Ersan Üstündag

A dedicated superbend x-ray microdiffraction beamline for materials, geo-, and environmental sciences at the advanced light source

A new facility for microdiffraction strain measurements and microfluorescence mapping has been built on beamline 12.3.2 at the advanced light source of the Lawrence Berkeley National Laboratory. This beamline benefits from the hard x-radiation generated by a 6 T superconducting bending magnet (superbend). This provides a hard x-ray spectrum from 5 to 22 keV and a flux within a 1 microm spot of approximately 5x10(9) photons/s (0.1% bandwidth at 8 keV). The radiation is relayed from the superbend source to a focus in the experimental hutch by a toroidal mirror. The focus spot is tailored by two pairs of adjustable slits, which serve as secondary source point. Inside the lead hutch, a pair of Kirkpatrick-Baez (KB) mirrors placed in a vacuum tank refocuses the secondary slit source onto the sample position. A new KB-bending mechanism with active temperature stabilization allows for more reproducible and stable mirror bending and thus mirror focusing. Focus spots around 1 microm are routinely achieved and allow a variety of experiments, which have in common the need of spatial resolution. The effective spatial resolution (approximately 0.2 microm) is limited by a convolution of beam size, scan-stage resolution, and stage stability. A four-bounce monochromator consisting of two channel-cut Si(111) crystals placed between the secondary source and KB-mirrors allows for easy changes between white-beam and monochromatic experiments while maintaining a fixed beam position. High resolution stage scans are performed while recording a fluorescence emission signal or an x-ray diffraction signal coming from either a monochromatic or a white focused beam. The former allows for elemental mapping, whereas the latter is used to produce two-dimensional maps of crystal-phases, -orientation, -texture, and -strain/stress. Typically achieved strain resolution is in the order of 5x10(-5) strain units. Accurate sample positioning in the x-ray focus spot is achieved with a commercial laser-triangulation unit. A Si-drift detector serves as a high-energy-resolution (approximately 150 eV full width at half maximum) fluorescence detector. Fluorescence scans can be collected in continuous scan mode with up to 300 pixels/s scan speed. A charge coupled device area detector is utilized as diffraction detector. Diffraction can be performed in reflecting or transmitting geometry. Diffraction data are processed using XMAS, an in-house written software package for Laue and monochromatic microdiffraction analysis.

Texture and strain analysis of the ferroelastic behavior of Pb(Zr,Ti)O3 by in situ neutron diffraction

In situ uniaxial compression experiments on Pb(Zr,Ti)O3 or PZT-based polycrystalline electroceramics were conducted using time-of-flight neutron diffraction. Elastic lattice strain and texture evolution were observed in PZT’s near the edge of the morphotropic phase boundary (with tetragonal and rhombohedral phases present). Multiphase Rietveld analysis yielded anisotropic lattice strain evolution curves in directions parallel and perpendicular to the loading axis for both phases. A quantitative analysis of the domain switching under applied stress was possible through application of a March–Dollase model for texture.

Compressive yielding of tungsten fiber reinforced bulk metallic glass composites

In-situ uniaxial compression tests were conducted on four tungsten fiber reinforced bulk metallic glass matrix composites using neutron diffraction. The results were interpreted with a finite element model. Both phases were seen to approximately obey the von Mises yield criterion. The fibers were observed to yield first and then transfer load to the matrix.

Modeling and Measurement of Residual Stresses in a Bulk Metallic Glass Plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr41.2Ti13.8Cu12.5Ni10Be22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.

Time-resolved and orientation-dependent electric-field-induced strains in lead zirconate titanate ceramics

Electric-field-induced lattice strains in a tetragonal ferroelectric lead zirconate titanate bulk ceramic are characterized under application of subcoercive cyclic electric fields using neutron diffraction and a stroboscopic data collection technique. At a driving electric field equal to half of the coercive field, the field-induced lattice strains are found to be a function of orientation with the greatest electric-field-induced strain coefficient of 680pm∕V in crystal orientations such that the 211 pole is parallel to the electric field. A time dependence of the 111 strain was also observed. Suggestions as to the nature of these dependences are discussed.

