Measured intensity in high-energy monochromatic X-ray diffraction (HEXD) experiments provides information regarding the microstructure of the crystalline material under study. The location of intensity on an areal detector is determined by the lattice spacing and orientation of crystals so that changes in the heterogeneity of these quantities are reflected in the spreading of diffraction peaks over time. High temporal resolution of such dynamics can now be experimentally observed using technologies such as the mixed-mode pixel array detector (MM-PAD) which facilitates in situ dynamic HEXD experiments to study plasticity and its underlying mechanisms. In this paper, we define and demonstrate a feature computed directly from such diffraction time series data quantifying signal spread in a manner that is correlated with plastic deformation of the sample. A distinguishing characteristic of the analysis is the capability to describe the evolution from the distinct diffraction peaks of an undeformed alloy sample through to the non-uniform Debye–Scherrer rings developed upon significant plastic deformation. We build on our previous work modeling data using an overcomplete dictionary by treating temporal measurements jointly to improve signal spread recovery. We demonstrate our approach in simulations and on experimental HEXD measurements captured using the MM-PAD. Our method for characterizing the temporal evolution of signal spread is shown to provide an informative means of data analysis that adds to the capabilities of existing methods. Our work draws on ideas from convolutional sparse coding and requires solving a coupled convex optimization problem based on the alternating direction method of multipliers.

Titanium alloys are processed to develop a wide range of microstructure configurations and therefore material properties. While these properties are typically measured experimentally, a framework for property prediction could greatly enhance alloy design and manufacturing. Here a microstructure-sensitive framework is presented for the prediction of strength and ductility as well as estimates of the bounds in variability for these properties. The framework explicitly considers distributions of microstructure via new approaches for instantiation of structure in synthetic samples. The parametric evaluation strategy, including the finite element simulation package FEpX, is used to create and test virtual polycrystalline samples to evaluate the variability bounds of mechanical properties in Ti-6Al-4V. Critical parameters for the property evaluation framework are provided by measurements of single crystal properties and advanced characterization of microstructure and slip system strengths in 2D and 3D. Property distributions for yield strength and ductility are presented, along with the validation and verification steps undertaken. Comparisons between strain localization and slip activity in virtual samples and in experimental grain-scale strain measurements are also discussed.

Fatigue crack growth in Ti-6Al-4V has been explored through the combined use of high-energy x-ray diffraction (HEXD) analysis, electron backscatter diffraction analysis, and fractography to draw a connection between the driving forces and micromechanisms of fatigue crack growth. Samples were prepared from a forging with two configurations, viz. with the crack plane either perpendicular or parallel to the primary extension direction of the forging. Crack growth was examined in situ by mapping the lattice strain fields using HEXD for loading at R = 0.1 and R = 0.7. The crack growth as assessed by HEXD was contrasted with predictions of linear elastic fracture mechanics. The variation in the lattice strain field with crystallographic orientation was assessed.At the microscale, a clear correlation between the availability of c-axes along the loading direction and facet formation was found.

The accuracy of the stresses predicted from crystal plasticity-based finite element formulation depends on estimation and control of the errors associated with the discretization. In the current work, the errors in the stress distribution are estimated in virtual polycrystalline samples of α-phase titanium (hexagonal close-packed phase of Ti–6Al–4V). To estimate the error, the stress field, which does not possess inter-element continuity, is smoothed over a grain using an $$L_2$$ projection, thereby providing continuous stress distributions with inter-element continuity. The differences between the continuous (smooth) and discontinuous (raw) stress fields are calculated at individual Gauss quadrature points and used to estimate errors for corresponding elements and grains. Error estimations are performed for a Voronoi-tessellated microstructure, an equiaxed microstructure, and two microstructures with varying grain sizes for tensile loading extending into the fully plastic regime ($$\approx $$ 5% extension). Magnitudes of the errors are found to depend on microstructural characteristics, particularly the shape and size of grains. Samples having variations in grain size or having less spherical grains exhibited higher errors than samples with uniformly sized, equiaxed grains, with the size variations having a more pronounced effect. Errors correlate with proximity to grain boundaries at small (elastic) strains and with deformation-induced features (deformation bands) at large strains.

