Browsing by Author "Anderoglu, Osman"
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Item Nanoscale Growth Twins in Sputtered Copper Films(2011-08-08) Anderoglu, OsmanThe focus of this research is the development of high strength, high conductivity copper films. Pure copper is soft and traditional strengthening mechanisms cause substantial decrease in conductivity. To address the challenge, epitaxial nanotwinned copper films are synthesized on HF etched Si (110) substrates. These films show high hardness (~ 2.8 GPa) due to high density of coherent twin boundaries (CTBs) which effectively block the motion of dislocations similar to grain boundaries (GBs). Resistivity of CTBs is calculated to be an order of magnitude lower than that of GBs. Hence, conductivity of nanotwinned copper is still comparable to that of pure copper. In addition, it is shown that average twin spacing can be controlled by adjusting deposition rate. Analytical studies together with experimental evidence show that nanotwins can improve the strength-to-resistivity ratio significantly in copper. In general, nanocrystalline metals suffer from low ductility. To study plastic deformation via rolling, thick polycrystalline nanotwinned copper foils are sputtered on SiO2 and then peeled off the substrate. Despite the high strength, room temperature rolling experiments show that nanotwinned copper films exhibit stable plastic flow with no shear localization or fracture even at thickness reduction of over 50%. Postdeformation studies of microstructure reveals that the plastic deformation is facilitated by the migration of CTBs normal to the twin boundary plane due to the glide of twinning dislocations in the twin plane. X-ray pole figure measurements show insignificant out of plane rotation as a result of 50% rolling thickness reduction. Thermal stability of nanocrystalline metals is also a concern. Free standing nanotwinned polycrystalline copper films show remarkable thermal stability after annealing at 800 degrees C. The driving force for twin growth is much lower than that for grain coarsening because the energy stored in CTBs is an order of magnitude lower than that of GBs. As a result, the average twin spacing stays below 20 nm after annealing. Such high thermal stability of nanotwins leads to the retention of hardness of 2.2 GPa. Low energy twin boundary may provide a unique way to achieve both high strength and high temperature thermal stability in certain metallic materials.Item Residual stress measurement using X-ray diffraction(Texas A&M University, 2005-02-17) Anderoglu, OsmanThis paper briefly describes the theory and methods of x-ray residual stress measurements. Residual stresses can be defined as the stresses which remain in a material in the absence of any external forces. There are many stress determination methods. Some of those methods are destructive and some are nondestructive. X-ray residual stress measurement is considered as a nondestructive method. X-ray diffraction together with the other diffraction techniques of residual stress measurement uses the distance between crystallographic planes as a strain gage. The deformations cause changes in the spacing of the lattice planes from their stress free value to a new value that corresponds to the magnitude of the residual stress. Because of Poisson?s ratio effect, if a tensile stress is applied, the lattice spacing will increase for planes perpendicular to the stress direction, and decrease for planes parallel to the stress direction. This new spacing will be the same in any similarly oriented planes, with respect to the applied stress. Therefore the method can only be applied to crystalline, polycrystalline and semi-crystalline materials. The diffraction angle, 2θ, is measured experimentally and then the lattice spacing is calculated from the diffraction angle, and the known x-ray wavelength using Bragg's Law. Once the d-spacing values are known, they can be plotted versus 2 sin ψ, ( ψ is the tilt angle). In this paper, stress measurement of the samples that exhibit a linear behavior as in the case of a homogenous isotropic sample in a biaxial stress state is included. The plot of d vs. 2 sin ψ is a straight line which slope is proportional to stress. On the other hand, the second set of samples showed oscillatory d vs. 2 sin ψ behavior. The oscillatory behavior indicates the presence of inhomogeneous stress distribution. In this case the xray elastic constants must be used instead of E and ν values. These constants can be obtained from the literature for a given material and reflection combination. It is also possible to obtain these values experimentally. Calculation of the residual stresses for these samples is beyond the scope of this paper and will not be discussed here.