The back scattered electron (BSE) image of different samples S1 to S5 are shown inFig. 1 It was observed that sample S1was having highest density having compact structure, while the compactness decreased in sample-S2. Sample-S3 had larger grains and lesser compaction of grains. Also, the grain size of sample S1was very less in comparison to that of sample-S2 and sample S3. Sample-S4 is having a veryfine grained structure with good compaction, but lesser than sample-S1. Sample-S5 was found to have highly porous structure. Grains were though smaller but they are not properly sintered as a result of which the porosity is very high.
Fig. 1. SEM micrographs of polished samples S1, S2, S3, S4 and S5.
Fracture surface micrograph reveals the grain size and densification of all the samples (Fig. 2). Similar to polished surface micrograph, here again increase of grain size from sample S1 to S3 was observed. Sample-S4 showed the least grain size of the order of 1–2 μm and it was maximum for sample-S3 about 7–8 μm. The grain size variation may be attributed to the combustion temperature and percentage titanium silicide formed. The adiabatic temperature was maximum for sample-S3 i.e. 3140 K and minimum for sample-S5 with 2144 K. Though the adiabatic temperatures are theoretical calculated and for combustion to proceed the actual combustion temperature may become around 200–250 K lesser than calculated ones due to different environmental and porosity in the sample. In addition, the combustion temperature measurement during in-situ reaction and densification was not possible as it will need thermocouple insertion and leading to stresses in the composite. The high temperature is responsible for grain growth and thus grain size of sample-S3 is highest. The plot of average grain size calculated by measuring statistically from SEM micrograph with the increase of adiabatic temperature is shown in Fig. 3. As the adiabatic temperature decreases, the grain size also decreases, it is due to the fact that with decrease of temperature the mass transfer phenomenon will be slower and hence the grain growth. From the phase diagram of Ti-Si, and Ti-C it is known that at a temperature greater than 2200 K, no phase of titanium silicide exists in solid state. All the phases TiSi, TiSi2,T i5Si4,T i5Si3,T i3Si have low melting point for any composition taken So during reaction they remain in liquid state in the above reactions. But, in phase diagram of Ti-C, TiC does not go into liquid state until the temperature rises to around 3300 K. Thus, the melt phase consists of titanium silicide phase and the solid grains consist of TiC. The liquid phase formation though was at times difficult to ascertain in all cases as SEM observations were done after sample has cooled down. However, the fracture surfaces showed the melted and solidified structure of grains at some places and are marked by arrow in Fig. 2. Such liquid phase formation have also been reported by other researchers.
Fig. 2. Fracture surface SEM micrograph of sample S1, S2, S3, S4 and S5
Fig. 3. Variation of grain size of composite S1–S5 with adiabatic temperature.
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