Department of Metallurgical and Materials Engineering

Wide band gap metal oxides are ideally suited for inorganic optoelectronic devices. While zinc oxide is a commonly used n-type material, there is still a lot of ongoing work for finding suitable p-type oxides. In this work, we describe a two-step route to formulate a stable and conducting p-type nickel oxide (NiO) nanofluid. NiO nanoparticles were synthesised using a bottom-up wet chemical approach and dispersed in ethylene glycol to form a nanofluid. The viscosity and surface tension of the nanofluid were optimised for printing. The printing was done using an extrusion-based direct writer. The NiO nanofluid was printed onto an aluminum-doped zinc oxide layer and annealed at different temperatures. Electrical characterisation of the junction was used to extract the junction barrier for carriers across the interface. The resulting heterojunction was found to exhibit rectifying behaviour, with the highest rectification ratio occurring at an annealing temperature of 250 °C. This annealing temperature also resulted in the lowest junction barrier height, and was in excellent agreement with theoretically predicted values. The development of a printed p-type ink will help in the realisation of oxide-based printed electronic devices.

Recent advances in alumina ceramics are focused toward innovative processing routes to improve their mechanical reliability while retaining their superior wear resistance, which might be possible if a thin layer of dense alumina can be formed on a metallic substrate such as Ti-6Al-4V with high mechanical strength. For this purpose, we propose a new two-step process in which a dense layer of Al deposited on the Ti alloy by cold metal transfer method, formed a dense Al3Ti gradient reaction layer at their interface to improve adhesion in a single step. Subsequent micro-arc oxidation treatment transformed Al layer to a graded alumina layer in which γ-alumina decreased and α-alumina increased with increasing depth. Abrasion of outer regions revealed underlying pure α-alumina regions with high Vickers hardness matching with that of sintered alumina. The designed alumina/Ti alloy hybrid can be a potential candidate for wear resistance applications.

This paper describes the tensile properties, flow and work-hardening behavior of four metastable β-titanium alloys Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al-5Mo-8.6V-1.5Cr (A3), and Ti-5Al-3.5Mo-5V-3.94Cr (A4) in α+β hot-rolled condition. The decreasing order of average strength parameters (σYS and σUTS) is A4, A2, A1, and A3. The maximum strength observed in alloy A4 is due to the presence of highest wt. fraction of Cr. The elongation is the maximum and minimum in alloys A3 and A4, respectively. These alloys display moderate to high percent in-plane anisotropy (AIP) and reasonably low anisotropic index (δ) values. Both the AIP and δ values are maximum and minimum in alloys A1 and A3, respectively. The yield locus plots also exhibit the presence of anisotropy due to relatively large differences in yield strength values along tension and compression directions. The flow behavior of alloys A1, A2, and A4 follows Swift equation, while the alloy A3 displays best fit with Holloman equation. The presence of prestrain (ε0) in hot-rolled materials before tensile testing has an important bearing on the flow curves of A1, A2, and A4 alloys. The instantaneous work-hardening rate curves of the alloys A1, A2, and A3 exhibit all the three typical stages (stage I, stage II, and stage III) in RD samples, while the alloy A4 shows the presence of only stage I and stage III. The 45 deg to RD and TD samples of alloys A1, A2, and A4 display only stage I. The stages I and III as well as I and II are present in alloy A3 in 45 deg to RD and TD samples, respectively. Dislocation-controlled strain hardening occurs in all the three stages of these alloys in the absence of stress-induced martensitic transformation (α″) and twinning. Slip is the predominant deformation mechanism during tensile testing. Three types of slip lines, i.e., planar, wavy, and intersecting have been observed close to fracture surfaces of post tensile-tested specimens.

Creep-rupture behaviour of IN718 with deposit of 87.5 wt.%Na2SO4+5 wt.%NaCl+7.5 wt.%NaVO3 (3SM) in the stress range of 550–850 MPa at 650 °C is investigated. 3SM shortens the creep-rupture time by ≈70–90 %. Comprehensive microstructural analysis suggests that oxygen-induced-dynamic- embrittlement, sulphidation followed by oxidation, and vanadic-hot corrosion operate during creep. Creep-hot corrosion interaction influence on rupture strain is explained by introducing strain-associated-time (SAT) and Δtε plots, for the first time. These plots accompanied by detailed microstructural examination demonstrate that deleterious influence of 3SM increases with decreasing stress. Further, SAT plots and EPMA analysis confirm that oxygen-induced-dynamic-embrittlement commences from ∼9% strain under 3SM at 550 MPa.

