Ultrafast X-Ray Diffraction Visualization of B1-B2 Phase Transition in KCl under Shock Compression.

The classical B1(NaCl)↔B2(CsCl) transitions have been considered as a model for general structural phase transformations, and resolving corresponding phase transition mechanisms under high strain rate shock compression is critical to a fundamental understanding of phase transition dynamics. Here, we use subnanosecond synchrotron x-ray diffraction to visualize the lattice response of single-crystal KCl to planar shock compression. Complete B1-B2 orientation relations are revealed for KCl under shock compression along ⟨100⟩_{B1} and ⟨110⟩_{B1}; the orientation relations and transition mechanisms are anisotropic and can be described with the standard and modified Watanabe-Tokonami-Morimoto model, respectively, both involving interlayer sliding and intralayer ion rearrangement. The current study also establishes a paradigm for investigating solid-solid phase transitions under dynamic extremes with ultrafast synchrotron x-ray diffraction.

Multiscale measurements with adjustable x-ray spot size for in situ imaging and diffraction.

A large field of view is normally desired for synchrotron x-ray imaging, while a small x-ray spot size is required for x-ray diffraction. A multiscale measurement system with an adjustable x-ray spot size is developed to accommodate different spot size requirements for in situ phase-contrast imaging and diffraction. The centers of a diffraction scintillator with a through-hole and an imaging scintillator are collinear with the x-ray beam. With the proof-of-principle experiments on a magnesium alloy under uniaxial tension, we demonstrate the feasibility of the multiscale measurement system for full azimuthal range diffraction measurements with improved resolution and large field of view strain field measurements via x-ray digital image correlation.

Capture Deformation Twinning in Mg during Shock Compression with Ultrafast Synchrotron X-Ray Diffraction.

Deformation twinning plays a vital role in accommodating plastic deformation of hexagonal-close-packed (hcp) metals, but its mechanisms are still unsettled under high strain rate shock compression. Here we investigate deformation twinning in shock-compressed Mg as a typical hcp metal with in situ, ultrafast synchrotron x-ray diffraction. Extension twinning occurs upon shock compression along ⟨112[over ¯]0⟩ and ⟨101[over ¯]0⟩, but only upon release for loading along ⟨0001⟩. Such deformation mechanisms are a result of the polarity of deformation twinning, which depends on directionality and relative magnitude of resolved shear stress and may be common for Mg and its alloys in a wide range of strain rates.

Evidence for Octupole Correlations in Multiple Chiral Doublet Bands.

Two pairs of positive-and negative-parity doublet bands together with eight strong electric dipole transitions linking their yrast positive- and negative-parity bands have been identified in ^{78}Br. They are interpreted as multiple chiral doublet bands with octupole correlations, which is supported by the microscopic multidimensionally-constrained covariant density functional theory and triaxial particle rotor model calculations. This observation reports the first example of chiral geometry in octupole soft nuclei.