Formation of Mo − Si − Ti Alloys by Self − propagating Combustion Synthesis

Test specimens with nominal compositions MoSi2, (Mo0.9Ti0.1)Si2, (Mo0.8Ti0.2)Si2, (Mo0.7Ti0.3)Si2, (Mo0.6Ti0.4)Si2, (Mo0.5Ti0.5)Si2 and (Mo0.4Ti0.6)Si2 were prepared by combustion synthesis. The combustion mode, propagation velocity of combustion wave, combustion temperature and product structure were investigated. Specimens MoSi2, (Mo0.9Ti0.1)Si2, (Mo0.8Ti0.2)Si2, (Mo0.7Ti0.3)Si2, underwent spontaneously self−propagating combustion synthesis. However, the (Mo0.6Ti0.4)Si2 and (Mo0.5Ti0.5)Si2 specimens required a sustainable energy supply to complete the combustion synthesis reaction. There was no combustion synthesis reaction in specimen (Mo0.4Ti0.6)Si2. The combustion wave propagated along a spiral trajectory from top to the bottom of the specimen compacts in a layer by layer mode. The propagation velocity of the combustion wave reduced with the addition of titanium. The X-ray diffraction analysis showed that the Cllb-MoSi2 and C40-(Mo,Ti)Si2 type phases were formed during combustion synthesis. The intensity of diffraction peaks of C40-(Mo,Ti)Si2 phase increased with Ti content.


Introduction
Molybdenum disilicide is a cheap, non−toxic and environmentally benign intermetallic.It is regarded as a competitive high temperature structural material owing to its high melting point of 2303K, excellent oxidation resistance up to 1873K, moderate density of 6.24g•cm −3 , low thermal expansion coefficient of 8.1×10 −6 K −1 , good thermal and electrical conductivities of 50 W•m −1 •K −1 and 21.5×10 −6 Ω•cm, respectively [1][2][3][4][5] .However, the applications of MoSi 2 are limited due to its low fracture toughness at room temperature, poor strength at high temperature and degradation by a phenomenon known as "pesting" (destruction of intermetallic by oxidation into powder) in the temperature range of 673-873 K 5,6 .The key material issues are to improve the fracture toughness and high temperature strength [1][2][3] .
The room temperature fracture toughness and high temperature strength of MoSi 2 can be improved by alloying and composite approaches [1][2][3][4]7,8 . Allying promotes the dislocation plasticity in MoSi 2 at low temperatures and lowers the brittle−to−ductile transition temperature (BDTT) and results in an increase in room temperature fracture toughness [2][3][4]7 .The effects of various alloying elements on the ductility of MoSi 2 single crystals are reported from first principles calculations 2,9 .The calculations suggest that the alloying addition of Mg, V, Nb, Tc, or Al to the C11 b MoSi 2 crystal structure may enhance ductility whereas the addition of Ge, P, or Re is predicted to have an opposite effect 2,9 .
Titanium disilicide, TiSi 2 , has found wide applications as thin film in microelectronics [10][11][12] .In Ti-Si system, five silicide compounds: Ti 3 Si, Ti 5 Si 3 , Ti 5 Si 4 , TiSi, and TiSi 2 exist 13 .TiSi 2 and MoSi 2 are of AB 2 type structure with the same atomic percent of silicon.There are TiSi 2 and MoSi 2 with hexagonal C54 and tetragonal Cll b structures respectively.The disilicides with C54 and Cll b crystal structures are closely related by different stacking sequence 14 .The Mo−Si−Ti system can form a ternary compound like (Mo,Ti)Si 2

