The Influence of Al Content on the Ignition and Flame Propagation Behavior of Ti_1−xAl_x Alloys in Enriched-Oxygen Environment

Highlights

What are the main findings?

• The critical ignition temperature and oxygen pressure of Ti_1−xAl_x alloys increase as Al content increases from 20 at% to 70 at%.
• The combustion rate of Ti_1−xAl_x alloys increases from 11.85 ± 0.13 mm·s⁻¹ to 14.05 ± 0.09 mm·s⁻¹ as Al content increases from 20 at% to 70 at%.
• The influence of Al content on the ignition conditions and combustion rate is attributed to multiple factors involving bonding energy, melting temperature, and heat release.

What are the implications of the main findings?

• A higher Al content increases the volume fraction of intermetallic phases, which improves the ignition conditions by strengthening the bonding character.
• A higher Al content accelerates the combustion kinetics by increasing the heat release during oxidation and reducing the melting temperature at the solid–liquid interface.
• These findings provide theoretical and data support for the safe use of intermetallic compounds and the design of new generation intermetallic compounds.

Abstract

Titanium aluminide intermetallics have gained considerable attention as high-temperature structural materials for aerospace applications, but are susceptible to “titanium fire” under extreme service conditions. The role of Al elements on the combustion behavior of titanium aluminide intermetallics remains not fully understood. Herein, the influence of Al content on the ignition critical condition and burning rate of Ti_1−xAl_x alloys was investigated by using promoted ignition combustion (PIC) tests under oxygen-enriched atmosphere. Results indicated that the critical oxygen pressure of Ti_1−xAl_x alloys increases from 0.11 MPa to 0.23 MPa, and the ignition temperature under oxygen pressure of 0.41 MPa increases from 1059.5 ± 4.8 K to 1120.4 ± 2.5 K as Al content increases from 20 at% to 70 at%. However, the combustion rate increases from 11.85 ± 0.13 mm·s⁻¹ to 14.05 ± 0.09 mm·s⁻¹ as Al content increases from 20 at% to 70 at%. Moreover, the activation energy for ignition increases from 105.44 kJ·mol⁻¹ to 153.04 kJ·mol⁻¹ as Al content increases from 20 at% to 70 at%. According to the microstructure analysis after combustion, the influence of Al content on the ignition activation energy and burning rate is attributed to multiple factors involving bonding energy, melting temperature, and heat release.

1. Introduction

Titanium alloys are extensively employed in key components of aircraft engines due to their excellent strength, high temperature stability, and low density [1,2,3]. However, they exhibit high combustion sensitivity under extreme service conditions involving high temperatures, high pressure, and high-speed rubbing owing to their high chemical activity, substantial oxidation enthalpy, and low thermal diffusivity [4,5]. This combustion, often termed “titanium fire,” can propagate within seconds after ignition, leading to catastrophic failure and severely compromising engine safety and reliability. Consequently, combustion resistance remains a major challenge limiting the broader application of titanium alloys in advanced propulsion systems.

To enhance the combustion resistance of titanium alloys, research has focused on understanding how alloying elements influence ignition and flame propagation [6,7,8,9,10]. Elements such as Cr and V have garnered particular attention due to their beneficial effects on oxidation resistance. For instance, Shao et al. [10] reported that increased Cr content reduces combustion velocity, which they attributed to the enrichment of Cr and V in the melt zone. Similarly, Mi et al. [11] proposed that higher Cr levels retard flame propagation by forming mixed oxides (e.g., Cr₂O₃ and V₂O₅) on frictional surfaces, thereby improving lubrication. Chen et al. [12] further reported that during the burning of Ti40 alloy, the formation of V₂O₅ and Cr₂O₃ oxides densifies the oxide layer, hindering sustained combustion. In addition, in Ti-Cu systems, Shao et al. [9] observed that Ti-22Cu burns 32% slower than Ti–2Cu, attributing this to Cu-induced formation of the Ti₂Cu phase, which acts as a barrier to oxygen diffusion. Additionally, Mo enrichment at the solid–liquid interface (the boundary between the heat-affected zone and the melt zone) has been shown to raise the local melting point, thereby suppressing interface migration and slowing combustion [13].

