A systematic investigation of fracture mechanisms in Al-Si based eutectic alloy-Effect of Si modification

The fracture characteristics of Al-Si based eutectic alloy are investigated in the unmodified and modified conditions under compression. The investigations are carried out at different strain rates and temperatures. Fracture of the alloy starts with eutectic Si particle fracture and modification plays an important role in particle fracture. The fraction of fractured particles is found to be always lesser in the modified condition than in the unmodified condition. Particle fracture increases with increase in strain. It is found that the Si particle fracture shows an increase with increase in strain rate and decreases with increase in temperature at 10% strain. Large and elongated particles show a greater tendency for fracture in the unmodified and modified conditions. Particle orientation plays an important role on fracture and the cracks are found to occur almost in a direction normal to the tensile strain imposed upon the particles by the deforming matrix in the unmodified alloy. The modified alloy shows a random distribution of fractured particles and crack orientation. The criteria of fracture based on dislocation pile-up mechanism and fiber loading explain the observed difference in particle fracture characteristics due to modification. The particle fracture for the modified alloy is also discussed in terms of Weibull statistics and the existing models of dispersion hardening. Particle/matrix interface decohesion is observed at higher strain rates and temperatures in the modified alloy. Dendritic rotation of 10 degrees is also observed at higher strain rates, which can increase the amount of particle fracture. (C) 2013 Elsevier B.V. All rights reserved.

Comparative Evaluation of Switching and Average Models of a DC-DC Boost Converter for Real-Time Simulation

This paper presents a comparative evaluation of the average and switching models of a dc-dc boost converter from the point of view of real-time simulation. Both the models are used to simulate the converter in real-time on a Field Programmable Gate Array (FPGA) platform. The converter is considered to function over a wide range of operating conditions, and could do transition between continuous conduction mode (CCM) and discontinuous conduction mode (DCM). While the average model is known to be computationally efficient from the perspective of off-line simulation, the same is shown here to consume more logical resources than the switching model for real-time simulation of the dc-dc converter. Further, evaluation of the boundary condition between CCM and DCM is found to be the main reason for the increased consumption of resources by the average model.

Effect of chemical treatment on surface characteristics of sputter deposited Ti-rich NiTi shape memory alloy thin-films

NiTi thin-films were deposited by DC magnetron sputtering from single alloy target (Ni/Ti: 45/55 aL.%). The rate of deposition and thickness of sputter deposited films were maintained to similar to 35 nm min(-1) and 4 mu m respectively. A set of sputter deposited NiTi films were selected for specific chemical treatment with the solution comprising of de-ionized water, HF and HNO3 respectively. The influence of chemical treatment on surface characteristics of NiTi films before and after chemical treatment was investigated for their structure, micro-structure and composition using different analytical techniques. Prior to chemical treatment, the composition of NiTi films using energy dispersive X-ray dispersive spectroscopy (EDS), were found to be 51.8 atomic percent of Ti and 48.2 atomic percent of Ni. The structure and morphology of these films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD investigations, demonstrated the presence of dominant Austenite (110) phase along with Martensite phase, for untreated NiTi films whereas some additional diffraction peaks viz. (100), (101), and (200) corresponding to Rutile and Anatase phase of Titanium dioxide (TiO2) along with parent Austenite (110) phase were observed for chemically treated NiTi films. FTIR studies, it can be concluded that chemically treated films have higher tendency to form metal oxide/hydroxide than the untreated NiTi films. XPS investigations, demonstrated the presence of Ni-free surface and formation of a protective metal oxide (TiO2) layer on the surface of the films, in both the cases. The extent of the formation of surface oxide layer onto the surface of NiTi films has enhanced after chemical treatment. (C) 2014 Elsevier B.V. All rights reserved.

Effect of nanoscale boron carbide particle addition on the microstructural evolution and mechanical response of pure magnesium

In this study, the effect of nano-B4C addition on the microstructural and the mechanical behavior of pure Mg are investigated. Pure Mg-metal reinforced with different amounts of nano-size B4C particulates were synthesized using the disintegrated melt deposition technique followed by hot extrusion. Microstructural characterization of the developed Mg/x-B4C composites revealed uniform distribution of nano-B4C particulates and significant grain refinement. Electron back scattered diffraction (EBSD) analyses showed presence of relatively more recrystallized grains and absence of fiber texture in Mg/B4C nanocomposites when compared to pure Mg. The evaluation of mechanical properties indicated a significant improvement in tensile properties of the composites. The significant improvement in tensile ductility (similar to 180% increase with respect to pure Mg) is among the highest observed when compared to the pure Mg based nanocomposites existing in the current literature. The superior mechanical properties of the Mg/B4C nanocomposites are attributed to the uniform distribution of the nanoparticles and the tendency for texture randomization (absence of fiber texture) achieved due to the nano-B4C addition. (C) 2013 Elsevier Ltd. All rights reserved.

