Nano-grained nickel produced by electroforming technique was investigated for its microstructure evolution and mechanical properties upon annealing. It was found that during low temperature annealing (T<250 oC), electroformed nano-grained nickel showed scattered and isolated abnormal grain growth, followed by a major abnormal grain growth at 320 oC. A secondary abnormal grain growth, featuring faceted grain boundaries, was observed at a higher annealing temperature (T=528 oC). A semi-in-situ observation using optical microscopy was conducted to track the movement of the faceted grain boundaries, and it was found that these boundaries were mostly immobile. The mechanical properties under various annealing conditions were studies using hardness and tensile testing. The hardness was observed to decrease with increasing annealing temperature. The material became very brittle after annealing at 320 oC or higher temperatures. Fractography investigation showed that the brittleness is caused by intergranular fracture.

Nano-grained nickel produced by electroforming technique was investigated for its microstructure evolution and mechanical properties upon annealing. It was found that during low temperature annealing (T<250 oC), electroformed nano-grained nickel showed scattered and isolated abnormal grain growth, followed by a major abnormal grain growth at 320 oC. A secondary abnormal grain growth, featuring faceted grain boundaries, was observed at a higher annealing temperature (T=528 oC). A semi-in-situ observation using optical microscopy was conducted to track the movement of the faceted grain boundaries, and it was found that these boundaries were mostly immobile. The mechanical properties under various annealing conditions were studies using hardness and tensile testing. The hardness was observed to decrease with increasing annealing temperature. The material became very brittle after annealing at 320 oC or higher temperatures. Fractography investigation showed that the brittleness is caused by intergranular fracture.

Indiana University-Purdue University Indianapolis (IUPUI) / Sintering is a primary particulate manufacturing technology to provide densification and strength for ceramics and many metals. A persistent problem in this manufacturing technology has been to maintain the quality of the manufactured parts. This can be attributed to the various sources of uncertainty present during the manufacturing process. In this work, a two-particle phase-field model has been analyzed which simulates microstructure evolution during the solid-state sintering process. The sources of uncertainty have been considered as the two input parameters surface diffusivity and inter-particle distance. The response quantity of interest (QOI) has been selected as the size of the neck region that develops between the two particles. Two different cases with equal and unequal sized particles were studied. It was observed that the neck size increased with increasing surface diffusivity and decreased with increasing inter-particle distance irrespective of particle size. Sensitivity analysis found that the inter-particle distance has more influence on variation in neck size than that of surface diffusivity. The machine-learning algorithm Gaussian Process Regression was used to create the surrogate model of the QOI. Bayesian Optimization method was used to find optimal values of the input parameters. For equal-sized particles, optimization using Probability of Improvement provided optimal values of surface diffusivity and inter-particle distance as 23.8268 and 40.0001, respectively. The Expected Improvement as an acquisition function gave optimal values 23.9874 and 40.7428, respectively. For unequal sized particles, optimal design values from Probability of Improvement were 23.9700 and 33.3005 for surface diffusivity and inter-particle distance, respectively, while those from Expected Improvement were 23.9893 and 33.9627. The optimization results from the two different acquisition functions seemed to be in good agreement with each other. The results also validated the fact that surface diffusivity should be higher and inter-particle distance should be lower for achieving larger neck size and better mechanical properties of the material.

The mechanical performance evaluation of asphalt binder has always been a challenging issue for pavement engineers. Recently, the Phase Field Method (PFM) has emerged as a powerful computational tool to simulate the microstructure evolution of asphalt binder. PFM analyzes the structure from the free energy aspect and can provide a view of the whole microstructure evolution process. In this dissertation, asphalt binder performance is analyzed by PFM in three aspects: first, the relationship between asphalt chemistry and performance is investigated. The components of asphalt are simplified to three: asphaltene, resin and oil. Simulation results show that phase separation will occur under certain thermal conditions and result in an uneven distribution of residual thermal stress. Second, asphalt cracking is analyzed by PFM. The traditional approach to analyze crack propagation is Classic Fracture Mechanics first proposed by Griffith, which needs to clearly depict the crack front conditions and may cause complex cracking topologies. PFM describes the microstructure using a phase-field variable which assumes positive one in the intact solid and negative one in the crack void. The fracture toughness is modeled as the surface energy stored in the diffuse interface between the intact solid and crack void. To account for the growth of cracks, a non-conserved Allen-Cahn equation is adopted to evolve the phase-field variable. The energy based formulation of the phase-field method handles the competition between the growth of surface energy and release of elastic energy in a natural way: the crack propagation is a result of the energy minimization in the direction of the steepest descent. Both the linear elasticity and phase-field equation are solved in a unified finite element frame work, which is implemented in the commercial software COMSOL. Different crack mode simulations are performed for validation. It was discovered that the onset of crack propagation agrees very well with the Griffith criterion and experimental results. Third, asphalt self-healing phenomenon is studied based on the Atomic Force Microscopy (AFM) technology. The self-healing mechanism is simulated in two ways: thermodynamic approach and mechanical approach. Cahn-Hilliard dynamics and Allen-Cahn dynamics are adopted, respectively. / Ph. D.

