A constitutive model for quasicrystal plasticity

High-pressure X-ray diffraction studies on gadolinium sesquioxide (Gd₂O₃) have been carried out up to a pressure of ～25 GPa in a diamond-anvil cell at room temperature. Gadolinium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase at around 12 GPa. The high-pressure phase has been identified as a hexagonal La₂O₃-type structure. The bulk modulus and its pressure derivative of this phase have been calculated.     

Thermodynamic fluctuations in magnetic states: Fe3Pt as a prototype

To understand the Invar anomalies, such as negative thermal expansion and spontaneous magnetization, we have applied our recently developed thermodynamic framework for a system with itinerant-electron magnetism to the ordered Fe₃Pt. The framework has coherently predicted the finite temperature intermixing between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs). We have also discovered a tri-critical point at which a high-temperature second-order phase transition, between the fully ordered FM phase and the paramagnetic phase which is disordered due to SFCs, becomes first order at low temperatures.     

Growth of carbon nanotubes: effect of Fe diffusion and oxidation

The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO₂ and Al₂O₃/SiO₂ have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO₂/Si and Fe/Al₂O₃/SiO₂/Si samples were subjected to thermal chemical vapour deposition (CVD) at ～650°C for ～30?min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al₂O₃/SiO₂/Si samples was relatively high (～9.5–11?µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ～650°C for ～30?min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x _Fe, and magnitude of the diffusivity, D _Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al₂O₃/SiO₂/Si samples, the magnitudes of x _Fe and D _Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al₂O₃ layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al₂O₃/SiO₂/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples.     

Log-normal nanograin-size distributions in nanostructured composites

A key challenge in the fabrication of superhard nanocomposite films is how to control the distribution of grain sizes in these materials. High-resolution transmission electron microscopy has been used to measure nanograin-size distributions in the Ti–B–N films with various B contents. The results show that the mean grain size decreases with increase of B content and the grain-size distribution conforms to a log-normal function when the hardness approaches a maximum value. The transition from normal to log-normal distributions can be determined by analysis in terms of a minimum information criterion. The origin of a log-normal size distribution probably results from heterogeneity arising from a diffusion-drift process.     

Growth orientation of one-dimensional silicon nanowires prepared by thermal evaporation

One-dimensional silicon nanowires have been grown by thermal evaporation and their growth orientations determined by transmission electron microscopy studies. The nanowires, which are often highly curved in morphology and heavily twinned in microstructure, are crystallographically separated into several sections, each with a characteristic crystallographic orientation along the wire axis. Straight nanowires, or straight sections in a curved nanowire, are found to have non-unique crystallographic orientations when {111} twinning occurs.     

Determination of electrostatic characteristics at a 24°, [001] tilt grain boundary in a SrTiO3 bicrystal by electron holography

The electrostatic potential and associated space charge across a 24°, [001] tilt boundary of SrTiO₃ ceramic have been determined using electron holography. The results reveal a positive charged interface with a negative space charge on either side. The form of the double Schottky barrier and the local charge-density distribution at the interface are derived from the results.     

Mechanical-milling-induced amorphization of Se: A crystallite destabilization model

Complete solid-state amorphization has been realized in elemental Se by means of mechanical milling of crystalline Se powder. Quantitative X-ray diffraction and thermal analyses were employed to characterize the amorphization process and indicated that the amorphization onset corresponds to a critical crystallite size and a drop in microstrain. During the major amorphization process, the remaining crystallite size remains unchanged with a constant lattice expansion. A new kinetics model of crystallite destabilization is proposed for the solid-state amorphization which satisfactorily explains the experimental observations.     

Pop-in phenomenon in MgO and LiF: Observation of dislocation structures

This letter is based on recent progress in the observation of dislocation distributions around nanoindentations by chemical etching. This so-called nanoetching technique is used to determine the dislocation mechanisms associated with the pop-in phenomenon in MgO and LiF. Successive stages of highly controlled chemomechanical polishing have revealed that these dislocations are half-loops lying in the classical slip systems of MgO or LiF. However, they do not extend on the surface in the classical rosette-arms pattern but stay concentrated around the imprint. A mechanism of dislocation interactions, enhanced by the fact that the dislocations are suddenly nucleated in a small volume, is proposed to explain this specific distribution.     

Crystal growth velocity in deeply undercooled Ni–Si alloys

The crystal growth velocity of Ni₉₅Si₅ and Ni₉₀Si₁₀ alloys as a function of undercooling is investigated using molecular dynamics simulations. The modified imbedded atom method potential yields the equilibrium liquidus temperatures T _L?≈?1505 and 1387?K for Ni₉₅Si₅ and Ni₉₀Si₁₀ alloys, respectively. From the liquidus temperatures down to the deeply undercooled region, the crystal growth velocities of both the alloys rise to the maximum with increasing undercooling and then drop slowly, whereas the athermal growth process presented in elemental Ni is not observed in Ni–Si alloys. Instead, the undercooling dependence of the growth velocity can be well-described by the diffusion-limited model, furthermore, the activation energy associated with the diffusion from melt to interface increases as the concentration increases from 5 to 10?at.% Si, resulting in the remarkable decrease of growth velocity.     

Fractals and scaling in fracture induced by microcrack coalescence

A numerical model is proposed to simulate fracture induced by the coalescence of numerous microcracks, in which the condition for coalescence between two randomly nucleated microcracks is determined in terms of a load-sharing principle. The results of the simulation show that, as the number density of nucleated microcracks increases, stochastic coalescence first occurs followed by a small fluctuation, and finally a newly nucleated microcrack triggers a cascade coalescence of microcracks resulting in catastrophic failure. The fracture profiles exhibit self-affine fractal characteristics with a universal roughness exponent, but the critical damage threshold is sensitive to details of the model. The spatiotemporal distribution of nucleated microcracks in the vicinity of critical failure follows a power-law behaviour, which implies that the microcrack system may evolve to a critical state.