Effects of Ti/Al Ratio on Formation of Ti-Al Intermetallics/TiB₂ Composites by SHS from Ti-Al-B Powder Mixtures

Abstract

Ti-Al intermetallics/TiB₂ composites were prepared from elemental powder mixtures by the method of self-propagating high-temperature synthesis (SHS). Reactant mixtures were formulated to contain two parts; one group was (2Ti + 4B) to form 2TiB₂ and the other group was (Ti + xAl) to produce Ti-Al intermetallic compounds. The content of Al ranged between x = 0.33 and 3.0, which was equivalent to the Ti/Al atomic ratio from Ti-25% Al to Ti-75% Al in the (Ti + xAl) group. The results showed that the increase of Al percentage reduced the overall combustion exothermicity and led to a slower self-sustaining combustion wave speed and a lower combustion temperature. Apparent activation energy of the Ti-Al-B solid-state combustion reaction was determined to be 114.7 kJ/mol by this study. Based on the XRD analysis, Ti-Al intermetallics/TiB₂ composites featuring Ti₃Al, TiAl, TiAl₂, and TiAl₃ as the dominant aluminide phase were respectively synthesized from the samples of Ti-25%~40% Al, Ti-50%~60% Al, Ti-71.4% Al, and Ti-75% Al. For the samples of Ti-25% Al and Ti-30% Al, Ti₃Al was the only aluminide formed. The microstructure of the composites exhibited that TiB₂ grains with a columnar shape of 2–3 μm in length were well distributed and embedded in the aluminide matrix. This study demonstrated an effective and energy-saving fabrication route for producing Ti-Al intermetallics/TiB₂ composites with different dominant aluminide phases.

1. Introduction

Titanium aluminide-based alloys and composites have attracted much attention for high-temperature structural applications in the automotive, aerospace, and nuclear industries [1,2]. Ti-Al intermetallics possess many unique properties, such as low density, high strength, excellent oxidation resistance, and creep resistance at elevated temperatures [1,2,3]. Alloying additions of 1–3 at.% of Cr, Nb, Ta, Mn, V, Mo, and Zr led to an increase in the low-temperature ductility and high-temperature strength as well as oxidation resistance of the Ti-Al intermetallics [3,4]. Formation of TiAl-based composites by adding ceramic particles has been shown to improve the mechanical and tribological properties of the Ti-Al intermetallics [5]. Ceramic compounds, for example TiB₂, Al₂O₃, B₄C, SiC, Ti₅Si₃, and Ti₂AlC, are thermally and chemically stable with the Ti-Al matrix and have been considered as the effective reinforcements [6,7,8,9,10,11,12,13,14,15]. Among most of the ceramic reinforcements, the coefficient of thermal expansion (CTE) of TiB₂ is similar to that of γ-TiAl alloys. Moreover, TiB₂ possesses high elastic modulus and outstanding wear resistance, and shows equal density to, and excellent chemical compatibility with, γ-TiAl alloys [6]. Fine TiB₂ particles with special morphology and induced bridging effect contributed to the notable fracture toughness of the TiAl/TiB₂ composite [7,8]. Therefore, in situ TiB₂ particles were found to significantly enhance the strength of TiAl alloys and have been adopted as a promising reinforcement for γ-TiAl based composites. For the TiB₂/TiAl composites prepared by the method of hot isostatic pressing, Li et al. [7] confirmed the strengthening effect of TiB₂ particulates and obtained the tensile stress of 653.4 ± 39.2 MPa and strain of 2.1 ± 0.33% for the TiB₂/TiAl composites. Han et al. [8] fabricated TiB₂-strengthened TiAl-based alloys by the induction melting method and reported that such composites at 700 °C showed fracture toughness (K_IC) of 21.89 ± 0.84 MPa m^1/2, ultimate tensile strength (UTS) of 545 ± 19 MPa, and elongation-to-failure (ε_f) of 3.9% ± 0.18.