Transition metal-based carbo-nitride solid solution phases have wide application in electrical, electronic, automotive, and refractory industries due to the excellent combination of metallic and ceramic properties. Mainly, carbo-nitride phases were synthesized by high-temperature solid-state diffusion, carbothermal reduction-nitridation, HIP, SHS, pressureless sintering at high temperature, etc. All the methods required considerable investment and production costs, limiting their application in the larger area of research. To overcome all the problems related to the synthesis, one simple, cost-effective, energy-efficient, and environmentally friendly method is required where synthesis will be carried out at a lower temperature with a shorter soaking time. Molten salt-assisted synthesis method fulfils these requirements by accelerating the diffusion rate for phase formation through a chemically inert liquid medium. In this manuscript, carbo-nitride TiC0.5N0.5 and MAX phases Ti2AlC0.5N0.5 and Ti3AlCN have been synthesized using this method with a different atmosphere (Ar, N2, and Air) at a relatively lower temperature with a shorter period of time and characterized by XRD, SEM, particle size analyzer, and HRTEM. The relative density was measured by Archimedes’ principle in case of all the samples and electrical conductivity of the respective samples was determined by the four-probe method.

Purging of argon gas in the molten metal bath is a process that is regularly involved in secondary steel making operations. The injected gas imparts momentum to the liquid metal, which induces high turbulence in the molten metal and helps in homogenization of the bath composition and temperature, and facilitates the slag metal interactions. In this study, a computational fluid dynamics (CFD) based numerical investigation is carried out on an argon gas stirred ladle to study the flow and interface behavior in a secondary steel making ladle. A transient, three phase coupled level-set volume of fluid (CLSVOF) model is employed to track the slag-metal, gas-metal and slag-gas interfaces. The transient behavior of slag layer deformation and open eye formation is studied for different slag layer to metal bath height ratios at various argon gas flow rates.

The ductility of Magnesium alloys is often limited due to the strong basal texture. Initial attempts were made to reduce the intensity of the basal textures by adding rare earth(RE) elements. However, owing to the cost and scarcity of RE elements, alloys with calcium and tin were recently introduced. Among them, Mg-2Sn-2Ca alloys are the most promising, with high strength reported in the extruded condition. In this work, we report, for the first time, the mechanical properties of the alloy in the sheet form. In plane mechanical anisotropy in compressive behaviour is studied along with the detailed characterisation of texture, microstructure and the deformation twins in the material. Unlike strongly basal textured Mg alloys, deformation twinning was also observed in samples loaded along the normal direction. Crystal plasticity simulations were performed to confirm the slip and twin activity. Furthermore, a detailed analysis of deformation twins revealed that the so called Schmid twins accommodate strain by both twin growth and slip, whereas the non-Schmid twins accommodate the strain exclusively by crystallographic slip.

Diborides of Zr, Ti and Hf have potential applications as ultrahigh temperature ceramics, armor material, MEMS, substrates for optoelectronic applications and solar absorbers. Previous reports suggest that crystallographically textured diboride compacts have better mechanical and functional properties. Current methods of fabricating textured compacts are either cumbersome or energy and time intensive. We demonstrate a novel method for fabrication of bulk textured TiB2 by reactive spark plasma sintering (RSPS) of Ti-B mixtures having graphene nanoplatelets (GNP) as reinforcement. GNP acted as a template resulting in formation of large TiB2 grains with plate type morphology. X-ray diffraction and Electron backscattered diffraction analysis revealed presence of basal texture in TiB2 – GNP compacts. In comparison to GNP, carbon nanotube addition is shown to result in randomly oriented equiaxed grains. The improvement in indentation fracture toughness indicates GNP is better toughening additive compared to CNT.

For the first time, this study shows that distortion in a crystal structure due to magnetic effects is possible in a lattice with extreme chemical disorder. The transition metal-high entropy oxide (TM-HEO), (Co,Cu,Mg,Ni,Zn)O, has been attracting a lot of attention due to its unique application potential in many fields including electrochemical energy storage. In the present investigation, nanocrystalline TM-HEO was synthesised by three bottom-up methods. The presence of distortion in the rocksalt crystal structure, revealed by X-ray diffraction and Raman spectroscopy, and correlated with magnetic measurements from Superconducting quantum interference device (SQUID) and Electron paramagnetic resonance (EPR) studies could be attributed to the additive effects of exchange striction (from the magnetic constituents) and magnetic anisotropy (from the decreased crystallite size). Iron has been added to the TM-HEO to show that a higher amount of magnetic constituent increases the distortion in the lattice. Nanocrystalline TM-HEO also showed a “core-shell” magnetic behaviour below the bifurcation temperature arising from the uncompensated or canted spin at the surface. Néel temperature of the nanocrystalline TM-HEO is reported for first time to be as high as 700 K. This study helps unravel the structure and magnetic properties of such high entropy materials, and augurs a definite scope for better understanding of the factors influencing the crystal structure in high entropy oxides.

The effect of strain rate and nitrogen content on cyclic deformation and substructural changes in 316LN stainless steel is investigated at temperatures 773, 823 and 873 K. Dynamic strain aging (DSA) and/or thermal-recovery processes are observed to control cyclic deformation, and the regimes of their predominance are mapped. An increase in nitrogen content and DSA enhanced cyclic stress and are found to offset thermal-recovery induced cyclic strength reduction. In addition, strain localization in the form of slip-bands impinging on grain boundary is observed. The predominance of thermal-recovery over DSA manifested as dislocation-poor channels, dislocation cells within and in-between planar slip-bands.