Department of Metallurgical & Materials Engineering

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The present study explores the possibility of using ultrasonic treatment for the conversion of dendritic microstructure of in-situ A356/TiB2 composite into non-dendritic globular structure. A356/2TiB2 in-situ composite has been subjected to high intensity ultrasonic treatment in liquid metal state as well as during the process of solidification. Microstructural analysis shows that the ultrasonic treatment during the process of solidification is an effective technique for the transformation of dendritic morphology into fine globular structure along with the modification of Si needles. © 2015, The Indian Institute of Metals - IIM.

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Transactions of the Indian Institute of Metals

Role of Ultrasonic Treatment on Microstructural Evolution in A356/TiB2 In-Situ Composite

MgAl2O4 particles were in-situ synthesised in Al-Mg melts using SiO2 precursor assisted by ultrasonic treatment (UT). 7 to 8 Fold reduction in grain size is achieved in Al-1Mg-0.1SiO2 alloy subjected to 5 min UT. Reason behind the grain reduction is explained in terms of ultrasonic cavitation activated SiO2 reduction reaction, thereby increasing the number density of MgAl2O4 particles and their wettability. © 2015 Elsevier B.V. All rights reserved.

Materials Letters

Ultrasonic assisted grain refinement of Al-Mg alloy using in-situ MgAl2O4 particles

Metallic glasses have emerged as promising biodegradable materials due to their excellent corrosion resistance properties. However, processing constraints for achieving desired section thickness have limited their real world applications. This study involves the development of partially amorphous bio-compatible and bio-degradable Mg66Zn30Ca4 and Mg60Zn35Ca5 alloys with adequate section thickness for bio-medical components. The corrosion rates of these alloys are analyzed through in-vitro corrosion studies. The combined contributions of the amorphous structure and alloying elements have been used to explain their corrosion behaviors. © 2015 Elsevier Ltd.

Materials and Design

Role of partially amorphous structure and alloying elements on the corrosion behavior of Mg-Zn-Ca bulk metallic glass for biomedical applications

We present control over the ambipolar nature of Te in ZnO and the corresponding optical band gap in epitaxial thin film grown by pulsed laser deposition. Experimentally, the decrease in band gap is found to be strongly dependent on the Te concentration for TeO compared to TeZn. Emission at 3eV for TeZn, irrespective of Te content is explained by first principle calculation considering Zn vacancy in the lattice. An experimental band bowing parameter of ∼7eV is obtained for TeO and is in agreement with modified Becke-Johnson potential (mBJLDA) based theoretical calculation. The physical insight into the nature of band bowing is elucidated. © 2015 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

physica status solidi (b)

Ambipolar behavior of Te and its effect on the optical emission of ZnO: Te epitaxial thin film

We report on the observation of Co vacancy (VCo) induced charge ordering and ferromagnetism in CoO epitaxial thin film. The ordering is associated with the coexistence of commensurate, incommensurate, and discommensurate electronic phases. Density functional theory calculation indicates the origin of ordering in Co atoms undergoing high spin to low spin transition immediately surrounding the VCo(16.6 at. %). Electron magnetic chiral dichroism experiment confirms the ferromagnetic signal at uncompensated Co spins. Such a native defects induced coexistence of different electronic phases at room temperature in a simple compound CoO is unique and adds to the richness of the field with the possibility of practical device application. © 2015 AIP Publishing LLC.

Applied Physics Letters

Native defect induced charge and ferromagnetic spin ordering and coexisting electronic phases in CoO epitaxial thin film

One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ChemSusChem

Hydrogen storage materials for mobile and stationary applications: current state of the art

A combined experimental and numerical method is developed to estimate the continuously evolving cyclic plastic strain amplitudes in plastically deformed subsurface regions of a case-hardened M50 NiL steel rod subjected to rolling contact fatigue (RCF) over several hundred million cycles. The subsurface hardness values measured over the entire plastically deformed regions and the elastoplastic von Mises stresses determined from the three-dimensional (3D) Hertzian contact finite element (FE) model have been used in conjunction with Neuber's rule to estimate the evolved cyclic plastic strain amplitudes at various points within the RCF-affected zone. The cyclic stress-strain plots developed as a function of case depth revealed that cyclic hardening exponent of the material is greater than the monotonic strain-hardening exponent. Effective S-N diagram for the RCF loading of the case-hardened steel has been presented and the effect of compressive mean stress on its fatigue strength has been explained using Haigh diagram. The compressive mean stress correction according to Haigh diagram predicts that the allowable fatigue strength of the steel increases by a factor of two compared to its fatigue limit before mean stress correction, thus potentially allowing the rolling element bearings to operate over several hundred billion cycles. The methodology presented here is generalized and can be adopted to obtain the constitutive response and S-N diagrams of both through- and case-hardened steels subjected to RCF. Copyright © 2015 by ASME.

Journal of Tribology

Cyclic constitutive response and effective S-N diagram of M50 NiL case-hardened bearing steel subjected to rolling contact fatigue

Using repeated or cyclic annealing process a recrystallised ultrafine grained microstructure possessing high strength and a reasonable ductility was obtained from a heavily cold rolled Austenitic Stainless Steel. Johnson Mehl Avrami and Kolmogorov model for predicting recrystallisation process was used to optimize the process parameters for the formation of ultra-fine grained recrystallised microstructure. The microstructure evolving during thermal cycling was analyzed by electron microscopy. The tensile properties of various ultrafine grained microstructures were determined. Electron microscopy studies on tensile deformed specimens showed that there is a narrow dispersion in grain size distribution of ultrafine grained microstructure which promotes shear banding or strain localization causing impairment of the ductility or poor plastic strain hardenability. © 2014 Elsevier Ltd.

Ultrafine grained microstructure tailoring in austenitic stainless steel for enhanced plasticity

Calibre rolling of Mg-3Al-1Zn alloy at 300 °C led to development of fine grain size of 3 μm. Subsequent annealing, from 5 to 6000 minutes at 300-450 °C, revealed faster grain growth initially up to 60 minutes, which became sluggish on prolonged annealing. The time exponent for grain growth kinetics (n) suggests bi-linear behaviour with n = 0.11 and 0.008 over these time scales. The activation energy, based on various n values, varied over wide ranges that made the understanding of the mechanisms for grain growth difficult. This problem is explained by concurrent evolution of texture and grain boundary structure. The effect of grain growth on hardness at ambient temperature was found to follow the H-P type relationship. © 2015.

Fulltext

Journal of Magnesium and Alloys

Grain growth in calibre rolled Mg-3Al-1Zn alloy and its effect on hardness

The high temperature deformation behaviour of cast and homogenised near α titanium alloy Titan 29A (M/s MIDHANI, India), equivalent to IMI 834 (M/s TIMET, UK) was studied towards developing a processing map. The processing map is interpreted in terms of the microstructural changes occurring during deformation, based on the values of a dimensionless parameter η which represents the energy dissipation through microstructural evolution processes. An instability criterion (ξ &lt; 0) is applied to demarcate the flow instability regions in the processing map. Both the parameters (η and ξ) were computed from the experimental data generated by hot isothermal compression tests conducted at various temperature (T) and strain rate (έ) combinations T = 850-1100 °C and έ = 3 × 10−4 to 10 0/s. The deterministic domains observed in the investigated temperature and strain rate regime are attributed to continuous dynamic recrystallization of α lamella and dynamic recrystallization of β grains. © 2013, Springer-Verlag France.