G.K. Dey

Precipitation stages in a 316L austenitic stainless steel

Abstract Detailed studies of precipitation in type 316L (UNS S31603) austenitic stainless steel were carried out by using a combination of differential scanning calorimetry, electrical resistivity measurement and transmission electron microscopy. Four distinct stages of precipitation were identified––coherent precipitation, its coarsening and initiation of grain boundary precipitation, σ phase and finally M 23 C 6 precipitation.

Deformation twinning in zircaloy 2

Abstract Fully recrystallised zircaloy 2 samples were subjected to different degrees of uniaxial compression. Grains of high Taylor factors showed {1012}〈1011〉 deformation twins, noticeable up to 13–16% compression. Twinning strongly affected the crystallographic texture and also brought in clear differences in stored energy and residual stress between the suspected parent and product grains/orientations of twinning. At later stages of deformation, where presence of twinning was insignificant, aforementioned heterogeneity was further supplemented by heterogeneity in microstructure – clear presence of fragmenting and non-fragmenting grains. Direct observations on twin fraction, twin deviation and twin continuity had shown an apparent peak in twinning by ∼7·5% compression, an observation explainable through a simple model of twin decay by in grain misorientation development.

Deformation texture and microtexture developments in a cold rolled single phase hexagonal Zircaloy 2

Abstract A single phase hexagonal close packed Zircaloy 2 was cold deformed to different reductions by laboratory rolling. Systematic characterisations of the structural developments were carried out. Bulk texture developments were gradual, strongest developments being noticed at the highest strain. Although formation of well defined deformation fibre(s) could not be identified, overall developments in deformation texture were best captured through Taylor type models incorporating only prismatic slip. Strain localisations were observed as single or double walled dislocation structures at approximately 45 and 60° to the rolling direction. Such strain localisations were always associated with significant lattice reorientations or misorientation developments. Relative softening in lattice strain, observed at the higher reductions, can possibly be explained by the appearance of extensive strain localisations and the associated concurrent local dynamic recovery. The grains or orientations with dominant presence of strain localisations could be indirectly related to negative textural softening.

Disordered bcc γ-phase to δ-phase transformation in Zr-rich U-Zr alloy

The transformation mechanism of hexagonal delta phase from the disordered bcc gamma phase has not been reported before in the Zr-rich U–Zr alloy system. With the help of X-ray diffraction, transmission electron microscopy (TEM) and high-resolution TEM analyses it was shown that the gamma to delta conversion takes place by the lattice collapse mechanism of omega transformation. It was also ascertained that a higher aging temperature or time promotes the growth of all four variants of the delta phase within a parent gamma grain. In addition, ab initio electronic structure calculations showed that the bcc to hexagonal transformation, involving partial ordering of the parent bcc phase followed by (111) plane collapse, is energetically favorable.

Nanoparticles of uranium oxide occluded in MCM-41 silica host: Influence of synthesis condition on the size and the chemisorption behavior

The paper presents a comparative study on the characteristics of uranium oxide crystallites deposited in the mesopores of MCM-41 by two different methods, one involving the repeated cycles of wet impregnation of template-free MCM-41 with uranyl acetate and the other by way of exchanging the template cations in an as-synthesized host matrix. XPS, DRUV visible and XRD studies revealed that both the preparation procedures resulted in initial binding of uranyl groups with the Si–OH sites of host matrix, a part of which converted to U3O8 particles upon calcination. The size of the U3O8 crystallites, however, depended upon the synthesis conditions. Thus, more uniform and smaller size (<3 nm) particles of uranium oxide were formed in the case of the impregnation-prepared samples, as compared to the crystallites of 3–15 nm size formed using a template-exchanged procedure. Furthermore, the post-impregnation drying of a sample under the conditions of vacuum and room temperature was vital for the uniformity and for the small size of these crystallites. XRD, TEM and N2 sorption results showed that the U-loading resulted in a decrease in the unit cell parameter and also in a decrease in the long range ordering of the host matrix while the mesoporosity was preserved in both kinds of samples. At the same time, whereas the crystallites of U3O8 were largely dispersed within the pore system of MCM-41 in the case of impregnation-prepared samples , a large fraction existed at the external surface of the samples synthesized through template exchange route. This is attributed to the restriction imposed by template cations to the transport of uranyl species into the pore system. The monitoring by in situ IR spectroscopy revealed that the methanol molecules undergo dehydrogenation as a result of room temperature adsorption over uranium oxide crystallites, giving rise to oxymethylene (–OCH2) species as primary products which transform subsequently over larger size crystallites to form polyoxymethylene, i.e. (–OCH2)n, species. Smaller size crystallites of U3O8, on the other hand, promoted the further oxidation of oxymethylene groups resulting thereby in the formation of formate-type complexes.

Immobilization of High Level Nuclear Wastes: The Indian Scenario

Nuclear power stands as an immediate and sustainable soluton for satisfying the emerging energy crisis in India. Successful execution of any national ‘nuclear power program’ is keyed to its effective ‘high level nuclear waste’ management strategy. Towards this, India has recently developed sodium-barium-borosilicate glass matrix to immobilize sulfate containing high level waste. Currently, efforts are underway to explore the possibilities of using the same matrix or its modified versions to condition nuclear wastes likely to be generated from ‘closed thorium fuel cycle’. Apart from conventionally used ‘hot wall induction furnace technology’, India has recently acquired expertise in operations of indegineously developed ‘Joule heated ceramic melter’ and ‘Cold crucible induction melter’ for development of suitable inert glass matrices.

