Department of Metallurgical & Materials Engineering

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A commercial Zirconium alloy was subjected to different thickness reductions (20%, 40% and 60%) by cold rolling. A through-thickness gradient in microstructure, crystallographic texture and residual stress was observed. This gradient was till 1/8th of the specimen thickness, and implied a corresponding anisotropy in the imposed strain state. An elasto-plastic FE (finite element) model was developed to capture such through thickness deformation gradients. A reasonably good agreement was observed between the experimental and predicted residual stress distributions when the material anisotropy was accounted for. Through-thickness residual stress evolution was shown to be significantly affected by material anisotropy and to a lesser extent by the rolling parameters (coefficient of friction and rotational speed). © 2017 CIRP

World University root node

CIRP Journal of Manufacturing Science and Technology

Experimental characterization and finite element modeling of through thickness deformation gradient in a cold rolled zirconium sheet

Fully recrystallized commercial Zirconium plates were subjected to uniaxial tension. Tests were conducted at different temperatures (123 K - 623 K) and along two plate directions. Both directions were nominally unfavorable for deformation twinning. The effect of the working temperature on crystallographic texture and in-grain misorientation development was insignificant. However, systematic variation in work hardening and in the area fraction and morphology of deformation twins was observed with temperature. At all temperatures, twinning was associated with significant near boundary mesoscopic shear, suggesting a possible linkage with twin nucleation. A binary tree based model of the polycrystal, which explicitly accounts for grain boundary accommodation and implements the phenomenological extended Voce hardening law, was implemented. This model could capture the measured stress-strain response and twin volume fractions accurately. Interestingly, slip and twin system hardness evolution permitted multiplicative decomposition into temperature-dependent, and accumulated strain-dependent parts. Furthermore, under conditions of relatively limited deformation twinning, the work hardening of the slip and twin systems followed two phenomenological laws proposed in the literature for non-twinning single-phase face centered cubic materials. © 2016 Acta Materialia Inc.

Acta Materialia

Temperature dependence of work hardening in sparsely twinning zirconium

We investigate the thermoelectric properties of the half-Heusler alloys XCoSb (X: Ti,Zr,Hf) by solving Boltzmann transport equations and discuss them in terms of the electronic band structure. The rigid band approximation is employed to address the effects of doping. While many half-Heuser alloys show excellent thermoelectric performance, the materials under study are special by supporting both n- and p-doping. We identify the reasons for this balanced thermoelectric transport and explain why experimentally p-doping is superior to n-doping. We also determine the spectrum of phonon mean free paths to guide grain refinement methods to enhance the thermoelectric figure of merit. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Physica Status Solidi (B) Basic Research

Thermoelectric Properties of the XCoSb (X: Ti,Zr,Hf) Half-Heusler alloys

We solve numerically the Boltzmann transport equations of the phonons and electrons to understand the thermoelectric response in heterostructures of M2CO2 (M: Ti, Zr, Hf) MXenes with transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding, contribute significantly to the thermal transport, and compensate for the reduced contributions of the acoustic phonons (increased scattering cross-sections in heterostructures), such that the thermal conductivities turn out to be similar to those of the bare MXenes. Our results indicate that the important superlattice design approach of thermoelectrics (to reduce the thermal conductivity) may be effective for two-dimensional van der Waals materials when used in conjunction with intercalation. © 2016 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Thermal response in van der Waals heterostructures

The effects of die channel angle (Φ) in hot (~623 K) equi-channel angular pressing (ECAP) on microstructure, and tensile and compressive flow properties of AZ80 Mg alloy were investigated. Two solid ECAP dies, having Φ of (1) dual 60° and 120° in a single die and (2) 90° in another die, were designed for this purpose. Grain refinement with more than 40% reduction in average grain size along with submicron size second-phase β-precipitates was achieved after single-pass ECAP. A great variation in β-Mg17Al12 phase morphology with increasing flow stresses in tension and compression are found with decreasing value of angle Φ. There found an increasing effect on strain to failure with decrease in porosity and second-phase precipitate modification. However, there appears flow asymmetry between tension and compression with the latter exhibiting greater flow stress and strain to failure. © 2017, ASM International.

Journal of Materials Engineering and Performance

Effect of Solid Die Equi-Channel Pressing Angle on β-Mg17Al12 Phase Morphology and Mechanical Behavior of AZ80 Mg alloy

This paper presents a systematic investigation of the electric properties of epitaxial PMN-33PT thin film as a function of temperature and frequency. Complex impedance measurements were performed over the temperature range of 30–500 K and frequency range 20 Hz–1 MHz. Obtained results were corrected for the SRO series resistance. Ferroelectric hysteresis loops and electric field tunability of the real part of dielectric permittivity were measured in the 300 K–450 K temperature range and at a frequency of 1 kHz. The results of electrical field tunability of complex dielectric permittivity were approximated by Landau-Ginsburg-Devonshire theory frame by the Johnson relation. Meanwhile, ferroelectric hysteresis loop measurements of epitaxial PMN-33PT thin film shows a good hysteresis property with a remnant polarization of 2Pr (Formula presented.) 10 µC/cm2 and coercive field of 2Ec (Formula presented.) 12 kV/cm at temperature range from 300 K to 380 K. © 2017 Taylor & Francis Group, LLC.

