In Situ Synthesis of Titanium Carbide in Pure Aluminium

The present work reports on the mechanism of formation of TiC in pure Aluminium melt. A halide salt of Al3TiF6, graphite powder and pure Al were used to prepare in situ Al-TiC Metal Matrix Composites (MMCs). Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were used to determine the microstructural characteristics of the composite. XRD was further used to determine the phases involved in the composite for confirming the formation of TiC. Further to assess the mechanism involved in the in situ reactions, DTA/TGA thermograms were used to analyze the reactions between molten Al, halide salt and graphite powder. Tensile tests were conducted to study the fracture behavior of the in situ prepared MMCs.


Introduction
With the ever increasing commercial applications of particulate reinforced composites, studies are carried out to find prospective ways of different processing methods of composites [1]. During processing such composites, a number of ceramic phases can be included as particulate reinforcements in Aluminium Metal Matrix Composites (AMCs) [2]. Some of these include Al 2 O 3 , TiO 2 , SiC, B 4 C, BN, TiB 2 , Si 3 N 4 , AlN. A fine and ultra-grain structure usually results in improved mechanical properties and owing to the grain refining efficiency of Al-TiC, a number of researchers have studied on different techniques to synthesize the composite [3] [4].
Titanium Carbide (TiC) particulates offer good wettability with Aluminium.
The other reasons to consider TiC are due to its high temperature stability, high hardness, low weight and low chemical reactivity [5]. Among the several me-How to cite this paper: Boppana thods of preparing TiC reinforced composite, like liquid-solid reactions, liquid-liquid reactions, reaction gas injection, exothermic dispersion and self propagating high-temperature synthesis, the in situ method of preparing the composite is becoming popular due to homogeneous distribution of reinforcement throughout the matrix [6]. TiC reinforced-aluminium composites were also produced by adding elemental carbon to Al-Ti molten alloy, while observing that a temperature of 1200˚C was responsible for the formation of Al 3 Ti particles along with TiC. However, increasing the temperatures by 100˚C would reveal only TiC phases [7]. Al-TiC composite was also fabricated through powder metallurgy route by using elemental powders of Ti, C and Al through compaction.
Spherical TiC particles were observed in the matrix; however Al 3 Ti phases were also seen [8].
Yucel Birol [9] worked on the thermal response of composites prepared by using Al-Ti and graphite powder through milling and blending. Differential Thermal Analyzer (DTA) scans on the composites exhibited endothermic and exothermic peaks. The X-ray Diffractometer (XRD) patterns had also shown the presence of Al 3 Ti and Al 4 C 3 phases and could have also influenced the thermograms.
The mechanical behavior of TiC reinforced aluminium based metal matrix composites processed with the addition of TiC particles in molten Aluminium (99.7% Al) was investigated by A. E. Karantzalis et al. [10]. Some of the castings were extruded to rods, while others were used in as-cast condition. The Ultimate Tensile Strength (UTS) of the specimen increased with the increasing reinforcement content and attributed to grain refining process, while X.C. Tong [11] investigated on the tensile fracture surfaces of Al-TiC composites processed through ingot metallurgy by melting a mixture of Al, Ti and graphite powder of the size of 45 microns in an induction furnace under a cover of argon gas. A ductile mode of fracture was observed during the tensile tests while the strengthening mechanism was affirmed to the micromechanics approach.
Several researchers have been investigating to know the microstructure and property particularly at the interface site [12]. Since the Al-Ti-C system is gaining popularity due to its relevance as grain refiners, researchers have been trying to continuously predict the reactions occurring through ternary phase diagrams [13] [14] [15] [16] [17], but very little has been analyzed on the formation of TiC at higher temperatures. Hence an attempt has been made to study the effect of temperature on the in situ formation of reinforced particle.

Preparation of Composites
The related work involves preparing Al-TiC MMCs using an in situ technique. A measured quantity of K 2 TiF 6 halide salt and graphite powder, according to a known stoichiometric ratio was taken in an Aluminium foil and preheated to drive off any moisture present, using a muffle furnace at a temperature of 300˚C.

S. B. Boppana Journal of Materials Science and Chemical Engineering
Pure Aluminium in the form of ingots were pickled to remove impurities present using NaOH solution and further cleaned by using nitric acid and methanol. A calculated amount of ingots corresponding to Al-5 wt% TiC were then placed in a SiC crucible and melted at 660˚C and super heated to reach 800˚C. Halide salt and graphite powder were then added into the melt. A chromel-alumel thermo couple was used to control the temperature digitally with an accuracy of ±5˚C. A cover flux was poured into the crucible to prevent oxidation of liquid aluminium. Hexachloro ethane tablets were then added to the melt, to remove hydrogen gas formed during the reaction. The melt is carefully stirred at regular intervals with a graphite rod until the reaction completes for 30 min. The spent salt is decanted from the surface of the melt, and the melt is finally poured into a preheated permanent mould. The molten material was poured into the dies at temperatures of 800˚C and 930˚C. After solidification, the castings were removed.