TiAl-based composites composed of in situ TiB₂ particles and 3 at.% Cr were prepared by hot pressing (HP) sintering from mixed Ti, Al, B, and Cr powders at 10 MPa and 1300 °C, and composites with a significant improvement in the yield strength and compressive strength were attained [6]. Li et al. [7] fabricated TiAl/TiB₂ composites alloyed with Nb, Cr, and C by hot isostatic pressing (HIP) at 1150 °C and 120 MPa for 3 h and showed that the tensile strength and strain of the composites increased with TiB₂ content. Lazurenko et al. [9] utilized spark plasma sintering (SPS) with Ti and Al foils and TiB₂ and TiC particles as the starting materials at 1250 °C to produce Ti₃Al/TiAl matrix composites reinforced by TiB₂ and Ti₂AlC. The composites displayed an improved compressive strength and better creep resistance [8]. With the use of the HP method operating at 1280 °C and 30 MPa for 2 h, a TiAl/Ti₃Al/Al₂O₃ composite was produced from pre-alloyed Ti, Al, Nb₂O₅, Nb, and B powder mixture [10]. In situ synthesis of a TiAl/Al₂O₃ composite from Ti, Al, Nb, W, and TiO₂ mixture was conducted by mechanical milling with subsequent spark plasma sintering [11]. Tan et al. [12] prepared Ti₃Al/TiAl composites with added SiC fibers by vacuum arc melting. The addition of SiC led to the formation of Ti₅Si₃ and Ti₂AlC, both of which contributed to better strength and ductility of the products [12]. Wang et al. [13] fabricated Ti₂AlC/TiAl composites with excellent room-temperature compression strength and fracture strain by the SPS technique using pre-alloyed Ti-48Al-2Cr-2Nb powders and graphene nanosheets as raw materials and operating at different temperatures of 1200–1320 °C for 10 min. TiAl/Ti₅Si₃ composites characterized by superplastic deformation were produced by high-energy ball milling of Ti, Al, and Si powders, followed by HIP at 1050–1150 °C and 300 MPa for 2 h [14]. Shu et al. [15] synthesized TiB₂- and Ti₅Si₃-reinforced TiAl composites from elemental powder blends by the thermal explosion method. TiB₂ particles were found to enhance the ultimate compression strength and Ti₅Si₃ particles to increase the strength and ductility of the composite [15]. Through combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS), Yeh and Li [16] studied the formation of TiAl/Ti₅Si₃ and TiAl/Al₂O₃ composites. From elemental powder compacts, TiAl/Ti₅Si₃ composites were produced and the increase of the Ti₅Si₃ content facilitated sustainability of a steady combustion process. TiAl/Al₂O₃ composites were prepared from Ti, Al, and TiO₂ powders by the SHS process involving a thermite reaction in which TiO₂ was reduced by Al. The increase of Al₂O₃ lowered combustion exothermicity and was responsible for combustion wave propagation in a pulsating mode [16].

Of various fabrication methods, the SHS technique takes advantage of the highly exothermic reaction and is an energy-efficient, time-saving, facile, and low-cost approach. Moreover, the SHS method is an in situ fabrication route for preparation of composite materials [16,17]. The objective of this study was to investigate in situ formation of Ti-Al intermetallics/TiB₂ composites by the SHS process from elemental powder mixtures consisting of Ti, Al, and boron. According to the Ti-Al phase diagram [18,19,20], Ti-Al intermetallic compounds include α₂-Ti₃Al, γ-TiAl, TiAl₂, and TiAl₃. Ti₃Al has a wide homogeneity range from 22 at.% Al to approximately 35 at.% Al. Depending on temperature, the TiAl phase has a composition of 49–66 at.% Al. Two-phase alloys (γ-TiAl + α₂-Ti₃Al) exist between 35 at.% and 49 at.% Al. For the Al-rich compounds, TiAl₂ has a narrow homogeneity range of 64.5–67 at.% Al and TiAl₃ is a line compound. In this study, the effects on the combustion wave temperature and velocity of the Ti/Al atomic ratio from Ti-25 at.% Al to Ti-75 at.% Al were explored, as well as those on the Ti-Al intermetallic phases formed in the composite. Unlike most of the previous studies focusing only on the γ-TiAl phase, this work represents the first attempt to investigate the formation of Ti-Al intermetallics/TiB₂ composites featuring four different dominant aluminide phases. The activation energy of solid-state combustion of the Ti-Al-B system was deduced from combustion kinetics. Microstructure and composition of the synthesized composites were examined.

