The thermoelastic martensitic transformation inβ′ Ni-Al alloys: I. Crystallography and morphology

This investigation describes a study of the crystallographic features of the thermoelastic martensitic transformation inβ′ Ni-Al alloys. Experimental measurements of the habit plane, shape strain and orientation relationship have been made, and the results are found to be in good agreement with the predictions of the Bowles-Mackenzie phenomenological theory, assuming δ = 1.000, p₂', and {110}_β', and d₂ = <110> _β'. The martensite habit plane normals are close to {2, 14, 15}_β' and are typically clustered in self-accommodating groups of four crystallographically equivalent variants centered around {011}_β' poles. The experimental shape strain is found to be exactly an invariant plane strain with the displacement direction lying ∼92 deg from the habit plane normal.     

Parallel adaptive simulations of dynamic fracture events

Finite element simulations of dynamic fracture problems usually require very fine discretizations in the vicinity of the propagating stress waves and advancing crack fronts, while coarser meshes can be used in the remainder of the domain. This need for a constantly evolving discretization poses several challenges, especially when the simulation is performed on a parallel computing platform. To address this issue, we present a parallel computational framework developed specifically for unstructured meshes. This framework allows dynamic adaptive refinement and coarsening of finite element meshes and also performs load balancing between processors. We demonstrate the capability of this framework, called ParFUM, using two-dimensional structural dynamic problems involving the propagation of elastodynamic waves and the spontaneous initiation and propagation of cracks through a domain discretized with triangular finite elements.     

Electro-conducting glass-ceramics produced by ion-exchange and reduction treatments

The crystallization characteristics of some bismuth-containing soda-lime-silica glasses have been studied. The addition of bismuth reduces the glass transition temperature as well as the crystallization temperatures of these glasses. Electrically conducting layers have been induced in such glass-ceramics by subjecting them to a Na⁺ Ag⁺ ionexchange reaction followed by a reduction treatment in hydrogen. Resistances of the surfaces vary from 0.08 /square to 14.76 /square depending on the glass composition as well as the reduction parameters. Induced surface conductance tends to rise with the increase of volume of the crystalline phase in the parent glass. The TCR values range between 400 and 2300 p.p.m. K^–1. The thicknesses of these layers are about 130 m. The high surface conductivity arises from the percolation of the silver metallic phase in the glass-crystal boundary region. The glass-crystal interface is believed to act as heterogeneous nucleation sites.     

Electrical conductivity of ion-exchanged oxide glasses

Bulk glasses in the system Na₂O-CaO-Al₂O₃-B₂O₃-SiO₂ have been subjected to a sodium silver exchange reaction. The application of a suitable combination of electric field and temperature brings about switching to a high conducting state (HIE). The critical electric field,E _c, has a value in the range 2 to 20 V cm^–1 andE _c values decrease as a function of temperature. The resistivity in the HIE state is of the order of 10 cm and the activation energy is 0.01 eV. Wagner's asymmetric polarization cell measurements show that the conductivity is ionic in nature. No switching is observed in the case of glasses subjected to a sodium copper exchange reaction carried out on these glasses at 525 ° C for 12h. This is ascribed to the microstructure exhibited by such exchanged samples which consists of copper-rich droplet phases dispersed within a copper-deficient matrix. The rather large inter-droplet separation makes it difficult for the copper-rich phase to grow to its percolation limit.     

Organometallic route to nanocomposite synthesis

The nature and scope of different varieties of nanocomposite materials are described. The practical and basic aspects of the physical properties of nanocomposites are discussed. The principles underlying the preparation of diphasic materials involving either ceramic-metal or glass-metal combinations are delineated. The sol-gel technique involving suitable organometallic compounds has been shown to be a versatile method for making these materials. A brief survey is given about the different materials made to date following the organometallic route.     

Multiferroic behavior in glass-crystal nanocomposites containing Te2NiMnO6

Nanocomposites containing nanocrystals of Te₂NiMnO₆ were synthesized by suitable heat treatment of a glass with composition 2 TeO₂·NiO·MnO (molar ratio). The crystallites had dimensions in the range 17-41 nm. X-ray diffraction data of the specimens were analyzed by using a TREOR computer programme. Lattice parameters extracted by this method indicated that the crystal symmetry was monoclinic. The nanocomposites exhibited weak ferromagnetism in the temperature range 2-300 K. They also showed ferroelectric hysteresis at room temperature with a remanent polarization of 0.015 μC/cm². The specimens showed a magnetodielectric (MD) behavior with dielectric constant increasing as a function of applied magnetic field. The MD parameter obtained in the present system was 0.55%.