In recent decades, ordered titanium aluminide intermetallics have gained considerable attention as high-temperature structural materials for aerospace applications, owing to their favorable combination of low density, excellent creep strength, and good oxidation resistance [14]. However, like conventional titanium alloys, they remain susceptible to “titanium fire” under extreme service conditions. To date, limited research has focused on the combustion behavior and flame-retardant mechanisms of titanium aluminides. Some studies suggest these intermetallics possess inherent fire resistance due to their mixed metallic/covalent bonding. For instance, Wu et al. [15] reported that the ignition temperature of TiAl under laser heating in oxygen reaches 1557 °C under a laser power of 400 W, where the value exceeds its melting temperature (1460 °C). Ouyang et al. [16] and Zhu et al. [17] attributed improved flame retardancy to the formation of a continuous, protective Al₂O₃-rich layer via selective oxidation of Al during combustion. Additionally, Mo-doped TiAl alloys with medium Nb content have shown enhanced flame resistance, linked to rapid Al enrichment in the solid–liquid zone during burning [18].

Conversely, other studies report that Ti₂AlNb intermetallics combust approximately 30% faster than TC11 alloy, as higher Al content increases the heat release of the oxidation reaction [19,20]. This suggests that the flame will propagate faster than that of commercial titanium alloys once the Ti-Al intermetallics are ignited, which brings a high risk for their safe use. These controversial findings highlight the complex and not yet fully understood role of Al in the ignition and flame propagation of titanium-based materials. As titanium aluminides are increasingly deployed in extreme environments, a fundamental understanding of their combustion mechanisms becomes imperative for ensuring safe and reliable performance.

In this work, a series of Ti_1−xAl_x alloys (x = 0.2, 0.3, 0.5, and 0.7) was designed to explore their ignition and combustion behaviors. The influence of Al content on the ignition critical condition and burning rate of Ti_1−xAl_x alloys is investigated by using promoted ignition combustion (PIC) tests under oxygen-enriched atmosphere. The microstructure post-combustion on the Ti_1−xAl_x alloys is observed by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA). Based on the above analysis, the role of Al content on the ignition and flame propagation of Ti_1−xAl_x alloys is further discussed.

2. Materials and Methods

Alloy ingots with a nominal composition of Ti₈₀Al₂₀, Ti₇₀Al₃₀, Ti₅₀Al₅₀, and Ti₃₀Al₇₀ used in this study were prepared by the BAOTI Group Co., Ltd. in Baoji, Shaanxi, China, through an induction skull melting process in an argon atmosphere. Combustion samples of Ti₈₀Al₂₀, Ti₇₀Al₃₀, Ti₅₀Al₅₀, and Ti₃₀Al₇₀ with a length of 70 mm and diameters of 1, 3, 5, 8, 10, and 12 mm were cut from the ingots into rods, respectively. These specimens were wire-cut and surface-polished using sandpaper with the mesh sizes of 800 and 1000, and the specimens were cleaned by ultrasonic cleaning in acetone before the combustion experiment.

