• S Banerjee

Articles written in Bulletin of Materials Science

• Electron microscopy of defect clusters produced by radiation damage

The methods of characterization of different types of radiation-induced defect clusters bytem have been reviewed. Point defects produced in irradiated materials agglomerate in two or three dimensional clusters to reduce the strain energy associated with them. Two-dimensional clusters assume the configuration of vacancy or interstitial type dislocation loops which can be resolved if the size of the loops is large compared to the extinction distance associated with the imaging reflection. The small loops give rise to a black dot contrast under the kinematical and a black-white contrast under the dynamical imaging conditions. The method of characterization of dislocation loops which include the determination of the nature of the loop, the Burgers vector and the loop plane normal is discussed taking examples from the work done on the ion irradiated Ni4Mo samples. A summary of available experimental results on the characterization of dislocation loops in different metals and alloys having fcc, bcc and hcp structures is presented. The contrast from stacking fault tetrahedra which form in some fcc metals and alloys after a certain degree of annealing is also discussed. The optimum conditions for imaging three-dimensional clusters or voids are derived on the basis of the contrast theory proposed for such defects. Special reference is made to the usefulness of “through focus analysis” in the imaging of very small cavities (with diameters as small as about 10 Å).

It is shown that the formation of disordered zones resulting from displacement cascades in the ordered matrix can be utilized in determining the shape and the volume of cascades in the virgin state. The importance of different contributing factors like the strain contrast and the structure factor contrast in producing the overall contrast from the disordered zones is discussed. Detailed observations on the shape of the disordered zones are shown to be important to establish the occurrence of the replacement collision sequence and the formation of sub-cascades.

• Instabilities in first order phase transitions

Majority of the metallurgical phase transformations are first-order transitions which occur by the nucleation and growth process near equilibrium conditions. In recent years, homogeneous transformation has been reported in some of these cases at conditions significantly away from those of equilibrium. In this paper some of these transformations will be discussed. In the first part of the presentation the thermodynamic and the mechanistic distinctions between first and higher order phase transformations will be discussed and a comparison made between homogeneous and heterogeneous modes of phase transformations and those of deformation. Based on Landau’s free energyvs generalised order parameter plots, an instability temperature is defined for first order phase transformations below which the transformation can occur by a continuous amplification of a concentration or a strain fluctuation. In the second part experimental evidence in support of the continuous mode of transformations in two ordering reactions are presented. These are: (i) a transition from the short range to the long range chemical order in Ni4Mo (D1a structure) and (ii) a hybrid displacive-replacive ordering in Zr2Al (B82 structure). In order to make the continuous mode operative in these first order transformations (which is possible at a high “supercooling”), radiation in the former and rapid quenching in the latter were employed. In the last part, the martensitic transformation and the shape memory effect is described in terms of Landau’s plots and the mechanical and thermodynamical consequences of the model are discussed.

• Applications of superconducting magnetic energy storage in electrical power systems

Fast-acting energy storage devices can effectively damp electromechanical oscillations in a power system, because they provide storage capacity in addition to the kinetic energy of the generator rotor, which can share the sudden changes in power requirement. The present paper explores the means of reducing the inductor size for this application so that the use of high-Tc superconducting materials becomes feasible.

• Effect of cold working and annealing on the texture of Zr-2·5Nb pressure tubes

Texture plays an important role in the commercial acceptability of Zr-2·5 wt% Nb pressure tubes used in nuclear reactors. A modified flow sheet for the fabrication of these pressure tubes involves a few additional steps viz. stress relieving, cold working and annealing, as compared to the conventional route. The evolution of texture during sequential fabrication steps of extrusion, stress relieving, cold working and annealing was studied in terms of texture coefficients and inverse pole figures. It was observed that crystallographic texture primarily developed during hot extrusion and the additional steps of stress relieving, cold working and annealing did not alter the texture significantly. The texture developed was one having a majority of the basal plane normals along the tangential direction of the tube.

• Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.

• Self-propagating high-temperature synthesis of titanium borides

The application of self-propagating high-temperature synthesis (SHS) to prepare a few borides of titanium was investigated. Using the plane wave propagation mode, the synthesis of titanium borides in the cold-pressed cylindrical specimens of the component powder mixtures was effected and was studied as a function of boron content in the initial mix and the specimen size. SHS reaction in compacts having diam. of 6 mm or less and high bulk density could not be initiated and/or sustained and was considered to be a result of rapid heat dissipation.

• Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ⩾ 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.

