Department of Metallurgical & Materials Engineering

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Magnesium (Mg)-based metallic glasses are considered as possible candidates in orthopedic implant applications. This paper aims to theoretically predict the glass-forming ability (GFA) in Mg-Zn-Ca alloy using a newly proposed thermodynamic model (PHHS), and the consistency of this model is verified through experimental analysis. PHHS is based on thermodynamic parameters such as enthalpy of chemical mixing, elastic enthalpy, and configurational entropy, thus incorporating the pivotal effects, i.e., electron transfer effects, effect of atomic size mismatch, and effect of randomness, which aid to high GFA. In essence, PHHS can be visualized as the energy barrier that exists between the transformations of random atomic structure of glass to ordered crystalline structure. When the PHHS value is more negative, the energy barrier will be high, supporting easy glass formation. Various Mg-Zn-Ca metallic glass compositions displayed almost an expected and supporting trend, where the critical diameter of the metallic glass rod increased with a more negative PHHS value. Among the predicted Mg-Zn-Ca systems, the Mg60Zn35Ca5 composition shows deviation from the expected trend. This discrepancy has been clearly elucidated using a eutectic phase diagram. In addition to the consistency of the PHHS parameter to verifying the GFA of various compositions, the unique ability of this model is to predict unexplored Mg-Zn-Ca glass-forming compositions using contour development. Thus, proving PHHS parameter to be used as an efficient tool in predicting new glass-forming compositions. © 2015, The Minerals, Metals & Materials Society and ASM International.

World University root node

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Exploring Mg-Zn-Ca-Based Bulk Metallic Glasses for Biomedical Applications Based on Thermodynamic Approach

Electron magnetic chiral dichroism (EMCD) in a transmission electron microscope is an element-specific magnetic characterization technique and is extremely powerful for understanding magnetism of materials at the nanoscale. However, quantitative EMCD remains a challenge. In the present paper, we have highlighted and overcome major difficulties associated with the technique. For example, the experimentally observed low dichroic signal and imbalance between the L3 and L2 edge have been explained based on the oscillatory nature of electron propagation through the crystal thickness and specific momentum resolved signal detection, respectively. With this advancement in understanding, site-specific quantitative EMCD has been accomplished in epitaxial thin films of two important ferrimagnetic spinel oxides, NiFe2O4 (NFO) and CoFe2O4 (CFO), with varying degree of cation mixing and A site cation defects. A simple model based on phenomenological absorption has been developed for different site-specific signal contributions for the inverse spinel structure. The experimental moment values for NFO and CFO obtained using EMCD are in good agreement with first principle based theoretical calculations and the results strengthen the promise of utilizing EMCD as a routine nanoscale magnetic characterization technique. © 2015 American Physical Society.

Physical Review B

Effect of Bloch wave electron propagation and momentum-resolved signal detection on the quantitative and site-specific electron magnetic chiral dichroism of magnetic spinel

High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1-xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300-773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and TAGS. © 2015 American Chemical Society.

Chemistry of Materials

High Thermoelectric Performance and Enhanced Mechanical Stability of p-type Ge1–xSbxTe

We have utilized spatially resolved high resolution electron energy loss spectroscopy to quantify the relative percentage of ferromagnetic order in the core and the surface regions of CuCr2S4 nanoparticles with nanocube and nanocluster morphology. The organic capping layer is found to play a significant role in restoring magnetic order at the surface. The technique is based on recording the fine features of the Cr L3 absorption edge and matching them with the theoretical spectra. The nanoscale probing technique we have developed is quite versatile and can be extended to understand magnetic ordering in a number of nanodimensional magnetic materials. © 2015 AIP Publishing LLC.

Applied Physics Letters

Spatially resolved quantitative magnetic order measurement in spinel CuCr2S4 nanocrystals

Deformation twinning was directly observed in three commercial zirconium alloy samples during split channel die plane-strain compression. One pair of samples had similar starting texture but different grain size distributions, while another pair had similar grain size distribution but different starting textures. Extension twinning was found to be more sensitive to the starting texture than to the grain size distribution. Also, regions of intense deformation near grain boundaries were observed. A hierarchical binary tree-based polycrystal plasticity model, implementing the Chin-Hosford-Mendorf twinning criterion, captured the experimentally observed twinning grains’ lattice orientation distribution, and the twin volume fraction evolution, provided the critical resolved shear stress for extension twinning, $$ \tau_{0} , $$τ0, was assumed much larger than any of the values reported in the literature, based on the viscoplastic self-consistent model. A comparison of the models suggests that $$ \tau_{0} $$τ0 obtained using the present model and the viscoplastic self-consistent models physically correspond to the critical stress required for twin nucleation, and twin growth, respectively. © 2015, The Minerals, Metals & Materials Society and ASM International.

Deformation Twinning in Zirconium: Direct Experimental Observations and Polycrystal Plasticity Predictions

High-energy ball milling is utilized in creating microstructural changes (such as change in lattice parameter, lattice strain) within micro-ceria, which were quantified using line profile analysis by X-ray diffraction. Crystallite size significantly decreased from 85 to 11 nm when subjected to 4 h of high-energy ball milling, and then remained same with increase in milling duration (up to 16 h of milling). Three different methods, namely Nelson–Riley function, Cohen’s method, and Pawley fitting, were used to calculate the lattice parameter and a lattice expansion from 5.4082 to 5.4147 Å was observed for a duration of 0 up to 12 h. The effect of milling on generation of residual strain and the probability of stacking fault in affecting the lattice parameter are delineated in the current work. Non-monotonic change of lattice parameter with saturation of crystallite size is attributed to the vacancy-induced stresses that generate at inter-crystallite regions and render excessive free volume during high-energy ball milling. © 2015, Springer Science+Business Media New York.

