Shiteng Zhao

High-velocity deformation of Al0.3CoCrFeNi high-entropy alloy: Remarkable resistance to shear failure

The mechanical behavior of a single phase (fcc) Al0.3CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to shear localization was studied by dynamically-loading hat-shaped specimens to induce forced shear localization. However, no adiabatic shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to shear localization, which makes this material an excellent candidate for penetration protection applications such as armors.

Directional amorphization of boron carbide subjected to laser shock compression

Significance When crystalline solids are stressed quasi-statically, dislocation slip, twinning, and phase transformations are the predominant mechanisms to dissipate the imparted elastic energy. Under shock, high hydrostatic and shear stresses promptly build up at the shock front, favoring fast energy dissipation mechanisms. Amorphization, which may only involve localized atomic arrangements, is therefore an additional potential candidate. Shock-induced amorphization has now been reported in various materials and hence should be incorporated as a deformation/damage mechanism of crystals subjected to high-strain-rate loading. Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laser-driven shock compression, unprecedented high strain rates can be achieved; here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45∼50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High-resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B4C.

Supersonic Dislocation Bursts in Silicon

Dislocations are the primary agents of permanent deformation in crystalline solids. Since the theoretical prediction of supersonic dislocations over half a century ago, there is a dearth of experimental evidence supporting their existence. Here we use non-equilibrium molecular dynamics simulations of shocked silicon to reveal transient supersonic partial dislocation motion at approximately 15 km/s, faster than any previous in-silico observation. Homogeneous dislocation nucleation occurs near the shock front and supersonic dislocation motion lasts just fractions of picoseconds before the dislocations catch the shock front and decelerate back to the elastic wave speed. Applying a modified analytical equation for dislocation evolution we successfully predict a dislocation density of 1.5 × 1012 cm−2 within the shocked volume, in agreement with the present simulations and realistic in regards to prior and on-going recovery experiments in silicon.

Structural characterization and viscoelastic constitutive modeling of skin

A fascinating material, skin has a tensile response which exhibits an extended toe region of minimal stress up to nominal strains that, in some species, exceed 1, followed by significant stiffening until a roughly linear region. The large toe region has been attributed to its unique structure, consisting of a network of curved collagen fibers. Investigation of the structure of rabbit skin reveals that it consists of layers of wavy fibers, each one with a characteristic orientation. Additionally, the existence of two preferred layer orientations is suggested based on the results of small angle X-ray scattering. These observations are used to construct a viscoelastic model consisting of collagen in two orientations, which leads to an in-plane anisotropic response. The structure-based model presented incorporates the elastic straightening and stretching of fibrils, their rotation towards the tensile axis, and the viscous effects which occur in the matrix of the skin due to interfibrillar and interlamellar sliding. The model is shown to effectively capture key features which dictate the mechanical response of skin. STATEMENT OF SIGNIFICANCE Examination by transmission and scanning electron microscopy of rabbit dermis enabled the identification of the key elements in its structure. The organization of collagen fibrils into flat fibers was identified and incorporated into a constitutive model that reproduces the mechanical response of skin. This enhanced quantitative predictive capability can be used in the design of synthetic skin and skin-like structures.

Generating gradient germanium nanostructures by shock-induced amorphization and crystallization

Significance Amorphization and nanocrystallization are two powerful methods to tailor material properties by altering their microstructure without changing the overall chemistry. Using powerful laser-driven shocks, we demonstrate that amorphization and nanocrystallization can be achieved within a time scale that is considerably shorter than other conventional techniques. Our results provide compelling insights into pressure/shear amorphization and propose a route to fabricate gradient semiconducting nanostructures using lasers. Additionally, shear-driven amorphization is demonstrated as the dominant deformation mechanism in this extreme regime. Gradient nanostructures are attracting considerable interest due to their potential to obtain superior structural and functional properties of materials. Applying powerful laser-driven shocks (stresses of up to one-third million atmospheres, or 33 gigapascals) to germanium, we report here a complex gradient nanostructure consisting of, near the surface, nanocrystals with high density of nanotwins. Beyond there, the structure exhibits arrays of amorphous bands which are preceded by planar defects such as stacking faults generated by partial dislocations. At a lower shock stress, the surface region of the recovered target is completely amorphous. We propose that germanium undergoes amorphization above a threshold stress and that the deformation-generated heat leads to nanocrystallization. These experiments are corroborated by molecular dynamics simulations which show that supersonic partial dislocation bursts play a role in triggering the crystalline-to-amorphous transition.