Melting and solidification characteristics of Zr-, Ni-, and Mn-containing 354-type Al-Si-Cu-Mg cast alloys

This study investigated the effects of adding Zr, as a base alloying element, besides Ni and Mn in different amounts and combinations on the melting and solidification characteristics of 354-type Al-Si-Cu-Mg alloys. Differential scanning calorimetry (DSC) was used to characterise the sequence of reactions occurring during the heating and/or cooling cycles; whereas scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to observe and identify existing intermetallic phases. Nickel proved to have a retarding effect on the kinetics of precipitation of the α-Al network and the eutectic Al-Si structure. Also, the presence of Ni consumed a considerable amount of Cu to form Al-Cu-Ni particles instead of Al₂Cu particles. Results revealed that solution treatment at 495°C for 5?h was sufficient to dissolve a large amount of Al₂Cu particles in the α-Al matrix, which is mandatory for a successful aging treatment of the alloys studied. Additions of these transition elements produced new intermetallic phases such as (Al,Si)₃(Ti,Zr), (Al,Si)₃Zr, Al₉FeNi, Al₃Ni, Al₃CuNi, and Al₉FeSi₃Ni₄Zr, in addition to the other phases, namely α-Al, eutectic silicon, Al₂Cu, Mg₂Si, Q-phase (Al₅Cu₂Mg₈Si₆), commonly observed in 354-type alloys, and Fe-based intermetallic phases including β-Al₅FeSi, α-Al₁₅(Fe, Mn)₃Si₂, and π-Al₈FeMg₃Si_6. Superheating the melt at 800°C instead of 750°C had an advantageous effect in that Al₃Zr particles originating from the Al-15%Zr master alloy were dissolved and hence coarse Zr-containing particles were barely spotted in the microstructures examined.     

First principle calculation of accurate native defect levels in CaF<Subscript>2</Subscript>

We report on the first principle density functional calculation of the charge transition levels of native defects (vacancies and interstitials) in CaF₂ structure. The transition level was defined as the Fermi level where two charge states of given defect have the same formation energy. The common error in the band gap inherited to semiclocal density functional has been accounted for by incorporating the hybrid density functional method, leading to correct placement of the transition levels within the band gap. The band gap size from hybrid calculation has been validated using the full potential, Linearized Augmented Planewave method with the Modified-Becke-Johnson exchange potential. Prior to level calculations, we ensured that an agreement between the formation energies from small (95–97 atoms) and large (323–325 atoms) supercells was achieved after applying the Makov-Payne correction method. Our calculated transition level for the anion vacancy was 2.97 eV below the conduction band, agreeing with the experimental optical absorption band at 3.3 eV associated with the electron transition from the ground state F-center to the conduction band in CaF₂.     

Evanescent-wave in perturbed optical fibers

Two adjacent components may express the total electromagnetic fields of an optical fiber. The first one is defined as a summation over bound modes describing the spatial steady state where the light energy is guided along a non-absorbing waveguide. The second component is the radiation field, which describes the spatial transient. Since total internal reflection on a straight fiber is malfunctioned by bending, therefore the evanescent wave which represents the second component has been investigated in a bent single-mode fiber. The decay constant and phase propagation constant of the evanescent field are determined for the first time as a function of curvature at two standard operating wavelengths 1300 and 1550 nm using an interferometric technique.     

Pointwise Efficiency in Multiobjective Stochastic Linear Programming

In most of the approaches to the Multiobjective Stochastic Linear Programming problem that have been proposed in the literature, the notion of quality of a solution is not adequately defined. We reconsider this problem from a decision point of view, in contexts where either the decision maker's preference structure cannot be described by a utility function, or where this structure is expressed by an unknown non-decreasing utility function and the probability distribution of the random parameters is unknown. We define a fundamental set of ‘pointwise admissible’ solutions, as well as several subsets of particular interest. We discuss the relevance of these various pointwise efficient sets, their interrelations, and their practical identification.     

Positivity and Stability for a Population Equation with Diffusion on 1

Using positivity, abstract extrapolation spaces, and a generalization of the Jörgens–Vidav-Voigt perturbation theorem for the essential spectral radius of strongly continuous semigroups given in [19], we study the asymptotic behavior of a linear age-dependent population problem with spatial diffusion where the birth and the death rates depend on the spatial variable.