PROPENSITIES OF AMINO ACID RESIDUES IN PROTEINS FOR DIFFERENT REGIONS OF THE RAMACHANDRAN MAP

Using a dataset of 1164 crystal structures of largely non-homologous proteins defined at a resolution of 1.5 angstrom or better, we have investigated the (phi,psi) preferences of 20 residue types by considering the residues which occur in loops. Propensities of residue types to occur in the loops with (phi,psi) values in the aa region of the Ramachandran map has a poor correlation coefficient of 0.48 to the Chou-Fasman propensities of the residue types to occur in the a-helical segments. However the correlation coefficient between propensities of residues in loops to adopt beta conformations and those in beta-sheet is much higher (0.95). These observations suggest that a-helix formation is well influenced by the local amino acid sequence while intrinsic preference of residue types for beta-sheet plays a major role in the formation of beta-sheet. The main chain polar groups of residues in loops, that can affect the (phi,psi) values, can be involved in intra-molecular hydrogen bonding. Therefore we investigated further by considering subset of residues in loops with low (0 to 2) number of intra-molecular hydrogen bonds per residue involving main chain polar atoms. For this subset, the correlation coefficients between propensities for alpha-helix and alpha(R) region and between beta-sheet and beta-region are 0.26 and 0.64 respectively. This reiterates higher intrinsic tendency of beta-region favouring residues to adopt beta-sheet than alpha(R) region favouring residues to adopt alpha-helical structure.

Confinement enhances dispersion in nanoparticle-polymer blend films

Polymer nanocomposites constitute an important class of materials whose properties depend on the state of dispersion of the nanoparticles in the polymer matrix. Here we report the first observations of confinement-induced enhancement of dispersion in nanoparticle-polymer blend films. Systematic variation in the dispersion of nanoparticles with confinement for various compositions and matrix polymer chain dimensions has been observed. For fixed composition, strong reduction in glass transition temperature, T-g, is observed with decreasing blend-film thickness. The enhanced dispersion occurs without altering the polymer-particle interactions and seems to be driven by enhanced matrix-chain orientation propensity and a tendency to minimize the density gradients within the matrix. This implies the existence of two different mechanisms in polymer nanocomposites, which determines their state of dispersion and glass transition.

Digestive ripening and green synthesis of ultra-small (r < 2 nm) stable ZnO quantum dots

Given the recent reports pertaining to novel optical properties of ultra-small quantum dots (QDs) (r <2 nm), this nanomaterial is of relevance to both technology and science. However it is well known that in these size regimes most chalocogenide QD dispersions are unstable. Since applications often require use of QD dispersions (e.g. for deployment on a substrate), stabilizing these ultra-small particles is of practical relevance. In this work we demonstrate a facile, green, solution approach for synthesis of stable, ultra-small ZnO QDs having radius less than 2 nm. The particle size is calculated using Brits' equation and confirmed by transmission electron micrographs. ZnO QDs reported remain stable for > 120 days in ethanol (at similar to 298-303 K). We report digestive ripening (DR) in TEA capped ZnO QDs; this occurs rapidly over a short duration of 5 min. To explain this observation we propose a suitable mechanism based on the Lee's theory, which correlates the tendency of DR with the observed zeta potentials of the dispersed medium. To the best of our knowledge this is the (i) first report on DR in oxide QDs, as well as the first direct experimental verification of Lee's theory, and (ii) most rapid DR reported so far. The facile nature of the method presented here makes ultra-small ZnO readily accessible for fundamental exploration and technologically relevant applications. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Stress Engineering using Si3N4 for stiction free release of SOI beams

We report on the effect of thin silicon nitride (Si3N4) induced tensile stress on the structural release of 200nm thick SOI beam, in the surface micro-machining process. A thin (20nm / 100nm) LPCVD grown Si3N4 is shown to significantly enhance the yield of released beam in wet release technique. This is especially prominent with increase in beam length, where the beams have higher tendency for stiction. We attribute this yield enhancement to the nitride induced tensile stress, as verified by buckling tendency and resonance frequency data obtained from optical profilometry and laser doppler vibrometry.

