Elastic moduli inheritance and the weakest link in bulk metallic glasses.

We show that a variety of bulk metallic glasses (BMGs) inherit their Young's modulus and shear modulus from the solvent components. This is attributed to preferential straining of locally solvent-rich configurations among tightly bonded atomic clusters, which constitute the weakest link in an amorphous structure. This aspect of inhomogeneous deformation, also revealed by our in situ neutron diffraction studies of an elastically deformed BMG, suggests a rubberlike viscoelastic behavior due to a hierarchy of atomic bonds in BMGs.

A concept of a broadband inverted geometry spectrometer for the Second Target Station at the Spallation Neutron Source.

BWAVES is an acronym for Broadband Wide-Angle VElocity Selector spectrometer, indicating that a novel WAVES (Wide-Angle VElocity Selector) device will be used to select the velocity/wavelength of the detected neutrons after they are scattered by the sample. We describe a conceptual design of BWAVES, a time-of-flight broadband inverted-geometry neutron spectrometer for the Second Target Station at the Spallation Neutron Source operated by Oak Ridge National Laboratory. Being the first inverted geometry spectrometer where the energy of the detected neutrons can be chosen by a WAVES device mechanically, irrespective of the limitations imposed by the crystal analyzers or filters, BWAVES will feature a uniquely broad, continuous dynamic range of measurable energy transfers, spanning 4.5 decades. This will enable measurements of both vibrational and relaxational excitations within the same, continuous scattering spectra. Novel approaches that are necessary for the implementation of a WAVES device at the BWAVES spectrometer will result in a spectrometer with the design and characteristics much different from those displayed by the neutron spectrometers in existence today.

Power-law scaling and fractal nature of medium-range order in metallic glasses.

The atomic structure of metallic glasses has been a long-standing scientific problem. Unlike crystalline metals, where long-range ordering is established by periodic stacking of fundamental building blocks known as unit cells, a metallic glass has no long-range translational or orientational order, although some degrees of short- and medium-range order do exist. Previous studies have identified solute- (minority atom)-centred clusters as the fundamental building blocks or short-range order in metallic glasses. Idealized cluster packing schemes, such as efficient cluster packing on a cubic lattice and icosahedral packing as in a quasicrystal, have been proposed and provided first insights on the medium-range order in metallic glasses. However, these packing schemes break down beyond a length scale of a few clusters. Here, on the basis of neutron and X-ray diffraction experiments, we propose a new packing scheme-self-similar packing of atomic clusters. We show that the medium-range order has the characteristics of a fractal network with a dimension of 2.31, and is described by a power-law correlation function over the medium-range length scale. Our finding provides a new perspective of order in disordered materials and has broad implications for understanding their structure-property relationship, particularly those involving a change in length scales.

Single crystal to polycrystal neutron transmission simulation.

A collection of routines for calculation of the total cross section that determines the attenuation of neutrons by crystalline solids is presented. The total cross section is calculated semi-empirically as a function of crystal structure, neutron energy, temperature, and crystal orientation. The semi-empirical formula includes the contribution of parasitic Bragg scattering to the total cross section using both the crystal's mosaic spread value and its orientation with respect to the neutron beam direction as parameters. These routines allow users to enter a distribution of crystal orientations for calculation of total cross sections of user defined powder or pseudo powder distributions, which enables simulation of non-uniformities such as texture and strain. The spectra for neutron transmission simulations in the neutron thermal energy range (2 meV-100 meV) are presented for single crystal and polycrystal samples and compared to measurements.

Temperature-dependent elastic anisotropy and mesoscale deformation in a nanostructured ferritic alloy.

Nanostructured ferritic alloys are a new class of ultrafine-grained oxide dispersion-strengthened steels that have promising properties for service in extreme environments in future nuclear reactors. This is due to the remarkable stability of their complex microstructures containing numerous Y-Ti-O nanoclusters within grains and along grain boundaries. Although nanoclusters account primarily for the exceptional resistance to irradiation damage and high-temperature creep, little is known about the mechanical roles of the polycrystalline grains that constitute the ferritic matrix. Here we report an in situ mesoscale characterization of anisotropic responses of ultrafine ferrite grains to stresses using state-of-the-art neutron diffraction. We show the experimental determination of single-crystal elastic constants for a 14YWT alloy, and reveal a strong temperature-dependent elastic anisotropy that leads to elastic softening and instability of the ferrite. We also demonstrate, from anisotropy-induced intergranular strains, that a deformation crossover exists from low-temperature lattice hardening to high-temperature lattice softening in response to extensive plastic deformation.

Enhancing glass-forming ability via frustration of nano-clustering in alloys with a high solvent content.

The glass-forming ability (GFA) of alloys with a high-solvent content such as soft magnetic Fe-based and Al-based alloys is usually limited due to strong formation of the solvent-based solid solution phase. Herein, we report that the GFA of soft magnetic Fe-based alloys (with >70 at.% Fe to ensure large saturation magnetization) could be dramatically improved by doping with only 0.3 at.% Cu which has a positive enthalpy of mixing with Fe. It was found that an appropriate Cu addition could enhance the liquid phase stability and crystallization resistance by destabilizing the α-Fe nano-clusters due to the necessity to redistribute the Cu atoms. However, excessive Cu doping would stimulate nucleation of the α-Fe nano-clusters due to the repulsive nature between the Fe and Cu atoms, thus deteriorating the GFA. Our findings provide new insights into understanding of glass formation in general.

Strain incompatibility and residual strains in ferroelectric single crystals.

Residual strains in ferroelectrics are known to adversely affect the material properties by aggravating crack growth and fatigue degradation. The primary cause for residual strains is strain incompatibility between different microstructural entities. For example, it was shown in polycrystalline ferroelectrics that residual strains are caused due to incompatibility between the electric-field-induced strains in grains with different crystallographic orientations. However, similar characterization of cause-effect in multidomain ferroelectric single crystals is lacking. In this article, we report on the development of plastic residual strains in [111]-oriented domain engineered BaTiO(3) single crystals. These internal strains are created due to strain incompatibility across 90° domain walls between the differently oriented domains. The average residual strains over a large crystal volume measured by in situ neutron diffraction is comparable to previous X-ray measurements of localized strains near domain boundaries, but are an order of magnitude lower than electric-field-induced residual strains in polycrystalline ferroelectrics.

Deformation crossover: from nano- to mesoscale.

In situ synchrotron and neutron diffraction were used to study deformation mechanisms in Ni over a broad range of grain sizes. The experimental data show that unlike in coarse-grained metals, where the deformation is dominated by dislocation slip, plastic deformation in nanocrystalline Ni is mediated by grain-boundary activities, as evidenced by the lack of intergranular strain and texture development. For ultrafine-grained Ni, although dislocation slip is an active deformation mechanism, deformation twinning also plays an important role, whose propensity increases with the grain size.

In situ synchrotron study of phase transformation behaviors in bulk metallic glass by simultaneous diffraction and small angle scattering.

We have used a new approach involving simultaneous diffraction and small angle scattering to study the amorphous-to-crystalline phase transformation in Zr-based bulk metallic glass. In situ, time-resolved data provided the first direct demonstration of a phase separation prior to crystallization. There is evidence that nucleation and growth of the crystalline phase occur in separate stages, with different kinetics. Our data support the view that crystalline nucleation is achieved via short-range diffusion of small atoms (e.g., Ni), whereas the growth is dictated by long-range diffusion.