The combustion tests were performed in a promoted ignition combustion (PIC) device designed according to the ASTM G-124 standard [13,21], as shown in Figure 1. In each trial, a sample was vertically loaded on the bracket, and one end of the sample was wound with a copper wire with a 1 mm diameter. Air was subsequently evacuated from the reactor to obtain a vacuum of 10⁻² Pa, and then the container was filled with gaseous oxygen in a pressure range of 0.07–1.01 MPa. Subsequently, the samples were ignited using a copper wire as a promoter, and temperature variation during the ignition process was recorded by a thermal imager (MCS640, LUMASENSE TECHNOLOGIES, Santa Clara, CA, USA, frame rate 2000–5000 fps, temporal resolution of 60 Hz) with a test accuracy of ±5 K, through an observation window on the pressure reactor. The emissivity of Ti-Al alloys was calibrated to be 0.8 by dual color thermometer. In this paper, the critical pressure of the specimen was defined as the maximum oxygen pressure that does not ignite continuously for five times, and the ignition temperature was determined as the abrupt starting point within the temperature curve. To test the burning rate of Ti-Al alloys, a sample 40 mm away from the combustion end was tightly covered by a glass tube, and the combustion process was stopped once the combustion encountered the glass tube due to insufficient oxygen. The flame propagation of Ti-Al alloys was recorded using a high-speed camera (PCO.DIMAX S4, PCO AG, Kelheim, Germany, frame rate 3500 fps). Thus, the burning rates of the specimens could be determined after cooling according to the length of the rod burned in unit time and were determined as the average value of three tests.

The specimens obtained by PIC tests were cut along the longitudinal section, as shown in Figure 2. These specimens were ground, polished, and etched in a mixed solution containing 10 vol.% HF (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), 30 vol.% HNO₃ (Shanghai Aladdin Biochemical Technology Co., Ltd.), and 60 vol.% H₂O to observe the microstructures. The microstructure observation of combustion areas was conducted by field-emission scanning electron microscopy (SEM, Zeiss SUPRATM 55, Boston, MA, USA) operated in back-scattered electron (BSE) mode with an energy-dispersive X-ray spectroscopy (EDS) under an operating voltage of 20 keV. The phase structure of the reaction area was identified by X-ray diffraction (XRD, Rigaku TTRIII, Tokyo, Japan) using Cu target radiation with a step of 0.02° and a counting time of 1 s/step. The chemical composition distribution of different combustion areas was analyzed by electron probe microanalysis (EPMA, JEOL JXA-8100, Tokyo, Japan).

3. Results

3.1. The Effect of Al Content on Critical Ignition Conditions of Ti-Al Alloys

The XRD patterns of Ti₈₀Al₂₀, Ti₇₀Al₃₀, Ti₅₀Al₅₀, and Ti₃₀Al₇₀ alloys are shown in Figure 3. The XRD patterns confirm the presence of Ti₃Al, TiAl, and TiAl₃ phases, respectively, in the alloys. Moreover, the peak intensity of TiAl and TiAl₃ phases increases as the Al content increases.

Figure 4 presents thermal imaging sequences of the ignition behavior for Ti_1−xAl_x alloys with varying Al contents under 0.45 MPa. The combustion process, revealed by thermal analysis, comprises four distinct stages: oxidation (I), ignition (II), flame propagation (III), and stable combustion (IV). For consistent comparison, the frame immediately preceding ignition is defined as t = 0.00 s. During the ignition stage (Figure 4b,f,j,n), the intensity and frequency of observed sparks increase progressively with higher Al content. Concurrently, the time to ignition is delayed as Al content rises. In the flame propagation stage (Figure 4c,g,k,o), both flame luminosity and the ejection of molten droplets become more pronounced with increasing Al content, indicating more vigorous combustion as Al content increases from 20 to 70 at.%. Finally, in the stable combustion stage, all alloys form an upward-expanding flame and a stable molten pool, which undergoes periodic growth and detachment under gravity (Figure 4d,h,l,p).

Figure 5 details the temperature evolution during the ignition of Ti_1−xAl_x alloys, specifically the ignition temperature and heating rate, as captured by thermal imaging. The ignition temperatures for Ti₈₀Al₂₀, Ti₇₀Al₃₀, Ti₅₀Al₅₀, and Ti₃₀Al₇₀ under 0.45 MPa are determined as 966.10 K, 997.27 K, 1076.20 K, and 1175.61 K, respectively. These values reveal an approximately linear increase in ignition temperature with rising Al content, indicating enhanced combustion retardancy. Furthermore, the heating rate during ignition also escalates progressively with higher Al content, as illustrated by the derivative (red curve) of the temperature profiles in Figure 5. This can be attributed to the increased heat released from oxidation due to higher Al content, which consequently enhances the heating rate.

