P. Ramachandrarao

Sintering studies on ultrafine ZrB 2 powder produced by a self-propagating high-temperature synthesis process

A detailed study on the sintering behavior of zirconium diboride powder produced by the self-propagating high-temperature synthesis (SHS) process was carried out in the temperature range of 1500-1800 degreesC. The fine powder prepared by the SHS process exhibited excellent sinterability and could be sintered at 1800 degreesC for 1 h to approximately 94% of the theoretical density. The apparent activation energy of densification in the range of 1500-1800 degreesC was estimated to be 248 +/- 4 kJ mol(-1). A zirconium dioxide layer formed on the surface of the sintered body and was attributed to boron oxide formation during sintering and concurrent surface oxidation by the oxygen generated from the reduction of boron oxide in the carbonaceous atmosphere. Sintering aids like Fe and Cr appeared to help in densification of ZrB2 powder.

A metastable phase Al3Cu2

A new metastable intermediate phase having a trigonal unit cell belonging to the space groupP¯3ml has been detected in a rapidly solidified aluminium-45 at. % copper alloy. The unit cell dimensions area=4.106 andc=5.094 å. With five atoms per unit cell, the observed structure can be regarded as an isotype of Al3Ni2.

Quenching from the mushy state — a new technique for the production of metastable phases

Abstract Liquids entrapped in solids are known to undercool drastically. This principle has been utilized in producing metastable phases in an Al-6.8 at.% Ge alloy. By holding the specimens at temperatures at which solid and liquid coexist and quenching them into water baths, undercooling is induced and two metastable phases with rhombohedral (a = 0.7672 nm and α = 96.55°) and monoclinic (a = 0.6734 nm, b = 0.5818 nm, c = 0.4282 nm and β = 88.96°) unit cells are produced. The metastable phases are identical to those obtained by splat quenching.

Synthesis of Nanosized Copper Powder by an Aqueous Route

Nanostructured, crystalline copper powder was produced at ambient temperature by aqueous reduction of a copper salt by sodium borohydride. Conditions were optimized to produce boron-free copper powder with an average particle diameter of 200 nm, surface area of 5.48 m * m/g, and oxygent content of 0.155%. The effects of different experimental conditions on average particle size of the powder were also studied. The sintering phenomenon exhibited by produced copper particles at 100°C may be attributed to the low oxygen content of the powder along with other size variation-dependent factors.

Thermodynamic and constitutional studies of the PbTe-GeTe system

Some of the uncertainties concerning phase equilibria in the pseudo-binary system PbTe-GeTe have been clarified in this thermodynamic and constitutional investigation. The heats of fusion of PbTe, GeTe and Pb0.2 Ge0.8 Te have been determined in a differential thermal analysis calorimeter. Based on these results and solution thermodynamics, the phase diagram of the system has been synthesized. Evidence for spinodal decomposition in this system has been presented for the first time with the aid of X-ray diffraction as well as transmission and scanning electron-microscopic data. The results are in confirmity with a pseudo-isomorphous system having a solid state miscibility gap.

Rapid solidification and decomposition of a hypomonotectic Al-Cd alloy

Rapid solidification of Al-1.5at % Cd alloy can lead to a wide variety of morphologies. The sequence of the morphologies with increasing foil thickness is single phase → elongated cell → hexagonal cell → dendrite. Monotectic reaction ahead of the solid-liquid interface leads to a unique distribution of the cadmium particles. Models are proposed to explain the genesis of the distribution of these particles. The study of the decomposition behaviour confirms the existence of a preprecipitation stage. The G.P. zones are found to be spherical and lead to a precipitate-free zone near grain/cell boundaries. The coarsening of the precipitates at liquid temperatures takes place by the migration and coalescence of the droplets.

Solidification of undercooled cadmium-zinc eutectic melts

Molten specimens of Cd-Zn eutectic alloy (26.50 at.% Zn) were undercooled by the glass-slag technique using glass-forming ZnCl2 as the slag. The microstructure of the solidified samples, weighing about 20 g each, was found to be a function of the undercooling produced. At lower undercoolings the microstructure consisted of supersaturated primary cadmium dendrites co-existing with the eutectic, while at higher coolings duplex structures were observed. Possible causes for the differences in microstructure are discussed with due importance given to the recorded recalescence effects.

Metastable phase formation and decomposition in a rapidly solidified aluminium-platinum alloy

Abstract The metastable phase formation and decomposition behaviour of a rapidly solidified Al-2 at.% Pt alloy were studied by electron microscopy and X-ray diffraction techniques. Two metastable phase were detected. One of them has a cubic structure with a = 5.67 A and can be considered as a metastable extension of the equilibrium Al2Pt phase. The other phase, of composition Al6Pt, has an orthorhombic structure with lattice parameters a = 15.762 A , b = 12.103 A and c = 8.318 A and is isostructural with the equilibrium phase Ga6Pt. In the as-splat foils, the Al2Pt phase forms at the interdendritic regions whereas Al6Pt forms at the predendritically solidified regions. After prolonged aging the Al6Pt phase grows with the dissolution of the Al2Pt phase. The mechanisms of formation of the Al6Pt phase during artificial aging and natural aging are different. While normal nucleation processes control the decomposition during artificial aging, sympathetic nucleation of Al6Pt is the dominant mode of decomposition in the naturally aged foils and leads to a star-like morphology. The reasons for the formation of the above metastable phases are also discussed.

Free energies of undercooled phases

A novel approach for the estimation of the free energies of metastable liquid and allotropie phases has been presented. The method is based on the Taylor series expansion of the free energy difference between stable and metastable phases. The procedure has been applied to arrive at allotropie transformation temperatures in iron and also the isentropic temperature for any two coexisting phases. Results obtained strongly validate the expressions derived which involve only the use of equilibrium heat capacity and entropy data.

Self-propagating high-temperature synthesis of a zirconium diboride-alumina composite: a dynamic X-ray diffraction study

A time-resolved X-ray diffraction study during the formation of a zirconium diboride (ZrB2)–alumina (Al2O3) composite from zirconium dioxide, Al and boron oxide by self-propagating high-temperature synthesis (SHS) has been carried out. During the formation of the composite, solid solutions of Al in ZrB2 form, with a slight change in the lattice parameters of ZrB2, during the SHS reaction and finally form a ZrB2–Al2O3 composite powder. Dendritic growth of ZrB2 was also observed during the synthesis.