Approved for public release; distribution is unlimited / Large shear stresses often develop at the interface between dissimilar materials in microelectronic devices, when they are subjected to thermo-mechanical excursions. These stresses can facilitate diffusionally accommodated interfacial sliding, or creep. A driving factor for these stresses is the thermal expansion mismatch between the adjoining materials. For narrow thin film lines, these stresses may exist over a large fraction of the film-substrate interface. This thesis explores methodologies to measure the kinetics of interfacial creep at model Al thin film/silicon substrate interfaces. A method of sample production, which involved diffusion bonding a polished Si substrate to the surface of a thin Al film deposited on a second Si substrate was developed (Si/Al/Si sandwich). When loaded edge-wise in compression, the Al thin film - Si interface are loaded in shear. By measuring the relative displacements between the two Si substrates, the interfacial displacement rates at varying temperatures and stresses were experimentally determined. In accordance with previous results, the kinetics was given by a diffusional creep law with a threshold stress, and an activation energy representing interfacial diffusion. The activation energy was found to be unusually low, and further experimental and modeling studies are needed to better understand its origin. / Lieutenant Commander, United States Navy

The success in fabrication of various nano-sized powders, wires or tubes has arisen the new possibility in modifying the existing commercial materials in terms of their functional or structural characteristics. In this study, the AZ61 Mg alloy is adopted as the matrix, and nano-sized SiO2 particulates are introduced into the alloy by means of casting, powder metallurgy, or spray forming processes to fabricate a high performance Mg matrix composite. The strengthening mechanisms, fracture toughness and bending toughness of the AZ61 Mg based composites are examined. The composites were prepared either by spray forming, ingot metallurgy, or powder metallurgy, followed by severe hot extrusion. The spray formed composites exhibit the best nano particle distribution and toughness, but the volume fraction of the nano particles that can be inserted is limited. The nano composites fabricated through the powder metallurgy method possess the highest strength due to the extra strengthening effect from the MgO phase. Strengthening analysis based on the Orowan strengthening mechanism can predict well the composite strength provided that the nano particles are in reasonably uniform dispersion. For composites containing higher nano particle volume fractions greater than 3%, the experimental strength data fall well below the theoretical predictions, suggesting poor dispersion of the reinforcement. The creep properties of the composites are also explored. The specimens are subjected to tensile loading at temperatures 200 to 400oC and strain rates 1x10-3 to 1x10-1. The creep mechanism is identified as dislocation creep controlled with the rate controlling diffusion step being the magnesium lattice diffusion at low strain rates and grain boundary diffusion at high strain rates.

The roles of residual stresses distribution and creep deformation in the post-weld-shifts (PWS) of a laser model packaging are investigated in this dissertation. The temperature dependent material properties are employed to calculate the distribution of the residual stresses introduced in the solidification of soldering joints and lasering joints respectively. A power law proposed by Norton is applied to the creep deformation calculation. The post-weld-shifts of fiber-solder-ferrule (FSF) introduced in the aging and temperature cycling tests are simulation. A finite element package ¡V MARC is used to module the fiber-solder-ferrule joint and laser joint respectively. Experimental results of the PWS of a FSF joint are compared with the calculated shifts. Results indicate that the redistribution of residual stresses in joint and the creep deformation under high temperature load may affect the PWS significantly. A good agreement between the simulated and the measured results indicate the proposed model is feasible in the laser module packaging analysis.

The thermal stresses, residual stresses distributions in a vertical cavity surface emitting laser (VCSEL) are investigated in this thesis. The post-weld shifts of the laser light sources in the VCSEL under the re-flow, temperature cycling test and high temperature aging test have also been simulated and studied. The creeping effects of the 63Sn/37Pb solder on the stresses distributions and post-weld shifts are compared and discussed. The finite element package MARC is used for the stresses and deformation simulations of a VCSEL under different load cases. The heat transfer, the non-linear couple elastic-plastic increment and the creep models are employed in the numerical simulations. The temperature dependent material properties of the 63Sn/37Pb and 96.5Sn/3.5Ag solder are used for the temperature cycling and aging test. The effect of the circle and square solder pad designs on the stresses distributions in a VCSEL is also studied in this thesis. Numerical results indicate that the residual stresses introduced in the solidification is significant in stress and post-weld analyses of a VCSEL.

