M. Grujicic

Computational Analysis of the Interfacial Bonding Between Feed-Powder Particles and the Substrate in the Cold-Gas Dynamic-Spray Process

The cold-gas dynamic-spray process is analyzed by numerical modeling of the impact between a single spherical feed-powder particle and a semi-infinite substrate. The numerical modeling approach is applied to the copper‐aluminum system to help explain experimentally observed higher deposition efficiencies of the copper deposition on aluminum than the ones associated with the aluminum deposition on copper. To properly account for the high strain, high strain-rate deformation behavior of the two materials, the appropriate linear-elastic rate-dependent, temperature-dependent, strain-hardening materials constitutive models are used. The results obtained indicate that the two main factors contributing to the observed higher deposition efficiency in the case of copper deposition on aluminum are larger particle/substrate interfacial area and higher contact pressures. Both of these are the result of a larger kinetic energy associated with a heavier copper feed-powder particle. The character of the dominant particle/substrate bonding mechanism is also discussed in the present paper. It is argued that an interfacial instability which can lead to the formation of interfacial roll-ups and vortices can play a significant role in attaining the high strength of interfacial bonding. # 2003 Elsevier Science B.V. All rights reserved.

Nitrogen strengthening of a stable austenitic stainless steel

Abstract The low-temperature flow stress of mechanically stable austenitic stainless steels increases with increasing concentration of nitrogen in solution and with decreasing temperature. This phenomenon has been studied in a series of FeNiCrMo alloys with nitrogen contents between 0.04 and 0.36 wt% by measuring the flow stress and the thermal activation parameters for plastic flow as a function of stress, plastic strain and nitrogen concentration in stress relaxation and strain-rate change experiments. Care is taken, when analyzing the data, to distinguish between athermal and thermal effects. The significant increase of the athermal flow stress with increasing nitrogen concentration is attributed to short-range ordering of chromium and nitrogen atoms. The thermally activated component of the flow stress is also dependent on the nitrogen concentration and is thought to be due to localized, predominantly modulus interactions between lattice disturbances in the immediate vicinity of nitrogen atoms and slip dislocations. The thermally activated component is suitably described by Friedels model of solid solution strengthening.

Strain aging of austenitic Hadfield manganese steel

Abstract Strain aging of Hadfield steel is discussed in terms of the interstitial octahedron, local-order model, which defines order as the probability that a C atom in an octahedral cluster of metal atoms has n (an integer between 0 and 6) Mn nearest neighbors. Equilibrium order is assessed by a Monte Carlo procedure using pair exchange energies derived from an established thermodynamic database and a Boltzmann distribution function. The disorder produced by the passage of a slip dislocation, the resulting change in free energy and, consequently, the stress opposing dislocation motion are calculated both for a single isolated dislocation and for a sequence of dislocations moving on the same slip plane. The model is extended to analyze aging effects involving diffusion of carbon before or during deformation. It is assumed that, during aging, atoms on the metal sublattice are frozen on sites determined either by the high-temperature equilibrium anneal or by prior deformation. Only diffusion of carbon is allowed. The fully aged condition at selected aging temperatures is simulated using a Monte Carlo procedure to assess local order when the free energy of the system is minimum (para-equilibrium). It is shown that the increase in strength on aging is a direct result of the relatively small thermal energy at the aging temperature favoring an increase in the number of Mn–C atom pairs. The predictions of the model are supported by the results of static aging experiments and the model provides a complete phenomenological description of dynamic strain aging in Hadfield steel.

UV-light enhanced oxidation of carbon nanotubes

Abstract Ab initio density functional theory (DFT) calculations of the interactions between selected semiconducting and metallic single-walled carbon nanotubes (SWCNTs) (as well between single and double graphene sheets) and single oxygen molecules are carried out in order to provide a rationale for the recent experimental observations of UV-light accelerated oxidation of carbon nanotubes and the accompanying changes in the thermoelectric power. The computational results obtained show that these experimental findings can be related to UV-light excitation of oxygen molecules from their ground spin-triplet state into a higher-energy spin-singlet state. Such excitation lowers the activation energy for molecular-oxygen chemisorption to a nanotube, increases the adsorption energy and promotes charge transfer from the nanotube to the oxygen molecule. Lattice defects such as 7-5-5-7 and Stone–Wales defects are found to play a critical role in enhancing oxygen molecule/nanotube bonding and in affecting the extent of charge transfer. Contrary to this, the effects of nanotube diameter and chirality and the number of walls appear to be less significant.

Determination of effective elastic properties of functionally graded materials using Voronoi cell finite element method

A Voronoi cell finite element method (VCFEM) has been implemented into the commercial finite element package ABAQUS and used to determine the effective microstructure-dependent elastic properties (the Youngs modulus and the Poissons ratio) of functionally graded materials (FGMs). Before applying VCFEM to FGM, the microstructure of FGM has been idealized as follows. At low volume fractions of either of the two materials in FGM, the microstructure is taken to consist of discrete particles embedded in a continuous matrix. In the region of FGM where the volume fractions of the two materials are comparable, the microstructure is taken to consist of intertwined clusters of the materials. In addition, for both types of microstructures, void inclusions are introduced to account for the porosity in FGM. Application of VCFEM to Ni/MgO and Ni3Al/TiC yielded the values of the effective elastic properties which are in significantly better agreement with the experimental values than the values predicted by the commonly used Eshelbys equivalent inclusion method and the self consistent method.