Crystallization of ternary Zr-based glasses—Kinetics and microstructure

The effect of ternary addition on the thermal stability and the sequence and the kinetics of crystallization of metallic glasses Zr 76 Fe (24− x )Ni x ( x = 0, 4, 8, 12, 16, 20, 24) have been examined. It has been found that the surface crystallization occurs in the composition range 16 x 3 Zr cubic phase, followed by the transformation of the bulk to a mixture of α−Zr and Zr 2 Ni. Crystallization of alloys containing 12 to 20% Fe occurs at lower temperatures by primary crystallization of Zr 3 (Fe, Ni), followed by decomposition of the remaining amorphous matrix by eutectic crystallization giving rise to α−Zr + Zr 2 Ni. At higher temperatures these alloys transform polymorphically to Zr 3 (Fe, Ni) in which Ni partially substitutes Fe in the Zr 3 Fe lattice. Copious nucleation of Zr 3 (Fe, Ni) phase in these alloys, leading to the formation of a nanophase structure, has been observed. This is consistent with the prediction of increasing nucleation rate for Fe-rich compositions. The crystal nucleation and growth kinetics have been examined for primary, eutectic, and polymorphic crystallization processes. The observed nucleation and growth behaviors have been rationalized by considering the role of the quenched in nuclei and the activation energies of nucleation and growth.

Ageing characteristics of the metastable gamma phase in U–9 wt.% Mo alloy: experimental observations and thermodynamic validation

Ageing characteristics of the metastable bcc γ-phase in U–9 wt.% Mo alloy, a candidate for high uranium density nuclear fuel for research and test reactors, have been investigated in this study. Analyses of the aged microstructures, employing X-ray diffraction and various microscopy techniques, revealed the decomposition mechanism of the metastable γ-phase to the stable α-U and γ′ (U2Mo) phases. A discontinuous precipitation reaction, leading to the generation of partially transformed cellular colonies with lamellae comprising of either the α-phase or the γ′-phase in γ-phase matrix, was found to be operative. The in situ transformation of γ interlamellar regions to the γ′-phase was noticed occasionally within the γ + α lamellar colonies. Thermodynamic analysis of the U–Mo system, using free energy–composition diagrams, could associate the observed attributes of the decomposition of γ-phase to the extent of Mo segregation and the chemical driving force required for the nucleation of α- and γ′-phases in the γ-matrix.

Microstructural Studies of Heat Treated Zr-2.5Nb Alloy for Pressure Tube Applications

Zr-2.5Nb alloy is used as pressure tube material in pressurized heavy water reactors (PHWR). Generally, these pressure tubes are used in the cold drawn condition. Heat treated Zr-2.5Nb alloy pressure tubes are used in reaktor bolshoy moshchnosti kanalniy and FUGEN type of reactors. In recent times, there has been a greater interest toward increasing the life of pressure tubes in advanced reactors. In the present work, fabrication parameters were optimized to manufacture heat treated Zr-2.5Nb alloy tube. A quenching dilatometer study was performed to establish the continuous cooling transus temperature for the alloy used in this study. Heat treatment under controlled condition in a dilatometer was performed to study microstructure at different soaking temperatures and cooling rates. In the dilatometer, during gas quenching, quenching rates were varied from 0.06 to 100°C/s to assess the effect of cooling rate on resulting microstructures. Soaking temperature and cooling rates were varied to obtain martensitic microstructure with appropriate volume fractions of primary α. On the basis of the results obtained during controlled heat treatments performed in the quenching dilatometer, 883°C was selected as the soaking temperature and water as the quenching medium for the α + β quenching operation for large dimension tubes. The α + β quenched microstructures, consisting of fine martensite phase along with 20%–25% primary α volume fraction, were used for further cold deformation and subsequent aging below the recrystallization temperature. Aging at 540°C produced fully recovered and tempered structure consisting of βNb of equilibrium composition. Mechanical properties of the finished heat treated pressure tube (aged at 515°C/24 h) produced with the present route were similar to the cold work pressure tube. Tube produced with 540°C/24 h aging exhibited substantially higher yield strength value at reactor operating temperature (300°C). The bulk texture at different stages of fabrication was evaluated. The volume fraction of the primary α phase significantly controls the final texture.

A study of morphological and compositional evolution of nanoprecipitates in the Zr–Nb system and their transformational behavior

The metastable transformational behavior (both martensitic and omega) along with compositional and morphological evolution of bcc β precipitates, dispersed in the hcp α matrix of a Zr–1 wt% Nb alloy, were studied as a function of temperature and time. The evolution of the chemical composition of the β phase suggested preference towards metastable compositions having Nb content higher than the equilibrium value. Thermodynamic analysis showed that the metastable chemical compositions are the driving force for the nucleation of such β precipitates. The β to martensite transformation was observed to be possible only if β precipitate size exceeded a critical value of 160 nm. Micromechanical modeling was performed to estimate the critical size of β precipitate required to induce martensite transformation and the model predictions were in close agreement with the experimental observations. The omega transformation, on the other hand, showed less size dependence.