Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centered cubic metals

We propose a mesoscopic model that explains the factor of two to three discrepancy between experimentally measured yield stresses of BCC metals at low temperatures and typical Peierls stresses determined by atomistic simulations of isolated screw dislocations. The model involves a Frank-Read type source emitting dislocations that become pure screws at a certain distance from the source and, owing to their high Peierls stress, control its operation. However, due to the mutual interaction between emitted dislocations the group consisting of both non-screw and screw dislocations can move at an applied stress that is about a factor of two to three lower than the stress needed for the glide of individual screw dislocations.Comment: 4 pages, 2 figures; RevTex4; submitted to PR

Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale

Dislocation multi-junctions and strain hardening

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects--dislocations. First theorized in 1934 to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed only two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening: a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions tying dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed hereafter multi-junctions. The existence of multi-junctions is first predicted by Dislocation Dynamics (DD) and atomistic simulations and then confirmed by the transmission electron microscopy (TEM) experiments in single crystal molybdenum. In large-scale Dislocation Dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in BCC crystals

Collinear interactions of dislocations and slip systems

he collinear interaction arises between dislocations of same Burgers vector gliding in their slip and cross-slip planes. It is the strongest of all interactions between dislocations or slip systems. Its impact on the mechanical properties of fcc single crystals with high symmetry orientations, is examined by both dislocation dynamics simulations and continuum modellin

Comment on "Portevin-Le Chatelier effect"

Instabilities of plastic How in alloys and the associated deformation patterns are currently attracting a lot of attention. We comment on a recent investigation by Franklin et al. [Phys. Rev. E 62, 8195 (2000)] on one such type of instability, the Portevin-Le Chatelier effect, attempting to clarify a few points about the instability mechanism as well as the reported experimental results

Statistical and multifractal analysis of the Portevin–Le Chatelier effect

The stress time series associated with the Portevin–Le Chatelier (PLC) effect in Al–2.5%Mg polycrystals deformed in constant applied strain rate have been analyzed by statistical and multifractal methods. The objective was to check the conjecture that jerky flow should exhibit distinct ynamical regimes. Statistical distributions of stress drops and drop durations exhibit a well-defined sequence as the applied strain rate increases. Peaked distributions are followed by monotonically decreasing ones at higher strain rate. For a large grain size, the high strain rate distributions are consistently fitted by power laws suggesting a self-organized critical (SOC) dynamics. At smaller grain sizes, a multifractal fit of the distributions is more appropriate. Multifractality increases with strain rate. A sharp upturn is observed at the boundary between type B and type A PLC bands. The correlation between independant statistical and multifractal analyses suggests a fundamental crossover of the underlying dynamics. It is also an indication that the multifractal analysis has the ability to provide sensible quantitative characterizations of jerky flow

Crossover from chaotic to self-organized critical dynamics in jerky flow of single crystals

We report a crossover from chaotic to self-organized critical dynamics in the Portevin-Le Chatelier effect in single crystals of Cu-10% Al in tension as a function of the applied strain rate. For low and intermediate strain rates, we provide an unambiguous support for the existence of chaotic stress drops by showing the existence of a finite correlation dimension and a stable positive Lyapunov exponent. A surrogate data analysis rules out the possibility that the time series is due to a power law stochastic process. As the strain rate is increased, the distributions of stress drops and the time intervals between the stress drops change from peaked to power law type with an exponent close to unity reminiscent of self-organized critical state. A scaling relation compatible with self-organized criticality relates the various exponents. The absence of a finite correlation dimension and a stable positive Lyapunov exponent at the highest strain rate also supports the evidence of crossover. [S1063-651X(99)11011-0]

Crossover in the dynamics of Portevin–Le Chatelier effect from chaos to SOC

We demonstrate the existence of a crossover in the dynamics of the PLC effect from chaotic to self-organized critical nature by analysing the stress–strain curves obtained from single crystals of Al–10%Cu subjected to three levels of constant strain rate deformation. At the lowest and intermediate strain rates, we provide an unambiguous support for the existence of a chaotic regime. For these two strain rates, the distributions of stress drops are peaked. At the highest strain rate, we find that the distributions of stress drops and their durations exhibit a power law behaviour reminiscent of a self-organized critical state. We also show that scaling relations are obeyed by the various exponents