2. Materials and Methods

The materials used in this study included Ti (Alfa Aesar, Ward Hill, MA, USA, <45 μm, 99.8%), Al (Alfa Aesar, <45 μm, 99.8%), and amorphous boron (B) (Noah Technologies, San Antonio, TX, USA, < 1 μm, 93.5%). Reactant mixtures were formulated as in Equation (1):

$$\left(2\mathrm{T}\mathrm{i}+4\mathrm{B}\right)+\left(\mathrm{T}\mathrm{i}+\mathrm{x}\mathrm{A}\mathrm{l}\right)\to 2\mathrm{T}\mathrm{i}{\mathrm{B}}_2+({\mathrm{T}\mathrm{i}}_3\mathrm{A}\mathrm{l},\mathrm{T}\mathrm{i}\mathrm{A}\mathrm{l},\mathrm{T}\mathrm{i}{\mathrm{A}\mathrm{l}}_2,\mathrm{a}\mathrm{n}\mathrm{d}/\mathrm{o}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{A}\mathrm{l}}_3)$$

(1)

As expressed in Equation (1), the amount of Ti in the reactant blend was divided into two groups. One part of Ti is to react with boron (i.e., 2Ti + 4B) to produce TiB₂, which is a highly exothermic phase. The other part of Ti is to react with Al to produce Ti-Al intermetallic compounds. The phase of titanium aluminide depends on the content of Al in the (Ti + xAl) group. As shown in

Table 1. Ti/Al atomic ratio and Ti-Al intermetallics of Equation (1) under different values of x.

x	Ti/Al Atomic Ratio	Ti-Al Intermetallics
0.33	Ti-25% Al	1/3 Ti3Al
0.43	Ti-30% Al	1/3 Ti3Al
0.67	Ti-40% Al	1/6 Ti3Al + 1/2 TiAl
1.0	Ti-50% Al	TiAl
1.5	Ti-60% Al	1/2 TiAl + 1/2 TiAl2
2.0	Ti-66.7% Al	TiAl2
2.5	Ti-71.4% Al	1/2 TiAl2 + 1/2 TiAl3
3.0	Ti-75% Al	TiAl3

, the parameter x varies from 0.33 to 3.0, which corresponds to the Ti/Al atomic ratio from Ti-25 at.% Al to Ti-75 at.% Al. The expression of Ti-25% Al means that the atomic percentage of Al in the (Ti + xAl) group with x = 0.33 is 25%. Likewise, Ti-75% Al represents 75% of Al in the (Ti + xAl) group for the case of x = 3.0. This type of expression for the Ti/Al atomic ratio was used in the following text. Based on stoichiometric balance under different values of x, Table 1 lists either sole or mixed Ti-Al intermetallic compounds to be formed as the products. It should be noted that the sample of x = 0.43 (Ti-30% Al) is an Al-rich condition to produce Ti₃Al, because Ti₃Al exists in a wide homogeneity range from 22 to 35 at.% Al.

The combustion exothermicity of Equation (1) was evaluated by calculating the enthalpy of reaction (∆H_r) at 298 K and adiabatic combustion temperature (T_ad) based on the intermetallic compositions presented in Table 1. Equation (2) is an energy balance equation for computing T_ad [21]. ∆H_r was calculated from the heats of formation (∆H_f) of the reactants and products. The required thermochemical data including ∆H_f and heat capacity c_p(P_i) of the product species are listed in

Table 2. Heat of formation (∆H_f) and specific heat (c_p) of product species.

Product Species	Heat of Formation ∆Hf (kJ/mol)	Specific Heat, cp (J/mol·K)
TiB2	–315.9	$56.38+25.86·{10}^{-3}·\mathrm{T}-1.75·{10}^6·{\mathrm{T}}^{-2}$
Ti3Al	–97.9	$103.3+15.90·{10}^{-3}·\mathrm{T}-1.73·{10}^6·{\mathrm{T}}^{-2}$
TiAl	–75.3	$49.2+8.85·{10}^{-3}·\mathrm{T}-0.75·{10}^6·{\mathrm{T}}^{-2}$
TiAl2	–90.4	$62.2+10.33·{10}^{-3}·\mathrm{T}-0.96·{10}^6·{\mathrm{T}}^{-2}$
TiAl3	–146.4	$98.9+19.34·{10}^{-3}·\mathrm{T}-1.66·{10}^6·{\mathrm{T}}^{-2}$

[22].

$$∆H_r+{\int }_{298}^{T_{ad}}\sum n_j{c}_p\left(P_j\right)dT+\sum _{298-T_{ad}}{n}_{j}L\left(P_j\right)=0$$

(2)

where n_j is the stoichiometric coefficient of the product and L(P_j) is the latent heat of the product.

