, and their oxidation behavior was studied at 900-1300 °C [3-10]. Zhou et al. reported that Ti₃Al_xSi_1-xC₂ (x=0.05-0.15, 0.75-0.95) displayed good oxidation resistance at 900-1100 °C, because of formation of a continuous, protective α-Al₂O₃ layer, in addition to the less protective TiO₂ [3-5]. The oxidation resistance of Ti₃Al_xSi_1-xC₂ decreased at 1200-1350 °C owing to additional formation of Al₂TiO₅ through the reaction of α-Al₂O₃ and TiO₂ [4]. Besides α-Al₂O₃ and TiO₂, the formation of a small amount of protective SiO₂ in oxide scales during oxidation at 900-1100 °C was also reported by Liu et al [6]. Lee et al. reported that the oxidation of Ti₃Al_xSi_1-xC₂ at 900-1200 °C led to formation of oxide scales that consisted of an outer (rutile-TiO₂, α-Al₂O₃)-mixed layer, a thin, intermediate Al₂O₃ layer, and an inner (rutile-TiO₂, amorphous SiO₂)-mixed layer [7-10]. They suggested that the formation of semi-protective TiO₂ and the escape of carbon deteriorated the oxidation resistance. However, further studies on the oxidation of Ti₃Al_xSi_1-xC₂ are needed, because the purity and size of the starting powders, the sintering temperature/time/method, and the composition (x in Ti₃Al_xSi_1-xC₂) in the powder metallurgically synthesized samples greatly influence the oxidation kinetics, mechanism, and scale structures. This study aimed to characterize the oxidation behavior of Ti₃Al_xSi_1-xC₂ (x=0.3, 0.5, 0.7) at 400-800 °C for 6 months, which has not yet been adequately investigated. In this study, the oxidation characteristics of Ti₃Al_xSi_1-xC₂ were compared with those of pure titanium.

Polycrystalline Ti₃Al_0.3Si_0.7C₂, Ti₃Al_0.5Si_0.5C₂, and Ti₃Al_0.7Si_0.3C₂ were synthesized by mixing TiC_x (x=0.6, <45 µmφ), Al (<45 µmf, 99.9% purity), and Si (<70 µmφ, 99.9% purity) powders at molar ratios of 3:0.7:0.3, 3:0.5:0.5, and 3:0.3:0.7, respectively, in a shaker mill for 10 min, and hot pressing into 19 mmφ × 10 mm pellets at 1400 °C under a pressure of 25 MPa for 60 min in flowing Ar gas [7-10]. Hot pressed pellets were cut into the required dimensions of 10 × 10 × 5 mm³, ground with 1000 grit SiC paper, ultrasonically cleaned in acetone and methanol, and oxidized at 400, 600, and 800 °C for 0.5-6 months in air atmosphere using a muffle furnace. For comparison purposes, pure titanium coupons were simultaneously oxidized. The specimens were analyzed using a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS), a field-emission electron probe microanalyzer (EPMA), and a high-power X-ray diffractometer (XRD) with Cu-Kα radiation at 40 kV and 150 mA.

Figure 1 shows the SEM microstructures of synthesized Ti₃Al_xSi_1-xC₂. These specimens consisted of lamellar or somewhat equiaxed grains of micrometer size, which was typical in isostructural Ti₃SiC₂ [1,11], Ti₃AlC₂ [12], and Ti₃Al_xSi_1-xC₂ [7-10]. Average grain size was about 7.5 μm in length and 2.5 μm in thickness for Ti₃Al_0.7Si_0.3C₂, 4 μm in length and 2 μm in thickness for Ti₃Al_0.5Si_0.5C₂, and 3 μm in length and 1.5 μm in thickness for Ti₃Al_0.3Si_0.7C₂ Apparently, grains refined with the addition of Si, which might be closely related with the reactivity and diffusivity of starting materials during hot pressing. Using the image analyzer, the volume fraction of pores was measured as 1.5% for Ti₃Al_0.7Si_0.3C₂, 1.3% for Ti₃Al_0.5Si_0.5C₂, and 1% for Ti₃Al_0.3Si_0.7C₂ respectively, which suggested that the average density of Ti₃Al_xSi_1-xC₂ was 98.7%.

In general Ti₃AlC₂, Ti₃Al_xSi_1-xC₂, and Ti₃SiC₂ have a layered hexagonal structure. Silicon atoms partially substituted into the Al sites in Ti₃AlC₂ to form Ti₃Al_xSi_1-xC₂. Monolithic Ti₃Al_xSi_1-xC₂ was successfully synthesized, without any impurities under the detection limit of the XRD technique, as shown in Fig 2. Minor impurities in synthesized carbides are usually Ti₂AlC and Al₄C₃ in case of Ti₃AlC₂, TiC, Ti₅Si₃ and SiC in case of Ti₃SiC₂, and unreacted raw materials such as SiC and graphite [13-16].

