[en] Highlights: ► The paper describe the effect of cryo rolling process on solution treated bulk Al 6063. ► The cryo rolled sheets and solutionized sheets were subjected to aged to induce precipitation. ► We investigated that the effects of ageing duration and cryo rolled at different strains on the refinement of alloys. ► We observed that after cryo rolling, the strength is increasing due to the presence of second phase Mg₂Si. ► The sub-grains are heavily deformed after cryo rolling. - Abstract: The microstructural characterization and mechanical properties of precipitation hardenable 6063 Al alloy subjected to cryo rolling (CR) are reported in this present work. The cryo rolled sheets and solutionized sheets were subjected to aged to induce precipitation. Microstructural and mechanical properties evaluation were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), conventional test and portable ball indentation technique test. The effect of two types of ageing treatment (peak aging and over aging) on solutionized and cryo rolled samples were investigated in this work. Ultrafine grained microstructure with improved strength was obtained after solutionized and cryo rolling. The mechanical properties of before and after cryo rolling at different ageing treatment were compared.

[en] Polyacenes in their armchair geometry (phenacenes) have recently been found to possess appealing electronic and optical properties with higher chemical stability and comparatively larger band gap as compared to linear polyacenes. They also behave as high-temperature superconductors upon alkali metal doping. Moreover, the optical properties of crystalline picene can be finely tuned by applying external pressure. We investigated the variation of optical gap as a function of altering the interplanar distances between parallel cofacial phenacene dimers. We employed both time-dependent density functional theory and density matrix renormalization group (DMRG) technique to investigate the lowest singlet excitations in phenacene dimer. Our study showed that the lowest singlet excitation in these systems evolved as a function of interplanar separation. The optical excitation energy gap decreases as a function of inverse interplanar separation of the phenacene dimer. The distant dependent variation of optical absorption at the dimer level may be comparable with experimental observation in picene crystal under pressure. DMRG study also demonstrates that besides picene, electronic properties of higher phenacenes can also be tunable by altering interplanar separation

[en] We studied the nature of the ground state and low-lying excited states of armchair polyacene oligomers (Polyphenanthrene) within long-range Pariser-Parr-Pople model Hamiltonian with up to 14 monomers using symmetrized density matrix renormalization group technique. The ground state of all armchair polyacenes studied is found to be singlet. The results show that lowest singlet dipole allowed excited state has higher energy for armchair polyacenes as compared to linear fused polyacenes. Moreover, unlike linear fused polyacenes, the lowest singlet excited state of these oligomers is always found to lie below the lowest dipole forbidden two-photon state indicating that these armchair polyacene oligomers strongly fluoresce. The calculations of low-lying excitations on singly and triply electron doped armchair polyacene oligomers show a low energy band with strong transition dipole moment that coupled to charge conductivity. This implies armchair polyacene posses novel field-effect transistor properties

[en] One of the most important issues of standard conventional test methods for determining mechanical properties of ductile materials is that the requirement of test materials should be adequate and it cannot be applied to in-service components. Therefore, in this study, an instrumented ball indentation technique has been applied to determine a few mechanical properties of a 6063 Al alloy in order to establish a correlation between microstructure and its mechanical properties. The as-received 6063 alloy sample was in cast and homogenized condition. It was solutionized at 520 deg. C with two aging treatments. It was found that solutionized material with subsequent aging at 220 deg. C for 3 h does not produce any noticeable variation in mechanical properties. Whereas, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) on solutionized samples after 7 h aging at 220 deg. C of the same material revealed precipitation of sub-micron size (of the order of 60 nm) Mg₂Si phase and this subsequently resulted in variation in the mechanical properties of the alloy.

[en] In this present investigation, the structures of SO₂, SO₂⁻, CO₂ and CO₂⁻ as well as the adiabatic electron affinities of the corresponding SO₂ and CO₂ neutral parents are computed using the infinite-order coupled-cluster method with all singles and doubles and non-iterative inclusion of triple excitations (CCSD(T)) and the Møller–Plesset perturbation theory up to the second order (MP2). The molecular structure, fundamental frequencies with their corresponding intensities and electron localization functions of SO₂ and SO₂⁻ are interpreted from respective ground-state optimized electronic structures calculated and compared with other simple tri-atomic molecules CO₂ and CO₂⁻, respectively. The electron localization function are then quantified and the second-order perturbation energies for different oxygen lone pairs (n) to σ^* and π^* interactions of S–O and C–O bond orbitals have been calculated by carrying out NBO analysis and the results are compared. The change in the electronic structure of the molecule after the attachment of a low-energy (≤15 eV) electron, thus forming a transient negative ion (TNI), can be interpreted from the n → σ^* and n → π^* interactions. The potential energy curves of SO₂–SO₂⁻ and CO₂– CO₂⁻ pairs are calculated as a function of S–O and C–O bond distance and bending angle. The adiabatic electron affinity of SO₂ is calculated to be positive while that of CO₂ is negative. Graphical abstract:

.

