Phase Field Modeling of Crack Growth in Shape Memory Ceramics

Shape memory ceramics (SMCs) are promising candidates for actuators in extreme environments such as high temperature and corrosive applications. Despite outstanding energy dissipation, compared to metallic shape memory materials, SMCs suffer from a sudden brittle fracture. While the interaction of crack propagation and phase transformation in SMCs has been the subject of several experimental and theoretical studies, mainly at the macroscale, the fundamental understanding of the dynamic interaction of crack propagation and martensitic transformation is poorly understood. This dissertation attempts to provide a mathematical model for crack propagation in transformable zirconia to address the shortage of classical methods. This dissertation uses the phase field framework to fully couple the martensitic transformation to the variational formulation of brittle fracture. Firstly, the model is parameterized for single crystal zirconia, which experiences tetragonal to monoclinic transformation during crack propagation. For mode I of fracture, the opening mode, crack shows an unusual propagation path that is in good agreement with the experiments and indicates the significant role of phase transformation on the crack propagation path. The investigation on the effect of lattice orientation on crack propagation shows that the lattice orientation has a significant influence not only on the crack propagation path but also on the magnitude of the transformation toughening. Secondly, the model is parameterized for tetragonal polycrystalline zirconia, and the experimental data from literature were used to validate the model. The model predicts the three dominant crack propagation patterns which were observed experimentally, including the secondary crack initiation, crack branching, and grain bridging. The model shows the critical role of texture engineering in toughening enhancement. Polycrystalline zirconia samples with grains that make low angles between the a-axis in the tetragonal phase and the crack plane, show higher transformation toughening, due to maximum hydrostatic strain release perpendicular to the crack tip. The model also shows the grain boundary engineering as a way to enhance the transformation toughening. The maximum fracture toughness occurs at a specific grain size, and further coarsening or refinement reduces the fracture toughness. This optimum grain size is the consequence of the competition between the toughening enhancement and MT suppression with grain refinement. Finally, we parameterized the model for the 3D single crystal zirconia, which experienced stress- and thermal-induced tetragonal to monoclinic transformation. The developed 3D model considers all 12 monoclinic variants, making it possible to acquire realistic microstructures. Surface uplifting, self-accommodated martensite pairs formation, and transformed zone fragmentation were observed by the model, which agrees with the experimental observations. The influence of the crystal lattice orientation is investigated in this study, which reveals its profound effects on the transformation toughening and crack propagation path.

Localization of plastic deformation in stretching plates: effect of crystalline structure

The integration of the polycrystalline structure in the simulation of stretching plates, performed by the random generation of a grain aggregate and a set of lattice orientations, gives new insights into the phenomenon of plastic strain localization in the form of necking, albeit well predicted at the scale of continuum by instability analysis. A transition is displayed from initial grain scale heterogeneity, with some connection to crystal lattice orientation towards stress axes, to the onset of macroscopic localization patterns. Depending on the number of grains and on the stretching rate, it seems that a competition emerges between a “weakest link” process for which the final necks are located in places where initial deformation is high and an instable mode controlled process during which larger patterns emerge driving the location of the final necks. At high stretching rates, it seems that the second effect is enhanced with, accordingly, a pattern size almost insensitive to the grain size and a limited variability with the grain structure occurrence.

Stable anisotropic single-layer of ReTe2: a first principles prediction

In order to investigate the structural, vibrational, electronic, and mechanical features of single-layer ReTe2first-principles calculations are performed. Dynamical stability analyses reveal that single-layer ReTe2crystallize in adistorted phase while its 1H and 1T phases are dynamically unstable. Raman spectrum calculations show that single-layer distorted phase of ReTe2exhibits 18 Raman peaks similar to those of ReS2and ReSe2. Electronically, single-layerReTe2is shown to be an indirect gap semiconductor with a suitable band gap for optoelectronic applications. In addition,it is found that the formation of Re-units in the crystal induces anisotropic mechanical parameters. The in-plane stiffnessand Poisson ratio are shown to be significantly dependent on the lattice orientation. Our findings indicate that single-layer form of ReTe2can only crystallize in a dynamically stable distorted phase formed by the Re-units. Single-layer ofdistorted ReTe2can be a potential in-plane anisotropic material for various nanotechnology applications.

Sculpted grain boundaries in soft crystals

Engineering the grain boundaries of crystalline materials represents an enduring challenge, particularly in the case of soft materials. Grain boundaries, however, can provide preferential sites for chemical reactions, adsorption processes, nucleation of phase transitions, and mechanical transformations. In this work, “soft heteroepitaxy” is used to exert precise control over the lattice orientation of three-dimensional liquid crystalline soft crystals, thereby granting the ability to sculpt the grain boundaries between them. Since these soft crystals are liquid-like in nature, the heteroepitaxy approach introduced here provides a clear strategy to accurately mold liquid-liquid interfaces in structured liquids with a hitherto unavailable level of precision.