Srikumar Banerjee

Application of free energy-composition diagrams in predicting the sequences of phase transformations in Zr-Nb alloys

The free energy composition diagrams for the Zr-Nb alloy system have been computed from the phase diagram employing Rudman’s technique of phase diagram analysis. The influence of the clustering tendency in the β phase on the sequence of phase transformations in both Zr-rich and Nb (Cb)-rich alloys has been examined. It has been possible to predict different sequences of phase reactions in different ranges of composition and temperature. The experimentally observed phase reactions as reported earlier have been rationalized by the calculated free energy composition plots.

ACTIVE EUTECTOID DECOMPOSITION IN Zr-3 wt.% Fe

The eutectoid reaction is exhibited by many binary Zr and Ti base alloys. In many of these systems, the eutectoid compositions correspond to low levels of solute concentration, implying a large equilibrium volume fraction of eutectoid product, which consists of a mixture of the a (h.c.p.) phase and an appropriate intermetallic phase. In some of these eutectoid systems, called “active” eutectoid systems, the decomposition of the b (b.c.c) phase into a lamellar aggregate of two phases cannot be suppressed, in alloys of near eutectoid composition, even on rapid b quenching. Some of these alloy systems are: Ti-Cu, Ti-Ni, Zr-Ni, Zr-Cu and Zr-Fe [1–6]. It has been suggested that the active eutectoid decomposition would occur when the eutectoid temperature is high, the eutectoid composition is solute lean and the intermetallic phase resulting from the reaction is rich in the base metal. The non-equilibrium two phase aggregate formed on quenching often comprises lamellae of the a phase and of a metastable phase which is structurally similar but not identical to the appropriate equilibrium intermetallic phase. On subsequent aging at temperatures below the eutectoid temperature, a cellular reaction may occur and the fine lamellar structure may be replaced by a coarser structure. This reaction may be the result of incomplete partitioning of the solute during the rapid eutectoid reaction and also of the tendency to minimize the interfacial energy by decreasing the interlamellar interfacial area [7]. Active eutectoid decomposition and martensite formation can be competitive processes in these alloys [4]. For compositions away from the eutectoid composition a large supercooling may generally be needed to initiate active eutectoid decomposition and this supercooling may be sufficient to bring about martensitic transformation as well. In near eutectoid alloys, however, the degree of supercooling needed for triggering active eutectoid decomposition is generally too small to induce martensitic transformation. The eutectoid decomposition can occur rapidly because the diffusion distances are small, interface diffusion can occur and solute partitioning can be incomplete. The Zr-rich side of the binary Zr-Fe phase diagram shows the eutectoid reaction: b 3 a 1 Zr3Fe. The eutectoid composition is 4 at.%Fe (;2.5 wt.%Fe) and the eutectoid temperature is 730°C. The Zr3Fe phase, which is the intermetallic phase richest in Zr in this system, has a base centred orthorhomic structure (Re3B type structure, space group Cmcm), with a 5 0.332 nm, b 5 1.095 nm and c 5 0.881 nm [8,9]. The next Zr enriched intermetallic phase in this system is the Zr2Fe phase which has a Al2Cu type structure which is body centred tetragonal with a 5 0.636 nm and c 5 0.582 nm [8,9]. In the work reported here, the microstructural features associated with the active eutectoid decomposition of a near eutectoid alloy (Zr-3 wt.%Fe) on b quenching were examined. The effects of aging after b quenching, and of furnace cooling from the b phase field, on the microstructure of the alloy were also studied. The microstructural characterization was carried out using optical microscopy, scanning Pergamon Scripta Materialia, Vol. 40, No. 6, pp. 723–728, 1999 Elsevier Science Ltd Copyright

Active eutectoid decomposition in a near-eutectoid zirconium-copper alloy

The decomposition of the beta-phase into the alpha- and the Zr2Cu-phases has been investigated in a Zr-1.6 wt pct Cu alloy. It has been observed that the decomposition process is extremely rapid and cannot be suppressed even by rapid beta quenching. The product structure obtained on fast cooling consists of a lamellar aggregate of the alpha-phase and a metastable phase which does not have the stoichiometry of the Zr2Cu-phase. Structurewise, this phase is similar but not identical to the equilibrium phase. A mechanism for this rapid decomposition of the beta-phase has been proposed. It has also been found that the equilibrium Zr2Cu-phase emerges after short aging treatments at sufficiently high temperatures in the α+Zr2Cu field.

The effect of sintering atmosphere on the dielectric constant and density of PMN ceramics

Lead magnesium niobate (PMN) is a ferroelectric compound with relaxor characteristics. PMN powder prepared by semi-wet hydroxide route was compacted and sintered at 1000 C in air and PbO atmosphere. The loss of PbO, during sintering, leads to the formation of pyrochlore phase which lowers the dielectric constant and density of PMN ceramics. The effect of sintering atmosphere on the dielectric properties and the density of PMN ceramics is reported in this study.

Effect of Manganese Oxide Doping on the Dielectric Properties of Lead Iron Niobate Ceramics

Lead Iron Niobate (PFN) powder, prepared by semi-wet-hydroxide route, was doped with different concentrations of Mn2O3. The powders were compacted into pellets and these were then sintered at 1000°C in PbO atmosphere. The dielectric constant and loss tangent of these ceramics were measured in the temperature range, 25°-130°C. The effects of different concentrations of Mn2O3 doping on the microstructure, dielectric constant, loss tangent and Curie temperature of PFN are reported in this study.

Nanostructural Studies on Lead Magnesium Niobate and Lead Iron Niobate by TEM

PMN and PFN are very interesting ferroelectric ceramics finding many useful applications in multilayer ceramic capacitors and electrostrictive devices. These ceramics prepared by semi-wet hydroxide route were observed by TEM for their nanostructural features such as ordering-disordering, grain boundary phase etc.

Consolidation of YBaCuO powders by shock loading

Superconducting oxide-copper monoliths have been fabricated by shock-wave loading using cylindrical and plane geometries. Bulk densities up to 96% T.D. have been obtained by varying detonation parameters. The superconducting properties, interface bonding and microstructure of the compacts have been evaluated.