Figure 3 shows long-time oxidation curves of Ti₃Al_xSi_1-xC₂, which were obtained by measuring weight gains due to oxidation using a microbalance. Each data point was obtained from one specimen. Generally, weight increased more with the increment of oxidation time and temperature. At 400 °C, Ti₃Al_0.7Si_0.3C₂ displayed the smallest weight gains for 0.5-3 months, but oxidized completely to powder at 6 months. At 600 °C, Ti₃Al_0.7Si_0.3C₂ displayed modest weight gains for 0.5-3 months, but again oxidized to powder at 6 months. As such, pulverization of Ti₃Al_0.7Si_0.3C₂ during oxidation at (400, 600 °C)/(6 months) resulted from oxidationinduced microcracking of oxide scales, as will be explained later. When microcracking of oxide scales and subsequent pulverization of samples were absent during oxidation at (400-600 °C)/(6 months), simply the thickening of oxide scales proceeded, being similar to the oxidation of common metals. At 800 C, all Ti₃Al_xSi_1-xC₂ oxidized relatively fast at similar rates, without pulverization. Ti₃Al_0.3Si_0.7C₂ oxidized faster at 400 °C than at 600 °C. Such anomalous oxidation was also observed during oxidation of Ti₃AlC₂ at 400-900 °C [17,18], Ti₂AlC at 500-900 °C [19], Ti₃AlC₂/TiC composites at 550–950 °C [20], and Ti₃SiC₂ at 650 °C [21].

Basically, the oxidation behavior of Ti₃Al_0.3Si_0.7C₂, Ti₃Al_0.5Si_0.5C₂, and Ti₃Al_0.7Si_0.3C₂ at each test temperature of 400, 600, and 800 °C is considered to be similar, considering anomalous oxidation kinetics, and experimental error that arouse owing to the diverse size and alignment of matrix grains and density difference in each specimen (Fig 1). Ti₃Al_xSi_1-xC₂ oxidizes according to the following equation [7-10].

The protectiveness of Ti₃Al_xSi_1-xC₂ mainly depends on the continuity and compactness of extremely slow-growing Al₂O₃ and SiO₂. These oxides could not grow fast enough to suppress the growth and transformation of titanium oxides, and the microcracking of oxide scales at 400-600 °C (see Figs 7b and 11b). The origin of oxidation-induced microcracking is the large volume expansion arisen owing to the eq. (1), which yielded large stress that could disintegrate Ti₃Al_xSi_1-xC₂ into powder at 400-600 °C. At 800 °C, Ti₃Al_xSi_1-xC₂ did not suffer microcracking due to the stress relaxation (see Figs 8 and 9). These figures show that Ti₃Al_xSi_1-xC₂ formed thick TiO₂-rich oxide scales due mainly to fast oxidation of titanium, although Al₂O₃ and SiO₂ still grew slowly at 800 °C. The result obtained Fig 3 may be summarized as follows. The long-time oxidation resistance of Ti₃Al_xSi_1-xC₂ was poor due to microcracking of oxide scales and subsequent pulverization of samples at 400-600 °C as well as fast oxidation of titanium in Ti₃Al_xSi_1-xC₂ at 800 C.

For comparison, titanium was subjected to the same oxidation conditions given in Fig 3, and the weight gains due to oxidation were measured as shown in Fig 4. Each data point was obtained from one sample. Titanium oxidized quite slowly at 400 °C, moderately at 600 °C, and extensively at 800 °C. Hence, the oxidation limit of Ti is known to be 600 °C [22]. Oxidation kinetics of Ti changed from parabolic to linear at 600-1000 °C, transitioning earlier with the increment of temperature [23,24]. Titanium, which forms semi-protective TiO₂, is not oxidation-resistant. It displayed better oxidation behavior than Ti₃Al_xSi_1-xC₂ at 400 and 600 °C without pulverization. However, it oxidized much faster than Ti₃Al_xSi_1-xC₂ at 800 °C (Figs 3 and 4).

Figure 5 shows XRD patterns of Ti₃Al_0.5Si_0.5C₂ after (400-800 °C/3 months)-oxidation. At 400 °C, strong matrix peaks and weak (rutile-TiO₂, Ti₃O)-peaks were detected, because of the thin scale formation. At 600 °C, Ti₃O disappeared while rutile-TiO₂ peaks became slightly stronger, implying that transient Ti₃O transformed to stable rutile-TiO₂. At 600 °C, matrix peaks were strong, because the scale was still thin. Alumina and silica, which formed according to eq. (1), were undetected due to their small amount or amorphous nature. When oxidized further at 800 °C, matrix peaks were no longer detectable because a thick rutile-TiO₂ scale covered the surface [17,18].

