A nanocrystalline surface layer of a Cu-Zn alloy was developed by electropulsing (ECP) surface treatment. The average grain size was about 20 nm on the top surface layer and was gradually augmented with the increase in depth from the top surface. Nanoindentation measurements showed that the hardness was significantly enhanced on the top surface layer compared with the as-annealed Cu-Zn sample. The mechanism for the evolution of this structure and property was related not only to a solid-state phase transformation, but also to the effect of the enhancement of the nucleation rate and the skin effect during the ECP treatment. Therefore, the ECP surface treatment provides a promising method for obtaining surface self-nanocrystallization materials.
We demonstrate an approach to double the optical efficiency of virtual reality (VR) systems based on a directional backlight and a diffractive deflection film (DDF). The directional backlight consists of a commercial collimated light-emitting diode (LED) array and a two-layer privacy film, while the DDF is a three-domain Pancharatnam-Berry (PB) phase lens. Such a PB phase lens was fabricated by the zone exposure and spin-coating method. The focal length of each domain is designed according to the imaging optics of the VR system. Our approach works well in both Fresnel and “pancake” VR systems. We also build the corresponding models in LightTools, and the simulation results are in good agreement with experiment. In experiment, we achieved a 2.25x optical efficiency enhancement for both systems, which agrees with the simulation results (2.48x for Fresnel and 2.44x for “pancake” systems) well. Potential application for high efficiency VR displays is foreseeable.
The mechanical properties and corresponding microstructure development of the AZ31 Mg alloy after treatment with equal channel angular pressing (ECAP) and subsequent electropulsing (ECP) was investigated. Comparing the ECAP+ECP-treated AZ31 alloy with the ECAP-treated alloy, the elongation to failure was improved significantly, while the yield stress and the ultimate tensile strength were not decreased, the grain sizes were slightly increased and more homogeneous, and the texture was barely changed. The main mechanism for the evolution of the structures and properties might be ascribed to the increased nucleation rate on recrystallization and the decreased dislocation density during the ECP treatment. It was reasonable to expect that the ECAP+ECP treatment would provide a promising approach for enhancing the mechanical properties of the Mg alloys.
Elastic constants are among the most fundamental properties of materials. Simulations of microstructural evolution and constitutive/micro-mechanistic modeling of materials properties require elastic constants that are predominately measured from single crystals that are labor intensive to grow. A facile technique is developed to measure elastic constants from polycrystalline samples. The technique is based upon measurements of the surface acoustic wave velocities with the help of a polydimethylsiloxane film grating that is placed on a polished surface of a polycrystalline sample to confine surface acoustic waves that are induced by a femtosecond laser and measured using pump-probe time-domain thermoreflectance. Electron backscatter diffraction is employed to measure the crystallographic orientation along which the surface acoustic wave propagates in each grain (perpendicular to the polydimethylsiloxane grating). Such measurements are performed on several grains. A robust mathematical solution was developed to compute the surface acoustic wave velocity along any crystallographic orientation of any crystal structure with given elastic constants and density. By inputting various starting values of elastic constants to compute the surface acoustic wave velocities to match experimental measurements in several distinct crystallographic orientations using an optimization algorithm, accurate elastic constant values have been obtained from seven polycrystalline metal samples to be within 6.8% of single-crystal measurements. This new technique can help change the current scenario that experimentally measured elastic constants are available for only about 1% of the estimated 160,000 distinct solid compounds, not to mention the significant need for elastic constants of various solid solution compositions that are the base of structural materials.
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