Figure 6 illustrates the influence of Al content on the critical ignition conditions (critical pressure and ignition temperature) for Ti_1−xAl_x alloys. Both the critical pressure and ignition temperature increase in an approximately linear manner as the Al content rises from 20 at.% to 70 at.%. To determine the combustion reaction order and activation energy, the dependencies of the critical oxygen pressure on specimen diameter and of the ignition temperature on oxygen pressure were evaluated, as shown in Figure 7a. The critical oxygen pressure increases with specimen diameter, a trend consistent with previous work [22]. Notably, as the Al content increases from 20 at.% to 70 at.%, the critical pressure rises more steeply with increasing specimen diameter, indicating enhanced resistance to ignition. Furthermore, Figure 7b shows that the ignition temperature decreases progressively with rising oxygen pressure for all Ti_1−xAl_x alloys.

3.2. The Effect of Al Content on Flame Propagation Process

Figure 8 presents high-speed camera records of the flame propagation in Ti_1−xAl_x alloys with varying Al contents under 0.45 MPa. Ignition is defined as t = 0.00 s. At this moment, spark ejection is observed, with the intensity and frequency of sparks increasing with Al content (Figure 8a,e,i,m). During flame propagation, the time required to form a stable molten pool decreases from approximately 2.26 s for Ti₈₀Al₂₀ (Figure 8b–d) to 1.84 s for Ti₃₀Al₇₀, indicating markedly accelerated flame spread with higher Al content. Notably, sustained droplet splashing—accompanied by apparent vaporization—is observed during combustion of the Ti₃₀Al₇₀ alloy (Figure 8n–p), consistent with the thermal imaging results in Figure 4.

Figure 9 presents the dependence of the burning rate on oxygen pressure for Ti_1−xAl_x alloys with different Al contents. As shown in Figure 9a, the burning rate rises as the oxygen pressure increases. In particular, this trend becomes more pronounced when the Al content exceeds 50 at.%. Furthermore, at a given oxygen pressure, the burning rate increases systematically with higher Al content (Figure 9b). Correlating this finding with high-speed imaging observations indicates that the acceleration in burning rate occurs primarily during the stable combustion stage, suggesting that increased Al content enhances the migration velocity of the solid–liquid interface. The increased burning rate suggests that the flame is easily propagated once the alloy is ignited, which is unfavorable for the application in aerospace.

3.3. Microstructure Analysis After Combustion of Ti-Al Alloys

Figure 10 presents the post-combustion SEM microstructures of Ti_1−xAl_x alloys. As shown in Figure 10a, the Ti₈₀Al₂₀ alloy exhibits a characteristic three-zone structure consisting of an oxide zone, a melt zone, and a heat-affected zone (HAZ), analogous to that observed in conventional titanium alloys. The oxide zone is primarily composed of oxides formed during combustion, the melt zone is primarily composed of oxygen-enriched solidification microstructure, and the HAZ region undergoes phase transformation or grain coarsening due to high temperature during combustion. With increasing Al content from 20 at.% to 50 at.%, the melt zone progressively widens while the HAZ becomes narrower (Figure 10a–c). At 70 at.% Al (Ti₃₀Al₇₀), only the oxide and melt zones remain visible, with the HAZ no longer distinctly resolvable (Figure 10d).

Figure 11 shows the oxide zones of Ti_1−xAl_x alloys with different Al contents. The oxide region exhibits numerous pores and cracks, likely resulting from intense thermal stress and significant volume expansion during combustion. In the Ti₈₀Al₂₀ alloy (Figure 11a), the oxide zone consists of three distinct phases: a black phase (Spot 1), a gray-white phase (Spot 2), and a light-gray phase (Spot 3). EPMA analysis (

Table 1. Chemical composition of different phases in the oxide zone for Ti-Al alloys with different Al contents, as shown in Figure 11.

