Hydrostatic stress

About: Hydrostatic stress is a research topic. Over the lifetime, 1568 publications have been published within this topic receiving 37773 citations.

Modeling of Oxygen Diffusion Along Grain Boundaries in a Nickel-Based Superalloy

Abstract: Finite element analyses of oxygen diffusion at the grain level have been carried out for a polycrystalline nickel-based superalloy, aiming to quantify the oxidation damage under surface oxidation conditions at high temperature. Grain microstructures were considered explicitly in the finite element model where the grain boundary was taken as the primary path for oxygen diffusion. The model has been used to simulate natural diffusion of oxygen at temperatures between 650C and 800C, which are controlled by the parabolic oxidation rate and oxygen diffusivity. To study the effects of mechanical stress on oxygen diffusion, a sequentially coupled deformation-diffusion analysis was carried out for a generic specimen geometry under creep loading condition using a submodeling technique. The material constitutive behavior was described by a crystal plasticity model at the grain level and a unified viscoplasticity model at the global level, respectively. The stress-assisted oxygen diffusion was driven by the gradient of hydrostatic stress in terms of pressure factor. Heterogeneous deformation presented at the grain level imposes a great influence on oxygen diffusion at 750C and above, leading to further penetration of oxygen into the bulk material. Increased load level and temperature enhance oxygen concentration and penetration within the material. At 700C and below, mechanical loading seems to have negligible influence on the oxygen penetration because of the extremely low values of oxygen diffusivity and pressure factor. In the case of an existing surface microcrack, oxygen tends to accumulate around the crack tip due to the high stress level presented near the crack tip, leading to localized material embrittlement and promotion of rapid crack propagation.

Stress distributions and energetics in the laterally ordered systems of buried pyramidal Ge/Si(001) islands: An atomistic simulation study

Abstract: Stress distributions in laterally ordered arrays of coherent Ge islands of shallow pyramidal shape buried in a Si(001) matrix are studied via large-scale atomistic simulations, using Stillinger-Weber Ge/Si systems as a vehicle. The existence of tensile hydrostatic stress regions is observed on the spacer surface, above the buried islands. Our previously reported finding [M. A. Makeev and A. Madhukar, Phys. Rev. Lett. 86, 5542 (2001)] that the hydrostatic stress at the spacer layer surface above the island apex is nearly inversely proportional to the spacer layer thickness is validated by a comparison with experimental data. The lateral variations of the hydrostatic stress on the spacer layer surface show ``bell-shape'' profiles, with the effective size of the tensile regions above the island apex varying as a power law with the spacer layer thickness, with the power exponent being greater than 1. Studies of the energetics of twofold stacks of island systems show that the elastic interaction energy between the islands is minimized for the vertically aligned geometry. The spacer layer thickness dependence of the hydrostatic and biaxial stress field distributions in the interior of the Si(001) matrix are presented as these define the behavior of the electron and hole three-dimensional confinement potentials that determine the electronic properties of the pyramidal island quantum dots.

Using finite element modelling to examine the flow process and temperature evolution in HPT under different constraining conditions

Abstract: High-pressure torsion (HPT) is a metal-working technique used to impose severe plastic deformation into disc-shaped samples under high hydrostatic pressures. Different HPT facilities have been developed and they may be divided into three distinct categories depending upon the configuration of the anvils and the restriction imposed on the lateral flow of the samples. In the present paper, finite element simulations were performed to compare the flow process, temperature, strain and hydrostatic stress distributions under unconstrained, quasi-constrained and constrained conditions. It is shown there are distinct strain distributions in the samples depending on the facility configurations and a similar trend in the temperature rise of the HPT workpieces.