Life of flame particles embedded in premixed flames interacting with near isotropic turbulence

Flame particles are surface points that always remain embedded on, by comoving with a given iso-scalar surface within a flame. Tracking flame particles allow us to study the fate of propagating surface locations uniquely identified throughout their evolution with time. In this work, using Direct Numerical Simulations we study the finite lifetime of such flame particles residing on iso-temperature surfaces of statistically planar H-2-air flames interacting with near-isotropic turbulence. We find that individual flame particles as well as their ensemble, experience progressively increasing tangential straining rate (K-t) and increasing negative curvature (kappa) near the end of their lifetime to finally get annihilated. By studying two different turbulent flow conditions, flame particle tracking shows that such tendency of local flame surfaces to be strained and cusped towards pinch-off from the main surface is a rather generic feature, independent of initial conditions, locations and ambient turbulence intensity levels. The evolution of the alignments between the flame surface normals and the principal components of the local straining rates are also tracked. We find that the surface normals initially aligned with the most extensive principal strain rate components, rotate near the end of flame particles' lifetime to enable preferential alignment between the surface tangent and the most extensive principal strain rate component. This could explain the persistently increasing tangential strain rate, sharp negative curvature formation and eventual detachment. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

An Implementation of Partitioned Scheduling Scheme for Hard Real-Time Tasks in Multicore Linux with Fair Share for Linux Tasks

The correctness of a hard real-time system depends its ability to meet all its deadlines. Existing real-time systems use either a pure real-time scheduler or a real-time scheduler embedded as a real-time scheduling class in the scheduler of an operating system (OS). Existing implementations of schedulers in multicore systems that support real-time and non-real-time tasks, permit the execution of non-real-time tasks in all the cores with priorities lower than those of real-time tasks, but interrupts and softirqs associated with these non-real-time tasks can execute in any core with priorities higher than those of real-time tasks. As a result, the execution overhead of real-time tasks is quite large in these systems, which, in turn, affects their runtime. In order that the hard real-time tasks can be executed in such systems with minimal interference from other Linux tasks, we propose, in this paper, an integrated scheduler architecture, called SchedISA, which aims to considerably reduce the execution overhead of real-time tasks in these systems. In order to test the efficacy of the proposed scheduler, we implemented partitioned earliest deadline first (P-EDF) scheduling algorithm in SchedISA on Linux kernel, version 3.8, and conducted experiments on Intel core i7 processor with eight logical cores. We compared the execution overhead of real-time tasks in the above implementation of SchedISA with that in SCHED_DEADLINE's P-EDF implementation, which concurrently executes real-time and non-real-time tasks in Linux OS in all the cores. The experimental results show that the execution overhead of real-time tasks in the above implementation of SchedISA is considerably less than that in SCHED_DEADLINE. We believe that, with further refinement of SchedISA, the execution overhead of real-time tasks in SchedISA can be reduced to a predictable maximum, making it suitable for scheduling hard real-time tasks without affecting the CPU share of Linux tasks.

Phase formation and stability of alloy phases in free nanoparticles: some insights

This paper explores phase formation and phase stability in free nanoparticles of binary alloys. A procedure for estimating the size and composition dependent free energies incorporating the contributions from the interfaces has been presented. Both single phase solid solution and two phase morphology containing interphase interfaces have been considered. A free energy scenario has been evaluated for two binary alloy systems Ag-Ni and Ag-Cu to predict the microstructure of the alloy nanoparticles at different size ranges as a function of composition. Both Ag-Cu and Ag-Ni systems exhibit wide bulk immiscibility. Ag-Ni nanoparticles were synthesized using the wet chemical synthesis technique whereas Ag-Cu nanoparticles were synthesized using laser ablation of a Ag-Cu target immersed in distilled water. Microstructural and compositional characterization of Ag-Ni and Ag-Cu nanoparticles on a single nanoparticle level was conducted using transmission electron microscopy. Nanoparticle microstructures observed from the microscopic investigation have been correlated with thermodynamic calculation results. It is shown that the observed two phase microstructure consisting of Ag-Ni solid solution in partial decomposed state coexisting with pure Ag phases in the case of Ag-Ni nanoparticles can be only be rationalized by invoking the tendency for phase separation of an initial solid solution with increase in nanoparticle size. Smaller sized Ag-Ni nanoparticles prefer a single phase solid solution microstructure. Due to an increase in particle size during the synthesis process the initial solid solution decomposes into an ultrafine scale phase separated microstructure. We have shown that it is necessary to invoke critical point phenomenon and wetting transition in systems showing a critical point that leads to phase separated Ag-Ni nanoparticles providing a catalytic substrate for the nucleation of equilibrium Ag over it. In the case of the Ag-Cu system, we report the experimental observation of a core shell structure at small sizes. This can be rationalized in terms of a metastable solid solution. It is argued that the nucleation barrier can prevent the formation of biphasic morphology with an internal interface. In such a situation, demixing of the solid solution can bring the system to a lower energy configuration. This has lead to the observed core-shell morphology in the Ag-Cu system during room temperature synthesis.

