Initial sintering of cylindrical particles of different sizes by surface diffusion

Sintering of two cylindrical particles of different sizes by surface diffusion at the early stage is theoretically analysed. Formulae for the neck growth rate are derived using a variational principle and the continuity equation of vacancies. The size difference between two particles is found to influence the growth rate of the neck only when the difference is large.     

Creep behaviour of ice single crystals loaded in torsion explained by dislocation cross-slip

Torsion creep experiments are carried out in order to understand the physics of ice plasticity. A dislocation spreading mechanism based on double cross-slip of basal dislocations is proposed to explain the strong plastic anisotropy and the power law relationship between stress and strain rates. The scenario is tested using three-dimensional dislocation dynamics simulations. Numerical investigations give a stress exponent n?=?2.3 in agreement with experimental measurements. This dislocation spreading mechanism sheds a new light on the interpretation of former experimental observations.     

Connection between deformation-induced dislocation substructures and martensite formation in stainless steel

We report a quantitative connection between the strain amplitude dependencies of the organisation of dislocation cell substructures with the formation of deformation-induced martensite during cyclic plasticity of austenitic stainless steel at ambient temperature.     

Anisotropic thermal properties of single-wall-carbon- nanotube-reinforced nanoceramics

Dense single-wall-carbon-nanotube-(SWCNT)-reinforced alumina nanocomposites have been fabricated by a novel spark-plasma-sintering technique. Anisotropic thermal properties have been found in carbon nanotube composites. The introduction of ropes of SWCNTs gives rise to a decrease in the transverse thermal diffusivity with increasing carbon nanotube content while it does not change the in-plane thermal diffusivity. This is scientifically interesting and technologically important for the development of materials for novel thermal barrier coatings.     

TEM in situ straining experiments in Fe at low temperature

In situ transmission electron microscopy straining experiments were carried out in pure Fe to investigate the origin of the discontinuity observed at 250 K in the temperature variation of the deformation activation parameters. The results show that the motion of screw dislocations is steady at 300 K, in agreement with a kink-pair mechanism, but jerky at 110 K. This change has been attributed to a transition from a kink-pair mechanism to a locking–unlocking mechanism, similar to that observed previously in Ti.     

Determination of Peierls stress from acoustic harmonic generation

The interaction of acoustic waves with dislocations leads to aperiodic oscillations in the magnitude of the acoustic nonlinearity parameter β as a function of the acoustic drive amplitude σ _ampl. The magnitude and spacing of the oscillations depend on the value of the Peierls stress. A least-square curve fit of the β(σ _ampl) equation to experimental data taken of 99.999% pure aluminum monocrystals oriented for wave propagation along the [1?0?0] crystal axis yields the value 6.2?×?10⁴?Pa for the Peierls stress. The value is consistent with the smallest values reported in the literature for aluminum where for both theoretical and experimental studies the reported values range over three orders of magnitude.     

Interaction of a dislocation with an elliptical hole in icosahedral quasicrystals

The interaction between a dislocation and an elliptical hole in icosahedral quasicrystals is considered. An explicit expression for the complex potential is derived using the extended Stroh formalism. Based on the conformal mapping method and a perturbation technique, closed-form solutions are obtained. The field intensity factors at a crack tip and the image forces on the dislocation arising from the crack are calculated. The effects of phonon–phason elastic coupling on the mechanical behavior are also observed.     

The energetics and interactions of random dislocation walls

Abstract

Regular dislocation walls, where dislocations are spaced equally along the wall direction, are a popular idealization for modelling the arrangement and interactions of excess dislocations in plastic deformation. The assumption of regular walls is motivated by the fact that such walls represent minimum energy arrangements for dislocations of the same sign, and it allows to use the analytically known short-ranged stress fields of such walls for analyzing the structure of plastic boundary layers. In order to critically evaluate the physical robustness of models based on regular walls, we investigate their random counterparts and demonstrate that the energetics and interactions of such non-periodic dislocation arrangements differ completely from the periodic wall arrangements. Implications of our results for the modelling of plastic boundary layers and dislocation-grain boundary interactions are discussed.     

Negative,zero and positive stiffness in extended Eshelby plates

Structures under loading can show positive and negative stiffness (during buckling) with zero stiffness just being one point of transition. Material structures with extended zero stiffness regimes have also been reported. Using the example of an extended Eshelby plate with a positive and negative edge dislocation, we show the existence of finite regimes of positive, zero and negative stiffness in a single system. Given that stable states/configurations of negative stiffness cannot exist in an isolated system; we show the existence of metastable states of negative stiffness along with unstable states of positive stiffness. The system also exhibits one point of unstable equilibrium.