1. Introduction

There is currently extensive research on intermetallics for high temperature applications, especially for the automotive, aviation, and cosmic applications [1,2,3]. Titanium aluminides are of the greatest importance for the automotive industry, when turbochargers made of TiAl intermetallic alloys have already started to be used in passenger cars [4]. The use of these alloys in aircraft engines (for low-pressure turbine blades) is very interesting [4,5]. Today’s titanium alloys and titanium aluminides account for a third of the weight of aircraft engines and are the second most widely used material for engine components after nickel superalloys [6].

Cold extreme tool steels or high speed steels are most commonly used today for extreme adhesive and abrasive wear [7,8,9]. High temperature applications use nickel superalloys in aviation-related applications, such as engine turbines. Another widely used material is heat-resistant steels, the great advantage of which is the relatively simple production [10]. However, the problem is their high density [11]. In aviation, there is an effort to replace nickel [12] and heat-resistant steels with lighter materials to decrease the weight of the aircraft structure and to improve engine performance [10,11]. The strength-to-weight ratio could be significantly improved by significantly reducing weight by the means of a substitution of nickel-based superalloys with intermetallic alloys, such as titanium aluminides [6].

Titanium alloys with other light elements (aluminum, silicon) are therefore very promising materials for applications at higher temperatures [13,14], especially for use in structural applications operating under static load and at the same time high temperatures [15]. Their biggest advantage is the low density, which ranges around 4 g·cm⁻³ [16,17].

2. History of Using TiAl Alloys

In the second half of the last century, TiAl alloys began to be used as technical materials. Rolls-Royce, P&W, and GE tested automotive engines made from TiAl intermetallic alloy. Advantageous properties of TiAl motors include better specific tensile strength compared to other titanium alloys and higher strength at temperatures up to 800 °C compared to steels or nickel superalloys. However, a major disadvantage is the low room-temperature strain, usually up to 1% (1% is generally an acceptable minimum ductility value, cast samples seldom reach this value). Another problem is the difficult production of the component [18].

In the 1970s, extensive research was carried out on Ti₃Al-based systems for the production of aircraft engines, but this production failed due to the high difficulty and the limit temperature of use of the component (600 °C) [5]. In the 1980s, research was shifted to γ–TiAl alloys, which had better creep and oxidation resistance, and the maximum application temperature was increased to 750 °C. In addition to aviation, alloys have potential use in the automotive and nuclear industries [5]. The US Air Force then stated that TiAl intermetallics provide unique weight savings as a replacement for nickel superalloys in the hot parts of low-pressure turbine jet engines [19]. However, problems with the use of TiAl alloys in aviation include large deviation of mechanical properties, difficult production and high costs [5]. On the other hand, the use of an aircraft engine made of TiAl alloy reduces the weight from 20% to 50% compared to conventional materials, but production costs increase by 10% to 100% [5].

In this millennium, research has focused on TiAl alloys with 44–48 at. % aluminum, but for some applications, a maximum of 42 at. % aluminum is required. The β-phase appears at aluminum concentrations lower than 42 at. %, which significantly improves the heat treatment [20]. However, this phase has lower mechanical properties, so most applications requiring high heat strength must contain about 46 at. % aluminum [18]. If the TiAl46 alloy (at. %) is cooled sufficiently quickly from the α-phase (the rates are similar to thin castings), a lamellar structure is formed, and γ-phases precipitate on the crystallographic plane (0001). If the sample is heat-treated in the region of a two-phase structure, a duplex structure formed by γ-phase with lamellar grains is formed during cooling [18,21].

TiAl alloys have already begun to be used as turbochargers in commercial vehicles [4]. The development of TiAl alloy turbochargers began in Japan, when a relatively small number (about 1000) of TiAl turbochargers were installed in 1998 on Lancer cars [18]. In 1999, turbochargers were shown to Mitsubishi Motors Inc. for the model Lancer 6 [19] and great success in the following years led to the addition of more than 20,000 cars in 2003 with a TiAl alloy turbocharger [18]. Today, some companies now manufacture turbochargers based on TiAl, and it seems possible to successfully use a TiAl alloy in this relatively simple application, where very low toughness is acceptable to designers [18].