For comparison, titanium was oxidized under the same conditions given in Fig 5, and XRD data were acquired, as shown in Fig 6. When oxidized at 400 °C, strong α-Ti matrix peaks and a faint anatase-TiO₂ peak were detected, indicating that oxidation progressed so slowly that a superficial oxide scale formed. Under the same 400 °C-oxidation, oxide peaks shown in Fig 5 were a little stronger than those shown in Fig 6, which suggested that Ti₃Al_0.5Si_0.5C₂ oxidized a little faster than Ti, as is noticeable from Figs 3 and 4. Also, anatase-to-rutile transformation occurred slightly faster in Ti₃Al_0.5Si_0.5C₂ than in Ti. Such an increased oxidation rate and enhanced anatase-to-rutile transformation of Ti₃Al_0.5Si_0.5C₂ at 400 °C might be affected by the simultaneous oxidation of Al, Si, and carbon. At 400 °C, TiO₂ grew slowly, Al₂O₃ and SiO₂ grew much more slowly, and CO₂(g) liberated, which could affect the oxidation kinetics and TiO₂-transformation in Ti₃Al_xSi_1-xC₂. The 600 °C-oxidation revealed strong (rutile-TiO₂, α-Ti)-peaks and relatively weak Ti₃O peaks (Fig 6), which indicated that the oxide scale was relatively thin, and metastable anatase that was detected at 400 °C transformed to stable rutile at 600 °C [25,26]. Transient Ti₃O was absent, and a thick scale consisting of rutile-TiO₂ formed after 800 °C-oxidation (Fig 6).

Surfaces of Ti₃Al_0.3Si_0.7C₂ after (400-800 °C/3 months)-oxidation were compared with those of Ti, as shown in Fig 7. For Ti at 400 °C, grinding grooves that formed during the sample preparation stage were visible because of the slow oxidation rate. Titanium oxides on Ti grew to micrometer sized round grains at 600 °C. Rutile-TiO₂ grains that formed at 800 °C on Ti were still fine despite lengthy oxidation, because they continued to nucleate at the surface through outward diffusion of Ti⁴⁺ ions from the matrix [23-25]. By contrast, Ti₃Al_0.3Si_0.7C₂ suffered from the microcracking of the oxide scale at 400 °C. The oxide scale that formed at 600 °C on Ti₃Al_0.3Si_0.7C₂ was thin; however, short microcracks were visible. Unlike Ti, Ti₃Al_xSi_1-xC₂ was prone to microcracking and subsequent pulverization at 400-600 °C (Fig 3). Oxidation-induced microcracking was similarly observed during the oxidation of Ti₂AlC at 500-600 °C for 20 h [19], Ti₃SiC₂ at 650 °C for 10 h [21], and Ti₃AlC₂/TiC composites at 500-660 °C for 20-40 h [17,20]. At 800 °C, rutile-TiO₂ on Ti₃Al_xSi_1-xC₂ grew to characteristic coarse, pillar-like grains, because not nucleation but grain growth of TiO₂ dominated owing to partial deterrence of the outward diffusion of Ti⁴⁺ ions from the matrix by simultaneously formed Al₂O₃ and SiO₂ [7-10]. The absence of Al₂O₃ and SiO₂ was the reason for the much faster oxidation of Ti compared to Ti₃Al_xSi_1-xC₂ at 800 °C (Figs 3 and 4).

Figure 8 shows EPMA results of Ti₃Al_0.7Si_0.3C₂ after (800 °C/3 months)-oxidation. The oxide scale consisted primarily of an outer TiO₂ layer having a small amount of Al₂O₃, an intermediate Al₂O₃ layer having a small amount of TiO₂, and an inner TiO₂ layer having a small amount of (SiO₂+Al₂O₃). There was sufficient Ti in Ti₃Al_0.7Si_0.3C₂ to form outer and inner TiO₂–rich layers. Since silicon oxidizes roughly in situ due to the high bonding energy of Si⁴⁺-O^-2 [27], SiO₂ can be treated as a marker to understand the oxidation mechanism [11,28]. Hence, it is seen that the flat interface between the intermediate Al₂O₃–rich layer and inner TiO₂–rich layer corresponds to the original sample surface. The thickness of the oxide scale was ~ 140 μm, which varied to a certain extent depending on the location. Voids formed more in the thicker part of the oxide scale. They formed due to the Kirkendall effect caused by the outward diffusion of Ti⁴⁺ and Al³⁺ ions, the evolution of CO₂, and anisotropic volume expansion during scaling [7-10]. Voids could act as easy-diffusion paths and stress concentrators.