The SHS experiments were performed in a windowed combustion chamber. The chamber was first purged with high-purity (99.99%) argon for 3 min and then filled with argon at 2 bars. Well-mixed powders were uniaxially compressed to form cylindrical test samples with a diameter of 7 mm, a height of 12 mm, and a relative density of 55%. The ignition of the powder compact was achieved by a heated tungsten coil with a voltage of 120 V and a current of 5 A. Based on the time series of recorded combustion images, the propagation velocity of the self-sustaining combustion wave (V_f) was determined from the time derivative of the combustion front trajectory. The exposure time of each recorded image was set at 0.1 ms. A beam splitter with a mirror characteristic of 75% transmission and 25% reflection was used to optically superimpose a scale onto the image of the sample in order to accurately measure the instantaneous locations of the combustion front. The combustion temperature was measured by a bare Pt/Pt-13%Rh (R-type) thermocouple with a bead diameter of 125 μm and the thermocouple was mounted at a position about 6–7 mm from the ignited top plane of the sample. Details of the experimental setup and approaches are presented elsewhere [23]. For the synthesized products, the phase composition was identified by a Bruker D2 X-ray diffractometer (XRD Phaser, Karlsruhe, Germany). Microstructure and elemental analysis of the final products were examined by scanning electron microscopy (Hitachi S-3400N, Tokyo, Japan) and energy-dispersive X-ray spectroscopy.

3. Results and Discussion

3.1. Analysis of Combustion Exothermicity

Variations in ∆H_r and T_ad of the test samples containing different amounts of Al in the (Ti + xAl) group of Equation (1) are presented in Figure 1. The reaction of Equation (1) is exothermic and the enthalpy of reaction increases from 664.4 to 778.2 kJ with an increase in Al percentage from 25% to 75%. Because TiB₂ and four different Ti-Al compounds were formed as exothermic phases, the increase of the total number of mole of Ti-Al intermetallic compounds produced in the composites was responsible for the increase of ∆H_r from Ti-25% Al to Ti-50% Al. For the samples containing more than Ti-50% Al, because an equal amount of Ti-Al compounds was produced, the increase of ∆H_r is ascribed to the formation of Al-rich phases, TiAl₂ and TiAl₃. As indicated in Table 2, the heat of formation between TiAl, TiAl₂, and TiAl₃ is in an ascending order.

In contrast, Figure 1 shows that the T_ad of Equation (1) decreased from 3329 to 2954 K with increasing Al content in the samples from Ti-25% Al to Ti-75% Al. The high adiabatic combustion temperatures provide sufficient exothermicity for a self-sustaining reaction. Based on the heat of formation and specific heat of each compound listed in Table 2, TiB₂ was the most exothermic phase to form in the composite. Within Equation (1), the reaction of Ti with boron is considered as not only the major heat-releasing step but also the initiation step. Although the interaction of Ti with Al liberates heat in producing Ti-Al intermetallics, its exothermicity is less than the formation of TiB₂. As a result, the overall reaction exothermicity is weakened when the mole fraction of Ti-Al compounds augmented in the composite. This explains the descent of T_ad for the samples containing Al from Ti-25% Al to Ti-50% Al. Further decline in T_ad for the samples with Al percentage from Ti-50% Al to Ti-75% Al was mainly caused by the formation of Al-rich phases, TiAl₂ and TiAl₃, both of which have large specific heats.