Thermal Analysis
The composite was subjected to Perkin-Elmer Simultaneous Thermal Analyzer (TGA/DTA) cell. The scan rate in the unit was fixed at 10˚C/min and the gas flow rate was fixed at 80 ml/min. The area under the DTA peak is chosen to calculate the enthalpy change for finding out the transformations that have taken place during melting. The weight changes of the sample were recorded during the test and the TGA thermograms were recorded.

Microstructural Characterization
A specimen of the size of about 8mm was cut from the bottom of the casting using hack saw machine. Using a series of rough and smooth files, it was polished followed by working on a belt grinder, to remove burrs if any. Polishing was again carried out using emery sheets of increasing fineness. Finally a disc polisher with a velvet cloth and Al 2 O 3 solution was used in order to get a mirror finish.
The specimen was etched with Kellers reagent after being washed with distilled water. Image Analyzer, SEM equipped with EDS and X-Ray Diffractometer were used to study the microstructure.

Tensile Test
In order to calculate the ultimate tensile strength and elongation, a 40 ton capacity servo controlled universal testing machine was used. The tensile specimen was prepared to follow the ASTM E8 standards. An extensometer was attached coinciding with the axis of the specimen. Load was applied gradually while simultaneously noting down load and strain at room temperature.

DTA/TGA Analysis
The thermogram (Figure 1) shows two exothermic peaks starting at 657˚C and ending at 670˚C while another peak starting at 789˚C and ending at 795˚C. The two peaks reveal the melting of pure Aluminium ingots and simultaneous formation of Al 3 Ti and TiC phases.
The mass of the sample while heating is recorded through a TGA curve. From Figure 2, it is observed that the curve starts to move in the downward direction beyond 670˚C, indicating melting of pure Aluminium, while at 795˚C, formation of Al 3 Ti and TiC phases takes place. The DTA and TGA curves relatively give the same data for the formation of in situ TiC.
Further after that, the curve gradually keeps coming down as the composite is heated with respect to time, revealing that the mass of the sample decreases with increasing temperature.

Microstructures of the Samples
The optical micrograph obtained through an Image Analyzer of the composite in Figure 3, reveals the presence of plate like Al 3 Ti particles throughout the matrix. However grayish dark spots were also seen here and there in the matrix.   The black spots that were found, might be due to excess carbon left out after reacting with Al 3 Ti particles. A reaction temperature of 800˚C might not have been sufficient for the complete reaction to take place and hence Al 3 Ti particles are seen in more numbers. Upon increasing the temperature to 930˚C, almost all the aluminide particles converted to TiC. Figure 4 clearly indicates the spherical TiC particles scattered in the matrix. Henceforth, the composite processed at a reaction temperature of 930˚C was only considered for further characterization.
In order to further investigate about the reinforcement at a magnification of 1000×, Scanning Electron Microscopy equipped with EDS was used. Figure 5 shows a TiC particle in a SEM micrograph, which is confirmed using Energy Dispersive Spectroscopy for elemental analysis through Figure 6.
In order to find out the phases involved, X-ray powder diffraction analytical technique was used. The peaks from Figure 7, suggested the cubic structure of Aluminum and traces of TiC throughout the matrix.

Tensile Strength and Fracture Analysis
The Ultimate Tensile Strength (UTS) was recorded at 132 MPa while elongation was 9%. The increase in UTS compared to pure Aluminium is attributed to the presence of hard and dense intermetallic phases of TiC. Titanium acts as a grain refiner for the melt and promotes heterogeneous nucleation. Other reason for increase in strength is due to solid solution strengthening [8]. The sliding of grain boundary is altered by the presence of intermetallic phases, hence obstructs the dislocation movement. The specimen showed a ductile fracture. It is seen clearly from the micrographs ( Figure 8 & Figure 9) that there is no disbonding between TiC and matrix. The fracture could have occurred through nucleation, growth and propagation [11]. Figure 8 shows the micrograph of fractograhic sample at the point of necking. A small number of TiC particles are observed to be holding back and clinging in the dimples produced during fracture. Due to the grain refinement by Ti particles, the particle size decreases. It is well known that as the particle size decreases, ductility always increases. From the micrograph (Figure 9) we are able to observe fine particles of TiC, which again supports ductile fracture. The size of reinforcement greatly affects the fracture mechanism. During the application of tensile load, when the specimen is about to fracture through the formation of pores, it is observed that the pore height grows in the direction of applied load. All the neighboring interconnecting pores start to grow in the same direction, thereby leading to interconnection between a number of pores. Again, this is clearly evident in the micrograph (Figure 9) with a number of adjacent pores leading to ductile failure and thereby reducing sudden failure. Journal of Materials Science and Chemical Engineering

Conclusions
 Al-5TiC composites were successfully prepared by using a halide salt, graphite powder and pure Aluminium.