Mapping nanocrystal orientations via scanning Laue diffraction microscopy for multi-peak Bragg coherent diffraction imaging.

The recent commissioning of a movable monochromator at the 34-ID-C endstation of the Advanced Photon Source has vastly simplified the collection of Bragg coherent diffraction imaging (BCDI) data from multiple Bragg peaks of sub-micrometre scale samples. Laue patterns arising from the scattering of a polychromatic beam by arbitrarily oriented nanocrystals permit their crystal orientations to be computed, which are then used for locating and collecting several non-co-linear Bragg reflections. The volumetric six-component strain tensor is then constructed by combining the projected displacement fields that are imaged using each of the measured reflections via iterative phase retrieval algorithms.

Grain boundary velocity and curvature are not correlated in Ni polycrystals.

Grain boundary velocity has been believed to be correlated to curvature, and this is an important relationship for modeling how polycrystalline materials coarsen during annealing. We determined the velocities and curvatures of approximately 52,000 grain boundaries in a nickel polycrystal using three-dimensional orientation maps measured by high-energy diffraction microscopy before and after annealing at 800°C. Unexpectedly, the grain boundary velocities and curvatures were uncorrelated.

Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725.

Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy.

Comparison between diffraction contrast tomography and high-energy diffraction microscopy on a slightly deformed aluminium alloy.

The grain structure of an Al-0.3 wt%Mn alloy deformed to 1% strain was reconstructed using diffraction contrast tomography (DCT) and high-energy diffraction microscopy (HEDM). 14 equally spaced HEDM layers were acquired and their exact location within the DCT volume was determined using a generic algorithm minimizing a function of the local disorientations between the two data sets.

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.

Using x-ray reflectivity to determine the structure of surfactant monolayers.

Interactions among the multiple degrees of freedom of surfactant molecules cause fascinating richness in the structure of their monolayers. Beyond this scientific motivation for studying surfactant monolayers, the technological use of monolayers for interfacial control and molecular assembly demands a clear understanding of monolayer structure. X-ray and neutron reflectivity have become prime techniques for determining this structure.

Effect of substrate roughness on D spacing supports theoretical resolution of vapor pressure paradox.

The lamellar D spacing has been measured for oriented stacks of lecithin bilayers prepared on a variety of solid substrates and hydrated from the vapor. We find that, when the bilayers are in the L(alpha) phase near 100% relative humidity, the D spacing is consistently larger when the substrate is rougher than when it is smooth. The differences become smaller as the relative humidity is decreased to 80% and negligible differences are seen in the L(beta') phase.

Structure of gel phase saturated lecithin bilayers: temperature and chain length dependence.

Systematic low-angle and wide-angle x-ray scattering studies have been performed on fully hydrated unoriented multilamamellar vesicles of saturated lecithins with even chain lengths N = 16, 18, 20, 22, and 24 as a function of temperature T in the normal gel (L beta') phase. For all N, the area per chain Ac increases linearly with T with an average slope dAc/dT = 0.027 A2/degree C, and the lamellar D-spacings also increase linearly with an average slope dD/dT = 0.

Structure of the ripple phase in lecithin bilayers.

The phases of the x-ray form factors are derived for the ripple (Pbeta') thermodynamic phase in the lecithin bilayer system. By combining these phases with experimental intensity data, the electron density map of the ripple phase of dimyristoyl-phosphatidylcholine is constructed. The phases are derived by fitting the intensity data to two-dimensional electron density models, which are created by convolving an asymmetric triangular ripple profile with a transbilayer electron density profile.

X-ray structure determination of fully hydrated L alpha phase dipalmitoylphosphatidylcholine bilayers.

Bilayer form factors obtained from x-ray scattering data taken with high instrumental resolution are reported for multilamellar vesicles of L alpha phase lipid bilayers of dipalmitoylphosphatidylcholine at 50 degrees C under varying osmotic pressure. Artifacts in the magnitudes of the form factors due to liquid crystalline fluctuations have been eliminated by using modified Caillé theory. The Caillé fluctuation parameter eta 1 increases systematically with increasing lamellar D spacing and this explains why some higher order peaks are unobservable for the larger D spacings.

Anomalous phase behavior of long chain saturated lecithin bilayers.

X-ray scattering has been performed on fully hydrated unoriented multilamellar vesicles of lecithins with even chain lengths n from 16 to 24 as a function of temperature in chain ordered phases. The longer chain lengths, n > or = 20, show anomalous behavior compared to the shorter chain lengths, n < 20. This report concentrates on n = 24.

Small-angle x-ray scattering from lipid bilayers is well described by modified Caillé theory but not by paracrystalline theory.

X-ray scattering data at high instrumental resolution are reported for multilamellar vesicles of L alpha phase lipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine at 50 degrees C under varying osmotic pressure. The data are fitted to two theories that account for noncrystalline disorder, paracrystalline theory (PT) and modified Caillé theory (MCT). The MCT provides good fits to the data, much better than the PT fits.

Kinetics of subgel formation in DPPC: X-ray diffraction proves nucleation-growth hypothesis.

Wide-angle and low-angle X-ray diffraction data were obtained during the time course of the gel to subgel phase transformation in fully hydrated DPPC. When the system was kept close to equilibrium by following a T-jump protocol, the X-ray data unequivocally demonstrate the coexistence of growing subgel and shrinking gel domains. When the system was supercooled and held further from equilibrium as in previous studies, the kinetic behavior was more complicated.

Measurement of chain tilt angle in fully hydrated bilayers of gel phase lecithins.

The tilt angle theta tilt of the hydrocarbon chains has been determined for fully hydrated gel phase of a series of saturated lecithins. Oriented samples were prepared on glass substrates and hydrated with supersaturated water vapor. Evidence for full hydration was the same intensity pattern of the low angle lamellar peaks and the same lamellar repeat D as unoriented multilamellar vesicles.

Structure of the fully hydrated gel phase of dipalmitoylphosphatidylcholine.

X-ray diffraction intensities for lamellar repeats, h = 1 to 7, and wide-angle x-ray scattering were measured for the gel phase of fully hydrated dipalmitoylphosphatidylcholine. A hybrid model, which represents the electron density along the lamellar repeat direction as a continuous function composed of constant strips and superimposed Gaussians, is defined. The data were used to determine the best parameters in hybrid models and also in the older strip models.