Combustion synthesis of NiAl/TiC composite from mechanically activated elemental powders

The aim of this study was the synthesis of a homogenous distributed NiAl/TiC composite by combustion synthesis of elemental powders. The effect of the mechanical activation process was evaluated. The phase characterization and evaluation of the microstructure during milling and combustion synthesis were carried on by XRD and SEM-EDS and TEM-EDS, respectively. The XRD result of as-mixed powder compact after combustion synthesis showed that in addition to TiC 0.75 and NiAl, some undesired phases like Ni 3 Al and Ni 2 Al 3 formed. While after combustion synthesis of 3h mechanically activated powder compact, only AlNi and TiC formed. The microstructural evaluations by SEM-EDS and TEM-EDS analysis conrmed the synthesis of truly homogenized NiAl/TiC composite. The mechanism of NiAl/TiC formation consists of dissolution of nickel, titanium, and graphite in molten aluminum and precipitation of TiC primarily and then the formation of NiAl. The value of microhardness of synthesized NiAl/TiC was 1070±12 HV. The main reason for this result is a uniform distribution of spherical TiC reinforcement in the NiAl matrix. Therefore, the mechanical activation process facilitated the reactions and improved the mechanical properties of the nal composite.

Energy and time saving are the main properties of combustion synthesis which makes it become a considerable method to synthesize In-situ composite [1,[23][24][25][26]. The effect of many different parameters like heating rate, green density, etc. on the microstructure and properties of fabricated composite have been evaluated [27][28][29]. According to the author's knowledge, there is no study about the effect of mechanical activation on the synthesis of NiAl/TiC composite from elemental powders.
The mechanical activation is a short duration milling before the combustion synthesis [20,24,30].
Mechanical activation by the ball milling process is the simplest method for elemental and material dispersion. In this process, the structure and properties of powders modify and not only it decreases the crystallite and particle sizes of powders, but also increases the surface area of powders, forms structural defects and porosity. Also, this process destroys layer oxide of the reactant powders such as aluminum powder. All of these effects increase the kinetics of reactions and consequently, more homogeneous powder mixture forms after mechanical activation which raises reactivity and kinetics of reactions during combustion synthesis [8,20,31]. The entire procedure is named mechanical activation combustion synthesis.
Saadati et al. [1], Rahaie et al. [9] and Zhu et al. [27] synthesized NiAl/TiC composite by combustion synthesis of elemental powders. Their results showed that heterogeneous composite with different phases such as Al 3 Ni 2 , AlNi 3, and Al 3 Ni was formed. In this work, the effect of mechanical activation before combustion synthesis of elemental powders on the microstructure features of the synthesized NiAl/TiC composite was studied.

Experimental Procedure
The starting powder mixtures to synthesize NiAl/TiC composite contained nickel (99.99%, -45 µm), titanium (99.99%, -45 µm), aluminum (99.9%, -40 µm), and carbon (graphite) (99.0%, -10 µm). The powders were mixed with1:1:1:1 atomic ratio and mechanically activated by high-energy milling in a stainless steel container by stainless steel balls (different sizes) at 350 rpm speed which took 3 h under argon atmosphere. The balls to the powder mixture weight ratio were 15:1. After mixing and mechanically activation, as-mixed, and 3 h milled powder mixtures were pressed into compacts using a uniaxial force of 50 MPa by stainless steel die, and both compacts were subjected to furnace under vacuum to begin and complete combustion synthesis process. To determine phase development during milling and combustion synthesis, all powder samples were characterized by a Philips X'PERT XRD (x-ray diffraction) equipment with CuK α radiation (1.5404 Å). The morphology and microstructure of powder samples were studied by SEM (scanning electron microscopy, Philips XL30) equipped EDS (energy dispersive spectroscopy) and TEM (transmission electron microscopy, Tecnai G2 F20-200KV) equipped energy dispersive spectroscopy.
The synthesized NiAl/TiC sample was mounted in epoxy resin and polished for the microhardness test was carried out at 0.01N loading for 15 s. At least six tests were performed.

Results And Discussion
The combustion synthesis method consists of two different modes, TE (thermal explosion) which combustion is performed by heating the total compact at the desirable temperature [32,33] and SHS (self-propagating high temperature synthesis) which combustion is performed by heating one side of the compact and propagated to whole it [23,34]. To determine the kind of combustion synthesis mode, it must be calculated adiabatic temperature (T ad ) of reaction takes place during combustion synthesis.
Previous studies [15,25,29] proposed that the T ad must be more than 1800 K to yield SHS mode in a compact.
Eq. (2) can be used to calculate the T ad of reaction (1) with considering short reaction time and no heat exchanges between environment and compact; therefore, total formation heat of nal products can be consumed to raise the temperature of compact [1,28].