11
. MoSi 2 can be toughened by alloying approaches of titanium 15 .So, titanium has great potential as an alloying addition to MoSi 2 to improve its performance 15,16 .
Several studies have reported that the properties of MoSi 2 could be enhanced by alloying with titanium 11,[17][18][19] .Boettinger et al. 16 reported that the MoSi 2 composites containing 3mol% and 18mol% TiSi 2 have exhibited excellent oxidation resistance at 1300 °C.Yanagihara et al. 17 prepared the specimens with the nominal composition of (Mo 0.9 Ti 0.1 )Si 2 by arc−melting and reported that the pesting of (Mo 0.9 Ti 0.1 ) Si 2 was suppressed at temperatures between 673K−973K.(Mo 0.9 Ti 0.1 )Si 2 forms duplex dense and well adherent scale composed of amorphous SiO 2 and rutile at 1500 °C18 .
Combustion synthesis or self−propagating high−temperature synthesis (SHS) provides an attractive alternative to the conventional methods of producing advanced materials, such as ceramics and intermetallics.SHS offers advantages with respect to process economics and process simplicity.The underlying basis of SHS relies on the ability of highly exothermic reactions to be self−sustaining and, therefore, energetically efficient [20][21][22] .Combustion synthesis is an important method to prepare intermetallics compounds like MoSi 2 , Ti−Si and Nb−B [23][24][25][26][27] .However, there is no report in the literature on the combustion synthesis of Mo−Si−Ti alloys.In this study, we have investigated the effect of the alloying of Ti on the combustion synthesis of MoSi 2 .The combustion synthesis characteristics of the alloys with the nominal composition of MoSi 2 , Mo(Si 0.9 Ti 0.1 ) 2 , Mo(Si 0.8 Ti 0.2 ) 2 , Mo(Si 0.7 Ti 0.3 ) 2 , Mo(Si 0.6 Ti 0.4 ) 2 , Mo(Si 0.5 Ti 0.5 ) 2 , Mo(Si 0.4 Ti 0.6 ) 2 are reported.The combustion mode, propagation velocity of combustion wave, combustion temperature and product structure will be discussed.
The combustion synthesis experiments were conducted in a steel combustion chamber with a glass window, under an atmosphere of high purity argon (99.99%, 0.1MPa) (Figure 1).The ignition was accomplished by a heated molybdenum coil with a voltage of 18V and a current of 3A at room temperature.
A color CCD video camera was used to record the combustion synthesis images.The Windows Movie Maker software was applied to analyze the combustion synthesis images and the combustion mode as well as to calculate the propagation velocity of the combustion wave.Thermocouple wires (WRe3−WRe25 with a diameter of 0.1mm) was placed inside a hole drilled in bottom of the powder compacts to record the combustion synthesis temperature with a frequency of 300 Hz.The crystal structure of the combustion products were identified by a Rigaku D/Max−3B X−ray diffractometer (XRD) operating at 35 kV and 30 mA using Cu Kα (λ=0.15406nm) radiation.

Combustion characteristics
The typical, distinct and representative recorded combustion images (Figure 2) show that the combustion synthesis reaction was ignited locally at the contact point of the specimen with the ignition coil.After ignition, the combustion wave spreaded along a spiral trajectory from top to the bottom of the specimen.This spinning combustion wave is considered as an unstable combustion phenomenon and mainly caused by the lack of sufficient heat flux generated from the reaction to maintain the steady propagation of the planar front 28,29 .We refer it "normal combustion", compared to the explosion combustion; where the propagation velocity of the combustion wave can be up to 1000−4000 m/s 30 .The "normal combustion" refers to the combustion in a combustion chamber of an industrial furnace.The reactants are heated to the ignition temperature by a heater and ignite the combustion reaction locally at the contact point of the reactants with the heater.The local combustion reaction releases sufficient amount of heat to heat up the reactants in the surroundings.This process repeats until all the reactants are consumed.The "normal combustion" phenomenon corresponds to the experimental evidence shown in Figure 2.
The spreading mode of the combustion wave was similar for each specimen, however, the time duration to complete the combustion of the compacts increases with the Ti content.It is necessary to point out that the combustion synthesis reaction of (Mo 0.6 Ti 0.4 )Si 2 and (Mo 0.5 Ti 0.5 )Si 2 specimens was extinguished without a constant heat supply.The (Mo 0.4 Ti 0.6 ) Si 2 specimen was not ignited at room temperature, even though some auxiliary measures were adopted, such as preheating 23,28 , covering powders between the ignition coil and the green compact 31 .
For the formation of compounds, it has been demonstrated empirically that the reaction will not be self−propagating unless the adiabatic temperature is beyond 1800K (T ad ≥1800K) [19][20][21] .The formula of theoretical adiabatic temperature is as follow 20,29 : Where, is the heat of formation of the compound at 298K, T ad is adiabatic temperature of the combustion synthesis reaction, T o is initial temperature, C p is the heat capacity.
As shown in Table 1, the theoretical adiabatic temperature of MoSi 2 and TiSi 2 is 1943K and 1833K, respectively.MoSi 2 and TiSi 2 satisfy the requirement for self-propagating combustion synthesis theoretically (T ad ≥1800K).Thus MoSi 2 and TiSi 2 can be formed by combustion synthesis method.The adiabatic temperatures of (Mo 1−X ,Ti X )Si 2 alloys are calculated from the following reaction, In present work, the theoretical adiabatic temperature of seven specimens, including (Mo 0.4 Ti 0.6 )Si 2 , are higher than 1800K.Theoretically, (Mo 0.4 Ti 0.6 )Si 2 specimen can undergo combustion synthesis reaction but it did not show combustion synthesis reaction under our experimental conditions.The actual combustion temperature is always lower than the theoretical due to the mass or energy loss in the experiments.The green density of the compacts, furnace atmosphere and the preheating temperature would influence the actual reaction temperature.The alloys with theoretical adiabatic temperature slightly above 1800K, sometimes can not be synthesized by combustion synthesis method at T o =298K, such as SiC (T ad =1900K), TaSi 2 (T ad =1800K) 20,28,32 .