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Friction stir processing (FSP) has been developed based on the basic principles of friction stir welding (FSW), a solid-state joining process originally developed for aluminum alloys. What is attractive about FSP is that it can be incorporated in the overall manufacturing cycle as a post-processing step during the machining operation to provide localized modification and control of microstructures in near-surface layers of metallic components. FSP has emerged as an important post-processing technique, and has been identified as a process that may have a high impact, and perhaps is a disruptive manufacturing process. In this study, FSP has been applied to Al cast alloy A206, which is a high strength, widely used cast alloy in the manufacturing industry. Motivations behind this work are to (1) investigate the feasibility of FSP on manipulating the cast microstructure and strengthening the material, and (2) to explore the viability of FSP to produce a localized particle reinforced zone in cast A206 aluminum components. The thesis will show that we have optimized FSP for processing of Al alloys to locally manipulate the cast microstructure, eliminate casting defects, and attain grain refinement and second phase homogenization. We have established the mechanism leading to the microstructure evolution and have evaluated the resultant mechanical properties, i.e. hardness, tensile property and fatigue properties. We have also synthesized a localized composite material in the A206 work piece with three different reinforcement materials via FSP. These results will be presented and discussed.

All materials are heterogeneous at various scales of observation. The influence of material heterogeneity on nonuniform response and microstructure evolution can have profound impact on continuum thermomechanical response at macroscopic "engineering" scales. In many cases, it is necessary to treat this behavior as a multiscale process. This research developed a hierarchical multiscale approach for modeling microstructure evolution. A theoretical framework for the hierarchical homogenization of inelastic response of heterogeneous materials was developed with a special focus on scale invariance principles needed to assure physical consistency across scales. Within this multiscale framework, the second gradient is used as a nonlocal kinematic link between the response of a material point at the coarse scale and the response of a neighborhood of material points at the fine scale. Kinematic consistency between two scales results in specific requirements for constraints on the fluctuation field. A multiscale internal state variable (ISV) constitutive theory is developed that is couched in the coarse scale intermediate configuration and from which an important new concept in scale transitions emerges, namely scale invariance of dissipation. At the fine scale, the material is treated using finite element models of statistical volume elements of microstructure. The coarse scale is treated using a mixed-field finite element approach. The coarse scale constitutive equations are implemented in a finite deformation hyperelastic inelastic integration scheme developed for second gradient constitutive models. An example problem based on an idealized porous microstructure is presented to illustrate the approach and highlight its predictive utility. This example and a few variations are explored to address the boundary-value-problem dependent nature of length scale parameters employed in nonlocal continuum theories. Finally, strategies for developing meaningful kinematic ISVs, free energy functions, and the associated evolution kinetics are presented. These strategies are centered on the goal of accurately representing the energy stored and dissipated during irreversible processes.

Materials used in thermal power plants have to resist creep deformation for time periods up to 30 years. Material evaluation is typically based on creep testing with a maximum duration of a few years. This information is used as input when empirically deriving models for creep. These kinds of models are of limited use when considering service conditions or compositions different from those in the experiments. In order to provide a more general model for creep, the mechanisms that give creep strength have to be identified and fundamentally described. By combining tools for thermodynamic modelling and modern dislocation theory the microstructure evolution during creep can be predicted and used as input in creep rate modelling. The model for creep has been utilised to clarify the influence of aluminium on creep strength as a part of the European COST538 action. The results show how AlN is formed at the expense of MX carbonitrides. The role of heat treatment during welding has been analysed. It has been shown that particles start to dissolve already at 800ºC, which is believed to be the main cause of Type IV cracking in commercial alloys. The creep strength of these steels relies on minor additions of alloying elements. Precipitates such as M23C6 carbides and MX carbonitrides give rise to the main strengthening, and remaining elements produce solid solution hardening. Particle growth, coarsening and dissolution have been evaluated. By considering dislocation climb it is possible to determine particle strengthening at high temperatures and long-term service. Transient creep is predicted by considering different types of dislocations. Through the generation and recovery of dislocation densities an increase in work hardening during primary creep is achieved. The role of substructure is included through the composite model. Cavity nucleation and growth are analysed in order to explain the intergranular fracture and to estimate the ductility. / QC20100616