G.K. Dey

Microstructure and properties of a Cu-Cr-Zr alloy

Abstract Copper-based dilute Cu–Cr–Zr alloys and their minor modifications, because of their excellent thermal conductivity, strength and fatigue resistance, are commonly used in an aged condition in heat transfer elements. However, in comparison to dilute Cu–Cr binary alloys in which the precipitation of chromium has been studied extensively in the last two decades, attempts at delineating the morphology, composition and crystallography of precipitates in Cu–Cr–Zr alloys have been few and only partially successful. The role of zirconium has also remained largely unresolved. In the present work, the precipitation in an alloy having a nominal composition of Cu–1 wt% Cr–0.1 wt% Zr has been shown to take place through the formation of a metastable ordered fcc phase. Also, the improvement in fatigue resistance due to the addition of zirconium has been ascribed to lowering of the stacking fault energy (SFE) of the alloy.

Evolution of microstructure during fabrication of Zr-2.5 Wt pct Nb alloy pressure tubes

Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step, have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated α+ βI microstructure is required for obtaining the desired level of strength at 310 °C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.

Microwave sintering of zirconia ceramics

The potential of a microwave heating technique for the sintering of 3Y-TZP ceramics is demonstrated. High density samples were obtained by short duration firing in a domestic microwave oven. The ultrafine and monomodal size distribution of grains resulting from the process has important implications in terms of mechanical properties. The hardness and fracture toughness values compare very well with long duration, conventionally fired 3Y-TZP ceramics.

Precipitation in a Cu-Cr-Zr alloy

Transmission electron microscopy (TEM) investigations of the early stages of precipitation of a b.c.c. phase in f.c.c. matrix have been carried out in a dilute Cu-Cr-Zr alloy. The initial product of decomposition, characterized by a state of order, which mimics a cube-on-cube orientation relationship (OR) with the matrix appears prior to ordered b.c.c. phase precipitates showing Nishiyama-Wassermann (N-W) OR.

Explaining absence of texture development in cold rolled two-phase Zr–2.5 wt% Nb alloy

Abstract In the present study, two distinct starting microstructures of Zr–2.5 wt% Nb have been used: (1) single-phase α hcp martensitic structure; and (2) two-phase, 10% bcc β and rest hcp α, Widmanstatten structure. In the second case, two types of α were present—near grain boundary predominantly single-phase α (about 5% of the total α) and α plates in an apparently continuous β matrix. Both (1) and (2) had similar starting crystallographic texture of the hcp α phase and were deformed by unidirectional and cross rolling. In the two-phase structure the changes in the bulk texture on cold rolling was found to be insignificant, while in the single-phase material noticeable textural changes were observed. Taylor type deformation texture models predicted textural changes in single-phase structure but failed to predict the observed lack of textural development in the two-phase material. Microtexture observations showed that α plates remained approximately single crystalline after cold rolling, while the β matrix underwent significant orientational changes. Relative hardening, estimated by X-ray peak broadening, was observed mainly in β phase; while aspect ratio of α plates remained unchanged with cold rolling—indicating absence of effective macroscopic strain in the hcp α plates. Based on microstructural and microtextural observations, a simple model is proposed in which the plastic flow is mainly confined to the β matrix within which the α plates are subjected to ‘in-plane rigid body rotation’. The model explains the observed lack of textural developments in the two-phase structure.

Characterization of hot deformation behaviour of Zr---2.5Nb---0.5Cu using processing maps

The characteristics of hot deformation of beta-quenched Zr-2.5Nb-0.5Cu in the temperature range 650-1050 degrees C and in the strain rate range 0.001-100 s(-1) have been studied using hot compression testing. For this study, the approach of processing maps has been adopted and their interpretation done using the Dynamic Materials Model. The efficiency of power dissipation given by [2m/(m + 1)], where m is strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. The processing map for Zr-2.5Nb-0.5Cu within (alpha + beta) phase field showed a domain of dynamic recrystallization, occurring by shearing of alpha-platelets followed by spheroidization, with a peak efficiency of 48% at 750 degrees C and 0.001 s(-1). The stress-strain curves in this domain had features of continuous flow softening and all these are similar to that in Zr-2.5Nb alloy. In the beta-phase field, a second domain with a peak efficiency of 47% occurred at 1050 degrees C and 0.001 s(-1) and this domain is correlated with the superplasticity of beta-phase. The beta-deformation characteristics of this alloy are similar to that observed in pure beta-zirconium with large grain size. Analysis of flow instabilities using a continuum criterion revealed that the Zr-2.5Nb-0.5Cu exhibits flow localization at temperatures higher than 800 degrees C and strain rates higher than about 30 s(-1) and that the addition of copper to Zr-2.5Nb reduces its susceptibility to flow instability, particularly in the (alpha + beta) phase field.

