B. B. Rath

The effect of substrate on the elastic properties of films determined by the indentation test — axisymmetric boussinesq problem

Abstract T he elastic solution of an axisymmetric mixed boundary value problem is considered. An elastic layer is assumed to be either in frictionless contact or perfectly bonded to a semi-infinite elastic half-space. The elastic field caused by the indentation of the elastic layer, by a rigid indenter is solved for spherical, conical and flat-ended cylindrical indenters. The results are obtained by solving a Fredholm integral equation of the second kind with a continuous symmetrical kernel which depends on the bonding conditions. Numerical results are given for several combinations of film and substrate elastic moduli and film thicknesses. These results provide a guideline for selecting the appropriate film thickness and substrate in order to determine the elastic constants of thin films.

Thermoelastic stresses in bimaterials

Abstract The recently developed solution of the elastic equations for a bimaterial with an inclusion is used to study the fundamental thermoplastic problem in dissimilar media. The dissimilar medium consists of two semi-infinite isotropic solids of different elastic properties either perfectly bonded together or in frictionless contact with each other at the planar interface. The solutions are obtained by a method which is based on the integration of properly weighted centres of dilatation over the volume occupied by the body. The potential functions for the problem solved are the harmonic potential function of attracting matter filling the element of volume, which is identical to that for the solid of indefinite extent as in Goodiers theory of thermoelastic stress, and its mirror image. The results are applied to the case of an expanding (or contracting) inclusion of any shape embedded in one of the semi-infinite solids near the planar boundary. Numerical results for a spherical inclusion with pure dila...

Characteristics of plasma processed SiC nanocrystallites and nanorods

Nanoparticles and nanorods of SiC were synthesized using arc-plasma processing of coarse particles. X-ray diffraction and Raman spectroscopic studies showed the presence of β-SiC and carbon nanotubes in the starting coarse particles and SiC nanorods in the ultrafine particles produced by plasma processing. Scanning electron microscopy and transmission electron microscopy confirmed the presence of carbon nanotubes in the starting material and nanorods of SiC in the plasma-processed samples.

Elastic Fields Due to Defects in Transversely Isotropic Bimaterials

A method for obtaining the analytic solution of the elastic fields due to defects such as inclusions, dislocations, disclinations, and point defects in transversely isotropic bimaterials is presented. The bimaterial consists of two semi-infinite transversely isotropic solids either perfectly bonded together or in frictionless contact with each other at a planar interface which is parallel to the plane of isotropy of both solids. The elastic solution is expressed in terms of the hexagonal stress vectors for the double force and the double force with moment. Closed form solutions for inclusions with pure dilatational eigenstrain, straight dislocation and disclination lines, circular, dislocation loops, and point defects are presented.

High temperature structural studies of HgS and HgSe quantum dots

We report the structural investigations of β-HgS and HgSe quantum dots as a function of temperature between 300 and 600 K using x-ray diffraction. For both the chalcogenides, the zinc-blende structure remains stable up to 600 K without undergoing any phase transformation. The crystallite size increases as a function of temperature. However, for nanocrystallite ∼5.0 nm, lattice parameters show reduction in comparison to their bulk values. With increase in temperature, the lattice parameter increases and approaches the equilibrium value as the crystallite sizes grow to more than 10.0 nm. We attribute the temperature induced increase in crystallite size primarily to normal grain growth, a phenomenon observed in crystalline solids when the crystallite size undergoes gradual increase as function of time at elevated temperatures with accompanying recrystallization of new crystallite nuclei, and we rule out the possibility of size-dependent melting.

Coupling between Grain Growth and Grain Rotation

Grain boundary motion during grain growth or recrystallization is considered as a diffusion process of atomic movement across the boundary. It can be accompanied by subgrain rotation or nanograin rotation. However, grain boundary migration can be achieved also by dislocation motion or creep. The evidence is the power law relationship between driving force and boundary velocity for large driving forces and an activation energy which approaches that of self-diffusion at low driving forces and decreases with increasing driving force. The creep mechanism may or may not involve grain rotation. Experimental evidences and dislocation models are discussed in reference to coupling between boundary migration and grain rotation.

On the heterogeneous nucleation of martensite

Martensitic nucleation near inhomogeneities is modeled using linear elasticity. The coherent strain energy due to the formation of a martensite embryo decreases when the inhomogeneity is elastically stiffer than the matrix and vice versa. A maximum reduction of 20% in strain energy is calculated for the case when the embryo is formed near a free surface. The results are consistent with the experimental observations of preferential nucleation of martensite at a free surface. A possible explanation for the nature of the “preexisting martensite embryo” in the Kaufman and Cohen model of a nucleation site is also proposed: the dislocation loop in the parent phase is itself the site for the embryo such that it will transform into martensite during transformation. The calculated critical characteristics of this embryo are in good agreement with the model of Chen and Chiao and their experimental results.

Effects of Rolled Plate Thickness on Anisotropy, with Application to Acoustic Stress Measurement

Common structural materials which are normally idealized as isotropic for most engineering purposes must be considered anisotropic for purposes of acousto-elastic stress measurement. The degree of anisotropy affects the number of moduli, as well as their departure from the usual isotropic values. For instance, rolled plates made of the same alloy, with the same temper designation, may have different moduli for different amounts of rolling. It is the purpose of this paper to determine whether the acousto-elastic constants are identical for commercial rolled plates of the same alloy and temper but different thicknesses.

ACOUSTIC STRESS MEASUREMENT IN ALUMINUM AND STEEL CONSIDERING DIFFERENCES IN TEXTURE DUE TO ROLLED PLATE THICKNESS

Common structural materials normally idealized as isotropic for most engineering purposes must be considered anisotropic for purposes of acousto-elastic stress measurement. The degree of anisotropy affects the number of moduli, as well their departure from the usual isotropic values. For instance, rolled plates of the same alloy and temper but various thicknesses may have different moduli due to undergoing different amounts of rolling. It is the purpose of this paper to determine whether the acousto-elastic constants are identical for commercial rolled plates of the same alloy and temper but different thicknesses.

Fatigue properties of titanium alloy Ti–6Al–2Cb–1Ta–0.8Mo

Abstract The low cycle fatigue properties of a Ti–6Al–2Cb–1Ta–0.8Mo (Ti-6211) alloy have been studied by testing hourglass specimens, based on ASTM specification 606-92, under cyclic stresses in strain-controlled conditions. Results indicate that strain softening occurs under cyclic loading and that fatigue life can be related to either strain range or stress range. Both low cycle and high cycle fatigue data on Ti-6211 have been reviewed. A complete fatigue curve has been established which predicts fatigue life of the alloy for R =−1 from 100 to 100 million cycles.