Department of Metallurgical & Materials Engineering

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In this article, we report on the Co atom incorporation, secondary phase formation and composition-dependent magnetic and optical properties of mixed valence Co(II, III)xZn1-xO epitaxial thin films grown by pulsed laser deposition. The intended total Co concentration is varied between ~6-60 at.% with relatively higher concentration of +3 over +2 charge state. Mixed valence Co(II, III) shows high solubility in ZnO (up to 38 at.%) and ferromagnetism is observed in samples with total Co incorporation of ~29 and 38 at.%. Electron diffraction pattern and high resolution transmission electron microscopy images reveal single crystalline nature of the thin films with wurtzite structure. Co oxide interlayer, with both rock salt and spinel structure, are observed to be formed between the substrate and wurtzite film for total Co concentration at ~17 at.% and above. Magnetization shows composition dependence with a saturation moment value of ~93 emu cm-3 and a coercive field of ~285 Oe observed for ~38 at.% Co:ZnO films. Ferromagnetism was not observed for films with Co concentration 17 and 9 at.%. The Co oxide interlayer does not show any ferromagnetism. All the films are n-type with carrier concentration ~1019 cm-3. The observed magnetism is probably resulting from direct antiferromagntic exchange interaction between Co2+ and Co3+ ions favored by heavy Co alloying giving rise to ferrimagnetism in the system. © 2013 Elsevier B.V.

World University root node

Journal of magnetism and magnetic materials

Structural and magnetic characterization of mixed valence Co (II, III) xZn1− xO epitaxial thin films

We report here the growth of epitaxial Co-metal thin film on c-plane sapphire by pulsed laser deposition (PLD) using Co:ZnO target utilizing the composition inhomogeneity of the corresponding plasma. Two distinct plasma composition regions have been observed using heavily alloyed Co 0.6Zn0.4O target. The central and intense region of the plasma grows Co:ZnO film; the extreme tail grows only Co metal with no trace of either ZnO or Co-oxide. In between the two extremes, mixed phases (Co+Co-oxides+Co:ZnO) were observed. The Co metal thin film grown in this way shows room temperature ferromagnetism with large in-plane magnetization ~1288 emu cm-3 and a coercivity of ~230 Oe with applied field parallel to the film-substrate interface. Carrier density of the film is ~1022 cm-3. The film is epitaxial single phase Co metal which is confirmed by both X-ray diffraction and transmission electron microscopy characterizations. Planar Hall Effect (PHE) and Magneto Optic Kerr Effect (MOKE) measurements confirm that the film possesses similar attributes of Co metal. The result shows that the epitaxial Co metal thin film can be grown from its oxides in the PLD. © 2014 Elsevier B.V.

Journal of crystal growth

Epitaxial Co metal thin film grown by pulsed laser deposition using oxide target

The universal binding energy relation (UBER), derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleaved surfaces are allowed to relax. We suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of surfaces relaxed during cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in a number of crystal systems. Our results show that the total energies of all the relaxed surfaces lie on a single (universal) energy surface, that is given by the proposed functional form which contains an additional length-scale associated with structural relaxation. This functional form could be used in modelling the cohesive zones in crack growth simulation studies. We find that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occurring at higher displacements. © 2014 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Universal binding energy relation for cleaved and structurally relaxed surfaces

Ultrafine grain (UFG) microstructure of Al-30Si alloy formed by friction stir processing (FSP), was correlated with its corrosion property. Electrochemical impedance spectroscopy and anodic polarization studies of FSPed Al-30Si alloy in 3.5. wt.% NaCl solution showed nobler corrosion and pitting potential as well as reduction of corrosion current density with increasing number of FSP passes. The homogenization of UFG microstructure with refined primary silicon particles changed the semi-conducting nature of passive film formed on Al-30Si alloy from n to p type resulting in improved corrosion resistance. Correlation of grain and silicon particle size on corrosion properties reveals Hall-Petch type relationship. © 2014 Elsevier Ltd.

Corrosion Science

Effect of multipass friction stir processing on corrosion resistance of hypereutectic Al-30Si alloy

As-cast AZ80 magnesium alloy consists of α-Mg, eutectic product of α-Mg and γ-Mg17Al12, with the latter present in the form of partially and fully divorce eutectic. There occurs dissolution of harder γ-Mg17Al12 phase during homogenization treatment at 400 ° and 439 °C. The proportion of the α-Mg and γ-Mg17Al12 phase was varied by solutionizing the alloy for various lengths of time at these temperatures, in order to investigate the kinetics of phase transformation and to evaluate the effect of phase proportion, size and morphology on room temperature tensile properties. It was found that the yield strength decreases with the increase in solutionizing temperature from 400° to 439 °C and at the same time, ductility in general increases with the increasing solutionizing temperature. The variation in tensile properties and the nature of fractographs were analyzed in terms of the effects of microstructure. © (2014) Trans Tech Publications, Switzerland.