• Light emission from porous silicon

Room-temperature visible luminescence observed in porous silicon is one of the most significant discoveries of this decade as it opens up the possibility of silicon-based optoelectronics afresh. The exact mechanism of this different luminescence behaviour of porous silicon, compared to crystalline silicon, is not well established. In this paper results of a combination of infrared absorption, and photoluminescence emission and excitation spectroscopies will be described to show that the nanocrystallite nature of porous silicon and chemical environment at the surface are the important aspects of this novel luminescence behaviour.

• Studies on high temperature deformation behaviour of 3Y-TZP ceramics

The present work reports the results on the deformation behaviour of ZrO2-3 mol% Y2O3 (3Y-TZP) ceramics which were prepared by pressureless sintering at 1400°C. Dense, cylindrical samples were subjected to uniaxial compression tests under a constant stress of 15 MPa in the temperature range of 1200–1400°C. The ceramics exhibit considerable ductility, attaining over 60% true strain without any edge cracking. Microstructural changes due to interaction of grain boundary viscous phase with the ultrafine and equiaxed grains were analyzed by transmission electron microscopy. Results show the grain boundary sliding accompanied by a diffusion accommodation process as the predominant deformation mechanism in these ceramics.

• Fractography as well as fatigue and fracture of 25 wt% silicon carbide whisker reinforced alumina ceramic composite

Fatigue cracked and fast fractured regions in four-point bend specimens prepared from 25 wt% silicon carbide whisker reinforced alumina composite were examined by scanning electron microscopy. This composite was found to be susceptible to a fatigue crack growth phenomenon similar to that in the case of metallic materials, but with a higher crack growth exponent. In the fatigue region, the alumina matrix failed mainly in a transgranular mode and the whiskers mainly failed with a flat fracture surface but without their pullout. On the other hand, in the fast fracture region, the whiskers failed predominantly by pullout and the alumina matrix failed in a mixed mode with about half in transgranular and the other half in intergranular mode. Thus, to improve the fracture toughness of the material, the grain boundary strength of alumina and the matrix whisker interfacial bonding should be improved. To increase the resistance to fatigue, the fracture strength of the alumina grains should be improved by using finer α-alumina particles and the fatigue strength of the whisker have to be increased by improving the uniformity in distribution of β-SiC whiskers during hot pressing.

• Electrical resistivity and point contact studies on Al-Pd-Mn icosahedral quasicrystals

We report measurement of the electrical resistivityρ(T) and point contact differential conductance (dI/dV) for icosahedral Al70Pd30 −x (x=7,9 and 11) at low temperatures. Below 10 K, forx=11,ρ(T) follows a logT dependence. All the three compositions show a sharp minimum in the differential conductance near zero bias. Magnitude of the dip in the point contact conductance increases as the Mn concentration increases. The dip near zero bias also follows a logV dependence forV &lt; 30 mV.

• Interfaces in ordered intermetallics

In this paper, we have examined different types of interfaces that occur between orientational/translational variants generated during the ordering process. This has been illustrated citing examples of ordering of the FCC structure into DO22, Dla and Pt2Mo type structures in some nickel base alloys. Microstructures consisting of more than one ordered structure have also been investigated. Superlattice domains of DO22 and Pt2Mo type structures have also been found to coexist in a microscopic scale of mixing in Ni-V alloys while mixed domains of Dla and Pt2Mo type structures on a much finer scale have been observed in Ni-Mo alloys. The formation of different variants (rotational and translational) of ordered structure(s) from the disordered lattice has been explained on the basis of group theoretical and symmetry considerations.

• α → ω transition in shock compressed zirconium: A study on crystallographic aspects

In 1973, Usikov and Zilbershtein proposed that theα(hcp) →ω (a three atom hexagonal) transformation in Zr and Ti proceeds via theβ(bcc, a high temperature phase) intermediate. Based on this they derived two non-equivalent orientation relationships (OR) betweenα andω phases. Their transmission electron microscopy (TEM) study carried out on these elements, that wereαω-transformed under static high pressure, revealed only one of the two proposed ORs. Various TEM studies done thereafter on these elements and their alloys (ω transformed under static pressures) conform to either one of these ORs. In a recent TEM study by Song and Gray on Zr,ω-transformed under shock compression, a new OR has been observed which according to them is different than those given by UZ and they put forth the directαω transformation mechanism. In the present study, we have generated additional TEM data on shock compressed Zr samples and have reconciled the above conflicting results. We find all our ORs (which contain the OR of SG also) to be described by the OR reported by UZ. The latter OR (i.e. of SG) is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative both, under static and shock compression. Mechanism of the transition is discussed in terms of the required strains.

