Experimental and Computational Investigation of the Microstructure-Mechanical Deformation Relationship in Polycrystalline Materials, Applied to Additively Manufactured Titanium Alloys

Parts made out of titanium alloys demonstrate anisotropic mechanical properties when manufactured by electron beam melting, an emerging additive manufacturing technique. Understanding the process history dependent heterogeneous microstructure, and its effect on mechanical properties is crucial in determining the performance of additively manufactured titanium alloys as the mechanical behavior heavily relies on the underlying microstructural features. This thesis work focuses on combined experimental and computational techniques for microstructure characterization, synthetic microstructure generation, mechanical property measurement, and mechanical behavior modeling of polycrystalline materials, with special focus on dual phase titanium alloys. Macroscopic mechanical property measurements and multi-modal microstructure characterizations (high energy X-ray diffraction, computed tomography and optical microscopy) are performed on additively manufactured Ti-6Al-4V parts, revealing the heterogeneity of the microstructure and properties with respect to the build height. Because characterizing and testing every location within a build is not practical, a computational methodology is established in order to reduce the time and cost spent on microstructure-property database creation. First a statistical volume element size is determined for the Fast Fourier Transform based micromechanical modeling technique through a sensitivity study performed on an experimental Ni-based superalloy and syntheticW, Cu, Ni and Ti structures, showing that as the contrast of properties (e.g., texture, field localization, anisotropy, rate-sensitivity) increases, so does the minimum simulation domain size requirement. In all deformation regimes a minimum volume element is defined for both single and dual phase materials. The database is then expanded by generating statistically representative Ti structures which are modified for features of interest, e.g., lath thickness, grain size and orientation distribution, to be used in spectral full-field micromechanical modeling. The relative effect of the chosen microstructural features is quantified through comparisons of average and local field distributions. Fast Fourier transform based technique, being a spectral, full-field deformation modeling tool, is shown to be capable of capturing the relative contribution from varying microstructural features such as phase fractions, grain morphology/ size and texture on the overall mechanical properties as the results indicate that the mean field behavior is predominantly controlled by the alpha phase fraction and the prior beta phase orientation.