75 research outputs found
Abnormal subgrain growth in a dislocation-based model of recovery
Simulation of subgrain growth during recovery is carried out using
two-dimensional discrete dislocation dynamics on a hexagonal crystal lattice
having three symmetric slip planes. To account for elevated temperature (i)
dislocation climb was allowed and (ii) a Langevin type thermal noise was added
to the force acting on the dislocations. During the simulation, a random
ensemble of dislocations develop into subgrains and power-law type growth
kinetics are observed. The growth exponent is found to be independent of the
climb mobility, but dependent on the temperature introduced by the thermal
noise. The in-depth statistical analysis of the subgrain structure shows that
the coarsening is abnormal, i.e. larger cells grow faster than the small ones,
while the average misorientation between the adjacent subgrains remains nearly
constant. During the coarsening Holt's relation is found not to be fulfilled,
such that the average subgrain size is not proportional to the average
dislocation spacing. These findings are consistent with recent high precision
experiments on recovery.Comment: 17 pages, 11 figure
The role of weakest links and system size scaling in multiscale modeling of stochastic plasticity
Plastic deformation of crystalline and amorphous matter often involves
intermittent local strain burst events. To understand the physical background
of the phenomenon a minimal stochastic mesoscopic model was introduced, where
microstructural details are represented by a fluctuating local yielding
threshold. In the present paper, we propose a method for determining this yield
stress distribution by lower scale discrete dislocation dynamics simulations
and using a weakest link argument. The success of scale-linking is demonstrated
on the stress-strain curves obtained by the resulting mesoscopic and the
discrete dislocation models. As shown by various scaling relations they are
statistically equivalent and behave identically in the thermodynamic limit. The
proposed technique is expected to be applicable for different microstructures
and amorphous materials, too.Comment: 13 pages, 12 figure
Dynamic Length Scale and Weakest Link Behavior in Crystal Plasticity
Plastic deformation of heterogeneous solid structures is often characterized
by random intermittent local plastic events. On the mesoscale this feature can
be represented by a spatially fluctuating local yield threshold. Here we study
the validity of such an approach and the ideal choice for the size of the
representative volume element for crystal plasticity in terms of a discrete
dislocation model. We find that the number of links representing possible
sources of plastic activity exhibits anomalous (super-extensive) scaling which
tends to extensive scaling (often assumed in weakest-link models) if quenched
short-range interactions are introduced. The reason is that the interplay
between long-range dislocation interactions and short-range quenched disorder
destroys scale-free dynamical correlations leading to event localization with a
characteristic length-scale. Several methods are presented to determine the
dynamic length-scale that can be generalized to other types of heterogeneous
materials
Submicron plasticity: yield stress, dislocation avalanches, and velocity distribution
The existence of a well defined yield stress, where a macroscopic piece of
crystal begins to plastically flow, has been one of the basic observations of
materials science. In contrast to macroscopic samples, in micro- and
nanocrystals the strain accumulates in distinct, unpredictable bursts, which
makes controlled plastic forming rather difficult. Here we study by simulation,
in two and three dimensions, plastic deformation of submicron objects under
increasing stress. We show that, while the stress-strain relation of individual
samples exhibits jumps, its average and mean deviation still specify a
well-defined critical stress, which we identify with the jamming-flowing
transition. The statistical background of this phenomenon is analyzed through
the velocity distribution of short dislocation segments, revealing a universal
cubic decay and an appearance of a shoulder due to dislocation avalanches. Our
results can help to understand the jamming-flowing transition exhibited by a
series of various physical systems.Comment: 5 page
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