(see Equations 1 and 2 in the Supplementary Files)
Where ΔH˚2 98 is the whole formation heat of nal products (reaction 1) at room temperature, and c p is the heat capacity. Based on thermodynamic data and previous studies [1,27], the adiabatic temperature of reaction (1) was about 1911 K which is higher than 1800 K. Consequently, during ignition of as-mixed Ni/Ti/Al/C powder mixture, reaction (1) happens in self-propagating combustion mode.
The XRD spectra of as-mixed powder before and after combustion synthesis are shown in Fig. 1. As can be seen, the main phases of an as-mixed powder mixture are nickel (Reference code: 01-087-0712), titanium (Reference code: 00-044-1294), aluminum (Reference code: 01-085-1327) and graphite (Reference code: 01-075-2078). However, new phases such as TiC x , Ni 3 Al, Ni 2 Al 3 , and NiAl formed after the combustion synthesis of as-mixed powder. The formation of undesired phases like Ni 3 Al and Ni 2 Al 3 has been reported by Saadati et al. [1], Rahaie et al. [9], and Zhu et al. [27].
As reported previously, TiC x with the amount of x between 0.55 and 0.95 is a thermodynamically stable phase [14,23,35]. Based on lattice parameters, the amount of x can be calculated. The lattice parameter of synthesized TiC x is 4.310Ǻ which means that according to Ref. [14], the amount of x is about 0.75.
The atomic ratio of Ni and Al was equal, however, the undesired phases Ni 3 Al, and Ni 2 Al 3 formed. Zhu et al. [27] and Ozdemir et al. [36] reported that during the combustion synthesis of NiAl at T < 1000˚C, the reaction between Al and Ni is a dissolution of Ni in molten Al, with the formation of some intermediate Similar to the proposed mechanism by Curfs et al. [15] in the Ni-Ti-Al-C system, the reaction begins by melting of Al powder (because the selected temperature was 950 ˚C). After that, Ti, C, and Ni dissolved in molten aluminum and TiC x primarily appeared and after a moment, nickel aluminides appeared. Fig. 2 shows an SEM image for an as-mixed Ni-Ti-Al-C powder compact after combustion synthesis. As can be seen, some of the grey TiC x particles have precipitated in nickel aluminides matrix. But, the main characteristic of this microstructure is thick layer structure which EDS analysis con rmed that the white layer is nickel-aluminum rich and grey one is titanium-carbon rich. Due to short reaction time and heterogeneous distribution of elements, precipitation of NiAl and TiC, as one of the main parts of the mechanism, was not complete and therefore, the heterogeneous structure formed. These results (formation of TiC 0.75 and intermediate phases of the Ni-Al system) con rmed the heterogeneous distribution of elemental powders. Similar results have been reported by the previous studies [1,27]. Also, the size of starting powders must be reduced to increase the surface contact area between powders and reduce diffusion paths because the reaction time during combustion synthesis is too short and no enough time exists to elemental diffusion. To overcome this problem, the starting powder mixture was milled for 3 hours and mechanically activated. The mechanical activation of powders reduces crystallite and particle sizes of powders, fractured the oxide layer on the powders (especially Al powder) and increases surface contact area and interfaces between powders. These effects cause a more homogenous distribution of powders. Fig. 3 shows XRD results of 3 h mechanically activated powder mixture before and after combustion synthesis. XRD spectrum of 3 h milled powder shows the formation of no new phases and only the height of diffraction peaks reduced and peak broadening happened. These changes without new phase formation are related to crystallite size reduction and the increase of lattice strain of powders. However, after combustion synthesis of 3 h milled powder compact, new phases NiAl and TiC y formed. The amount of TiC y lattice parameter was 4.316 Ǻ and according to Ref. [14], the amount of y is about 0.95. The TiC 0.95 formation con rmed that the mechanical activation process caused better mixing of the elements and increases the contact of graphite and titanium particles. Therefore, more carbon reacted with titanium and TiC 0.95 formed. Also, no intermediate nickel aluminide phases were formed which these results con rmed that mechanical activation not only caused the homogeneous distribution of powders but also destroyed the oxide layer around Al powders which both of these effects facilitated NiAl-TiC composite formation.
The proposed mechanism is the molten Al distributed around more homogenized and ne nickel, titanium, and graphite powders uniformly and dissolved titanium, nickel, and graphite. Ni and Al reacted and NiAl formed. This reaction released energy and promote the reaction between Ti and graphite and consequently TiC 0.95 formed.
The microstructure of the 3 h milled powder after combustion synthesis was presented in Fig. 4. The matrix (NiAl) phase looks brighter than the TiC as reinforcement. The synthesized TiC is spherical and distributed homogeneously. The TiC boundaries indicate that TiC was formed before the NiAl formation. During the TiC formation, the heat released by this reaction can help further the formation of NiAl. Hence, TiC spherical particles surrounded NiAl. Also, the TEM image of the synthesized NiAl/TiC with chemical analysis in different points is shown in Fig. 5. These results also con rmed that the homogeneous distribution of NiAl and TiC phases was formed.
The hardness of the sole NiAl phase was reported between 330 HV 0.2 (synthesized by casting) [10] and 475 HV 1 (synthesized by sintering PPS) [37]. The addition of TiC (3000 HV) [38] increases the hardness of synthesized NiAl/TiC and Saadati et al. [1] reported that the hardness value of NiAl: TiC 1:1 composite is 838.5 HV. While the value of microhardness of synthesized NiAl/TiC composite in the present work was 1070±12 HV. The main reason for this difference is a uniform distribution of spherical TiC reinforcement in NiAl matrix in the present work. Therefore, the mechanical activation process facilitated the reactions and improved the mechanical properties of nal composite.

Conclusion
In this work, a homogenous NiAl/TiC composite was successfully synthesized after the mechanical activation of elemental powders. The product of combustion synthesis of a sample without mechanical activation was TiC 0.75 and NiAl, some other nickel aluminides such as Ni 2 Al 3 and Ni 3 Al formed. But, after the combustion synthesis of 3 h milled powder compact, no undesired phases were formed and a composite with a homogeneous distribution of NiAl and TiC was synthesized. The mechanical activation process decreased the crystallite and particle sizes of reactant powders which increased surface contact area of reactants and homogeneously distributed elements. The homogeneous distribution of TiC improved the mechanical properties of NiAl/TiC composite. The value of microhardness of synthesized NiAl/TiC composite was 1070 ± 12 HV. Figure 1 XRD patterns of as-mixed Ni-Ti-Al-C powder compact before and after combustion synthesis (CS).

Figure 3
The microstructure of as-mixed powder compact after combustion synthesis.   TEM image of NiAl/TiC composite synthesized after combustion synthesis of 3 h milled powder compact with chemical analysis of its different points.