3.2. Self-Propagating Combustion Wave Kinetics

Figure 2a–c illustrate typical time sequences of the SHS processes, which were respectively recorded from powder compacts formulated with Ti-25% Al, Ti-50% Al, and Ti-75% Al. It is evident that the combustion reaction started upon ignition and established a distinct combustion wave that propagated throughout the entire sample in a self-sustaining manner. Self-sustaining combustion was achieved for all samples conducted in this study. With the increase of Al percentage in the reactant mixture, as shown in Figure 2a–c, the burning luminosity decreased to some extent and the total spreading time of combustion wave was notably prolonged from about 1.50 s to 1.83 s and then to 3.97 s. These observations reflect the fact that combustion exothermicity decreased with increasing Al content in the reactant mixture.

The variation of combustion front velocity (V_f) with Al percentage in the (Ti + xAl) group of the reactant mixture is presented in Figure 3. The velocity decreased moderately from 6.6 to 5.6 mm/s as the Al content increased from Ti-20% Al to Ti-50% Al, beyond which a significant drop in combustion velocity was observed. The lowest flame velocity of about 2.2 mm/s was detected for the sample of Ti-75% Al. It is believed that energy transfer by heat conduction from the thin reaction zone to its adjacent unburned region is of critical importance to maintain a self-sustaining combustion process. As a result, the flame-front propagation velocity, in large part, is governed by the combustion wave temperature.

Figure 4 depicts four combustion temperature profiles measured from the powder compacts formulated with different Al percentages. Temperature profiles featured a sharp rise to a peak value followed by a continuous decline. This could be caused by the fact that the combustion propagation was speedy and synthesis reaction was normally confined in a hot thin layer. Moreover, the product was formed and cooled down after the passage of the combustion front. The peak value is considered as the combustion front temperature (T_c). As revealed in Figure 4, T_c reached 1655 °C for the sample of Ti-25% Al and decreased to about 1574 °C and 1442 °C with an increase in Al content to Ti-50% Al and Ti-66.7% Al, respectively. The sample of Ti-75% Al exhibited a less steep curve with T_c of about 1278 °C. More importantly, the dependency of combustion temperature on sample stoichiometry was consistent with that of combustion wave velocity.

According to combustion wave kinetics [23,24], a modified Arrhenius rate equation, Equation (3), was employed to deduce the apparent activation energy (E_a) of solid-state combustion reaction.

$$V_f^2=\frac{2\lambda }{\rho Q}\frac{{RT}_c^2}{E_a}{k}_o\exp (-E_a/R{T}_c)$$

(3)

where λ is the thermal conductivity, R is the universal gas constant, Q is the heat of the reaction, and k_o is a constant. Based on Equation (3), the relationship of ln(V_f/T_c)² with 1/T_c can be established by the measured data of V_f and T_c, and the slope of a linear line correlating ln(V_f/T_c)² and 1/T_c represents E_a/R. Figure 5 plots calculated values of −ln(V_f/T_c)² with respect to 1/T_c and a best-fitted linear line with the slope. Activation energy E_a equal to 114.7 kJ/mol was deduced for the Ti-Al-B combustion reaction. This value is much lower than the activation energy, 539 kJ/mol, obtained in the Ti + 2B combustion reaction [25]. It is believed that due to the presence of liquid-phase Al during combustion synthesis, the activation energy of the ternary Ti-Al-B combustion system was reduced. Combustion temperatures of this study shown in Figure 4 are higher than the melting point of Al. The existence of molten species could have facilitated the diffusion of solid reactants and substantially reduced the kinetic barrier of the reaction system. A similar example confirming the aid of molten species is that the activation energy of 102.5 kJ/mol was determined for the Ti-B₄C-Al-Ni combustion system to produce NiAl/TiB₂/TiC composites [26].

3.3. Phase Composition and Microstructure of Synthesized Products

Figure 6a–d present XRD patterns of Ti-Al intermetallics/TiB₂ composites synthesized from powder compacts with different Al percentages in the (Ti + xAl) group of reactant mixtures. The other mixture group in Equation (1) generated TiB₂. The XRD analysis revealed that TiB₂ was well produced from the elemental reaction between Ti and boron for all samples and the phase of the intermetallic compound was subject to the Al percentage in the (Ti + xAl) group. The sample of Ti-30% Al, shown in Figure 6a, produced only Ti₃Al, which was also the only aluminide produced from the sample of Ti-25% Al. This agrees with the fact that Ti₃Al has a wide homogeneity range from 22 to 35 at.% Al.