Combustion temperatures
As of the typical combustion temperature-time curve of MoSi 2 plot in Figure 3, 298K is the initial temperature, and 1797K is the recorded combustion temperature of MoSi 2 compact.This adiabatic temperature is higher than the measured value in our prior experiments 31 and it falls well within the temperature range (1615K−1886K) reported in the literature 23,24 .The recorded temperature is lower than the theoretical adiabatic temperature of 1943K.The difference in recorded and theoretical adiabatic temperature appears to be dependent on the composition of the raw material, reaction environment and heat exchange with the surrounding.Similar experimental evidences are reported earlier in the combustion synthesis of silicide, boride and carbide 25,26,33 .The theoretical adiabatic temperature of Ta 2 C, TiSi and NbB 2 are 2600K, 2000K and 2400K, respectively.The measured combustion temperatures reported were 1963K 33 , 1603K 25 and 1873K 26 , respectively, which are much lower than their theoretical adiabatic temperature.
The combustion temperature−time curves of (Mo 0.9 Ti 0.     observed in Figure 4.The combustion synthesis reaction of (Mo 0.9 Ti 0.1 )Si 2 , (Mo 0.8 Ti 0.2 )Si 2 , (Mo 0.7 Ti 0.3 )Si 2 , (Mo 0.6 Ti 0.4 ) Si 2 and (Mo 0.5 Ti 0.5 )Si 2 was sustained 0.62s, 0.76s, 0.75s, 0.95s and 1.09s (AB stage) respectively.The time duration of combustion synthesis reaction was prolonged and the combustion front became wider with the increase of Ti content (Figure 4).The rates of temperatures rise at the AB stage (Figure 4) were calculated through the temperature changes at AB stage (the initial temperature is 298 K, the maximum temperatures of 1 # -5 # are 1650, 1611, 1587, 1743 and 1714 K, respectively) division by the times of the AB stage.The calculated rates of temperatures rise of 1 # -5 # were 2181, 1728, 1719, 1521 and 1299 K/s, which suggest that the combustion reactions were ultra-fast and combustion rate decreased with the increase of Ti content.The experimental maximum combustion temperatures of (Mo 1−X ,Ti X )Si 2 specimens are shown in Figure 5.At least five iterations were tested for each composition.It shows that the maximum combustion temperature of MoSi 2 is the highest one.But there is no clear trend observed in combustion temperature for other specimens.

Flame−front propagation velocity
According to the combustion theory, the flame−front propagation velocity of combustion wave is a physical and chemical property of the combustible matter 20 .This is influenced by the chemical composition, diluent content and the density of the compact 20,21 .In present experiments, the specimens are only different in the atomic ratio of Mo−Si−Ti.The flame−front propagation velocity of compacts was deduced from the recorded combustion images.Figure 6 shows the flame−front propagation velocity of the six different specimens.The specimen with the nominal composition of MoSi 2 obtains the highest propagation velocity, 3.75 mm/s, and the propagation rate decreases slightly with increasing Ti content, and reaches to a minimum value of 2.50 mm/s at the nominal composition of (Mo 0.5 Ti 0.5 )Si 2 .In our case, it is found that the flame−front propagation velocity is significantly influenced by the starting composition.

X-ray diffraction
The X-ray diffraction (XRD) patterns of the combustion products were shown in Figure 7.It shows that the (Mo,Ti) Si 2 alloys were synthesized by combustion synthesis rapidly.
Figure 7a shows that a Cll b -type single-phase disilicide, MoSi 2 , is formed from the specimen 0 # with corresponding stoichiometry of Mo: Si = 1: 2.0. Figure 7b indicates that on adding the alloying element of Ti, the diffraction peaks of C40-type phase, (Mo , Ti)Si 2 appear.The nominal composition of (Mo 0.9 Ti 0.1 )Si 2 specimen consists of Cll b (MoSi 2 )+ C40((Mo,Ti) Si 2 ) type phases and it is in agreement with the experimental results of (Mo 0.9 Ti 0.1 )Si 2 prepared by the arc−melting process of Yanagihara et al. 17 Based on the MoSi 2 -TiSi 2 pseudo-binary    diagram, the Cll b structure has limited solubility of Ti in MoSi 2 34 .Svechnikov et al. studied the phase equilibria of Mo−Si−Ti system and found that the structure C40 phase was stabilized by the addition of Ti and extends into the ternary phase region to form the C40 (Mo,Ti)Si 2 phase 35 .In current work, dual phase materials with Cll b (MoSi 2 ) and C40((Mo,Ti) Si 2 ) were synthesized by in-situ combustion synthesis, this dual phase structure was confirmed by XRD measurement.It is evident that the dual phase structure can be manipulated into other morphologies and size scales 16 .Moreover, the fracture toughness of the dual phase Cll b (MoSi 2 )/C40(NbSi 2 ) were increased from 1.0-1.4MPa.m 1/2 (C40 single phase) to 3.7 MPa.m 1/2 (C11 b /C40 duplex-phase) 36 .With further increase in titanium content (Figure 7 Combustion synthesis is a rapid synthesis processing, another non-equilibrium processing technique, that can synthesize supersaturated solid solutions 37 .Therefore, the solid solubility level of Ti in MoSi 2 could be enhanced with the increase of the Ti content.

