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The monodispersed hard-sphere system is one of the simplest models for the study of phase transitions. Despite intensive studies of crystallization and melting of hard-sphere face-centered cubic (FCC) crystals, the phase transformations of hard-sphere body-centered cubic (BCC) crystals have not been explored because hard spheres cannot form a stable BCC lattice. In fact, unstable BCC hard-sphere crystals and their related phase transformations can be experimentally achieved. Here, we measured the kinetics of the melting and solid-solid transformations of BCC hard-sphere crystals at various volume fractions via molecular dynamics simulations. When the volume fraction ϕ < 0.494, the system melts catastrophically. At ϕ > 0.545, the BCC crystal transforms to a metastable polycrystal consisting of FCC and hexagonal close-packed (HCP) domains, which is different from those crystallized from supercooled liquids, and then slowly equilibrates toward the FCC crystal. At 0.494 < ϕ < 0.545, the BCC crystal transforms to an intermediate-order metastable state consisting of BCC and non-crystal particles without FCC and HCP symmetries and then equilibrates toward the coexistence of the FCC crystal and liquid. We further studied the melting and BCC-FCC transitions of crystals composed of soft spheres with potential u(r) = ϵ(r/σ)−n. The unstable BCC crystals at n = 12, 9, 8 exhibit similar melting and BCC-FCC transitions as hard-sphere BCC crystals, while the metastable BCC crystals at n = 5, 6, 7 melt quickly at low densities but take very long time for the BCC-FCC transition at high densities. We also estimate the BCC-FCC interfacial energy and critical nucleus size. These results cast light on the melting and solid-solid transformations of atomic BCC crystals, which exist widely in nature. [ABSTRACT FROM AUTHOR]

We studied the melting behavior of two-dimensional colloidal crystals with a Yukawa pair potential by Brownian dynamics simulations. The melting follows the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario with two continuous phase transitions and a middle hexatic phase. The two phase-transition points were accurately identified from the divergence of the translational and orientational susceptibilities. Configurational temperatures were employed to monitor the equilibrium of the overdamped system and the strongest temperature fluctuation was observed in the hexatic phase. The inherent structure obtained by rapid quenching exhibits three different behaviors in the solid, hexatic, and liquid phases. The measured core energy of the free dislocations, Ec = 7.81 ± 0.91 kBT, is larger than the critical value of 2.84 kBT, which consistently supports the KTHNY melting scenario. [ABSTRACT FROM AUTHOR]

Shear-induced amorphization is a phenomenon observed in polycrystalline NiTi shape memory alloys under severe plastic deformation, but the underlying mechanism of such a microstructural change remains elusive. To study the isothermal large-strain plastic deformation behavior and the associated mechanisms, uniaxial compression is performed on nanocrystalline NiTi cuboidal micropillars with initial grain sizes of 35 and 110 nm. It is found that the micropillars demonstrate high deformability with plastic strains up to 110% via shear-induced amorphization. It is shown that plastic strain localization in shear bands of the 35 nm-grain-size sample prompts notable amorphization up to an area fraction of 90% and crystal refinement down to 5 nm. High-resolution transmission electron microscope and molecular dynamics simulations reveal that the shear-induced amorphization starts from the local martensite phase near grain boundaries through accumulation of crystalline defects. The amorphization can lead to a reduction in the crystal size and an increase in the plastic flow stress. The large plastic deformation of the nanocrystalline NiTi micropillars via shear-induced amorphization is enabled by the suppression of crack nucleation through dynamic recovery of nanovoids. Our work provides new perspectives for producing crystalline-amorphous nanostructures in shape memory alloys at small scale. [Display omitted] [ABSTRACT FROM AUTHOR]