Al Contents	Region	Composition/(at%)
		O	Ti	Al
20%	Spot 1	58.18	3.21	38.61
	Spot 2	63.51	35.14	1.35
	Spot 3	49.43	50.01	0.56
30%	Spot 4	61.90	35.41	2.69
	Spot 5	57.90	5.41	36.69
	Spot 6	46.02	52.88	1.10
50%	Spot 7	58.01	2.61	39.38
	Spot 8	39.40	59.26	1.34
	Spot 9	61.24	12.58	26.18
70%	Spot 10	55.46	2.64	41.90
	Spot 11	54.01	5.69	40.30
	Spot 12	60.00	13.80	26.20

) identifies them as Al₂O₃, TiO₂, and TiO, respectively. A similar phase composition is observed in the Ti₇₀Al₃₀ alloy (Figure 11b). When the Al content exceeds 50 at.%, in addition to Al₂O₃ (Spot 7) and TiO₂ (Spot 8), a substantial amount of TiAl₂O₅ with dark-gray contrast (Spot 9) forms (Figure 11c). At 70 at.% Al (Ti₃₀Al₇₀), an even denser continuous layer of TiAl₂O₅ (Spot 12) is evident (Figure 11d).

Figure 12 presents the microstructures of the melt zone (MZ) and heat-affected zone (HAZ) in Ti_1−xAl_x alloys with varying Al contents. In Ti₈₀Al₂₀ (Figure 12a), the HAZ shows only grain coarsening relative to the base metal. EPMA analysis (

Table 2. Chemical composition of phases in the solid–liquid interface for Ti_1−xAl_x alloys.

Al Contents	Region	Composition/(at%)
		O	Ti	Al
20%	Spot 13	29.31	52.39	18.30
30%	Spot 14	31.90	47.41	20.69
	Spot 15	26.07	38.55	35.38
50%	Spot 16	38.81	31.61	29.58
	Spot 17	59.14	2.37	38.49
	Spot 18	3.64	48.37	47.99
	Spot 19	0.27	76.11	23.62
70%	Spot 20	2.91	47.24	49.85
	Spot 21	3.11	72.40	24.49

) indicates that the MZ is enriched in Ti and Al with an atomic ratio near 3:1, together with an oxygen content of 29.31 at.%. With 30 at.% Al (Figure 12b), a distinct solid–liquid interface appears between the MZ and HAZ. The MZ of Ti₇₀Al₃₀ consists of gray dendrites (Spot 14, identified as Ti₃Al) and lighter interdendritic regions (Spot 15, TiAl). At 50 at.% Al (Figure 12c), the MZ contains a light-gray phase (Spot 16, TiAl) and a black phase (Spot 17, Al₂O₃), while the adjacent HAZ comprises TiAl (Spot 18) and Ti₃Al (Spot 19), as confirmed by EPMA. Finally, at 70 at.% Al (Ti₃₀Al₇₀, Figure 10d), the Al₂O₃ content in the MZ increases markedly.

Figure 13 presents EPMA maps of the solid–liquid interface region for Ti1-ₓAlₓ alloys with varying Al content. In Ti₈₀Al₂₀, the heat-affected zone reveals Ti enrichment in the interdendritic regions and Al enrichment within the dendrites (Figure 13a,b). At 50 at.% Al, distinct Ti- and Al-rich regions corresponding to the Ti₃Al and TiAl phases, respectively, are observed in the heat-affected zone (Figure 13c). Finally, in Ti₃₀Al₇₀ (70 at.% Al), the intense heat release during combustion results in significant grain coarsening within the heat-affected zone (Figure 13d).

XRD analysis of the combustion products from Ti_1−xAl_x alloys is presented in Figure 12. For the Ti₈₀Al₂₀ and Ti₇₀Al₃₀ alloys (Figure 14a,b), the products consist primarily of aluminum oxide (Al₂O₃) and titanium oxides (TiO, Ti₂O₃, TiO₂), along with minor phases from Al-enriched droplets such as TiAl₂ and TiAl₃. When the Al content reaches 50 at.%, the diffraction patterns (Figure 14c,d) show not only increased peak intensities for the aforementioned oxides but also the distinct emergence of TiAl₂O₅. Concurrently, the peaks associated with TiAl₃ from solidified droplets become more pronounced with higher Al content.