• Ordering in Ni-Mo alloys—First-principles calculations versus experimental observations

An extensive amount of experimental work has been reported in the literature on the ordering behaviour of Ni-Mo alloys containing 8–33 at% of Mo, which exhibit both short-range and long-range ordering phenomena and a competition among several fcc-based long-range ordered structures. We have used local-density-based tight binding linear muffin-tin orbital (TB-LMTO) method in conjunction with ‘augmented space recursion + orbital peeling’ (ASR + OP) for the determination of ground state energies of these superstructures in terms of effective pair interactions up to the fourth nearest neighbour pairs. The ordering behaviour of the four competing fcc-based superstructures has been studied using the mean-field-based ‘static concentration wave’ (SCW) model in terms of the free energy-order parameter plots (Landau plots) and the free energy-composition plots. The instability domains with respect to concentration fluctuations, both short wavelength (ordering) and long wavelength (clustering) have been identified from these calculations. This information has been used to predict the sequence of transformation events in the Ni-Mo alloys undergoing ordering and/or clustering and the results are compared with those obtained experimentally.

• High resolution electron microscopy of alloys

High resolution electron microscopy (HREM) has emerged as a very powerful tool for probing the structure of metals and alloys. It has not only helped in unravelling the structure of materials which have been at the forefront of novel materials development such as quasicrystalline phases and high temperature superconducting compounds, but also is fast becoming a technique for solving some outstanding issues in the case of the commercial alloys thereby helping alloy development. In addition to the determination of the structures of phases, this tool is used for obtaining a first hand information of the arrangement of atoms around the various types of crystallographic defects and interphase interfaces. This mode of microscopy allows direct observation of orientation relationships between two phases across interfaces. HREM can be used for the direct examination of the prenucleation process. Initial stages of nucleation can also be studied readily in amorphous alloys, precipitation hardening alloys like maraging steels and in those systems where the formation of the omega phase occurs.

This presentation describes some results of HREM studies on various alloys, commercial as well as alloys of scientific interest, where some of the aforementioned aspects have been examined. The specific examples cited pertain to metallic glasses, NiTi shape memory alloys, Ni-Mo, Zr-Nb and Ti-Al alloys.

• Exploration of the role of disorder and the behaviour of the surface state in the three-dimensional topological insulator—Bi$_2$Se$_3$

Here we present an overview of some of our recent experimental investigation on the high conducting topologically protected surface state properties of a 3D topological insulator (TI), Bi$_2$Se$_3$, in both bulk and single-crystals form. Selenium (Se) vacancies in Bi$_2$Se$_3$ are natural bulk charge dopants, hence these vacancies act as non-magnetic defects in these materials. We use Bi$_2$Se$_3$ material as a prototype of a 3D TI) material with the disorder, for exploring the effects of non-magnetic disorder on the topological conducting surface states. Using a sensitive non-contact mutual inductance-based measurement technique, we identify distinct signatures of surface and bulk contributions to electrical conductivity in a TI Bi$_2$Se$_3$. We show a temperature-dependent transformation from surface to bulk dominated electrical conductivity and unravel the unusual resurfacing of a surface-dominated electrical conductivity at high temperatures. We show that the surface to bulk conductivity transformation is related to a unique activation energy scale ${\Delta}$ in the range of tens of meV, which is smaller than the bulk-insulating gap in Bi$_2$Se$_3$. This gap, we believe is related to a defect state created by the charge dopant Se vacancies in Bi$_2$Se$_3$. We also see interesting effects related to disorder-related coupling of the surface states. The Se vacancies which dope the bulk of the Bi$_2$Se$_3$ crystal lead to an unusual inductive-type coupling of the high conducting 2D-like surface states. The coupling leads to a critical thickness feature in bulk TI crystal, which is distinct from the direct coupling limit in TI’s. We find that if the thickness of the Bi$_2$Se$_3$ crystal exceeds this criticalthickness, then the surface states are no longer inductively coupled. To probe the temperature-related surface to bulk transformation in conductivity of Bi$_2$Se$_3$, we use a high sensitivity magneto-optical imaging technique to directly imagethe distribution of current in single crystal and a thin film of Bi$_2$Se$_3$. At low temperatures, we observed a strong sheet current from the topological surface state. Above 80 K, we report that the emergence of a temperature-dependentin homogeneous, grainy current distribution state in Bi$_2$Se$_3$ single crystals. The grainy state has mixed regions with high and low current densities. The observation of the emergence of a temperature-dependent inhomogeneous phase in the TI suggests the possibility of a disorder-driven spontaneous phase separation scenario.

• # Bulletin of Materials Science

Volume 45, 2022
All articles
Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020