Figure 9 shows EPMA results of Ti₃Al_0.3Si_0.7C₂ that oxidized under the same condition given in Fig 8. The oxide scale with a thickness of ~ 125 μm consisted primarily of the outer TiO₂ layer having some Al₂O₃, the intermediate Al₂O₃ layer having some TiO₂, and the inner TiO₂ layer having some Al₂O₃ and SiO₂. The oxide scales shown in Figs 8 and 9 formed through the follow process.

Upon exposure to oxygen, titania, alumina, and silica formed competitively on Ti₃Al_xSi_1-xC₂, according to eq. (1). Nonstoichimetric TiO₂ soon overgrew highly stoichiometric Al₂O₃ and SiO₂, and constituted the outer scale. This process depleted Ti underneath, and thereby enriched Al, leading to formation of the intermediate Al₂O₃ scale, which in turn depleted Al and thereby enriched Ti underneath, resulting in formation of the inner TiO₂ scale. It is noted that TiO₂ grew by either outward diffusion of interstitial Ti⁴⁺ ions or inward diffusion of O^2- ions via oxygen vacancies [29]. The outer oxide layer grew by outward diffusion of Ti⁴⁺ ions plus a small amount of Al³⁺ ions, the intermediate oxide layer grew by outward diffusion of Al³⁺ ions plus a small amount of Ti⁴⁺ ions, and the inner oxide layer containing some SiO₂ grew by inward diffusion of O^2- ions.

On the other hand, oxide scales that formed on Ti₃SiC₂ consisted of an outer TiO₂ layer, an intermediate SiO₂ layer, and an inner (TiO₂+SiO₂)-mixed layer [11], whereas those that formed on Ti₃AlC₂ consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃)-mixed layer [12]. In this study, the intermediate layer consisted of not SiO₂ but Al₂O₃, because of the low dissociation pressure of Al₂O₃ when compared to SiO₂.

For comparison, titanium was oxidized under the same condition given in Figs 8 and 9, and the SEM result is shown in Fig 10. A laminated rutile-TiO₂ oxide layer was seen. Its thickness varied in the range of 0.8-1.3 mm, depending on the location. Titanium gained the weight about six time more than Ti₃Al_xSi_1-xC₂ at 800 °C, because it could not form Al₂O₃ and SiO₂ (Figs 3 and 4).

Figure 11 shows EPMA results of Ti₃Al_0.3Si_0.7C₂ after (400 °C/3 months)-oxidation. This specimen displayed the weight gain of 1.8 mg/cm² (Fig 3). Its surface displayed oxidation-induced microcracks (Fig 7b). The oxide scale was 9 μm-thick (Fig 11a), and had microcracks, which could facilitate the inward diffusion of oxygen (Fig 11b). At 400 °C, Ti₃Al_xSi_1-xC₂ oxidized relatively slowly for 0.5-3 months (Fig 3). Unlike Figs 8 and 9, the oxide scale shown in Fig 11c could not develop into the triple-layered oxide scale, because of the slow diffusion and oxidation reaction. Additionally, the outward diffusion of cations was negligible, which could be recognizable from the location of SiO₂ in the oxide scale (Fig 11c). Such resulted in the formation of the TiO₂-rich oxide scale having rather uniformly distributed SiO₂ and Al₂O₃, which was plausible considering that the ratio of Ti:Al:Si was 3:0.3:0.7.

The oxidation behavior of Ti₃Al_xSi_1-xC₂ at 400-800 °C for 0.5-6 months was studied. During (400-600 °C/6 months)-oxidation, oxidation-induced microcracking could lead to pulverization of Ti₃Al_xSi_1-xC₂, because oxidation of Ti₃Al_xSi_1-xC₂ to Ti₃O, (anatase or rutile)-TiO₂, Al₂O₃, SiO₂, and CO₂(g) induced a large volume expansion and thereby high stress in the oxide scales. During (800 °C/0.5-6 months)-oxidation, microcracking and subsequent pulverization of Ti₃Al_xSi_1-xC₂ did not occur due to stress relaxation; however, Ti₃Al_xSi_1-xC₂ formed thick oxide scales consisting of an outer TiO₂-rich layer, an intermediate Al₂O₃-rich layer, and an inner TiO₂-rich layer owing to enhanced oxidation and diffusion rates.