4. Discussion

4.1. The Role of Al Content on the Ignition Behavior

Increasing Al content significantly alters the ignition behavior of Ti–Al alloys. With higher Al content, droplet ejection during ignition becomes more pronounced (Figure 2), which can be attributed to the reduced melting point and enhanced thermal conductivity [23], both of which facilitate bubble formation and promote splashing. According to the Ti-Al phase diagram, the melting points of Ti-Al alloys are as follows: Ti₈₀Al₂₀ (approximately 1700 °C) > Ti₇₀Al₃₀ (approximately 1650 °C) > Ti₅₀Al₅₀ (approximately 1483 °C) > Ti₃₀Al₇₀ (approximately 1394 °C) [24]. Furthermore, the critical ignition conditions, including oxygen pressure and ignition temperature, rise with increasing Al content, while the time to ignition is extended (Figure 3 and Figure 4). This trend stems from the higher oxygen affinity of Al relative to Ti (Δ_f$G_m^{\theta }$(α-Al₂O₃, s) = −1581.97 KJ/mol, Δ_f$G_m^{\theta }$(rutile-TiO₂, s) = −889.52 KJ/mol) [17,25], which promotes the formation of a protective Al₂O₃ layer during heating, thereby delaying ignition. However, once ignition occurs, the higher Al content leads to rapid oxidation accompanied by substantial heat release from the formation of Al₂O₃ and TiAl₂O₅, resulting in an accelerated combustion process.

In order to further understand the effect of Al content on the ignition thermodynamics of Ti-Al alloys, the thermodynamic characteristic parameters, including reaction order and activation energy for ignition, are obtained based on a modified Frank-Kamenetskii ignition model established in previous work [26]. An ignition criterion named ${\delta }_c$ was established basing on the assumption that temperature gradient exists within a sample and the heat transfer process follows Fourier’s law. The ignition criterion describing the condition of Ti-Al alloys can be expressed as follows.

$${\delta }_c={\displaystyle \frac{E_i{l_m}^2}{{\lambda }_{m}R{T}^2}}k{q}_r{\displaystyle \frac{{\left(C_i\frac{P}{P_a}\right)}^n}{1+\alpha {(1-C_i)}^n{\left(\frac{P}{P_a}\right)}^n}}exp\left({\displaystyle \frac{-E_i}{RT}}\right)$$

(1)

Here, ${\delta }_c$ is a nondimensional parameter, describing the ignition conditions of Ti-Al alloys, and can be expressed as $\delta =0.84(2+1/{(l_r/d)}^2)$ ($l_r$ is the total width of the resistance wire wrapped sample, and $d$ is the diameter of the sample). $E_i$ and $k$ are the activation energy and preexponent for ignition, respectively. $l_m$, $q_r$ and ${\lambda }_m$ are the length, reaction heat, and thermal conductivity of material, respectively. R is the molar gas constant, T is the ignition temperature, and $C_i$ is the oxygen concentration. $P$ and $P_a$ are the oxygen pressure and atmospheric pressure, respectively. $\alpha$ and $n$ are the adsorption coefficient and reaction order, respectively. The units of the above formula have been normalized.