Journal of materials science

Non-monotonic lattice parameter variation in ball-milled ceria

Solid oxide fuel cells (SOFC) have emerged as energy conversion devices in achieving high efficiency of over 70% with regeneration. The critical components of SOFC include anode, electrolyte, and cathode. However, for a stack of individual SOFCs, the evaluation of sealants and interconnects are also essential. In this review article, material selection, fundamentals of operation and underlying mechanisms, processing, microstructural and phase characterization, and the functionality and performance of individual SOFC components are presented in detail. The major challenges and complexity in functional section of SOFC include: (i) poisoning via sulfur and coke deposition, surface diffusion of adsorbate, and charge transfer at triple-phase boundary (TPB) in anode, (ii) hindered O2- migration that converts chemical energy into electrical energy in the solid electrolyte (thus, the creation of ion transfer channels, ease of O2- migration, dissociation of vacancy around dopants, straining of lattice, and other factors such as control of phase and its distribution, grain and grain boundary conductivity, become critical in designing the electrolytes for SOFCs), (iii) multiple rate determining factors such as geometry of active surfaces, and existence of overpotential, in cathode (thereby, comprehensive electrochemical impedance spectroscopy is required for the analysis of solid cathodes in SOFC), (iv) chemical incompatibility and instability in both oxidizing and reducing environments while matching the coefficient of thermal expansion (CTE) in the interconnects in order to sustain large number of thermal cycling during the operation of SOFC, and (v) isolation of the fuel and oxidizing gases while matching the CTE of the anode, cathode and interconnects, using sealant. Moreover, the glass-transition of sealant dictates the maximum allowable working temperature of SOFC. Thus, the necessitated temporal progress in material selection along with a detailed insight into the conceptual role of thermodynamics and kinetics of surface/cell reactions, effect of phases and microstructure on conductivity, fuel flexibility and deterioration in performance of individual fuel cell components, and evolution of new materials are coherently presented. This article provides a comprehensive review with respect to the structure, chemistry, design and selection of materials, underlying mechanisms, and performance of each SOFC component, and it opens up the future directions towards pursuing SOFC research. © 2015 Elsevier Ltd. All rights reserved.

Progress in Materials Science

Progress in material selection for solid oxide fuel cell technology: A review

Biocorrosion and biodegradation behavior of Mg–4Li–1Ca alloy were investigated for industrially important end product conditions, namely the homogenized, rolled, and rolled + annealed ones. Among the three, homogenized material showed the highest corrosion rate (27.2 mm/year) in a simulated body fluid (SBF) owing to its coarse grain structure containing long dumbbell-shaped eutectic phase. Rolled? annealed material exhibited the lowest corrosion rate (0.94 mm/year) corresponding to the highest corrosion resistance (1.854 kΩ cm2) in SBF. This higher corrosion resistance is associated with a uniform distribution of corrosion sites and a lower occurrence of twins in the microstructure. However, the rolled material showed a greater corrosion rate due to an appreciable volume fraction of {1011} compression twins, {1012} tension twins, and {1011}–{1012} double twins, which form galvanic couples with the adjacent grains that enhances localized corrosion. A mechanism of biodegradation at the alloy/SBF interface is proposed. It involves the formation of bone-like hydroxyapatite and metastable octa calcium phosphate, along with other degradation products, such as magnesium hydroxide and lithium hydroxide. © Springer Science+Business Media New York 2015.

Journal of Materials Science

Biocorrosion and biodegradation behavior of ultralight Mg–4Li–1Ca (LC41) alloy in simulated body fluid for degradable implant applications

Cu-1wt% Al2O3 nano-composites were prepared by three different methods, namely mechanical alloying, combustion synthesis and electrical explosion of wire, in order to investigate the effect of power processing technique on microstructure and ambient properties. The powders produced by these techniques were hydrogen reduced at 500°C and subsequently vacuum encapsulated in copper container followed by consolidation using hot extrusion at 800°C and 600MPa to nearly full density. As-synthesized composite powders as well as the extruded samples were characterized by scanning electron microscope, X-ray diffraction, transmission electron microscope, thermal conductivity and electrical conductivity. Microhardness and room temperature deformation behavior in compression at a strain rate of 10-4s-1 were measured. The mechanically milled powders, upon consolidation, exhibited a finer grain size of 146nm and a hardness of 164 HV1.0 compared to that obtained by two other techniques, which ranged between 179-390nm and 135-156 HV1.0, respectively. The physical and mechanical properties were found to vary with grain size according to the Hall-Petch type relationship, irrespective of the processing technique employed for getting these grain sizes. © 2013 Elsevier B.V.

Materials Science and Engineering A

Microstructure and properties of hot extruded Cu-1wt% Al2O3 nano-composites synthesized by various techniques

Differential strain rate compression tests were conducted to study flow behavior of hot rolled Al-5wt% B4C composite as a function of sample orientation (longitudinal and transverse) over the temperature and strain rate ranges of 25-500°C and 10-4 to 1s-1, respectively. The longitudinal samples are found to show lower flow stress than that shown by the transverse samples in the temperature range of 25-200°C. The reverse becomes true at higher temperatures of 300-500°C. The values of stress exponent (n) and activation energy for deformation (Q), based on applied stress, ranged from 10 to 46 and 307-416kJ/mol, respectively. However, by considering effective stress, these values were reduced to n=8 and Q=126-190kJ/mol. This stress exponent ofn=8 is further reduced to n=5 by considering substructural evolution, which suggests the dislocation climb creep mechanism to be favorable for deformation. © 2013 Elsevier Ltd.