Abstract Long-standing evidence suggests that plasticity in metals may proceed in an intermittent fashion. While the documentation of intermittency in plastically deforming materials has been achieved in several experimental settings, efforts to draw connections from dislocation motion and structure development to stress relaxation have been limited, especially in the bulk of deforming polycrystals. This work uses high energy x-ray diffraction measurements to build these links by characterizing plastic deformation events inside individual deforming grains in both the titanium alloy, Ti-7Al, and the magnesium alloy, AZ31. This analysis is performed by combining macroscopic stress relaxation data, complete grain stress states found using far-field high energy diffraction microscopy, and rapid x-ray diffraction spot measurements made using a Mixed-Mode Pixel Array Detector. Changes in the dislocation content within the deforming grains are monitored using the evolution of the full 3-D shapes of the diffraction spot intensity distributions in reciprocal space. The results for the Ti-7Al alloy show the presence of large stress fluctuations in contrast to AZ31, which shows a lesser degree of intermittent plastic flow.

Based on simulations of tensile tests, the macroscopic stiffness and strength of the α phase of Ti-6Al-4V are estimated. Sets of virtual samples are instantiated using characterization data from three-dimensional electron back-scattered diffraction (EBSD) scans collected with the TriBeam microscope and local properties extracted from in situ loading high-energy x-ray diffraction (HEXD) experiments. The simulations use a crystal-scale finite element framework to compute the mechanical response of the virtual samples under tensile loading. From the simulation data, mechanical properties are extracted from volume elements ranging in size from a fraction of the gage section to its entire length. To detect macroscopic yield, a flood-fill algorithm is used to identify a zone of plastically deformed finite elements extending through a volume element. Trends in the estimated properties as functions of the volume element size are examined. The lower bound of volume element size necessary to replicate experimentally measured macroscale properties is indicated. The technique and results provide a guideline for estimating macroscale property values in components designed with size smaller than the lower bound due to constraint.

The full three-dimensional stress state of 172 individual bulk grains in austenitic stainless steel 316 L at 0.1 and 1% sample elongation has been determined with sufficient accuracy to allow comparison with the theoretical Bishop-Hill stress states for plastically deforming grains as well as calculation of the resolved shear stresses on the individual slip systems. At 0.1%, the resolved shear stresses exhibit quite large variations between grains of similar orientation. When averaging over similarly oriented grains, the resolved shear stresses correspond to the Schmid factors for uniaxial tension. At 1%, only about half of the grains were close to a Bishop-Hill stress state. The stress state of the other half of the grains was closer to the applied uniaxial stress, in between Bishop-Hill states, or in some cases none of these. The orientation dependence of the assigned stress states deviate somewhat from the theoretical expectation. These deviations are found to originate from a larger tensile stress component than in the theoretical Bishop Hill stress states and to be associated also with deviations from axisymmetric plastic strain. This conclusion was supported by finite-element crystal plasticity simulations. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Engineering materials are typically made from transition metals and exist in polycrystalline form. The ability to characterize this class of materials, both ex situ and in situ, is particularly important as the demand for efficient, environment-friendly machines rises and the need to understand the material response at a smaller length scale increases. In the past decade, the use of high-energy synchrotron X-rays to characterize bulk engineering materials has increased significantly, with several high-energy beamlines and new characterization techniques coming online around the world.

Finite element simulations are carried out to follow the evolution of residual stresses in Ti-7Al (alpha-hcp) alloy, as developed under an applied stress gradient. A model built upon phenomenological mesoscopic field dislocation mechanics is employed to simulate the deformation behavior. Model predictions are validated with results generated from high energy X-ray diffraction experiments using synchrotron radiation. These experiments provide for important simulation input, viz. grain positions and orientations, and strain rate sensitivities of the prismatic and basal slip systems of Ti-7Al. X-ray diffraction data obtained from individual grains enabled calculation of strain rate sensitivities of the prismatic and basal slip systems and the values are estimated as similar to 0.04 and similar to 0.02 respectively. Residual stresses at the length scale of individual grains and subgrains are successfully predicted and validated against experimental data. A key achievement of the present work is the measurement and simulation of residual stress gradients within individual grains. Conclusions from this work are that grains deform mainly via prismatic slip, and accurate characterization of rate-sensitivity is needed to model the development of grain-level residual stresses. (C) 2017 Elsevier Ltd. All rights reserved.

