The damage and deformation behaviour of particle reinforced aluminum matrix composites can be highly sensitive to local variations in spatial distribution of reinforcement particles, which markedly depend on melt processing and solidification stages during production. The present study is aimed at understanding the mechanisms responsible for clustering of SiC particles in an Al-Si-Mg (A356) alloy composite during solidification process and establishing a model to predict the risk of cluster formation as a function of local solidification rate in a cast component. Special emphasis has been given to spatial characterization methods in terms of their suitability to characterize composite microstructures. Result indicate that methods that present a summary statistics on the global level of heterogeneity have limited application in quantitative analysis of discontinuously reinforced composites since the mechanical response of such materials are highly sensitive to dimensions, locations and spatial connectivities of clusters. The local density statistics, on the other hand, was observed to provide a satisfactory description of the microstructure, in terms of localization and quantification of clusters. A macrotransport - solidification kinetics model has been employed to simulate solidification microstructures for estimation of cluster formation tendency. Results show that the distribution of SiC particles is determined by the scale of secondary dendrite arms (SDAS). In order to attain the lowest amount of particle clustering, the arm spacings should be kept within the limit of 2dSiC &gt / SDAS &gt / dSiC, where dSiC is the average particle diameter.

The conventional route of materials development typically involves fabrication of numerous batches of specimens having a range of different microstructures generated via variations of process parameters and measurements of relevant properties of these microstructures to identify the combination of processing conditions that yield the material having desired properties. Clearly, such a trial and error based materials development methodology is expensive, time consuming, and inefficient. Consequently, it is of interest to explore alternate strategies that can lead to a decrease in the cost and time required for development of advanced materials such as composites. Availability of powerful and inexpensive computational power and progress in computational materials science permits advancement of modeling and simulations assisted materials design methodology that may require fewer experiments, and therefore, lower cost and time for materials development. The key facets of such a technology would be computational tools for (i) creating models to generate computer simulated realistic microstructures; (ii) capturing the process-microstructure relationship using these models; and (iii) implementation of simulated microstructures in the computational models for materials behavior. Therefore, development of a general and flexible methodology for simulations of realistic microstructures is crucial for the development of simulations based materials design and development technology. Accordingly, this research concerns development of such a methodology for simulations of realistic microstructures based on experimental quantitative stereological data on few microstructures that can capture relevant details of microstructural geometry (including spatial clustering and second phase particle orientations) and its variations with process parameters in terms of a set of simulation parameters. The interpolation and extrapolation of the simulation parameters can then permit generation of atlas of virtual microstructures that covers the complete range of variations of processing conditions of interest. These simulated and virtual microstructures can then be used in the micromechanical models such as FEM to analyze their constitutive properties

The creep behavior of unreinforced 2124 aluminum and 2124 aluminum reinforced with 15 w/o silicon carbide whiskers was studied at temperatures from 250 F to 500 F. Tensile tests were conducted to determine the basic mechanical properties, and microstructural and chemical anyalyses were performed to characterize the starting materials. The creep, tensile, and microstructural data for the 2124+SiC composite were compared with a similarly processed unreinforced 2124 aluminum alloy. Applying the basic theories for power law creep developed for common metals and alloys, the creep stress exponents and activation energies for creep were determined from the experimental data. The results were used to identify creep deformation mechanisms and compared to predicted values based on a parametric approach for creep analysis. The results demonstrate the applicability of traditional creep analysis on non-traditional materials. / Master of Science

No description available.