3. Properties of TiAl Alloys and Effect of Silicon on Them

As already mentioned, TiAl alloys excel in low density. Their other properties include good resistance to oxidation at temperatures of 600 to 800 °C [5,11], high specific strength at higher temperatures [22,23], and a favorable ratio between density and mechanical properties [12]. However, TiAl alloys have low ductility at room temperature (1–2%) [10,19,24] and fracture toughness [5,25]. Other properties of TiAl alloys are low thermal expansion and thermal conductivity comparable to nickel superalloys and refractory steels. The electrical conductivity is lower than that of commonly used alloys [26].

TiAl (50:50) alloy cannot be used above 827 °C, although it contains 50% aluminum, because the aluminum content is not enough to form a layer of alumina with protective effect. Approximately 75% aluminum is required to form the oxide layer [27]. Improvement of oxidation resistance can be achieved, for example, by surface modification or by adding silicon to TiAl powder in mechanical alloying [28]. By modifying the surface, silicon diffuses into the TiAl alloy, demonstrating the high affinity of silicon and titanium. The resistance to oxidation of TiAl alloys was also increased by the method of liquid phase siliconization with Al–Si alloy [28]. Silicon surface treatments are described, for example, in articles [29,30,31,32,33,34,35,36,37,38,39]. Siliconizing is a treatment to improve the chemical and mechanical properties of intermetallics based on TiAl system. Siliconizing methods can be used to obtain silicide layers on the surface of titanium-based alloys. These methods [30,32,34,35,36] combine in a siliconization process in the vapor phase and subsequent heat treatment [37], siliconization in the liquid phase combined with aluminizing in an Al–Si melt [19,38,40], heat treatment in quartz glass ampoules under low partial pressure of oxygen [39], or sol–gel coating in combination with heat treatment [30]. The silicide layers are very hard and prevent oxidation up to 950 °C.

The addition of silicon into the TiAl alloys improves oxidation and corrosion resistance at elevated temperatures, as well as creep resistance [41,42]. Silicon dissolves very little in TiAl alloys and therefore forms hard and brittle titanium silicides in the structure [17], mainly Ti₅Si₃ silicides, which have a strengthening effect [22,43]. A major problem of TiAl–Si alloys is the brittleness of titanium silicides, so they need to be as fine and fibrous as possible, which would increase the fracture toughness of the material [14]. Silicon is a light element, and the addition of more silicon to TiAl alloys improves the creep resistance and oxidation resistance due to the refinement of titanium dioxide particles formed on the surface of TiAl alloys. Silicon promotes the diffusion of aluminum in the oxidation layer and binds titanium to a stable silicide, then reduces the activity of Ti^IV and prevents the diffusion of Ti^IV outwards. The formation of titanium dioxide is thus limited [6]. The addition of silicon to TiAl alloys improves the oxidation resistance due to the formation of titanium nitride, which is formed in the form of a layer or isolated particles between the oxide layer and the base material and thus protects the base material from further oxidation [7].

High oxygen solubility in α-Ti (about 14.5 wt. %) leads to the formation of a brittle and hard subsurface layer enriched with oxygen, which cracks and causes deterioration of fatigue resistance and ductility [44,45,46,47]. Alloying by silicon improves oxidation resistance, oxide layer adhesion, and creep behavior at elevated temperatures [48,49]. High concentrations of silicon are more advantageous for improving resistance to oxidation than concentrations in the solubility range of silicon in α-Ti [50]. Kitashima et al. [51] showed similar oxidation behavior of an near-alpha alloy Ti-5Al-2Sn-2Zr-2Mo with 0.2 and 0.9 wt. % silicon [51]. However, commercial titanium alloys do not contain silicon in concentrations exceeding its solubility range in α-Ti (0.3 wt. %), which is mostly due to the negative effect of silicon on their ductility, especially when the silicon contents exceed 2 wt. % [52]. In addition, some researchers speculate that precipitation of silicides in aluminum-containing alloys may limit the formation of the heat-resistant phase Ti₃Al, thereby reducing high temperature strength. Mechanisms that increase oxidation resistance include reducing the depth of oxygen penetration into the metal substrate and dissolving the silicon in the oxide surface layer. Silicon dissolved in the oxide layer reduces the rate of oxygen diffusion through the layer and modulates the stress relaxation that leads to a more compact and less porous oxide layer [49,50]. In addition, the formation of silica (SiO₂) in the oxide surface layer inhibits the recrystallization of rutile (TiO₂) crystals. In the work of Novák et al. [53], the addition of silicon and aluminum has been found to reduce the oxidation rate of titanium to approximately 850 °C. Silica is formed in the oxide layer on titanium. In silica, oxygen diffuses more slowly than in rutile (titanium dioxide). In addition, the almost continuous sub-layer of silicides is formed beneath the oxide layer as a result of the depletion by aluminum, which diffuses to the oxidized surface [53]. This layer acts as the secondary protection and suppresses the diffusion of both oxygen and nitrogen towards the core of the material. As a result, titanium nitride TiN is formed below the oxide layer. It has been proved that TiN is due to the presence of silicon in the material, because TiN formation was not observed in the binary alloy TiAl [7].