In this work, the combustion is conducted in a pure oxygen environment, so the $C_i$ is 100%, and Equation (1) can be rewritten as Equation (2).

$${\delta }_c={\displaystyle \frac{E_i{l_m}^2}{{\lambda }_{m}R{T}^2}}k{q}_r{\left({\displaystyle \frac{P}{P_a}}\right)}^{n}exp\left({\displaystyle \frac{-E_i}{RT}}\right)$$

(2)

To investigate the effect of Al content on ignition thermodynamics, it is necessary to obtain the thermodynamic characteristic parameters of Ti-Al alloys. According to Equation (2) and the logarithm, the relationship between sample size and oxygen pressure can be written as follows:

$$lnP={\displaystyle \frac{1}{n}}\ln \left(d^2+A\right)+{\displaystyle \frac{1}{n}}\ln B$$

(3)

where $A=2{l_r}^2$, $B={\displaystyle \frac{{\lambda }_{m}R{T}^2}{E_i{l_m}^{2}k{q}_r{l_r}^{2}exp(-E_i/RT)}}$. Thus, A and B are the constants related to material. According to Equation (3), the reaction order can be obtained by fitting the relationship between sample size and pressure.

Moreover, according to Equation (2) and the logarithm, the relationship between oxygen pressure and ignition temperature can be written as Equation (4),

$$lnP={\displaystyle \frac{2}{n}}\ln T+{\displaystyle \frac{E_i}{nRT}}+\mathrm{C}$$

(4)

where $C={\displaystyle \frac{1}{n}}ln\left({\displaystyle \frac{{\delta }_c{\lambda }_{m}R}{E_i{l_m}^{2}k{q}_r}}\right)$, and $C$ is the constant related to material. According to Equation (4), the activation energy for ignition can be obtained by the above relationship.

According to Equation (3), substituting the value of $l_r$ as 0.005 mm, the relationship between specimen diameter and critical oxygen pressure was fitted to the experimental data, as shown in Figure 15a. The fitting result exhibits a correlation coefficient exceeding 0.97. The corresponding reaction orders (n) for Ti–Al alloys with varying Al contents, derived from this fitting, are listed in

Table 3. Thermodynamic parameters of ignition for Ti-Al alloys with different Al contents.

Alloy	Ti80Al20	Ti70Al30	Ti50Al50	Ti30Al70
Reaction order n	1.65	1.72	1.78	2.00
Activation energy E (kJ·mol−1)	105.44	115.34	133.75	153.04

. Furthermore, using Equation (4) and the obtained n values, the activation energies (E) for ignition were determined by fitting the dependence of ignition temperature on oxygen pressure (correlation coefficient > 0.98), as summarized in Table 3 and Figure 16. It can be found that both the reaction order n and the activation energy E approximately linearly increase as the Al content increases from 20 at% to 70 at%, which is consistent with the variation of ignition conditions, as shown in Figure 4.

The observed increase in reaction order and activation energy of Ti_1−xAl_x alloys with higher Al content can be explained by two key factors. First, the volume fraction of intermetallic phases such as Ti₃Al, TiAl, and TiAl₃ increases with Al content. These phases possess stronger bonding—a combination of metallic and covalent character [27,28], which raises the energy barrier for combustion reaction. This is supported by the detection of TiAl₂O₅ in the post-combustion oxide zone (Figure 11, Table 1), a phase with an orthorhombic crystal structure that retains Al–Ti bonding within its lattice [29]. The presence of TiAl₂O₅ has also been reported in the friction combustion of TiAl alloy and regarded as a substantial protective film for fire-proofing of TiAl alloy [17,18]. Second, the high affinity between Al and O promotes the preferential formation of a dense alumina surface film, which partially inhibits further oxidation by blocking oxygen dissociation and inward diffusion [30]. Consequently, both the reaction order and the activation energy for ignition increase with Al content.

4.2. The Role of Al Content on the Flame Propagation

The ignition area expands and the flame propagation time shortens as Al content increases (Figure 7). Concurrently, the burning rate of Ti–Al alloys rises progressively with higher Al content (Figure 8). Both thermal imaging and high-speed videography reveal that the formation of combustion zones and the ejection of droplets occur periodically. Consequently, the flame front propagates steadily upward along the vertical cylindrical specimen. Under these conditions, the burning rate corresponds directly to the migration velocity of the solid–liquid interface. The combustion process schematic is shown in Figure 17.