Effect of chromium addition on the ordering behaviour of Ni–Mo alloy: experimental results vs. electronic structure calculations

Abstract The ordering behaviour of a Ni-Mo alloy in the presence of a ternary additive, viz. Cr, has been studied. The sequence of ordering transformations in binary Ni–Mo alloys has been shown earlier to be controlled by a competition between several f.c.c.-based superlattices, viz. Ni 2 Mo (Pt 2 Mo type), Ni 3 Mo (DO 22 ), Ni 4 Mo (D1 a ) and the so-called short range ordered (SRO) structure characterized by the presence of diffraction intensity maxima at the 1 1 / 2 0 f.c.c. positions in reciprocal space. The effect of ternary addition of chromium in the selection of the superlattice structure has been examined in this paper in an alloy of composition Ni–24 at.% Mo–6 at.% Cr. The presence of Cr has been experimentally found to favour the formation of Ni 2 (Mo,Cr) (Pt 2 Mo-type) phase in preference to the Ni 3 Mo (DO 22 ) and Ni 4 Mo (D1 a ) superlattices. This leads to a sequence of transformation different from that obtained in binary alloys. The effect of Cr on the ground state phase stability is determined, in this alloy, using the first-principles local-density based full-potential augmented plane wave (FP-LAPW) method. From the differences in the electronic structures, densities of states and total energies of binary Ni 2 Cr, Ni 2 Mo (Pt 2 Mo type), Ni 3 Mo (DO 22 ) and Ni 4 Mo (D1 a ) phases and the corresponding ternary superlattice structures, an attempt has been made to predict the hierarchy of relative phase stabilities. Theoretical predictions based on these electronic structure calculations have been found to be consistent with the experimental microstructural observations of the evolutionary stages of ordering in the ternary Ni–24 at.% Mo–6 at.% Cr alloy.

Metastability of the β-phase in Zr-rich Zr-Nb alloys

Abstract The decomposition of the β-phase in Zr-rich Zr-Nb alloys by three processes viz., ω formation, α-formation and hydride precipitation has been examined. In the Zr-20Nb alloy, ω-formation has been examined after thermal treatment as well as after electron irradiation and a comparison has been made between the kinetics of ω-phase formation under these two conditions. The morphology of the α-precipitates and their internal structures has been found to depend upon the type of thermal treatment with step quenching from the β-phase field leading to an allotriomorphic morphology and quenching and aging leading to internally twinned Widmanstatten α. The different morphologies obtained due to change in thermal treatment and composition of the Zr Nb alloys has been rationalized. Hydride formation has been examined in α-Zr, β-Zr and in α + β microstructures. A comparison has been made between the mechanism of formation of hydride phase in these three types of microstructures and their morphology and internal structures have been explained.

Decomposition of the β-Phase in Zr-20%Nb

Abstract Decomposition of the β-phase in the Zr-20%Nb alloy for different thermal treatments was studied. The grains of the high temperature β-phase which could be fully retained on quenching showed a distribution of plate-like features in electropolished or chemically etched samples. These plates were identified to be of the γ-hydride phase. The crystallography and the mechanism of formation of γ-hydride in the β-phase have been discussed. The formation of the ω-phase during isothermal ageing at temperatures between 548 K and 698 K was studied. The appearance of ω-particles of cuboidal shape in periodic arrays along β directions and the pronounced tendency of alignment of clusters of ω-particles of the same variant have been explained in terms of a possible spinodal decomposition in the β-phase prior to ω-precipitation and the strain energy minimization tendency. Precipitation of α-plates was studied in two set of samples given the following thermal treatments: (i) quenched from 1173 K to 300 K followed by ageing at 773 K and (ii) rapidly cooled from 1173 K to 773 K and isothermally held. The former set showed a more homogeneous distribution of widmanstatten a-plates while in the latter, a majority of α-plates grew from the grain boundaries. Copious nucleation in the former set has been attributed to the presence of athermal co-particles or ω-like defects in samples quenched to 300 K. The periodic array of internal twins and the {334} β habit plane associated with the α-plates in the former set suggested the possibility of the operation of a shear in the α-precipitation process.

On the sequence of clustering and ordering in a meltspun Cu–Ti alloy

Abstract Although it is well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu–Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573–723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N 3 M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu 4 Ti (D1 a ).