Todd J. Turner

A rotational and axial motion system load frame insert for in situ high energy x-ray studies

High energy x-ray characterization methods hold great potential for gaining insight into the behavior of materials and providing comparison datasets for the validation and development of mesoscale modeling tools. A suite of techniques have been developed by the x-ray community for characterizing the 3D structure and micromechanical state of polycrystalline materials; however, combining these techniques with in situ mechanical testing under well characterized and controlled boundary conditions has been challenging due to experimental design requirements, which demand new high-precision hardware as well as access to high-energy x-ray beamlines. We describe the design and performance of a load frame insert with a rotational and axial motion system that has been developed to meet these requirements. An example dataset from a deforming titanium alloy demonstrates the new capability.

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

Abstract High-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.

Fiducial marker application method for position alignment of in situ multimodal X-ray experiments and reconstructions

An evolving suite of X-ray characterization methods are presently available to the materials community, providing a great opportunity to gain new insight into material behavior and provide critical validation data for materials models. Two critical and related issues are sample repositioning during an in situ experiment and registration of multiple data sets after the experiment. To address these issues, a method is described which utilizes a focused ion-beam scanning electron microscope equipped with a micromanipulator to apply gold fiducial markers to samples for X-ray measurements. The method is demonstrated with a synchrotron X-ray experiment involving in situ loading of a titanium alloy tensile specimen.

Deformation and Recrystallization during Thermomechanical Processing of a Nickel-Base Superalloy Ingot Material

Abstract : The deformation response and recrystallization behavior of a coarse, columnar-grain superalloy ingot material, Waspaloy, with a fiber texture were established. For this purpose, isothermal hot compression tests were performed on cylindrical and doublecone samples at supersolvus temperatures under both monotonic (constant strain rate) and multi-hit conditions. Plastic flow showed a noticeable dependence on test direction relative to the columnar-grain orientation; the observed anisotropy in peak flow stress and flow softening were explained on the basis of the evolution of crystallographic texture during recrystallization. Similarly, anisotropy in dynamic recrystallization kinetics with respect to test direction was interpreted in terms of the effect of initial texture on the plastic work imposed per increment of macroscopic strain. Nevertheless, the broad kinetics for the coarse-grain, ingot material deformed under both monotonic and multi-hit conditions were comparable to those previously measured for fine-grain, wrought Waspaloy. Such an effect was attributed to the beneficial influence of the nucleation of recrystallization at both grain boundaries and carbide particles in the ingot material. In addition, a spatial non-uniformity in recrystallization was found in the ingot material and was interpreted in the context of the grain-boundary character and non-uniform strain at the grain/intragrain scale. A suite of tools being developed to model recrystallization phenomena during the breakdown of superalloy ingots is described. These tools include a mechanistic cellular automata; a mesoscale, mechanism-based model; and the crystal-plasticity finite-element method.

Comparison of Residual Stress Characterization Techniques Using an Interference Fit Sample

Residual stress in an engineering component induced from processing is pervasive and can impact the component’s performance significantly. There are numerous destructive and non-destructive techniques that are available to determine the residual stresses in a component. In this work, an interference fit sample was manufactured from a titanium alloy. The sample was equipped with a set of strain gauges to measure the strains induced by the interference process used for sample assembly. Energy dispersive diffraction experiment using synchrotron radiation was conducted to measure the lattice strains in the interference fit sample. Hole drilling measurements were also conducted on the sample. The non-destructive X-ray result is compared with strain gauge measurements, and found to be in good agreement when appropriately averaged.

Data Infrastructure Developed for PW-8: Nickel Base Superalloy Residual Stress Foundational Engineering Problem

The PW-8: Nickel Base Superalloy Residual Stress Foundational Engineering Problem (FEP) is a program funded by the United States Air Force through the Metals Affordability Initiative (MAI) to address bulk residual stresses in Nickel-base superalloy engine disk components. These stresses can be induced during various manufacturing stages such as the heat treatment process or the forging process. Bulk residual stresses can be a problem and result in component distortion during the machining process and/or during elevated temperature service. Bulk residual stresses in aeroengine disks components are considered a Foundational Engineering Problem that affects both suppliers and Original Equipment Manufacturers (OEMs) and is an issue that must be addressed with a cross-functional team. The FEP addresses this problem by developing the infrastructure and tools needed to predict and incorporate bulk residual stress into the design and development of a turbine disk. In doing so, the FEP answers the challenge given in a 2008 report issued by the National Research Council in which the authors commented that addressing FEPs are an essential means to help establish the infrastructure needed to make Integrated Computational Materials Engineering (ICME) a reality. This paper will report on a key aspect of the ICME infrastructure; namely the infrastructure needed to manage physical and model data. In addition to discussing the infrastructure developed, this paper will document the lessons learned which can be applied to the ICME community as a whole.

Modeling Grain Boundary Interfaces in Pure Nickel

This work presents a three tiered modeling approach to examine grain boundary interfaces in a pure Nickel foil material utilizing a crystal plasticity based finite element model (CPFEM). The goal of this work is to calibrate a modeling approach through comparison to experimental data, and then use the models to gain insight into deformation at grain boundaries in Nickel and Nickel-base superalloy polycrystals. The first study utilizes a multi-crystal micro-tension specimen and simulations to calibrate the CPFEM model and examine the development of “hot-spots” or localized plasticity near the grain boundaries. Some orientation combinations exhibit localized plasticity along the boundary (bad-actor boundaries) while others do not. Insight from the deformation of this model is then used to instantiate simulations of Nickel bi-crystals which exhibit localized plasticity near the boundary. The third study embeds the grain boundary interfaces of interest, as determined from the bi-crystal simulations, into a larger polycrystalline simulation utilizing the same CPFEM framework. Using these interfaces we study deformation at these “characteristic” interfaces when subjected to the generalized loading conditions present in a polycrystalline microstructure.