In order to further study the effect of Al content on the combustion dynamics of Ti-Al alloys, the migration rate (v) of the solid–liquid interface can be calculated using the model by equating the heat flux on the liquid and solid sides, as Equation (5) [31]:

$$v={\displaystyle \frac{q_m}{\left\{{\rho }_m\left[c_m\left(T_m-T_o\right)+{\lambda }_m\right]\right\}}}·{\displaystyle \frac{A}{S}}$$

(5)

where $q_m$ is the heat release in the molten pool, and ${\rho }_m$ and $c_m$ are the density and specific heat capacity of oxide for Ti and Al, respectively, which can be regarded as constant. $T_m$ and $T_o$ are the temperature of the molten pool and environment, respectively, $A$ is the surface area of the solid–liquid interface, $S$ is the cross-sectional area of the sample, which is only related to the diameter of the sample. The effects of $A$ and $S$ are so small that they could be neglected. The $v_{Al20}$, $v_{Al30}$, $v_{Al50}$, and $v_{Al70}$ are defined as burning velocity of Ti₈₀Al₂₀, Ti₇₀Al₃₀, Ti₅₀Al₅₀, and Ti₃₀Al₇₀ alloys, respectively.

Based on the preceding analysis, the interface migration rate $v$ is primarily governed by the heat release per unit mass $q_m$ and the interfacial melting temperature $T_m$ across Ti–Al alloys, which is strongly related to the chemical composition. On one hand, aluminum exhibits a high specific heat release of 12.44 kJ/g upon oxidation [32]; thus, increasing Al content raises the total heat released in the molten pool. On the other hand, the melting temperature decreases with higher Al content according to the Ti–Al phase diagram [24]. For example, the melting points of TiAl and TiAl₃ are approximately 1456 °C and 1396 °C, respectively—about 200–300 °C lower than that of α-Ti [33]. Moreover, Al segregation at the solid–liquid interface can further reduce the local melting temperature, as observed in Figure 13b–d [13]. According to Equation (5), a lower melting point enhances the migration velocity v of the solid–liquid interface. Consequently, the burning rate of Ti_1−xAl_x alloys increases as the Al content increases.

Figure 18 presents a schematic diagram illustrating the influence of Al content on the ignition and combustion behavior of Ti_1−xAl_x alloys. With increasing Al content, the volume fraction of intermetallic phases (Ti₃Al, TiAl, TiAl₃) rises. These phases strengthen the bonding character (enhanced covalent contribution) and promote the formation of a dense alumina surface layer, thereby increasing the energy barrier for ignition—consistent with the observed rise in critical ignition pressure and temperature. Regarding combustion kinetics, higher Al content simultaneously raises the heat release during oxidation and lowers the melting temperature at the solid–liquid interface, both of which accelerate the burning rate as described by the combustion model. This research elucidates the influence of the Al element on the combustion characteristics of intermetallic Ti-Al compounds, providing theoretical and data support for the safe use of intermetallic compounds and the design of new generation intermetallic compounds.

5. Conclusions

(1)

The critical oxygen pressure of Ti_1−xAl_x alloys increases from 0.11 MPa to 0.23 MPa, and the ignition temperature under oxygen pressure of 0.41 MPa increases from 1059.5 ± 4.8 K to 1120.4 ± 2.5 K as Al content increases from 20 at% to 70 at%. Meanwhile, the burning rate increases from 11.85 ± 0.13 mm·s⁻¹ to 14.05 ± 0.09 mm·s⁻¹ as Al content increases from 20 at% to 70 at%.

(2)

The activation energy for ignition increases from 105.44 kJ·mol⁻¹ to 153.04 kJ·mol⁻¹ as Al content increases from 20 at% to 70 at%. Such an increase can be related to the increased bonding energy between Ti and Al, as well as the formation of the dense oxide layer at the surface.

(3)

According to the microstructure analysis after combustion, the accelerated combustion kinetics of Ti_1−xAl_x alloys as Al increases can be related to the segregation of Al content at the solid–liquid interface, which in turn decreases the melting temperature of the pool and increases heat release.