Papers by Author: Ersan Üstündag

Abstract: Bulk metallic glasses (BMGs) are attractive structural materials that develop residual stresses during processing due to thermal tempering. In this process, compressive surface stresses are generated within thin layers balanced with tension in the interior. In the present study, stress generation was analyzed experimentally and theoretically in a BMG plate and a cylinder. The residual stresses were measured using the crack compliance method. It was shown that high stresses can be attained in metallic glasses due to thermal tempering: over −300 MPa compression on the surface balanced by +150 MPa tension in the middle. The experimental data were then compared to the predictions of a viscoelastic model that took into account the equilibrium viscosity of BMG as a function of temperature. The model was shown to be accurate within 5 to 25% of the experimental stress data. It is therefore a powerful tool for estimating processing-induced residual stresses in bulk metallic glasses.

Abstract: Synchrotron x-rays are used for in situ determination of oxide strain, during oxide formation on a Kanthal A1 FeCrAlZr substrate at 1160°C. The measurements rely on use of high-energy (~80keV) x-rays and transmission geometry, and the methodology of the strain measurements is presented. Oxide growth strains at elevated temperature, relative to pure alumina, were seen to be small, while temperature excursions induced significant strains. Furthermore, significant strain relaxation was observed during isothermal holds, suggesting oxide creep as a major relaxation mechanism. Upon cooling to room temperature, significant residual strains developed, with a corresponding in-plane residual stress of -3.7 GPa.