Soil aging refers to the increase in strength and stiffness that is exhibited over time after it is disturbed. It is common in granular soils, such as sands, occurring over periods from hours to years. There have been relatively numerous laboratory studies on sand aging phenomena. However the majority of these studies were conducted on relatively clean sand (fines content <5%) and were performed under isotropic condition. In nature, granular soils with fines content > 5% are not uncommon. This research is an attempt to gain further insight and understanding of mechanical aging on silty sand by conducting laboratory studies mostly under K0 condition, which better reflects the field condition, at both macro-scale (triaxial test) and micro-scale (fabric test). As many factors (e.g. plasticity of fines, fines content, grain size composition, angularity and shape) affect silty sand behaviour and not all those factors could be investigated during the study period, this study focused on mechanical aging of non-plastic silty sand with 15% fines content. Triaxial tests have been conducted in this study in order to observe creep behavior under different density, initial fabric, and consolidation stress paths (K0 and isotropic). The tests were conducted at low effective confining stress stresses i.e. ’3= 30 – 120 kPa as this is relevant to many geotechnical aging problems (e.g. time effects on freshly deposited or disturbed soils such as in the case of hydraulic fills, mine tailings, and post-liquefaction state of soil behaviour following earthquakes). Creep induced aging effects on undrained shear behaviour at small-strain (<0.1% of shear strain), were investigated, as this strain range is most common in geotechnical structures under gravity-induced working loads. Aging effects on one way cyclic behaviour were also studied. Some new key findings from these tests are as follows: (1) Creep following K0 consolidation indicated that the soil tends to expand radially over time, resulting in a tendency of increasing horizontal stress with time even at low stress. (2) Following K0 consolidation, density appears to have more significant effect on creep compared to initial shear stress ratio and mean effective stress; as demonstrated by loose samples (low stress ratio and mean effectives stress) which exhibited greater creep compared to those of dense sample (higher stress ratio and mean effective stress) (3) For loose soils, there is a trade-off between high confining stresses driving aging and collapsing pore space. Generally higher confining stress was found to increase creep tendency thus enhancing aging, however there was also found to be a certain confining pressure where the aging effects became less due to local structure collapse. (4) Initial fabric plays an important role on creep development, thus aging. For instance, dense dry pluviated samples developed larger axial strain over time but also gained less increase in stiffness compared to dense moist tamped samples. This suggests the importance of specimen preparation for laboratory testing that replicates the field scenarios e.g. natural deposition and associated fabric; (5) Dense K0 consolidated samples produce more increase in stiffness with time than corresponding isotropically consolidated samples. Hence, as the K0 condition generally reflects the level-ground free field stress condition better, it is important to test under K0 if the degree of stiffness gain is important; (6) The number of cycles to trigger cyclic softening and liquefaction for one way cyclic loading increases with the aging duration. In addition there is tendency that the aging effect is more pronounced at lower cyclic stress ratios. Fabric tests under K0 consolidation with similar variables as the triaxial tests were also performed. Some new insights and contributions have been obtained as follows: (1) Moist tamped samples, have particles that are more clustered together and structured than dry pluviated samples; (2) In terms of particle orientation, a change in the degree of orientation for both sand particles and ‘fines’ under constant loading was observed with time. The dominant (i.e. most) rotated particles (sand or “fines’) depends on the initial fabric and density; (3) Over time, under constant loading, growth of micro voids was observed for dense samples while those of loose samples contracted; (4) A new parameter, variance to mean void ratio of void distance, was introduced as a measure of the degree of interlocking during aging. The variance to mean ratio of void distance for moist tamped samples tends to decrease whereas those of dry pluviated samples tends to increase with time. An increase in variance and variance to mean ratio for dry pluviated samples indicates that particles are more clustered together with time; (5) Original work on spatial void distance for the numerical analysis of creep induced aging based on Kang et al. (2012) was conducted (note: the model’s boundary condition allows lateral expansion, which is not the same as the fabric tests conducted). The analysis showed that mean void size in dense soil tends to increase with time under constant load while for loose sample it tends to decrease. However the particles also clustered together more – increasing structure. (6) A microstructural study of “undisturbed samples”, obtained by gel-push sampling, of clean sand (fines content = 4%) and silty sand (fines content = 30%), was conducted to investigate anisotropy of natural fabric of granular soils. The results show that dry pluviation reflects the field condition more, in terms of natural deposition, than moist tamping. In addition, spatial void distance qualitatively indicated the undisturbed samples are relatively “very young”, even in terms of engineering time, as indicated by similar variance to mean ratio and kurtosis with those of 1 hour and 1 week reconstituted samples. This research has shown that there was a relation between changes in the microstructure over time and changes in macro mechanical properties of non-plastic silty sand. Further improvement in theoretical modeling (e.g. numerical modeling of creep on polydisperse granular material) and experimental aspects (e.g. examining different grain size composition and angularity, different fines content, the influence of the shape of sand and fines and use of the photo-elastic method) will allow a better understanding of the sand aging phenomenon in silty sand.

Μηχανική συμπεριφορά και φαινόμενα ερπυσμού σε μαλακά αργιλικής σύστασης πετρώματα κάτω από συνθήκες σταθερής φόρτισης. / Rock creep at clays under conditions of high strength.

Zr-2.5Nb is used as pressure-tube material in CANDU (CANada Deuterium Uranium) reactors. Under reactor operating conditions, pressure tubes undergo anisotropic dimensional changes, and thermal creep contributes to this deformation. In a previous study, the limited textures available to Zr-2.5Nb significantly restricted the understanding of the relationship between texture and creep anisotropy. Moreover, there has been no research performed to investigate textures and stress states simultaneously for this material, which would provide a valuable resource for developing creep anisotropy models and optimizing textures to improve creep resistance. Cold-worked Zr-2.5Nb fuel sheathing (FS) and micro pressure tubes (MPT) with various textures and microstructures were used as experimental materials. The tubes were machined as thin-wall standard (ratio of axial to transverse stress 0.5) and end-loaded (ratio of axial to transverse stress = 0.25~0.75) capsules and were internally pressurized and sealed. Stress and temperature dependence tests were performed on standard capsules under transverse stresses of 100~325MPa at 300~400°C to establish a regime in which dislocation glide is the likely strain producing mechanism. An average stress exponent vaule of 6.4 was obtained, indicating that dislcation creep is the likely dominant mechanism. Texture and stress state dependence tests were performed on standard and end-loaded capsules under a nominal transverse stress of 300MPa at 350°C. It was evident that creep anisotropy strongly correlates with textures under different stress states. A self-consistent polycrystalline model SELFPOLY7 based only upon crystallographic texture was employed to simulate the creep anisotropy of the tubes. However, the model cannot fit all the experimental data well by using a uniform critical resolved shear stress (CRSS) ratio of the operating slip systems. A modification was made, by taking into account the pre-existing dislocation distributions generated during cold work, and an improvement was achieved. This work provides a valuable resource for understanding the effect of texture, stress states and microstructure on anisotropic creep of cold-worked Zr-2.5Nb tubes. The current research also provides a strategic direction to improve creep anisotropy predictions. The large sets of experimental data supply a database to evaluate and develop improved models. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-08-14 11:16:08.67

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