Structural crossover from nonmodulated to long-period modulated tetragonal phase and anomalous change in ferroelectric properties in the lead-free piezoelectric Na1/2Bi1/2TiO3-BaTiO3

The highly complex structure-property interrelationship in the lead-free piezoelectric (x) Na1/2Bi1/2TiO3 - (1 - x) BaTiO3 is a subject of considerable contemporary debate. Using comprehensive x-ray, neutron diffraction, dielectric, and ferroelectric studies, we have shown the existence of a new criticality in this system at x = 0.80, i.e., well within the conventional tetragonal phase field. This criticality manifests as a nonmonotonic variation of the tetragonality and coercivity and is shown to be associated with a crossover from a nonmodulated tetragonal phase (for x < 0.8) to a long-period modulated tetragonal phase (for x > 0.80). It is shown that the stabilization of long-period modulation introduces a characteristic depolarization temperature in the system. While differing qualitatively from the two-phase model often suggested for the critical compositions of this system, our results support the view with regard to the tendency in perovskites to stabilize long-period modulated structures as a result of complex interplay of antiferrodistortive modes Bellaiche and Iniguez, Phys. Rev. B 88, 014104 ( 2013); Prosandeev, Wang, Ren, Iniguez, ands Bellaiche, Adv. Funct. Mater. 23, 234 (2013)].

Hydrogen-induced mechanical properties of amorphous silicon thin films

Mechanical properties of thin films such as residual stress and hardness are of paramount importance from the device fabrication point of view. Intrinsic stress in sputtered films can be tensile or compressive as decided by the number density and the energy of the plasma species striking the growing film. In the presence of hydrogen we analyzed the applicability of idealized stress reversal curve for amorphous silicon thin films deposited by DC, pulsed DC (PDC) and RF sputtering. We are successfully able to correlate the microstructure with the stress reversal and hardness. We observed a stress reversal from compressive to tensile with hydrogen incorporation. It was found that unlike in idealized stress reversal curve case, though the energy of plasma species is less in DC plasma, DC deposited films exhibit more compressive stress, followed by PDC and RF deposited films. A tendency towards tensile stress from compressive stress was observed at similar to 13, 18 and 23 at%H for DC, PDC and RF deposited films respectively, which is in exact agreement with the vacancy to void transition in the films. Regardless of the sputtering power mode, the hardness of a-Si:H films is found to be maximum at C-H similar to 10 at%H. Enhancement in hardness with C-H (up to C-H similar to 10 at%H) is attributed to increase of Si-H bonds. Beyond C-H similar to 10 at%H, hardness starts falling. (C) 2015 Elsevier Ltd. All rights reserved.

Recent advances in engineering topography mediated antibacterial surfaces

The tendency of bacterial cells to adhere and colonize a material surface leading to biofilm formation is a fundamental challenge underlying many different applications including microbial infections associated with biomedical devices and products. Although, bacterial attachment to surfaces has been extensively studied in the past, the effect of surface topography on bacteria-material interactions has received little attention until more recently. We review the recent progress in surface topography based approaches for engineering antibacterial surfaces. Biomimicry of antibacterial surfaces in nature is a popular strategy. Whereas earlier endeavors in the field aimed at minimizing cell attachment, more recent efforts have focused on developing bactericidal surfaces. However, not all such topography mediated bactericidal surfaces are necessarily cytocompatible thus underscoring the need for continued efforts for research in this area for developing antibacterial and yet cytocompatible surfaces for use in implantable biomedical applications. This mini-review provides a brief overview of the current strategies and challenges in the emerging field of topography mediated antibacterial surfaces.

Evolution of Texture and Microstructure in Deformed and Annealed Copper-Iron Multilayer

The effect of multiple phases on the evolution of texture during cold rolling and annealing of a copper-iron multilayer, fabricated by accumulative roll bonding, has been studied. The presence of an iron layer affects the deformation texture of the copper layer only at very large strains. On the other hand, a strong effect of copper on iron is observed at both small and large strains. At smaller strains, the larger deformation carried by the copper suppresses the texture development in the iron, whereas, at higher strains, selection of specific orientation relationship at the interface influences the texture of the iron layer. Shear banding and continuous dynamic recrystallization were found to influence the evolution of texture in the copper layer. The influence of large plastic deformation on the recrystallization behavior of copper is demonstrated with the suppression of typical fcc annealing texture components, described as constrained recrystallization. Evolution of typical annealing texture component is suppressed because of the multilayer microstructure. The plane of the interface formed during deformation is determined by a combination of the rolling texture of individual phases, constrained annealing, and the tendency to form a low-energy interface between the two phases during annealing.