Microstructure of the products
The scanning electron microscope (SEM) micrographs (Figure 8) of the products show that the microstructure of the samples is uniform and consists of 2-5 μm (Mo,Ti)Si 2 particles and uniform arrays of pores.The volume change of samples before and after combustion process is negligible which suggests that the pores between powder particles in the green compacts do not collapse during the combustion synthesis process 38 .
Table 2 show the density, crystal structure and lattice parameters of MoSi 2 and (Ti,Mo)Si 2 phase.The density of (Mo , Ti)Si 2 is 4.54 g•cm −3 , which is only 72.4% of that of MoSi 2 .(Mo , Ti)Si 2 is an important phase in present experiments, even major phase in (Mo 1-X Ti X )Si 2 (x≥0.3)specimens.Thus, our study opens the way to synthesize light Mo based silicides for high-temperature structural applications.

Conclusions
This study presents an experimental investigation on the combustion synthesis characteristics of MoSi 2 , (Mo 0.9 Ti 0.1 ) Si 2 , (Mo 0.8 Ti 0.2 )Si 2 , (Mo 0.7 Ti 0.3 )Si 2 , (Mo 0.6 Ti 0.4 )Si 2 , (Mo 0.5 Ti 0.5 ) Si 2 and (Mo 0.4 Ti 0.6 )Si 2 powder compacts.The combustion synthesis reaction was ignited locally at the contact point of compact with the ignition coil, and it quickly spreads to the entire specimen section in a spiral way.It was found that the spinning combustion wave propagated.The flame−front propagation velocity of combustion wave reduced with the increase of the Ti content to MoSi 2 .The specimens (Mo 0.6 Ti 0.4 )Si 2 and (Mo 0.5 Ti 0.5 )Si 2 needed constant energy supply to keep a self-sustaining reaction.The specimen (Mo 0.4 Ti 0.6 )Si 2 did not ignite at room temperature.The time duration of combustion was prolonged and combustion front was gradually broadened with the Ti content.The crystal structure of products of the specimens with the Ti addition consisted of Cll b (MoSi 2 )+ C40((Mo,Ti)Si 2 ) type phases.The XRD peaks of Cll b phase were gradually weakened, while those of C40 phase were gradually enhanced with the increase of Ti.Alloying with the Ti opens up the way to synthesize light Mo based silicides for high-temperature structural applications.

Figure 1 .
Figure 1.Schematic diagram of experimental combustion synthesis setup.

Figure 2 .
Figure 2. Recorded typical, distinct and representative combustion images of the powder compacts with the nominal composition of (Mo 0.7 Ti 0.3 )Si 2 (3 # ).

Figure 3 .
Figure 3.The typical combustion temperature−time curve of MoSi 2 specimen.

Figure 5 .
Figure 5.The experimental maximum combustion temperatures of the six different molar ratio samples.

Figure 6 .
Figure 6.Variation of flame−front velocity of powder compacts with different starting nominal composition.

Figure 7 .
Figure 7. X−ray diffractions patterns of the combustion products with different starting nominal composition.
), the intensity of the diffraction peaks of Cll b -MoSi 2 phase decreases and increases C40-(Mo , Ti)Si 2 phase.It is difficult to identify the XRD peaks of Cll b -MoSi 2 phase in (Mo 0.5 Ti 0.5 )Si 2 specimen, and C40-(Mo , Ti)Si 2 is the major phase.It can be seen from Figure 7a-e, there is a systematic shift of the XRD peaks of C11 b −type MoSi 2 to lower angles with the increase in the Ti content.It indicates that Ti has made solid solution with MoSi 2 , and Cll b −type (Mo , Ti)Si 2 is formed.It may be due to the enhanced solid solubility of Ti in MoSi 2 .

Table 1 .
The thermodynamics and adiabatic temperature data of MoSi 2 and TiSi2.

Table 2 .
The density, crystal structure and lattice parameters of MoSi 2 and (Mo,Ti)Si2.