Snehanshu Pal

Role of W on the dislocation evolution in Ni-W alloy during tension followed by compression loading

In this paper, we report deformation behavior during tension followed by compression loading for Nickel and Nickel-Tungsten alloy (15 at% W) single crystals using molecular dynamics simulations to investigate the role of W on the dislocation evolution in Ni-W alloy. The stress-strain responses of single crystals under uniaxial tension followed by compression loading after different pre-strains (i.e. 0.10 and 0.20 true strains for pure Ni; 0.10 and 0.24 true strains for Ni-15 at% W alloy) are simulated at strain rate of 108 s−1 and at the temperature of 300 K. Dislocation mobility, dislocation-dislocation interaction and dislocation-twin interactions are thoroughly investigated to evaluate their influence on deformation behaviour during reverse loading. Slip dominated deformation mechanism prevails during forward loading but both twin and slip are found to be operative during reverse loading for Ni single crystal. It is observed that the dominant deformation mechanism is twin for both forward and reverse loading in case of Ni-15 at% W alloy single crystal.

Electrophoretic Deposition of Cu-SiO2 Coatings by DC and Pulsed DC for Enhanced Surface-Mechanical Properties

Abstract The present study explored the possibilities of improvement in the surface-mechanical properties of electrodeposited Cu-SiO2 composite coating and its underlying mechanism. Composite coatings were developed using SiO2-dispersed acidic copper sulfate electrolyte by direct current and pulse-current electro-codeposition techniques with variation of pulse frequencies at a fixed duty cycle. X-ray diffraction analysis of the coatings revealed information regarding the presence of various phases and crystallographic orientations of the deposited Cu matrix. Scanning electron microscopy and energy dispersive x-ray spectroscopy techniques were used to investigate the surface morphology and chemical composition of the coatings, respectively, and it was observed that SiO2 particles were uniformly distributed in the composite coatings. Surface roughness was found to be reduced with the increasing pulse frequency. The Vickers microhardness and ball-on-plate wear study showed improvement in surface-mechanical properties due to the formation of fine Cu matrix, dispersion strengthening due to homogeneously distributed SiO2 particles, and the preferred orientation of the Cu matrix. Marginal decrease in electrical conductivity with the increasing SiO2 content and pulse frequency was observed from the four-probe electrical conductivity measurement technique.

Experimental and Theoretical Studies on the Viscosity-Structure Correlation for High Alumina-Silicate Melts

Blast furnaces are encountering high Alumina (Al2O3 > 25 pct) in the final slag due to the charging of low-grade ores. To study the viscosity behavior of such high alumina slags, synthetic slags are prepared in the laboratory scale by maintaining a chemical composition of Al2O3 (25 to 30 wt pct) CaO/SiO2 ratio (0.8 to 1.6) and MgO (8 to 16 wt pct). A chemical thermodynamic software FactSage 7.0 is used to predict liquidus temperature and viscosity of the above slags. Experimental viscosity measurements are performed above the liquidus temperature in the range of 1748 K to 1848 K (1475 °C to 1575 °C). The viscosity values obtained from FactSage closely fit with the experimental values. The viscosity and the slag structure properties are intent by Fourier Transform Infrared (FTIR) and Raman spectroscopy. It is observed that increase in CaO/SiO2 ratio and MgO content in the slag depolymerizes the silicate structure. This leads to decrease in viscosity and activation energy (167 to 149 kJ/mol) of the slag. Also, an addition of Al2O3 content increases the viscosity of slag by polymerization of alumino-silicate structure and activation energy from 154 to 161 kJ/mol. It is witnessed that the activation energy values obtained from experiment closely fit with the Shankar model based on Arrhenius equation.

Stress-induced solid-state amorphization of nanocrystalline Ni and NiZr investigated by atomistic simulations

In this study, the feasibility of stress induced solid-state amorphization (SSA) of nanocrystalline (NC) Ni and NiZr alloys having ∼10 nm grain size has been investigated under constant tensile load (uniaxial and triaxial) via molecular dynamics simulations. In order to track the structural evaluation in both NC Ni and NiZr alloys during the SSA process, various types of analysis have been used, including simulated X-ray diffraction, centro-symmetry parameter, Voronoi cluster, common neighbor analysis, and radial distribution function. It is found that SSA in both NC Ni and NiZr alloys can only be achieved under triaxial loading conditions, and the hydrostatic tensile stress required for SSA is significantly lower when at. % Zr is increased in the NC NiZr alloy. Specifically, SSA in NC Ni and Ni-5 at. % Zr alloy was observed only when the temperature and hydrostatic tensile stress reached 800 K and 6 GPa, while SSA could occur in NC Ni-10 at. % Zr alloy under just 2 GPa of hydrostatic tensile stress at 300 K.

