Lattice continuum and diffusional creep

Diffusional creep is characterized by growth/disappearance of lattice planes at the crystal boundaries that serve as sources/sinks of vacancies, and by diffusion of vacancies. The lattice continuum theory developed here represents a natural and intuitive framework for the analysis of diffusion in crystals and lattice growth/loss at the boundaries. The formulation includes the definition of the Lagrangian reference configuration for the newly created lattice, the transport theorem and the definition of the creep rate tensor for a polycrystal as a piecewise uniform, discontinuous field. The values associated with each crystalline grain are related to the normal diffusional flux at grain boundaries. The governing equations for Nabarro–Herring creep are derived with coupled diffusion and elasticity with compositional eigenstrain. Both, bulk diffusional dissipation and boundary dissipation accompanying vacancy nucleation and absorption, are considered, but the latter is found to be negligible. For periodic arrangements of grains, diffusion formally decouples from elasticity but at the cost of a complicated boundary condition. The equilibrium of deviatorically stressed polycrystals is impossible without inclusion of interface energies. The secondary creep rate estimates correspond to the standard Nabarro–Herring model, and the volumetric creep is small. The initial (primary) creep rate is estimated to be much larger than the secondary creep rate.

High Temperature Brittleness of Cast Alloys / Kruchosc Wysokotemperaturowa Stopów Odlewniczych

Over-all mechanical properties of alloys are extremely low at the last stage of solidification where alloy exists at brittle temperature range (BTR). When the solidification process is completed a sudden and marked change in strength and ductility of metal is observed. It means that as long as liquid phase is present, metal will fail in a brittle manner. There are known different theories of brittleness of alloys in existence of liquid phase. The idea involved by authors of the paper is as follows: three major factors caused by presence of liquid may be taken into account: - decreasing the energy needed for crack nucleation, - increasing atomic diffusional flux out of the crack tip, - creating a path for abnormally quick diffusion of atoms from the crack tip.

Update on a Pharmacokinetic-Centric Alternative Tier II Program for MMT—Part II: Physiologically Based Pharmacokinetic Modeling and Manganese Risk Assessment

Recently, a variety of physiologically based pharmacokinetic (PBPK) models have been developed for the essential element manganese. This paper reviews the development of PBPK models (e.g., adult, pregnant, lactating, and neonatal rats, nonhuman primates, and adult, pregnant, lactating, and neonatal humans) and relevant risk assessment applications. Each PBPK model incorporates critical features including dose-dependent saturable tissue capacities and asymmetrical diffusional flux of manganese into brain and other tissues. Varied influx and efflux diffusion rate and binding constants for different brain regions account for the differential increases in regional brain manganese concentrations observed experimentally. We also present novel PBPK simulations to predict manganese tissue concentrations in fetal, neonatal, pregnant, or aged individuals, as well as individuals with liver disease or chronic manganese inhalation. The results of these simulations could help guide risk assessors in the application of uncertainty factors as they establish exposure guidelines for the general public or workers.

Ferriheme and ferroheme are isosteric inhibitors of fatty acid binding to rat liver fatty acid binding protein

In addition to fatty acids, liver fatty acid binding protein (L-FABP) also interacts with ferriheme, which it binds with an affinity approximately one order of magnitude greater than that for oleic acid. We have, therefore, examined the effect of ferroheme and ferriheme on the binding of oleate to rat L-FABP, also called heme-binding protein. Both oxidation states of heme behaved as isosteric inhibitors for the binding of the fatty acid confirming a common binding site. The reduced form of heme (Fe(II)) is a threefold better competitor of oleate binding than ferriheme. To show whether the diffusion of heme would be affected by the presence of the binding protein, we measured the effect of the fatty acid binding protein on the diffusional flux of a water-soluble heme derivative, iron-deuteroporphyrin. The diffusional flux of iron-deuteroporphyrin did not change in the presence of the protein. This suggested that the binding affinity of fatty acid binding protein for iron-deuteroporphyrin is too great to allow rapid equilibrium between bound and unbound ligand across the system in an appropriate time frame.Key words: fatty acid binding protein, heme, diffusion, liver, oleic acid.

The Effects of Accelerated Stress Conditions on Electromigration Failure Kinetics and Void Morphology

Accelerated life testing has long been used to measure the electromigration reliability of integrated circuit (IC) metallization systems. In order to establish or verify electromigration design rules for IC products the measured data is extrapolated over a wide range of operating conditions using phenomenological models. These models assume that the components of the diffusional flux, thermal stress and resulting void morphology are independent of test/operating condition. In this paper, electromigration void morphology and failure criteria are studied over a range of stress conditions and microstructures for non-layered AlCu and AlCuSi metallurgies. The failure criteria, defined as the average change in conductor resistance prior to an open circuit condition, was strongly dependent on test current density. Moreover, the nature of the relationship between failure criteria and current density was governed by the linewidth (W) relative to the median grain size (D50). This dependence can be explained qualitatively in terms of the Blech Effect.