Ferroelectrics

Electrical properties of PMN-33PT thin film at MPB

Brandon’s criterion is frequently used to quantify distribution of coincident site lattice (CSL) boundaries in studies related to grain boundary character distribution. This criterion is based on theoretical considerations and is meant to define a range within which special boundaries may exist. Experiments have repeatedly shown that this range includes boundaries which do not show special properties. A broad aim of this study is to explore if there exists a cut-off in deviation which includes only boundaries with special properties. While most other criteria in literature are based on theoretical dependence of secondary dislocation spacing, in this work we find the cut-off deviation through experimental data of recrystallised microstructures from in situ as well as ex-situ heating experiments. Deviation from structure was considered in terms of both, deviation of misorientation axis-angle as well as deviation of the boundary plane from the symmetric tilt orientation. Our results indicate that the deviation in terms of misorientation angle is more important than boundary plane deviation. We also show that the limiting deviation for various orders of twin boundaries (Σ3n) in a recrystallized microstructure is a constant and approximately 1° which is significantly lower than that defined by Brandon’s criterion. We show that this constant cut-off deviation for various Σ3n CSL boundaries can also be obtained by assuming that the secondary dislocations are spaced proportional to displacement shift complete lattice vector. Similar analysis was also carried out for non-Σ3n boundaries but due to limited statistics, no cut-off value could be deduced for these boundaries. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

Philosophical Magazine

Cut-off deviation for CSL boundaries in recrystallized face-centered cubic materials

High-resolution electron energy loss spectroscopy (HR-EELS) is utilized to probe the surface spin canting in nanoparticles of two technologically important magnetic materials, i.e. Fe3O4 and CoFe2O4 (CFO). A soft experimental technique has been developed that is capable of extracting EELS spectra with single atomic plane resolution recorded in a single frame. The technique yields information at different depths of the nanoparticle from the surface to the core regions with high signal-to-noise ratio and without beam damage. This enables comparing the fine structures between the surface and core regions of the nanoparticles. The results confirm earlier observations of uniformly oriented spin canting structure for CFO and provide additional information regarding atom site-selective spin canting information. In the case of Fe3O4, preferred canting orientation forming core and shell structure is deduced. Unlike earlier reports based on polarized spin-flip neutron scattering measurement, it is possible from the experimental spectra combined with the first principle-based calculations considering noncollinear magnetism to narrow down the canting angles for Fe3O4 (Td,Oh tilts 40°, 40°) and CFO (Td,Oh tilts 17°, 17°). In addition, the role of Dzyaloshinskii-Moriya interaction in stabilizing the spin canting at the nanoparticle surface is discussed. The results demonstrate that HR-EELS can be a powerful technique to probe the magnetic structure in nanodimensional systems and has advantages over neutron-based techniques in terms of superior spatial resolution, site-specific information, and ease of sample preparation. © 2017 American Physical Society.

Fulltext

Physical Review B

Surface spin canting in F e3 O4 and CoF e2 O4 nanoparticles probed by high-resolution electron energy loss spectroscopy

Proceedings of the seventeenth international conference on thin films: abstracts

Strain induced modifications of the magnetic moment in epitaxial multiferroic BiFeO3 and enhanced magneto-electric coupling in SrRuO3/BiFeO3 heterostructures

Inconsistent growth of tissues and poor osteoconduction performance on the metallic implant surfaces due to variation of surface energy are major contributing factors for failure of most metallic implant on account of lack of stronger attachment with surrounding bone or tissues. In this study, composite coating of hydroxyapatite, calcium hydrogen phosphate and MWCNT was developed on SS316 surface with varying amount of calcium phosphate-hydroxyapatite phase and crystallinity by pulsed electrodeposition. TEM study revealed that the MWCNTs were bonded strongly with the in situ deposition phases and thus act as reinforcement in the deposited coating similar to the collagen fiber in natural bone structure. Presence of MWCNT reinforcement increased the overall coating modulus of elasticity in the range of 6–10 GPa similar to that of natural bone. Different coating surfaces with varied amount of phase and crystallinity exhibits altogether different phenomena and growth geometry of apatite formation during osteoconduction period under contact with SBF. Coatings with highest amount of hydroxyapatite phase exhibit formation of porous spherical (~ 1 μm) and rod like (~ 600 nm) scaffold structure along with presence of nanopores (~ 100 nm) all along the contact surfaces. Cell proliferation study indicated uniform and fast spreading of cells over the coating surfaces as compared to bare metallic implant. EIS study illustrated five times high corrosion resistance capability along with formation of passivation layer under contact with SBF for coating comes with 66% of hydroxyapatite phase. © 2017 Elsevier B.V.

Surface and Coatings Technology

MWCNT reinforced bone like calcium phosphate—Hydroxyapatite composite coating developed through pulsed electrodeposition with varying amount of apatite phase and crystallinity to promote superior osteoconduction, cytocompatibility and corrosion protection performance compared to bare metallic implant surface