Joysurya Basu

Texturing of pure and doped CeO2 thin films by EBPVD through target engineering

In this paper, we report the effect of annealing temperature of target on the texture of thin films coated by electron beam physical vapor deposition method. Nanocrystalline cerium oxide (CeO2) and 20 mol% samarium doped cerium oxide (SDC) powders, compacted into pellets, were used as targets after annealing at 300, 500 and 800 °C. Grain size analysis of the target by X-ray diffraction showed a size range of 12–52 nm and 9–22 nm for CeO2 and SDC, respectively. Texture coefficient calculation from glancing incident X-ray diffraction showed a preferential orientation of (111) in CeO2 films. However SDC films exhibited (200) orientation grown at the expense of (111) which resulted in higher residual strain with annealing temperature. The pole figure analysis elucidated smaller in-plane misorientation in CeO2 than in SDC films. Under similar deposition conditions, difference in textured growth between CeO2 and SDC is primarily induced by vapor pressure modifications associated with the annealing temperature of the target. Raman and X-ray photoelectron spectroscopic studies of the films indicate the presence of higher oxygen vacancy concentration in SDC as well as a decrease in Ce3+ concentration with target annealing temperature.

Structure imaging and vanadium substitution in cubic TiCr2 Laves phase

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr2 Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces.

Icosahedral Cluster Energetics in Zr60Cu10Al15Ni15 Bulk Metallic Glass and Their Role on Solidification Behavior

The energetics behind transformation of liquid structure into subsequent intermediate phases during solidification is expected to play a decisive role in glass/crystal formation. A great deal of experimental and simulation work on supercooled liquids has indicated that, there exists a close link between the liquid structure and icosahedral clusters, especially for bulk metallic glass forming liquids. Pertinently, icosahedral clusters are also found to be energetically favorable to form upon devitrification of Zr60Cu10Al15Ni15 glassy alloy. Such evolution of icosahedral clusters upon devitrification in this alloy invariably proves their manifestation at the intermediate stage during transition of supercooled liquid into glass. Hence understanding the energetics behind restructuring of these clusters into glass or crystal during solidification, aids in microstructure optimization of glass/crystal composites for structural and functional applications. In this paper, it has been attempted to investigate the energetics behind the evolution of Zr–Ni and Zr–Al binary intermetallic phases during crystallization of Zr60Cu10Al15Ni15 glassy alloy. Ascalaph Designer Molecular Modeling Suite is used to generate different models of clusters to understand the formation of Zr–Ni and Zr–Al phases. We propose solidification mechanism in this alloy via two steps, namely, formation of intermediate Zr–Cu icosahedral clusters which is structurally restricted process and precipitation of crystalline phases as thermodynamically favorable process.

Vacancy-mediated structural changes in Au–Cu nanoparticles

ABSTRACT We report the formation of new phases in bimetallic Au–Cu nanoparticles. These phases were observed in nanoparticle synthesised by adopting a three-step protocol in a single pot. Nanoparticles at 180°C for 1 h led to the formation of single-phase solid solution of Cu in Au. Subsequent heat treatment at 290°C for 2 h of these Au–Cu nanoparticles revealed three new phases. One of them relates to the modification of occupancy of Cu in an ordered AuCu tetragonal phase (tP4). This cell although retains tetragonal symmetry but displays metrical properties akin to that of a cube. The other two relates to vacancy ordering along <111> directions in the {111} planes of an ordered AuCu3 cubic phase (cP4). On the one hand, statistical occupancy of vacancy on Cu site in this cell leads to the reduction of cell size from ∼3.75 Å to ∼3.5 Å whereas ordering of vacant layer on the other hand gives rise to symmetry breaking. Former continues to display cubic symmetry whereas latter transforms to a trigonal cell.

Direct structure imaging of partially collapsed omega domains in phase-separated V–Ti alloy through atom column contrast interpretation

The metastable ω phase stabilizes either in hexagonal (P6/mmm) or trigonal (

Exceptional resistance to grain growth in nanocrystalline CoCrFeNi high entropy alloy at high homologous temperatures

P\overline{3} m1

Thermal stability and grain boundary strengthening in ultrafine-grained CoCrFeNi high entropy alloy composite

P3¯m1) structures depending upon the extent of collapse of the atomic planes. The stability between these two structures depends on the atomic scale shear mechanism of the ω formation and also on the relative composition of the bcc stabilizer. Detailed interpretation of the bcc → ω phase transformation demands understanding of structure, chemistry and interface of ω phase along with the bcc matrix down to the atomic level. Present paper deals with the structural imaging of partially collapsed ω structure and its lattice correspondence with the bcc phase in a phase-separated V–Ti alloy. High-resolution TEM characterization with the aid of phase-contrast image simulation and atomic structure modeling has been systematically carried out to study the structural aspects of the nanostructured ω. The contrast of the collapsed atoms and the corner atoms are understood through systematic studies of the experimental images and their corresponding column intensities, as seen along simulated exit waves and their phase and amplitude parts along different zone axis. Attempts have also been made to understand the qualitative nature of the electron channeling behavior along the different atomic positions of the ω structure.

Micro indentation study on Cu60Zr20Ti20 metallic glass

An α-β alloy (β~8% in the stress relieved condition) of Ti-5Ta-1.8Nb has been subjected to severe plastic deformation (SPD) by cryo-rolling. The grain size of α-Ti could be reduced significantly from ~ 8µm to 100 nm and less by cryo-rolling. Extensive plastic deformation leads to grain fragmentation through the formation of defect clusters. The fragmented grains exhibit deformation texture. High resolution transmission electron microscopy (HRTEM) confirmed the presence of low and high angle grain boundaries. The role of substitutional atoms (Ta, Nb) in producing lattice strains and altering the projected potential from the atomic columns has been discussed. Although, the minor phase, β (bcc-Ti) is evident in the starting alloy, it was not observed after SPD, possibly due to extended solid solution formation (Gibbs–Thomson effect) in the fine grains or due to the stress induced transformation of the α-Ti phase.