Residual thermal stresses and calculation of the critical metal particle size for interfacial crack extension in metal-ceramic matrix composites

Abstract A combined experimental and theoretical study was carried out to analyze the effect of residual thermal stresses on the interfacial bonding in particulate metal-ceramic matrix composites, in particular, ductile phase toughened ceramics. The metal-ceramic microstructure studied is a two phase Ni Al 2 O 3 system produced by an in situ reduction processing technique. When a Ni particle exceeds the critical size, crack extension occurs along the particle/matrix interface upon cooling from the reduction temperature to room temperature. A closed form solution is derived for the case of thermal loading for the energy release rate and the phase angle for a crack along the interface of a cylindrical particle embedded in an infinite matrix. A theoretical estimate of the critical size for the nickel particle for crack extension is then computed based on the closed form solution and experimental data on the toughness of the Ni Al 2 O 3 interface.

Atomic pair distribution function analysis of materials containing crystalline and amorphous phases

Abstract The atomic pair distribution function (PDF) approach has been used to study the local structure of liquids, glasses and disordered crystalline materials. In this paper, we demonstrate the use of the PDF method to investigate systems containing a crystalline and an amorphous structural phase. We present two examples: Bulk metallic glass with crystalline reinforcements and Fontainebleau sandstone, where an unexpected glassy phase was discovered. In this paper we also discuss the refinement methods used in detail.

The effect of pressure on the structure of NiAl2O4

NiAl2O4 is known to transform from a normal spinel, (Ni2+)[Al 23+ ]O 4, to an inverse spinel, (Al3+)[Ni2+Al3+]O4, and vice versa. In this process, the larger Ni2+ ions which occupy the tetrahedral (8a) sites in normal spinel move to the octahedral (16d) sites in inverse spinel while half of the smaller Al3+ ions move in the opposite direction. The extent of this move is measured by the disorder or inversion parameter, I (the fraction of tetrahedral sites occupied by the Al ions; I = 0 for normal spinel and I = 1 for inverse spinel). Previous studies suggest that the lattice constant of spinel can decrease as the disorder parameter increases to better accommodate the Ni ions. In situ neutron diffraction studies performed by us indicate that this process is also occurring during the reduction of NiAl2O4 to Ni and Al2O3. It is possible that the compressive residual stresses generated during reduction play a role in the structural evolution of NiAl2O4. To systematically investigate the effect of pressure on the structure of NiAl2O4, x-ray diffraction studies at the X17 beamline of the National Synchrotron Light Source were performed. The pressure (up to 35 GPa) was applied via a diamond anvil cell and the experiments were conducted using a polychromatic x-ray beam. By comparing the relative intensities of certain spinel reflections that are sensitive to cationic disorder, a trend toward inverse spinel as a function of pressure was observed. The results are presented in comparison to previous studies on this material.

A Comparison of X-Ray Microdiffraction and Coherent Gradient Sensing in Measuring Discontinuous Curvatures in Thin Film: Substrate Systems

The coherent gradient sensor (CGS) is a shearing interferometer which has been proposed for the rapid, full-field measurement of deformation states (slopes and curvatures) in thin film-wafer substrate systems, and for the subsequent inference of stresses in the thin films. This approach needs to be verified using a more well-established but time-consuming grain orientation and stress measurement tool, X-ray microdiffraction (XRD). Both CGS and XRD are used to measure the deformation state of the same W film/Si wafer at room temperature. CGS provides a global, wafer-level measurement of slopes while XRD provides a local micromeasurement of lattice rotations. An extreme case of a circular Si wafer with a circular W film island in its center is used because of the presence of discontinuous system curvatures across the wafer. The results are also compared with a theoretical model based on elastic plate analysis of the axisymmetric biomaterial film-substrate system. Slope and curvature measurements by XRD and by CGS compare very well with each other and with theory. The favorable comparison demonstrates that wafer-level CGS metrology provides a quick and accurate alternative to other measurements. It also demonstrates the accuracy of plate theory in modeling thin film-substrate systems, even in the presence of curvature discontinuities.

Diffraction strain measurements in a partially crystallized bulk metallic glass composite containing ductile particles

In situ diffraction experiments were performed with high-energy synchrotron X-rays to examine load partitioning and high-stress relaxation during uniaxial compression of a bulk metallic glass composite containing both ductile tantalum particles and crystallized matrix material. The tantalum particles yielded at an applied stress of )800 MPa, while the matrix precipitates remained elastic up to the maximum applied stress of )1250 MPa. The von Mises effective stress in the tantalum particles at yielding was 1500 MPa, well in excess of typical tantalum yield stresses, which is attributed to a combination of solid-solution strengthening and the inhibition of dislocation motion in the 1–2 lm particles. A series of constant crosshead-position measurements made at )1250 MPa suggested the possibility of roomtemperature matrix relaxation under high applied loads. 2003 Elsevier Science B.V. All rights reserved.