Solid State Phenomena

Kinetics of γ-Mg17Al12 phase dissolution and its effect on room temperature tensile properties in as-cast AZ80 magnesium alloy

Mg-30Ca and Mg-14Li (wt %) master alloys were melted successively in the induction furnace to obtain a Mg-Li-Ca ternary alloy containing 3.99% Li and 1% Ca. The as-cast material of thickness 4 mm was homogenised at 350° C for 120 mins and subsequently rolled to 62.5% reduction in thickness at 300 °C to get 1.5 mm thick sheet. The microstructures of hot rolled samples were examined in as-rolled condition as well as after annealing at 350° C for various lengths of time. The presence of deformation twins was clearly seen in the as-rolled structure, whereas equiaxed twin-free grains were observed in the annealed condition. The average grain size was found to increase from 10 μm to 18 μm by annealing, according to the kinetics that follows a parabolic law. Tensile samples taken from rolled plate were deformed to failure at room temperature and a strain rate of 10-4 s-1. Ultimate tensile strength of as-rolled material increased to 213 MPa, while tensile elongation dropped to 6.5% from the initial values of 134 MPa and 8.5%, respectively. Annealing after rolling offered a good compromise between the enhanced tensile strength (160 MPa) and tensile ductility (9%) suggesting viability of the proposed thermomechanical treatment as a means for enhancing both strength and ductility of Mg -4Li-1Ca. © (2014) Trans Tech Publications, Switzerland.

Advanced Materials Research

Effect of rolling on microstructure and room temperature tensile properties of newly developed Mg-4Li-1Ca alloy

Cold worked and stress relieved Zr-2.5 wt%Nb alloy was annealed at 900 C for 5-90 min and quenched in water, which resulted in formation of α′ martensite and primary α phase. Reheating of the same samples to 700 C leads to α′ → αZr + βZr transformation following JMAK kinetics (n = 0.55 and k = 0.34). Tensile samples in annealed condition were deformed at 625-700 C over strain rates of 2 × 10-5-1 × 10-2 s-1. With the increase in annealing time, strain rate sensitivity and activation energy for deformation were found to change from 0.42 to 0.20 and 181.4-685 kJ/mol respectively. The maximum peak stress (136.5 MPa) obtained at 700 C from present results suggests higher load bearing ability of the Zr-2.5Nb alloy in annealed and quenched condition compared to that of cold-rolled and stress-relieved condition. © 2014 Elsevier B.V. All rights reserved.

Journal of Nuclear Materials

Microstructural evolution in and flow properties of Zr-2.5Nb pressure tube material at elevated temperature

The potential of WS2 as a thermoelectric material is assessed. The electronic contribution to the thermoelectric properties is calculated within the constant relaxation time approximation from the electronic band structure, whereas the lattice contribution is evaluated using self-consistently calculated phonon lifetimes. In addition, the dependence of the lattice thermal conductivity on the mean free path of the phonons is determined. © 2014 American Chemical Society.

Chemistry of Materials

WS2 as an excellent high-temperature thermoelectric material

Microstructure evolution after solutionizing and ageing treatment of cast AZ80 Mg alloy were investigated using optical and scanning electron microscopy. Effect of these treatments on grain size, β-Mg17Al12 intermetallic phase, mechanical behavior, and flow asymmetry were investigated. The initial continuous network of β-phase found to be reduced after solutionizing. The dissolution of β-phase and simultaneous grain growth are found to be interrelated. Mechanical properties including yield strength, maximum strength (ultimate compressive strength), and maximum strain attainable in compressive found almost twice than the corresponding values obtained in tension. The asymmetry in compressive and tensile properties is found to decrease with grain size at certain solutionizing duration. Particular heat treatment found to offer best combination of tensile compressive flow properties in AZ80 Mg alloy. Aging under certain conditions found to minimize the strength asymmetry. © ASM International.

Journal of Materials Engineering and Performance

Grain growth and β-Mg17Al12 intermetallic phase dissolution during heat treatment and its impact on deformation behavior of AZ80 Mg-alloy

The hot working behaviour of near-α titanium alloy - TITAN 29A (equivalent to IMI 834) with an acicular starting microstructure was evaluated by carrying out hot compression tests over a range of temperatures (850-1060°C) and strain rates (3×10-4-100/s). Using the flow curves, processing maps were generated to identify the safe processing window for the material. The material exhibits a deterministic domain between 920 and 1030°C at low strain rates of 3×10-4-10-3/s where it undergoes geometric dynamic recrystallisation (GDRX) or globularisation of α lamellae. The initiation and evolution of globularisation was investigated using the flow curve analysis method. The work hardening rate (θ)-flow stress (σ) curve was used to estimate the critical strain (εc) required for initiation of globularisation and the saturation stress (σsat) for dynamic recovery (DRV). The recrystallised or globularised volume fraction (X) was estimated from the difference between the calculated DRV and experimental DRX curves. The estimated globularised volume fraction modelled using Avrami equation was found to match with the microstructural observations. © 2014 Elsevier B.V.