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.     

Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.     

On elastic–viscoplastic modeling of nanotwinned metals based on coupled intra-twin and twin-boundary-mediated deformation mechanisms

An elastic–viscoplastic model has been developed for nanotwinned (nt) metals based on coupled intra-twin and twin-boundary-mediated (TBM) deformation mechanisms. The grain-size dependence of intra-twin plasticity was incorporated in the proposed model to determine the transitional twin thickness corresponding to the maximum strength. In addition, the joint distribution of grain size and twin thickness was also taken into account to simulate the microstructure of nt metals. The results obtained show that the TBM deformation mechanism dominated at low strain rate and small twin thickness, and that the grain-size and twin-thickness distributions had significant influence on the macroscopic behavior of nt metals. A linear relation between the transitional twin thickness and grain size is predicted by the proposed model, which is in good agreement with the results obtained from three-dimensional molecular dynamics simulations and experiments.     

Attainment of an exceptionally high strength in low-carbon steel along with modest ductility through a novel heat treatment route

ABSTRACT

Over the last few decades, the use of steel (the most significant structural engineering material) is facing a significant challenge due to its replacement by other materials (such as composites) possessing higher strength-to-weight ratio/specific strength. This necessitates further enhancement in the strength of steel. In particular, low-carbon steel, in the annealed condition, suffers from inherent problems of poor strength and discontinuous yielding. In this research work a novel heat treatment route of incomplete austenitisation-based cyclic ice-brine quenching has been adopted on low-carbon steel (AISI 1010 steel containing 0.1 wt.% C) without considering any costly alloying or thermo-mechanical treatment. Accordingly, exceptionally high strength (UTS?=?1.7?GPa) and specific strength (226?MPa?g^?1cm³) are achieved after three cycles along with a modest ductility (% Elongation?=?9). This is the highest strength reported so far for low-carbon steel containing 0.1 wt.% C. Yield point phenomenon is also eliminated. This is attributed to a novel microstructure consisting of highly sub-structured fine plate martensite crystals containing internal twin and dislocation tangles along with dispersion of nano-sized cementite particles and clusters of cementite particles.     

A criterion for determining the optimum value of twist in the topological model

In previous works, in which the topological model has been applied to martensitic phase transformations, the value of twist angle ω was determined based on the habit plane-(HP) matching method, where the physical realization of the so-predicted interfacial defect networks may require reorientations of defect line directions by short-range diffusion, though no long-range diffusion was needed. In the present work, a novel criterion for determining the optimum value of twist is proposed so that the predicted interface defects are not only able to fulfil the function of fully accommodating the coherency strains arising on the terrace plane, but also capable of reaching the required position at the HP without long- or short-range diffusions. A numerical analysis for an Fe–20Ni–5Mn alloy is demonstrated based on the newly proposed criterion, and the predictions so obtained are in good agreement with the results provided by the phenomenological theory and experimental measurements.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.