Figure 6b depicts the XRD spectrum of the product synthesized from the sample of Ti-50% Al. Formation of TiB₂ along with two aluminides, TiAl and Ti₃Al, was identified. The dominant intermetallic phase in Figure 6b is TiAl. The presence of Ti₃Al could be due to the evaporation loss of a small amount of Al during the SHS process, or because of TiAl having a composition range from 49 at.% Al to about 66 at.% Al. Both TiAl and Ti₃Al were also formed in the product from the sample with Ti-40% Al, which was consistent with the coexistence of TiAl and Ti₃Al in the composition between Ti-35 at.% Al and Ti-49 at.% Al. However, the governing aluminide phase was Ti₃Al in the product of Ti-40% Al.

The product composed of TiB₂, TiAl, and TiAl₂, as displayed in Figure 6c, is associated with the sample of Ti-66.7% Al. Since TiAl₂ has a narrow homogeneity range of 64.5–67 at.% Al and TiAl has a Al-rich composition of about 60–66 at.% Al at temperatures between 1200 °C and 1400 °C, a small evaporation loss of Al could result in the formation of TiAl. It is useful to mention that the formation of TiAl was more pronounced in the product from the sample of Ti-60% Al than that revealed in Figure 6c. Namely, TiAl dominated over TiAl₂ in the TiAl/TiAl₂/TiB₂ composite synthesized from the sample of Ti-60% Al, while TiAl and TiAl₂ were nearly comparable in the product from the sample of Ti-66.7% Al.

The XRD pattern of Figure 6d represents the product obtained from the sample of Ti-75% Al. The yield of TiAl₃ as the major aluminide is identified. The presence of a small amount of TiAl₂ in the product was because TiAl₃ is a line compound. Therefore, a slight deficiency of Al might lead to the formation of TiAl₂. It should be noted that a composite consisting of TiB₂, TiAl₂, and TiAl₃ was also synthesized from the sample of Ti-71.4% Al and the content of TiAl₂ relative to TiAl₃ was more than that observed in Figure 6d.

Table 3. Major and minor titanium aluminides of Ti-Al intermetallics/TiB₂ composites synthesized from Equation (1) with different Al percentages in the (Ti + xAl) group.

Al Percentage in Ti + xAl	Major Aluminide	Minor Aluminide
Ti-25% Al	Ti3Al	―
Ti-30% Al	Ti3Al	―
Ti-40% Al	Ti3Al	TiAl
Ti-50% Al	TiAl	Ti3Al
Ti-60% Al	TiAl	TiAl2
Ti-66.7% Al	TiAl2, TiAl	―
Ti-71.4% Al	TiAl2	TiAl3
Ti-75% Al	TiAl3	TiAl2

summarizes the major and minor phases of titanium aluminides formed in the TiB₂-containing composites from combustion of Equation (1) under different contents of Al. It is evident that Ti-Al intermetallics/TiB₂ composites featuring four different dominant aluminide phases were produced.

Figure 7 presents the SEM image and EDS spectrum, showing fracture surface microstructure and elemental composition of the product synthesized from the sample of Ti-30% Al. Most of the TiB₂ grains were formed in a columnar shape with a length of 2–3 μm and were embedded in the Ti-Al aluminide matrix. According to the formation mechanism of Ti-Al intermetallics [3,27], the stages involve the initial melting of Al, subsequent spreading of molten Al through channels of the capillary-porous medium, and diffusion of aluminum atoms into the titanium lattice. This resulted in the formation of the TiAl₃ compound, which can transform into TiAl₂, TiAl, and Ti₃Al as it is saturated with titanium. The formation of TiB₂ is governed by a dissolution/precipitation mechanism including solid-phase diffusion of boron into titanium and the precipitation of TiB₂ particles [28,29]. The EDS spectrum of Figure 7 confirms three constituent elements, from which an atomic ratio of B:Al:Ti = 55.84:8.94:35.22 was obtained. Low Al proportion was ascribed to this composite containing a Ti-rich aluminide Ti₃Al at a small mole fraction.

