Heterogeneous deformation in polycrystalline commercial purity titanium

Heterogeneous deformation is commonly viewed as a precursor to damage nucleation. Only if the heterogeneous deformation is reliably modeled can location where cracks form be properly predicted. In this research project, commercial purity titanium (hexagonal close-packed (hcp) metal) has been used as a model material to study different scales of heterogeneous deformation in four-point bending specimens and fatigued-to-crack specimens. A series of comprehensive experimental and modeling methods have been conducted to characterize the heterogeneous deformation within different microstructure patches.;Heterogeneous deformation within a microstructure patch was quantified by a new combined technique using atomic force microscopy (AFM), backscattered electron (BSE) imaging, and electron-backscattered diffraction (EBSD). Local shear distribution maps derived from z-displacement data of slip steps and/or twins measured by AFM were directly compared to results of a crystal plasticity finite element (CPFE) simulation that incorporates a phenomenological model of the deformation processes to evaluate the ability of the CPFE model to match the experimental observations. The CPFE model successfully predicted most types of active dislocation slip systems within the grains at the correct magnitudes, but the spatial distribution of the strains within grains differed between the measurements and the simulation. Since the difficulty of directly measuring the critical resolved shear stress (CRSS) of different deformation systems in titanium is a major cause of the deficiency of the match between CPFE modeling results and experimental measurements, nano-indentation technique is introduced to help study the values of CRSS and saturation resistance of different deformation systems for commercial purity titanium. Arrays of conical nano-indentations were performed on patches of grains. AFM scans and EBSD measurements on the patches indicate that the surface pile-up topography was strongly crystallographically dependent. By optimizing the modeling results of the nano-indentation behavior in different grains, a more reliable set of values for CRSS and saturation resistance has been determined using a non-linear optimization procedure. On a larger scale, a novel approach including BSE imaging and discrete Fourier transformation (DFT) processing is established to determine plastic zone shape and to measure plastic strain distribution within the plastic zone around a fatigue crack tip. The measured plastic zone size is in good correlation with the theoretically predicted zone sizes. This approach also holds promise to measure local heterogeneous deformation in other kinds of deformed samples.;This work has helped identity the needs for future experiments focused on the observation of dislocation activity near grain boundaries using ECCI, and a statistical study on the relationship of heterogeneous deformation in neighboring grains and damage nucleation at grain boundaries. Future simulation work should focus on developing a more comprehensive CPFE model, considering the information of slip/twin transfer and grain boundaries resistance, in order to simulate heterogeneous deformation within grain patches more reliably.