Shiu Fai Li

Crystal Plasticity Model Validation Using Combined High-Energy Diffraction Microscopy Data for a Ti-7Al Specimen

AbstractnHigh-Energy Diffraction Microscopy (HEDM) is a 3-d X-ray characterization method that is uniquely suited to measuring the evolving micro-mechanical state and microstructure of polycrystalline materials during in situ processing. The near-field and far-field configurations provide complementary information; orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provides intergranular strain tensors. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. Crystal Plasticity Finite Element Models (CPFEM) are one such class of micro-mechanical models. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. We utilize an HEDM dataset from an alpha-titanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4 and ~80 pct of the macroscopic yield strength. The evolution of the elastic strain state from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, while residual stress at the individual grain level is found to influence the intergranular stress state even upon loading. Implications for application of such an integrated computational/experimental approach to phenomena such as fatigue are discussed.

High-Energy Diffraction Microscopy Characterization of Spall Damage

The emerging characterization technique of high-energy diffraction microscopy (HEDM) was used to investigate ductile dynamic damage evolution in a Cu polycrystal. Experimental efforts were undertaken with the goal of elucidating correlations between microstructural features with preferred damage nucleation sites and the progression of damage at the localization stage. HEDM was used to microstructurally map the initial volume of a 1.2 mm-diameter Cu sample. HEDM in the near-field mode collects diffraction information from high-energy synchrotron radiation to non-destructively probe microstructure and orientation in three dimensions in volumes approaching the bulk scale. The Cu sample was subsequently planar shock-loaded in a plate-on-plate geometry and soft-recovered, using an assembly specially developed for sub-size samples. The ex situ shocked sample was then re-characterized by HEDM, providing data on the location of incipient spall voids with respect to the local microstructural neighborhood. In addition, diffraction quality and misorientation gradient data provide qualitative measures of the spatial distribution of stored work and indicate regions of plastic localization. This provides the potential for unprecedented insight as to the relative preference of spall nucleation sites and correlations between microstructure, damage, and plastic flow.

Shock induced damage in copper: A before and after, three-dimensional study

We report on the microstructural features associated with the formation of incipient spall and damage in a fully recrystallized, high purity copper sample. Before and after ballistic shock loading, approximately 0.8u2009mm3 of the samples crystal lattice orientation field is mapped using non-destructive near-field High Energy Diffraction Microscopy. Absorption contrast tomography is used to imagevoids after loading. This non-destructive interrogation of damage initiation allows for novel characterization of spall points vis-a-vis microstructural features and a fully 3D examination of microstructural topology and its influence on incipient damage. The spalled region is registered with and mapped back onto the pre-shock orientation field. As expected, the great majority of voids occur at grain boundaries and higher order microstructural features; however, we find no statistical preference for particular grain boundary types. The damaged region contains a large volume of Σ–3u2009(60°〈111〉) connected domains with a large area fraction of incoherent Σ-3 boundaries.

Correlation of Thermally Induced Pores with Microstructural Features Using High Energy X-rays

Combined application of a near-field High Energy Diffraction Microscopy measurement of crystal lattice orientation fields and a tomographic measurement of pore distributions in a sintered nickel-based superalloy sample allows pore locations to be correlated with microstructural features. Measurements were carried out at the Advanced Photon Source beamline 1-ID using an X-ray energy of 65 keV for each of the measurement modes. The nickel superalloy sample was prepared in such a way as to generate significant thermally induced porosity. A three-dimensionally resolved orientation map is directly overlaid with the tomographically determined pore map through a careful registration procedure. The data are shown to reliably reproduce the expected correlations between specific microstructural features (triple lines and quadruple nodes) and pore positions. With the statistics afforded by the 3D data set, we conclude that within statistical limits, pore formation does not depend on the relative orientations of the grains. The experimental procedures and analysis tools illustrated are being applied to a variety of materials problems in which local heterogeneities can affect materials properties.