On the role of intermetallic and interlayer in the dissimilar material welding of Ti6Al4V and SS 316L by friction stir welding

Joining titanium with stainless steel can lighten the structure of numerous industrial applications. However, a vast disparity of thermal, physical, and chemical properties between these alloys leads to defects in conventional arc welding techniques, viz., brittle intermetallic compounds, pores, cracks, etc. Friction stir welding (FSW) is a renowned solid-state joining technology for creating dissimilar material joints producing visco-plastic material flow at the interface. The present investigation compares the intermetallic layer thickness and properties as a function of the thickness of the Cu interlayer sandwiched in lap joints. Macrostructural and microstructural characterizations were carried out to understand the localized microstructural evolution comprising intermetallic, grain refinement, defects, etc. Mechanical properties were also evaluated for prepared lap joints

Comparison of hot-electron transmission in ferromagnetic Ni on epitaxial and polycrystalline Schottky interfaces

The hot-electron attenuation length in Ni is measured as a function of energy across two different Schottky interfaces viz. a polycrystalline Si(111)/Au and an epitaxial Si(111)/NiSi_2 interface using ballistic electron emission microscopy (BEEM). For similarly prepared Si(111) substrates and identical Ni thickness, the BEEM transmission is found to be lower for the polycrystalline interface than for the epitaxial interface. However, in both cases, the hot-electron attenuation length in Ni is found to be the same. This is elucidated by the temperature-independent inelastic scattering, transmission probabilities across the Schottky interface, and scattering at dissimilar interfaces.Comment: 5 pages, 5 figure

Electron-Hole Generation and Recombination Rates for Coulomb Scattering in Graphene

We calculate electron-hole generation and recombination rates for Coulomb scattering (Auger recombination and impact ionization) in Graphene. The conduction and valence band dispersion relation in Graphene together with energy and momentum conservation requirements restrict the phase space for Coulomb scattering so that electron-hole recombination times can be much longer than 1 ps for electron-hole densities smaller than $10^{12}$ cm$^{-2}$.Comment: 13 pages, 7 figure

Fragmentation of very high energy heavy ions

A stack of CR39 (C12H18O7)n nuclear track detectors with a Cu target was exposed to a 158 A GeV lead ion beam at the CERN-SPS, in order to study the fragmentation properties of lead nuclei. Measurements of the total, break-up and pick-up charge-changing cross sections of ultrarelativistic Pb ions on Cu and CR39 targets are presented and discussed.Comment: 4 pages, 3 EPS figures included with epsf, uses article.sty Talk presented by M. Giorgini at the Int. Conf. on Structure of the Nucleus at the Dawn of the Century, Bologna (Italy), May 29-June 3, 200

Influence of distinct tool pin geometries on aluminum 8090 FSW joint properties

Aluminum Lithium alloys are recuperating substantial interest from automotive and aerospace industries owing to their extraordinary specific strength as compared to conventional aluminum (2xxx, 6xxx, and 7xxx) alloys. The goal of the present investigation is to study AA 8090 joints produced with the unique solid-state welding technique friction stir welding (FSW). Tool pin profile induces remarkable influence on friction and further plastic deformation during FSW. Therefrom, the influences of three distinct but constant dynamic area conditioned tool pin geometries namely, square trapezoidal, hexagonal trapezoidal, and threaded taper on the resulting material flow patterns, mechanical properties, and the microstructure have been studied and discussed in detail. The FSW joint produced with hexagonal trapezoidal pin geometry delivered the highest joint resistance owing to grain refinement and almost flawless microstructure

Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.Comment: 4 pages, 5 figure

Probing the evolution of electronic phase-coexistence in complex systems by terahertz radiation

In complex oxides, the electrons under the influence of competing energetics are the cornerstone of coexistence (or phase-separation) of two or more electronic/magnetic phases in same structural configuration. Probing of growth and evolution of such phase-coexistence state is crucial to determine the correct mechanism of related phase-transition. Here, we demonstrate the combination of terahertz (THz) time-domain spectroscopy and DC transport as a novel strategy to probe the electronic phase-coexistence. This is demonstrated in disorder controlled phase-separated rare-earth nickelate thin films which exhibit metal-insulator transition in dc conductivity at around 180 K but lack this transition in terahertz (THz) dynamics conductivity down to low temperature. Such pronounced disparity exploits two extreme attributes: i) enormous sensitivity of THz radiation to a spatial range of its wavelength-compatible electronic inhomogeneities and ii) insensitivity to a range beyond the size of its wavelength. This feature is generic in nature (sans a photo-induced effect), depends solely on the size of insulating/metallic clusters and formulates a methodology with unique sensitivity to investigate electronic phase-coexistence and phase transition of any material system

Ultrastructural Patterns of Cell Damage and Death Following Gamma Radiation Exposure of Murine Erythroleukemia Cells

Radiation causes damage to cell surface membranes, cytoplasmic organelles, and the nuclear process of DNA synthesis and repair, and this eventually results in different modes of cell death. In this study we examined murine erythroleukemia (MEL) cells, exposed to 15 and 60 Gy of 10 MeV photonic energy, and left in culture for up to 96 hours. Electron microscopical analysis was performed on conventionally embedded samples and freeze-fracture replicas, in order to detect ultrastructural patterns of cell damage and death. Of interest was the observation of chromatin condensates, nuclear membrane associations and nuclear pore redistribution during early apoptosis. Pronounced rearrangements of transmembrane particles during late stages of cellular necrosis were also found. The morphological damage induced by both doses of radiation as a function of time after exposure was only quantitatively but not qualitatively different

Sample-Efficient Co-Design of Robotic Agents Using Multi-fidelity Training on Universal Policy Network

Co-design involves simultaneously optimizing the controller and agents physical design. Its inherent bi-level optimization formulation necessitates an outer loop design optimization driven by an inner loop control optimization. This can be challenging when the design space is large and each design evaluation involves data-intensive reinforcement learning process for control optimization. To improve the sample-efficiency we propose a multi-fidelity-based design exploration strategy based on Hyperband where we tie the controllers learnt across the design spaces through a universal policy learner for warm-starting the subsequent controller learning problems. Further, we recommend a particular way of traversing the Hyperband generated design matrix that ensures that the stochasticity of the Hyperband is reduced the most with the increasing warm starting effect of the universal policy learner as it is strengthened with each new design evaluation. Experiments performed on a wide range of agent design problems demonstrate the superiority of our method compared to the baselines. Additionally, analysis of the optimized designs shows interesting design alterations including design simplifications and non-intuitive alterations that have emerged in the biological world.Comment: 17 pages, 10 figure