J. Subrahmanyam

Self-propagating high-temperature synthesis

Self-propagating high-temperature synthesis (SHS) and processes based on SHS are currently being developed the world over for the production of powders and near-net shape components of advanced materials. The research activities that have been and are being carried out in this field are reviewed here. Theoretical principles underlying SHS process, such as equlibrium computation and kinetics involving heat and mass transfer are described. General concepts about the SHS reaction mechanisms with a few illustrative examples are presented. Along with a detailed description of the processing techniques such as powder production,in situ consolidation and casting, a few of the novel techniques based on SHS are also elaborated.

STUDIES ON THE FORMATION OF BLACK PARTICLES IN RICE HUSK SILICA ASH

Abstract The formation of black (carbon fixed) particles in rice husk silica ash has been studied in detail. The formation of black particles in the silica obtained by calcination of raw untreated rice husks, is higher than that in acid treated rice husks. The tendency to form black particles increases with increase in the heating rate and the temperature of calcination of the untreated rice husks. Potassium has been shown to cause the formation of black particles in rice husk silica. By treating with 3.0 N HCl acid the formation of black particles can be avoided. There is no effect of heating rate on the formation of black particles in silica from acid treated rice husks. Treatment with HCl acid was found to decrease the oxidation (burning) of carbon at lower temperature (400°C ).

Cyclic oxidation of aluminized Ti-14Al-24Nb alloy

Titanium aluminides are considered as replacements for superalloys in applications in gas turbine engines because of their outstanding properties. Ti3Al has a superior creep strength up to 815° C, but has poor oxidation resistance above 650° C. Two approaches can be followed to improve the oxidation resistance of Ti3Al above 650° C. One is alloying and the other obtaining a protective surface coating. Niobium was found to improve the oxidation resistance, when added as an alloying element. Recent investigations showed that a TiAl3 surface layer considerably improves the oxidation resistance of titanium. In the present work, a TiAl3 layer was obtained on a Ti-14Al-24Nb (wt%) alloy using a pack aluminizing process. The cyclic oxidation behaviour of aluminized and uncoated samples was evaluated.

Combustion synthesis of MoSi2WSi2 alloys

Abstract Thermochemical calculations were carried out for the self-propagating high temperature synthesis (SHS) of MoSi 2 WSi 2 alloys. Adiabatic temperatures and amounts of liquid phase of MoSi 2 and WSi 2 formed were calculated at various initial temperatures for the reactions forming MoSi 2 , WSi 2 and their alloys. These alloys were prepared using elemental powders of Mo, W and Si by the thermal explosion mode of SHS. The products were characterized by X-ray diffraction and scanning electron microscopy. The morphologies of the alloys are explained with the help of the thermomechanical calculations.

Formation of SiC from rice husk silica-carbon black mixture: Effect of rapid heating

Formation of SiC from rice husk silica and carbon black mixture with Co catalyst, and without any catalyst, has been studied by rapid heating to 1300–1600 °C. Formation of SiC in CoCl2 treated mixture was rapid at and above 1400 °C. However, in the untreated mixture considerable quantities of SiC has formed only above 1500 °C. Total SiC content and SiC whisker yield in the treated mixture were higher than that in the untreated mixture.

Formation of TiN whiskers through carbothermal reduction of TiO2

Formation of TiN whiskers through carbothermal reduction of TiO2 in nitrogen was studied. Effect of (i) addition of potassium (K2CO3) and nickel (NiCl2), (ii) reaction temperature on the formation of whiskers was studied. Addition of K2CO3 has strong influence on the formation whiskers. The yield of whisker was maximum at 920–1000°C. At higher temperatures, formation of particulates of TiN was the dominant process. Increase in K2CO3 concentration increased the formation of (i) whiskers at low temperatures 815°–920°C, (ii) particulates at high temperatures 1000°–1100°C. A vapor-liquid-solid growth mechanism of whisker formation was identified. K2CO3 has been identified as an essential constituent for the formation of a low melting liquid by reaction with TiO2 and NiCl2. Continuous supply of TiO, nitrogen and CO to this complex K-Ni-Ti liquid droplet lead to the precipitation of TiN whisker.

High temperature C/C–SiC composite by liquid silicon infiltration: a literature review

The ceramic matrix carbon fibre (CMC) reinforced composite has received great attention for use in aerospace engineering. In aerospace, the atmosphere is highly oxidative and experiences very high temperature. In addition to this, the materials require high thermal stability and high abrasion resistance in that atmosphere. The C/C–SiC composite meets with these requirements. In this paper, the C/C–SiC composite by liquid silicon infiltration is reviewed thoroughly.

Synthesis of TiC Whiskers through Carbothermal Reduction of TiO2

TiC whiskers were produced through carbothermal reduction of TiO2 in the presence of potassium (K2CO3) and nickel (NiCl2). The effect of potassium, nickel, and heating rate on the formation of whiskers was studied. Potassium was found to be an essential constituent for whisker formation. Nickel acts as a catalyst for TiC whisker formation only in the presence of potassium. The yield of whiskers was maximum at 1000–1200°C. At higher temperatures, formation of particulates of TiC was the dominant process. An increase in K2CO3 concentration during fast heating and decrease in K2CO3 concentration during slow heating was found to be beneficial in increasing the formation of TiC whiskers. A vapor–liquid–solid growth mechanism of whisker formation was explained.

Studies on the formation of whiskers and platelets of B4C and BN

The formation of whiskers and platelets of B4C and BN has been studied through carbothermal reduction of B2O3. In the absence of any additive, neither whiskers nor platelets have formed from B2O3 and carbon black. K2CO3 which forms a low melting liquid and NiCl2 which act as catalyst in gasification of carbon were used to facilitate the growth of whiskers and platelets. NiCl2, K2CO3, carbon black and B2O3 were reacted in a weight ratio (NiCl2:K2CO3:C:B2O3= 5:5:12:17.4) and studied the formation of B4C and BN in the temperature range of 940°C to 1500°C in 1-atm. argon and 1-atm. nitrogen respectively. Whiskers and platelets of different sizes have formed at 1100–1500°C. The whiskers have been observed to form by vapor-liquid-solid growth mechanism. The effect of NiCl2 and K2CO3 on the morphology of B4C and carbon has been studied. NiCl2 and K2CO3 have been found to accelerate the growth of whiskers and platelets.