Engineering topics

Dey, G. K.

Publications and source records attributed to Dey, G. K..

8 records

DOE OSTI2024

Comprehensive characterization of the structure of Zr-based metallic glasses

Structure of metallic glasses fascinates as the generic amorphous structural template for ubiquitous systems. Its specification necessitates determination of the complete hierarchical structure, starting from short-range-order (SRO) → medium-range-order (MRO) → bulk structure and free volume (FV) distribution. This link has largely remained elusive since previous investigations adopted one-technique-at-a-time approach, focusing on limited aspects of any one domain. Reconstruction of structure from experimental data inversion is non-unique for many of these techniques. As a result, complete and precise structural understanding of glass has not emerged yet. In this work, we demonstrate the first experimental pathway for reconstruction of the integrated structure, for and glasses. Our strategy engages diverse (×7) multi-scale techniques [XAFS, 3D-APT, ABED/NBED, FEM, XRD, PAS, FHREM] on the same glass. This strategy complemented mutual limitations of techniques and corroborated common parameters to generate complete, self-consistent and precise parameters. Further, MRO domain size and inter-void separation were correlated to identify the presence of FV at MRO boundaries. This enabled the first experimental reconstruction of hierarchical subset: SRO → MRO → FV → bulk structure. The first ever image of intermediate region between MRO domains emerged from this link. We clarify that determination of all subsets is not our objective; the essence and novelty of this work lies in directing the pathway towards finite solution, in the most logical and unambiguous way.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Corrosion Behavior of Wire Electrical Discharge Machined Surfaces of P91 Steel

The extensive usage of P91 steel in certain reactor components requires its precision fabrication by wire electrical discharge machining (WEDM). The WEDM cut surfaces consist of primarily re-melted and solidified layer (recast layer) of 2-8 µm. The recast layer is a result of fast removal of heat by unaffected metallic substrate and dielectric medium leading to rapid cooling. The WEDM surface is expected to affect the component performance in terms of its corrosion properties. The microstructural changes taking place due to EDM cutting of P91 steel bring about a drastic change in the electrochemical response of surface as compared to that for bulk P91 steel. The surface microstructure is shown to alter the passivation behavior and resistance to localized corrosion. Electrochemical polarization studies were done to evaluate passivation, pitting and intergranular corrosion (IGC) behavior and field emission gun—scanning electron microscopy was used for microstructural characterization of P91 steel surfaces. The passivation behavior for P91 steel WEDM cut surfaces was better than that for diamond polished surface. But pitting corrosion resistance decreased for the P91 steel WEDM surface which is a result of inhomogeneities in the microstructure of recast layer. The IGC resistance though increased due to the decreased grain size in the surface layer.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Precipitation-hardening of superalloy 693 and modeling of initial stages of hardening

Highlights: • Isothermal Time-Temperature-Precipitation diagram of superalloy 693 established. • At 973–1173 K, increase in hardness up to 1.5 h of ageing followed by drop noticed. • Initial hardening of alloy at 973–1173 K is modeled by Johnson-Mehl-Avrami equation. • XRD, TEM but not SEM-EDS indicate formation of fine γ′ precipitates in aged samples. • Initial stages of hardening is attributed to formation of fine γ′ precipitates. - Abstract: Isothermal time-temperature-hardness behavior for Ni{sub 3}Al type phase in austenitic superalloy 693 was investigated by micro hardness measurements of the aged specimens. Ageing of the alloy between 973 K and 1173 K indicated an initial increase in micro hardness with ageing time attaining a peak at 1.5 h followed by a drop of its value. The early stages of hardening for the alloy was modeled by the expression: ΔH = (Kt){sup n}, where ΔH, t, n and K are increase in hardness, ageing time (h; in hour), time exponent and temperature-dependent rate constant (h{sup −1}) respectively. X-ray diffraction of solution-annealed and aged specimens indicated formation of Ni{sub 3}Al type precipitation-hardening (γ′) phase, while scanning electron microscopy with energy dispersive X-ray spectroscopy could not clearly reveal the formation of γ′ phase. Transmission electron microscopy on peak-aged (1.5 h at 973 K, 1073 K, 1173 K) samples revealed formation of fine, face centered cubic, Ni{sub 3}Al type (γ′) precipitates, while over-aged (3 h at 973 K, 1073 K, 1173 K) specimens indicated coarsening of precipitates. The initial stages of hardening for the alloy (at 973 K, 1073 K, 1173 K) are attributed to formation of fine Ni{sub 3}Al type precipitates.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Development in corrosion resistance by microstructural refinement in Zr-16 SS 304 alloy using suction casting technique