[en] 6063 Al alloy is subjected to severe plastic deformation through high-pressure torsion (HPT) using disk samples. The values of the Vickers microhardness and equivalent strain were recorded along diameters in each disk. The microhardness of 6063 Al alloys increases strongly and continuously with increasing equivalent strain but levels off and enters into a steady-state where the hardness remains unchanged with further straining. It is confirmed that the yield and ultimate tensile strength also follows the same single role of the equivalent strain as the hardness. Transmission electron microscopy showed that a subgrain structure develops at an initial stage of straining with individual grains containing dislocations. When increasing the straining, the subgrain size decreases whereas the misorientation angle increases and more dislocations are formed within the grains. In the steady-state range, some recrystallized grains formed which are free from dislocations. The mechanism for the grain refinement is discussed in terms of dislocation mobility.

[en] We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope. The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (NGC 628 and NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R ₂₅). We estimated the sizes, star formation rates (SFRs), metallicities, and Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least 10 times smaller in area than those in the inner disk. The SFRs per unit area (Σ_SFR) in both regions have similar mean values, but the outer SFCs have a much smaller range of Σ_SFR. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (H i) distribution and detected within and near several H i holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q > 1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some nonluminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q < 1) to trigger local disk instabilities in the outer disk.

[en] An aluminum-copper alloy (Al-2024) was successfully subjected to high-pressure torsion (HPT) up to five turns at room temperature under an applied pressure of 6.0 GPa. The Al-2024 alloy is used as a fuselage structural material in the aerospace sector. Mechanical properties of the HPT-processed Al-2024 alloy were evaluated using the automated ball indentation technique. This test is based on multiple cycles of loading and unloading where a spherical indenter is used. After two and five turns of HPT, the Al-2024 alloy exhibited a UTS value of ∼1014 MPa and ∼1160 MPa respectively, at the edge of the samples. The microhardness was measured from edges to centers for all HPT samples. These results clearly demonstrate that processing by HPT gives a very significant increase in tensile properties and the microhardness values increase symmetrically from the centers to the edges. Following HPT, TEM examination of the five-turn HPT sample revealed the formation of high-angle grain boundaries and a large dislocation density with a reduced average grain size of ∼80 nm. These results also demonstrate that high-pressure torsion is a processing tool for developing nanostructures in the Al-2024 alloy with enhanced mechanical properties

[en] Triangulum-II, a newly discovered dwarf spheroidal galaxy, is a strong candidate for the indirect search of dark matter through the detection of γ-ray emission that could originate from the pair- annihilation of weakly interacting massive particles (WIMPs). We here report on the analysis of almost seven years Fermi Gamma-Ray Space Telescope data of Triangulum-II which was taken during its all sky survey operation mode. No excess γ-ray emission has been detected above 100 MeV from Triangulum-II. We derive the upper limits on γ-ray flux assuming both the power-law spectra and the spectra related to WIMP annihilation. In this work, we have considered several theoretical WIMP (neutralinos here) models envisioning both thermal and non-thermal production of WIMPs, and put limits on pair-annihilation cross section of WIMPs to constrain the parameter space related to those theoretical models.

[en] We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (H i) velocity dispersion (σ _{H i}). We have applied the method to nearby, gas-rich galaxies that have extended H i gas disks and have low inclinations. The galaxy sample includes four large disk galaxies, NGC 628, NGC 6496, NGC 3184, and NGC 4214, and three dwarf galaxies, DDO 46, DDO 63, and DDO 187. We have used archival H i data from The H i Nearby Galaxy Survey (THINGS) and the LITTLE THINGS survey to derive the H i gas distributions and Spitzer mid-infrared images to determine the stellar disk mass distributions. We examine the disk dynamical and baryonic mass ratios in the extreme outer disks where there is H i gas but no visible stellar disk. We find that for the large galaxies, the disk dynamical and H i gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total H i gas mass dominates the baryonic mass, i.e., M(H i) ≥ M(stars), the disk dynamical mass is much larger than the baryonic mass. For these galaxies, there must either be a very low-luminosity stellar disk which provides the vertical support for the H i gas disk or there is halo dark matter associated with their disks, which is possible if the halo has an oblate shape so that the inner part of the dark matter halo is concentrated around the disk. Our results are important for explaining the equilibrium of H i disks in the absence of stellar disks and is especially important for gas-rich, dwarf galaxies that appear to have significant dark matter masses associated with their disks.