Effect of temperature and stress on creep behavior of ultrafine grained nanocrystalline Ni-3 at% Zr alloy

In this paper, molecular dynamics (MD) simulation based study of creep behavior for nanocrystalline (NC) Ni-3 at% Zr alloy having grain size ~ 6 nm has been performed using embedded atom method (EAM) potential to study the influence of variation of temperature (1220-1450 K) as well as change in stress (0.5-1.5 GPa) on creep behavior. All the simulated creep curves for this ultra-fine grained NC Ni-Zr alloy has extensive tertiary creep regime. Primary creep regime is very short and steady state creep part is almost absent. The effect of temperatures and stress is prominent on the nature of the simulated creep curves and corresponding atomic configurations. Additionally, mean square displacement calculation has been performed at 1220 K, 1250 K, 1350 K, and 1450 K temperatures to correlate the activation energy of atomic diffusion and creep. The activation energy of creep process found to be less compared to activation energies of self-diffusion for Ni and Zr in NC Ni-3 at% Zr alloy. Formation of martensite is identified during creep process by common neighbour analysis. Presence of dislocations is observed only in primary regime of creep curve up till 20 ps, as evident from calculated dislocation density through MD simulations. Coble creep is found to be main operative mechanism for creep deformation of ultrafine grained NC Ni-3 at% Zr alloy.

Processing and refinement of steel microstructure images for assisting in computerized heat treatment of plain carbon steel

Abstract. Steels are alloys of iron and carbon, widely used in construction and other applications. The evolution of steel microstructure through various heat treatment processes is an important factor in controlling properties and performance of steel. Extensive experimentations have been performed to enhance the properties of steel by customizing heat treatment processes. However, experimental analyses are always associated with high resource requirements in terms of cost and time. As an alternative solution, we propose an image processing-based technique for refinement of raw plain carbon steel microstructure images, into a digital form, usable in experiments related to heat treatment processes of steel in diverse applications. The proposed work follows the conventional steps practiced by materials engineers in manual refinement of steel images; and it appropriately utilizes basic image processing techniques (including filtering, segmentation, opening, and clustering) to automate the whole process. The proposed refinement of steel microstructure images is aimed to enable computer-aided simulations of heat treatment of plain carbon steel, in a timely and cost-efficient manner; hence it is beneficial for the materials and metallurgy industry. Our experimental results prove the efficiency and effectiveness of the proposed technique.

Influence of Grain Boundary Complexion on Deformation Mechanism of High Temperature Bending Creep Process of Cu Bicrystal

Despite of substantial advancement, the effect of grain boundary (GB) complexions on high temperature creep deformation process has not been fully understood. In this paper, we have studied the high temperature bending creep deformation of copper bicrystal with various GB complexions under different loads using molecular dynamics simulation. It has been found that specimen with normal kite GB complexion have better creep resistance properties when subjected to comparatively lower applied load. In case of monolayer Zr segregation, a drastic decrease in creep strength as well as creep plasticity is observed due to inhibition of GB migration. On the other hand, deviation between creep properties for specimen with split-kite GB complexion and split-kite bilayer Zr segregation GB complexion is minimal. Enhanced creep plasticity is observed in case of split-kite bilayer Zr segregation GB complexion, which is due to formation of interpenetrating icosahedral clusters in the necking region. Fracture in specimen with monolayer Zr segregation GB complexion has occurred by means of slip phenomenon at lower deformation load whereas amorphization and necking is observed at higher deformation load. In case of specimen with bilayer Zr segregation GB complexion, it is found that fracture has occurred through amorphization and necking at all deformation loads due to higher GB thickness.

Analysis of deformation behaviour of Al–Ni–Co thin film coated aluminium during nano-indentation: a molecular dynamics study

ABSTRACT In the present work, molecular dynamics simulations of AlNiCo metallic film deposition on FCC Al substrate and subsequently nano-indentation on the same specimen considering different indenter velocities have been performed using Embedded Atom Method (EAM) potential. The mechanical properties and deformation behaviour of AlNiCo thin film deposited Al substrate is investigated subjected to simulated nano-indentation test. It has been found that indenter velocity significantly influences the calculated hardness of the thin film coated substrate specimen and faster indentation process increases the hardness of the specimen. This finding has been rationalised by correlating with the generation of various full and partial dislocations and their interactions during the nano-indentation process. Sessile dislocations such as stair-rod and Frank partials are found to aid/help the strain hardening phenomena. Furthermore, the effect of indenter velocity on the pile-up formation during the nano-indentation process is also investigated here and it is observed that the amount of pile-up reduces as indenter velocity increases.

Theoretical study of methanol as inhibitor and cyclopentane as stabilizer of dodecahedron methane hydrate cage

Density Functional Theory (DFT) based simulations have been performed to explain the role of methanol as an inhibitor and the role of cyclopentane as a promoter for methane hydrate. Interaction energy, Mullikan charges and electrostatic potential parameters for combined system of methanol and dodecahedron methane hydrate as well as cyclopentane and dodecahedron (512) methane hydrate cage are calculated using B3LYP functional (with and without dispersion function) and 6-31G(d) basis set. Methane hydrate formation inhibition by methanol and methane hydrate stabilization by cyclopentane is critically analyzed based on electrostatic potential and Mullikan charge. It is observed that hydrogen bond between water molecules of clathrate 512 cages become stronger in presence of cyclopentane and weaker in presence of methanol. It is also found that methanol breaks some hydrogen bonds of water molecules.