5. Preparation of TiAl–Si Alloys by Melting Metallurgy (MM)

The synthesis of the TiAl intermetallic compounds is one of the important directions in the development of new materials with heat resistance and high thermal stability [85]. Methods for producing TiAl alloys (and intermetallics in general) include conventional casting techniques, arc melting in an argon atmosphere, powder metallurgy, etc. [85].

The preparation of TiAl–Si alloy is very difficult using MM. This is due to the very high melting point of intermetallic compounds (e.g., titanium silicide Ti₅Si₃ melts at temperatures above 2100 °C [12,66]), high melt reactivity with crucibles [43], damage to crucibles (crucibles made of Y₂O₃ and ZrO₂ are reported to be required [40], which is more expensive than graphite or corundum crucibles), and melt contamination. Other disadvantages of the preparation of TiAl–Si alloys by MM include very poor casting properties of intermetallics (very frequent occurrence of casting defects, such as pores or microcracks) [11]. In order to eliminate defects, the castings are post-processed by hot isostatic pressing (HIP). This technology is used by GE for GEnx engines for Boeing airliners [86].

The preparation of TiAl alloys with addition of silicon by MM is limited to hypoeutectic and eutectic alloys based on α₂-Ti₃Al, as hypereutectic alloys are extremely brittle due to the formation of coarse primary silicides during solidification [22]. It is necessary to design the material so that the composition of the alloys is around the eutectic point, so that the eutectic phases are fine. The eutectic composition is advantageous, because the meltability and fluidity are improved during casting [14].

Melting and casting of TiAl–Si alloys produce brittle and hard titanium silicides Ti₅Si₃, and these coarse and randomly oriented sharp-edged silicide particles are undesirable in terms of deterioration of some mechanical properties (especially fracture toughness) [7,24]. Previous work has sought to eliminate it and to form an in-situ composite of elongated Ti₅Si₃ silicide particles in a tough matrix of TiAl or Ti₃Al titanium aluminides. However, there were cracks in the structure perpendicular to the solidification direction (due to the different thermal expansion coefficient of the Ti₅Si₃ titanium silicide in different crystal directions, therefore tensile stresses and crack initiation occurred). This results in a limited applicability of TiAl–Si alloys prepared by directional crystallization [7]. The stress in the material can generally be reduced by gradual slow heating and cooling, but for a polycrystalline material with high anisotropy of the coefficient of thermal expansion, it is not possible to reduce the residual stress [87].

TiAl–Si alloys prepared by MM have a very coarse microstructure with concentrated pores in the center of the material. The particles of titanium silicide, which are coarse, are oriented depending on the local direction of heat dissipation, which causes the elongated silicides, which was described in the article [17,87]. These silicides are cracked due to the high rate of cooling after melting process (Figure 1) [87]. The porosity in the center of sample is given by the production technology. The gases are trapped inside the material as a result of the elements evaporation (especially aluminum) during the exothermic reaction of intermetallic phases and the relatively fast cooling of the reacted sample. Gases cannot escape from the material due to the rapid solidification of the surface. Therefore, the porosity is concentrated in the center of the sample. The same applies to samples prepared by reactive sintering described, for example, in [14,88].

Figure 1. Microstructure of TiAl15Si15 prepared by vacuum induction melting.