Highlights: • Grain refinement was made in Zr–16 wt.% SS alloy while prepared by suction casting process. • Distribution of Laves phase, e.g., Zr{sub 2}(Fe, Cr) was raised in suction cast (SC) Zr–16 wt.% SS. • Corrosion resistance was improved in SC alloy compared to that of arc-melt-cast alloy. • Grain refinement in SC alloy assisted for an increase in its corrosion resistance. - Abstract: Zirconium (Zr)-stainless steel (SS) hybrid alloys are being considered as baseline alloys for developing metallic-waste-form (MWF) with the motivation of disposing of Zr and SS base nuclear metallic wastes. Zr–16 wt.% SS, a MWF alloy optimized from previous studies, exhibit significant grain refinement and changes in phase assemblages (soft phase: Zr{sub 2}(Fe, Cr)/α-Zr vs. hard phase: Zr{sub 3}(Fe, Ni)) when prepared by suction casting (SC) technique in comparison to arc-cast-melt (AMC) route. Variation in Cr-distribution among different phases are found to be low in suction cast alloy, which along with grain refinement restricted Cr-depletion at the Zr{sub 2}(Fe, Cr)/Zr interfaces, prone to localized attack. Hence, SC alloy, compared to AMC alloy, showed lower current density, higher potential at the breakdown of passivity and higher corrosion potential during polarization experiments (carried out under possible geological repository environments, viz., pH 8, 5 and 1) indicating its superior corrosion resistance.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Investigating the effect of V{sub 2}O{sub 5} addition on sodium barium borosilicate glasses

V{sub 2}O{sub 5} doped sodium barium borosilicate glasses were characterized by photoluminescence spectroscopy and electron probe microanalyzer (EPMA). The glass remains homogeneous for lower concentration of V{sub 2}O{sub 5} but a phase separation is observed when V{sub 2}O{sub 5} doping is increased beyond 5 mol%. Detailed microanalysis reveals that the phase separated glass consists of a phase containing V, Ba and Si and a separate Si rich phase within the glass matrix. The luminescence study demonstrated that at low concentration the vanadium mainly interacts with the structural units of B/Si while at higher concentrations, V-O-V/ V-O{sup −} Na{sup +}/Ba{sup 2+} linkages are formed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Ab-initio study of oxygen defects in pure ThO{sub 2}

First principles calculations using projector augmented wave potentials and generalized gradient approximations predicts the structural relaxations due to neutral and positively charged oxygen defects (+1 and +2) in bulk thoria leads to symmetric distortion around the vacancy site. Electronic Density of states (DOS) analysis shows presence of defects states mainly contributed by Th d and f states near the conduction band minima for the double positively charged oxygen vacancy which is having lowest energy of formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

A first principle study of the pressure dependent elastic properties of monazite LaPO{sub 4}

DFT based ab-initio simulations have been performed to study the effect of pressure on the elastic properties of monazite LaPO{sub 4} which is a promising host material for immobilization of high level nuclear waste. The phase is found to be stable up to 30 GPa. The calculated polycrystalline bulk, shear and Young moduli show an increasing trend as a function of pressure. The ductility and anisotropy in shear modulus of the material have been found to increase with pressure; whilethe bulk modulus anisotropy decreases with pressure.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Parameterization of interatomic potential by genetic algorithms: A case study

A framework for Genetic Algorithm based methodology is developed to systematically obtain and optimize parameters for interatomic force field functions for MD simulations by fitting to a reference data base. This methodology is applied to the fitting of ThO{sub 2} (CaF{sub 2} prototype) – a representative of ceramic based potential fuel for nuclear applications. The resulting GA optimized parameterization of ThO{sub 2} is able to capture basic structural, mechanical, thermo-physical properties and also describes defect structures within the permissible range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