Koteswararao V. Rajulapati

A New Electrochemical Approach for the Synthesis of Copper-Graphene Nanocomposite Foils with High Hardness

Graphene has proved its significant role as a reinforcement material in improving the strength of polymers as well as metal matrix composites due to its excellent mechanical properties. In addition, graphene is also shown to block dislocation motion in a nanolayered metal-graphene composites resulting in ultra high strength. In the present paper, we demonstrate the synthesis of very hard Cu-Graphene composite foils by a simple, scalable and economical pulse reverse electrodeposition method with a well designed pulse profile. Optimization of pulse parameters and current density resulted in composite foils with well dispersed graphene, exhibiting a high hardness of ~2.5 GPa and an increased elastic modulus of ~137 GPa while exhibiting an electrical conductivity comparable to that of pure Cu. The pulse parameters are designed in such a way to have finer grain size of Cu matrix as well as uniform dispersion of graphene throughout the matrix, contributing to high hardness and modulus. Annealing of these nanocomposite foils at 300°C, neither causes grain growth of the Cu matrix nor deteriorates the mechanical properties, indicating the role of graphene as an excellent reinforcement material as well as a grain growth inhibitor.

Process Optimization for Pulse Reverse Electrodeposition of Graphene-Reinforced Copper Nanocomposites

In the present study, processing of graphene-reinforced copper nanocomposite foils with homogenous dispersion of graphene throughout the matrix, exhibiting good mechanical properties by a simple, cost-effective, and scalable pulse reverse electrodeposition technique (PRED) with special focus on the influence of graphene content in the electrolyte to tailor the properties. A systematic approach has been adopted for enhancing the properties. Distribution of graphene nanosheets in the copper metal matrix and the microstructural properties have been studied by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). Interesting observations have been made from nanoindentation studies, where hardness (∼2.7 GPa) enhanced mainly with increase in graphene content (0–0.75 g/L), while maximum elastic modulus (∼139 GPa) is achieved for a graphene content of 0.5 g/L in the electrolyte. Four-point probe testing has been adopted to evaluate the electrical features. The major contribution in enhancement of properties is found to be the presence of graphene and its uniform individual dispersion and distribution as nanosheets in the copper matrix.

Strain rate sensitivity of bulk multi-phase nanocrystalline Al–W-based alloy

High-energy ball milling of conventional coarse-grained aluminium and nanocrystalline W in an Al-10 at.%W composition results in the formation of a two-phase mixture of Al and W with nanocrystalline features. Subsequent compaction of these powders using spark plasma sintering (SPS) at 748 K resulted in the formation of an Al12W phase in the nanocrystalline aluminium matrix. It is suggested that the mere attainment of nanocrystallinity was not enough to trigger a reaction between Al and W to form Al12W but that sufficient thermal activation was also required, as supplied during SPS. The second-phase particles (~175 nm in size) are uniformly distributed in the nanocrystalline Al matrix having a grain size of ~40 nm. The nanocomposite possessed a high hardness of 5.42 ± 0.33 GPa and an elastic modulus of 145 ± 5 GPa, both measured using depth-sensing nanoindentation. At room temperature, this novel nanocomposite exhibited a strain rate sensitivity (SRS) of 0.024 ± 0.001 and an activation volume in the range of 3.78–3.88 b3. Interfacial regions, viz. grain boundaries and triple junctions in the matrix and the reinforcement, matrix/particle boundaries, etc. could be influential factors in deciding the SRS and the activation volume. A scanning probe microscope image of the nanoindent shows a plastic flow region around the periphery of the indent.

Mechanical properties of in situ consolidated nanocrystalline multi-phase Al–Pb–W alloy studied by nanoindentation

Abstract Formation of chunks of various sizes ranging between 2 and 6 mm was achieved using high-energy ball milling in Al–1at.%Pb–1at.%W alloy system at room temperature during milling itself, aiding in in situ consolidation. X-ray diffraction and transmission electron microscopy (TEM) studies indicate the formation of multi-phase structure with nanocrystalline structural features. From TEM data, an average grain size of 23 nm was obtained for Al matrix and the second-phase particles were around 5 nm. A high strain rate sensitivity (SRS) of 0.071 ± 0.004 and an activation volume of 4.71b3 were measured using nanoindentation. Modulus mapping studies were carried out using Berkovich tip in dynamic mechanical analysis mode coupled with in situ scanning probe microscopy imaging. The salient feature of this investigation is highlighting the role of different phases, their crystal structures and the resultant interfaces on the overall SRS and activation volume of a multi-phase nc material.

Effect of Process Parameters on Microstructure and Hardness of Oxide Dispersion Strengthened 18Cr Ferritic Steel

Pre-alloyed ferritic 18Cr steel (Fe-18Cr-2.3W-0.3Ti) powder was milled with and without nano-yttria in high-energy ball mill for varying times until steady-state is reached. The milled powders were consolidated by upset forging followed by hot extrusion. Microstructural changes were examined at all stages of processing (milling, upset forging, and extrusion). In milled powders, crystallite size decreases and hardness increases with increasing milling time reaching a steady-state beyond 5 hours. The size of Y2O3 particles in powders decreases with milling time and under steady-state milling conditions; the particles either dissolve in matrix or form atomic clusters. Upset forged sample consists of unrecrystallized grain structure with few pockets of fine recrystallized grains and dispersoids of 2 to 4 nm. In extruded and annealed rods, the particles are of cuboidal Y2Ti2O7 at all sizes and their size decreased from 15 nm to 5 nm along with significant increase in number density. The oxide particles in ODS6 are of cuboidal Y2Ti2O7 with diamond cubic crystal structure (Fd

Adaption of a Green Technology (Friction Stir Welding) to Join Fusion Energy Materials

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Structure-Property Correlations in Cathodic Arc Deposited TiAlN Coatings

3¯m) having a lattice parameter of 10.1 Å and are semicoherent with the matrix. The hardness values of extruded and annealed samples predicted by linear summation model compare well with measured values.

Testing Techniques for Mechanical Characterization of Nanostructured Materials

Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic–martensitic steel at tool rotational speeds of 500 and 900 rev min−1. Comparator welds at 500 rev min−1 were also produced in 6 mm thick reduced activation ferritic–martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.