The SEM image and EDS spectrum presented in Figure 8 are associated with the product obtained from the sample of Ti-50% Al. The morphology clearly exhibited columnar TiB₂ grains well distributed in the Ti-Al aluminide matrix. The EDS intensity of the Al peak relative to that of Ti in Figure 8 is noticeably stronger than that observed in Figure 7. The atomic ratio of the scanned region was B:Al:Ti = 45.89:15.22:38.89, which is close to a composite consisting of TiB₂ and TiAl. It should be noted that the peak at around 2.1 keV is Pt (platinum). In this study, Pt was used to sputter coat SEM samples as a conductive layer.

For the sample of Ti-75% Al, the microstructural and elemental analyses of its resulting product are unveiled in Figure 9. TiB₂ grains were formed in a smaller size of about 1 μm and mixed closely with the Ti-Al aluminide matrix. When compared with that in Figure 8, an Al peak with a further increase in intensity was detected in the EDS spectrum of Figure 9. An atomic ratio of B:Al:Ti = 40.54:28.64:30.82 was obtained, which suggested TiAl₃ as the dominant aluminide in the Ti-Al intermetallics/TiB₂ composite. It is important to note that the major titanium aluminide revealed by the atomic ratio of constituent elements from the EDS analysis of Figure 7, Figure 8 and Figure 9 was in good agreement with the intermetallic phase identified by the XRD pattern.

The green powder compacts conducted by this work had a relative density of 55%. Due to volume expansion of the burned samples after the SHS process, the final products were porous and had an estimated porosity of about 50%. As a result, composite powders were easy to obtain. In view of practical applications, the size range and external morphology of Ti-Al intermetallics/TiB₂ composite powders are suitable as feedstock materials for powder metallurgy or in different thermal spraying systems for depositing protective coatings.

4. Conclusions

In situ formation of Ti-Al intermetallics/TiB₂ composites was investigated by the SHS technique from Ti-Al-B powder mixtures. The reactant mixtures comprised Ti and boron in a configuration of (2Ti + 4B) to produce 2TiB₂, as well as Ti and Al in a composition of (Ti + xAl) to generate titanium aluminide. The phase of aluminide compounds formed depended on the Al content in the (Ti + xAl) group. A value of x ranging from 0.33 to 3.0 was conducted, which corresponded to the Ti/Al atomic ratio from Ti-25% Al to Ti-75% Al for studying the formation of four different aluminides, Ti₃Al, TiAl, TiAl₂, and TiAl₃.

Combustion exothermicity of the reaction was sufficient to ensure progression of the synthesis process in the SHS manner. However, combustion exothermicity was weakened by an increasing Al percentage in the reactant mixture due to the increase of aluminide compounds formed and the high specific heats of Al-rich aluminide phases. As a result, the combustion wave velocity decreased from 6.6 to 2.2 mm/s and combustion temperature from 1655 °C to 1278 °C when the Al percentage in the sample increased from Ti-25% Al to Ti-75% Al. Based on combustion wave kinetics, the apparent activation energy of 114.7 kJ/mol was deduced for the Ti-Al-B combustion reaction. The XRD analysis of the final products indicated that Ti₃Al was the only intermetallic compound synthesized from samples of Ti-25% Al and Ti-30% Al. A composite containing TiB₂, Ti₃Al, and TiAl was produced from the sample of Ti-40% Al. TiAl was the dominant aluminide in the products from samples of Ti-50% Al and Ti-60% Al. For the samples with Ti-66.7% Al and Ti-75% Al, their resulting products were TiAl₂/TiAl/TiB₂ and TiAl₃/TiAl₂/TiB₂ composites, respectively. The transformation of the dominant aluminide from the Ti-rich to Al-rich phase was a consequence of the increase of Al in the samples, and the synthesized intermetallic compounds with respect to the Ti/Al ratio in the samples were essentially in good agreement with titanium aluminides in the Ti-Al phase diagram. The microstructure of the as-synthesized Ti-Al intermetallics/TiB₂ composites illustrated that columnar TiB₂ grains with a length of 2–3 μm were formed and well distributed and closely engaged in the aluminide matrix. The EDS analysis confirmed constituent elements and provided rational atomic ratios between Ti, Al, and B. In summary, this study demonstrated an effective synthesis route for fabricating Ti-Al intermetallics/TiB₂ composites featuring different dominant aluminide phases.