The effect of topological defects and oxygen adsorption on the electronic transport properties of single-walled carbon-nanotubes

Abstract Ab initio density functional theory (DFT) calculations of the interactions between isolated infinitely-long semiconducting zig-zag (10, 0) or isolated infinitely-long metallic arm-chair (5, 5) single-walled carbon-nanotubes (SWCNTs) and single oxygen-molecules are carried out in order to determine the character of molecular-oxygen adsorption and its effect on electronic transport properties of these SWCNTs. A Green’s function method combined with a nearest-neighbor tight-binding Hamiltonian in a non-orthogonal basis is used to compute the electrical conductance of SWCNTs and its dependence on the presence of topological defects in SWCNTs and of molecular-oxygen adsorbates. The computational results obtained show that in both semiconducting and metallic SWCNTs, oxygen-molecules are physisorbed to the defect-free nanotube walls, but when such walls contain topological defects, oxygen-molecules become strongly chemisorbed. In semiconducting (10, 0) SWCNTs, physisorbed O2-molecules are found to significantly increase electrical conductance while the effect of 7-5-5-7 defects is practically annulled by chemisorbed O2-molecules. In metallic (5, 5) SWCNTs, both O2 adsorbates and 7-5-5-7 defects are found to have a relatively small effect on electrical conductance of these nanotubes.

A computational analysis of the percolation threshold and the electrical conductivity of carbon nanotubes filled polymeric materials

Percolation of individual single walled carbon nanotubes (SWCNTs) and of SWCNT bundles dispersed in a non-interacting polymeric matrix has been analyzed computationally using an analytical model and a numerical simulation method. While the analytical model used is strictly valid only in the limit of an infinite length-to-diameter aspect ratio of the dispersed phase, good agreement is found between its predictions and the ones obtained using a computationally-intensive numerical method for the aspect ratios as small as 350. Since the aspect ratio of the individual SWCNTs is on the order of 1,000–10,000, this finding suggests that the analytical model can be used to study SWCNT percolation phenomena.An electrical network model is also applied to the percolating and near-percolating SWCNT clusters in order to compute the dc electrical conductivity of a CP2 polyimide + SWCNT composite material. A reasonably good agreement is obtained between the computational and the experimental results with respect to both the magnitude of the electrical conductivity and to its behavior in the vicinity of the percolation threshold.

Model-based control strategies in the dynamic interaction of air supply and fuel cell:

Abstract Model-based control strategies are utilized to analyse and optimize the transient behaviour of a polymer electrolyte membrane (PEM) fuel cell system consisting of air and fuel supply subsystems, a perfect air/fuel humidifier and a fuel cell stack at constant fuel cell temperature. The model is used to analyse the control of the fuel cell system with respect to maintaining a necessary level of oxygen partial pressure in the cathode during abrupt changes in the current demanded by the user. Maintaining the oxygen partial pressure in the cathode is necessary to prevent short circuit and membrane damage. The results obtained indicate that the oxygen level in the cathode can be successfully maintained through feedforward control of the air compressor motor voltage. However, the net power provided by the fuel cell system is compromised during the transients following abrupt changes in the stack current, suggesting a need for power management via the use of a secondary power source such as a battery.

The effect of degree of saturation of sand on detonation phenomena associated with shallow-buried and ground-laid mines

A new materials model for sand has been developed in order to include the effects of the degree of saturation and the deformation rate on the constitutive response of this material. The model is an extension of the original compaction materials model for sand in which these effects were neglected. The new materials model for sand is next used, within a non-linear-dynamics transient computational analysis, to study various phenomena associated with the explosion of shallow-buried and ground-laid mines. The computational results are compared with the corresponding experimental results obtained through the use of an instrumented horizontal mine-impulse pendulum, pressure transducers buried in sand and a post-detonation metrological study of the sand craters. The results obtained suggest that the modified compaction model for sand captures the essential features of the dynamic behavior of sand and accounts reasonably well for a variety of the experimental findings related to the detonation of shallow-buried or ground-laid mines.

Mobility of martensitic interfaces

Using a dislocation model of interfacial structure, kinetic theories of dislocation motion are adapted to predict the mobility of martensitic interfaces. Defining generalized driving forces and activation parameters, analytical models are developed which describe the kinetics of motion controlled by various types of obstacle interactions. The behaviors of martensitic interfaces and slip dislocations in identical microstructures are compared. For a lattice-invariant shear by slip, the martensitic interface behaves similarly to a collection of glide dislocations. The interface/obstacle interaction is much weaker if the martensite is internally twinned, giving a higher relative mobility.