John W. Gibbs

TIMBIR: A Method for Time-Space Reconstruction From Interlaced Views

Synchrotron X-ray computed tomography (SXCT) is increasingly being used for 3-D imaging of material samples at micron and finer scales. The success of these techniques has increased interest in 4-D reconstruction methods that can image a sample in both space and time. However, the temporal resolution of widely used 4-D reconstruction methods is severely limited by the need to acquire a very large number of views for each reconstructed 3-D volume. Consequently, the temporal resolution of current methods is insufficient to observe important physical phenomena. Furthermore, measurement nonidealities also tend to introduce ring and streak artifacts into the 4-D reconstructions. In this paper, we present a time-interlaced model-based iterative reconstruction (TIMBIR) method, which is a synergistic combination of two innovations. The first innovation, interlaced view sampling, is a novel method of data acquisition, which distributes the view angles more evenly in time. The second innovation is a 4-D model-based iterative reconstruction algorithm (MBIR), which can produce time-resolved volumetric reconstruction of the sample from the interlaced views. In addition to modeling both the sensor noise statistics and the 4-D object, the MBIR algorithm also reduces ring and streak artifacts by more accurately modeling the measurement nonidealities. We present reconstructions of both simulated and real X-ray synchrotron data, which indicate that TIMBIR can improve temporal resolution by an order of magnitude relative to existing approaches.

The Three-Dimensional Morphology of Growing Dendrites

The processes controlling the morphology of dendrites have been of great interest to a wide range of communities, since they are examples of an out-of-equilibrium pattern forming system, there is a clear connection with battery failure processes, and their morphology sets the properties of many metallic alloys. We determine the three-dimensional morphology of free growing metallic dendrites using a novel X-ray tomographic technique that improves the temporal resolution by more than an order of magnitude compared to conventional techniques. These measurements show that the growth morphology of metallic dendrites is surprisingly different from that seen in model systems, the morphology is not self-similar with distance back from the tip, and that this morphology can have an unexpectedly strong influence on solute segregation in castings. These experiments also provide benchmark data that can be used to validate simulations of free dendritic growth.

Segmentation of four-dimensional, X-ray computed tomography data

The rapidly improving temporal resolution of X-ray computed tomography (CT) imaging methods makes it ever easier to do in-situ, time-resolved (4D) experiments. This work describes a method of segmenting 4D X-ray CT data that works well for extracting information on the interfacial properties, such as interfacial curvature and velocity. As an example of this method, a segmentation is performed on data from an isothermal coarsening experiment of an Al-Cu solid/liquid mixture.

Integrated approach to the data processing of four-dimensional datasets from phase-contrast x-ray tomography

Phase contrast X-ray tomography (PCT) enables the study of systems consisting of elements with similar atomic numbers. Processing datasets acquired using PCT is nontrivial because of the low-pass characteristics of the commonly used single-image phase retrieval algorithm. In this study, we introduce an image processing methodology that simultaneously utilizes both phase and attenuation components of an image obtained at a single detector distance. This novel method, combined with regularized Perona-Malik filter and bias-corrected fuzzy C-means algorithm, allows for automated segmentation of data acquired through four-dimensional PCT. Using this integrated approach, the three-dimensional coarsening morphology of an Aluminum-29.9 wt% Silicon alloy can be analyzed.

From Solidification Processing to Microstructure to Mechanical Properties: A Multi-scale X-ray Study of an Al-Cu Alloy Sample

We follow an Al-12 at. pct Cu alloy sample from the liquid state to mechanical failure, using in situ X-ray radiography during directional solidification and tensile testing, as well as three-dimensional computed tomography of the microstructure before and after mechanical testing. The solidification processing stage is simulated with a multi-scale dendritic needle network model, and the micromechanical behavior of the solidified microstructure is simulated using voxelized tomography data and an elasto-viscoplastic fast Fourier transform model. This study demonstrates the feasibility of direct in situ monitoring of a metal alloy microstructure from the liquid processing stage up to its mechanical failure, supported by quantitative simulations of microstructure formation and its mechanical behavior.

4D model-based iterative reconstruction from interlaced views

X-ray tomography is increasingly being used for 4D spatio-temporal imaging of material samples at micron and finer scales. However, the temporal resolution of widely used 4D reconstruction methods is severely limited by the need to acquire a very large number of views for each reconstructed 3D volume. In this paper, we present a time interlaced model-based iterative reconstruction (TIMBIR) method which can significantly improve the temporal resolution of reconstructions. TIMBIR is a synergistic combination of two innovations. The first innovation, interlaced view sampling, is a novel approach to data acquisition which distributes the view angles more evenly in time. The second innovation is a 4D model based iterative reconstruction algorithm (MBIR) which can produce time resolved volumetric reconstructions of the sample from the interlaced views. Reconstructions of simulated data indicate that TIMBIR can improve the temporal resolution by an order of magnitude relative to existing approaches.

Martensite Fraction Determination Using Cooling Curve Analysis

This work presents a method of calculating the martensite fraction of an Fe-alloy, usingcooling curve analysis (CCA). It is based on a differential heat balance equation which takes intoaccount only convective exchange with the surroundings. By measuring a T(t) curve of an Fe-alloyand solving numerically the differential heat balance equation the martensite fraction can be calcu-lated. It is found that calculated martensite fraction using this methodology is comparable with resultsobtained using electron backscattering diffraction (EBDS).

From Alloy Processing to Performance: An In Situ Experimental and Modeling Effort

Solidification is present in almost all materials. It is influenced by grain size and shape, chemical homogeneity, defect type and density, and mechanical properties. During micro-mechanical testing, the following occur: 1) Micro-CT (as processed) - Map Initial 3D Microstructure 2) Nano-Radiography (In situ under Tension) - Observe of Damage Initiation/Propagation 3) Micro-CT (Post Mortem) - Global Fracture Study 4) Nano-CT (Post Mortem) - High-Resolution Fracture Study.

Imaging the Rapid Solidification of Metallic Alloys in the TEM

The macroscopic properties of a metal solidified from a liquid melt are strongly dependent on the final microstructure, which in turn is the result of the solidification conditions. With the growing popularity of laser-based additive manufacturing (AM), there is an increasing need to understand the microstructures that result from rapid solidification processes. Rapidly solidified alloy microstructures are typically far from equilibrium and therefore traditional thermodynamic approaches used to predict structure and composition (i.e., phase diagrams) must be extended to describe these deviations from equilibrium and ensuing metastable states. This work highlights progress toward corroborating predictive (phase-field) modeling capabilities [1] with in situ experimental observations [2] in order to better understand the non-equilibrium structures produced during rapid solidification following laser melting.