In-situ high energy diffraction microscopy (HEDM) experiments are carried out to analyze the state of combined bending and tension in a Ti-7Al alloy under room temperature creep. Grain-level elastic strain tensors are evaluated from HEDM data. Atomistic calculations are used to predict elastic constants of Ti-7Al, to be used in determination of stress from strain. The stress gradient and residual stresses are successfully determined, which allows the demarcation between macro-/micro-level residual stresses. A cluster of three neighboring grains are identified that highlight the variation of mean and effective stress between grains. Crystallographic orientations and slip characteristics are analyzed for the selected grains. It is inferred that the interfaces between loaded grains with markedly different stress triaxiality and slip tendency are potential spots for material damage.

Gas turbine combustion has been one of the principal sources for power generation and propulsion systems. Recent research thrust on flame monitoring for characterization of flame behavior has gained prominence for several reasons — notably for performance of combustor in aerospace propulsion and power plant applications, reduction in pollutant levels like NOx, and fire safety engineering, Lean air-fuel mixture leads to efficient combustion with lesser emissions, albeit with risk of Lean blow out (LBO). Flame monitoring is done to find out LBO point-which occurs by progressively varying the Air-fuel ratio or equivalence ratio. The current paper monitors the characteristics of lean premixed, swirl-stabilized, LPG fueled, dump combustor with the help of spectroscopy and high resolution camera images. Chemiluminescence is being used for determination of combustion characteristics. The spectroscopic peaks for chemical species like - OH* and CH* radicals and water vapor are found at varying parameters like air-fuel premixing and equivalence ratio. Blow off characteristics which occur in gas turbine combustor when going from rich to lean mixture are investigated. The comparison of the averaged red, green and blue (R, G, B) values has been done by graphical representation. The spectroscopic data are co-related with the RGB analysis results- and the location of spectroscopic peaks of intensities and their correspondence with electromagnetic spectrum in investigated. The behavior of peak intensities of Red, Green, Blue alongwith irradiation by chemical species – with the change in parameters like air flow rate, fuel flow rate or equivalence ratio and the extent of air-fuel premixing are investigated. Metrics for detecting the approach of impending LBO are proposed from the spectroscopic results.

Homogeneity in mixing of air and fuel in premixed combustion for a gas turbine combustor is a critical criterion to ensure efficient combustion and less environmental hazards. The current work deals with determining this homogenous characteristic of air-fuel mixture through computational simulation to specify homogeneity for a particular premixing length and equivalence ratio required for gas turbine combustion. A 3-D geometry of combustion chamber with combustion zone of internal diameter 6 cm is constructed. A premixing tube is augmented with the combustion chamber which has one air inlet port at the bottom and 3 fuel inlet ports. Air-fuel mixture is considered to enter the combustion zone with inlet swirl. The homogeneity of the mixture is found out at the dump plane and other important planes from simulation done with ANSYS FLUENT® for the meshed geometry. The results show whether mixing of air and fuel is full or partial and the extent of partial premixing. The parameters varied in the ANSYS FLUENT®. based simulation are the premixing length i.e. port of entry of fuel, the fuel flow rate i.e. the equivalence ratio and the air flow rate.

Cloud formation is a ubiquitous process and modeling it as a simplified experimental set-up may be an interesting and fruitful task. In this study different flow and thermal processes and fluid structures during cloud formation (especially Cumulus clouds) are investigated through an experimental setup. Vapor coming out of a 20cm×20cm water surface at temperature 60-800C interacts inside a square plexiglas tube of vertical height 60cm with dry, cold air passing from above the tube. An air conditioner is used to supply cold air at temperature 180C over the plexiglas tube. Various parameters like rate of evaporation, speed of cold air are regulated as per requirements. Turbulent plume structure is observed through a sheet of light. Optical visualization method is adopted for observing the condensed water droplets. Movies of vertical and horizontal views of plume structure are taken. Observation of this entrainment process reveals the turbulent nature of convection related to it and the gradual growth of cloud (aggregation of condensed water particles) with drop of temperature as clouds move up. Our present study is also important for prediction of type of Cumulus cloud formed for known environmental conditions.