Revision of Mechanics associated with grain-boundary diffusion and sliding in polycrystals and its application to nanocrystals from Wed, 2007-09-19 17:11

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Submitted by Yujie Wei on Wed, 2007-09-19 17:11.

As stated by Richard Vinci and Oliver Kraft in the announcement of
2008 Gordon Research Conference on Thin Film
and Small-Scale Mechanical Behavior, there is a
compelling need
to understand the critical roles of different deformation mechanisms in
structures with small characteristic dimensions, like nanocrystals and thin
films. We have recently studied deformation behaviors in nanostructured materials
and thin films with deformation mechanisms including grain-boundary diffusion,
grain-boundary sliding, and grain-interior plasticity. Some interesting
mechanical phenomena associated with heterogeneous grain-boundary properties
are found and summarized here.

If the diffusivity changes
abruptly at a point in the grain-boundary network and the load on the material
changes, the transient stress distribution develops crack-like stress
concentrations, and high stresses relax out of the system at a rate controlled
by the slow diffusion coefficient (See [1] );

Coble creep
changes character if heterogeneous grain-boundary diffusion exists (See [1]);

Creep deformation by heterogeneous grain-boundary
diffusion is partially recoverable
(See [2] ), which could
explain the recent experiments published in Science by Rajagopalan, Han, and Saif , (See [5] ).

There is
a transition from sliding and diffusion dominated creep in relatively small
grain sized nanocrystals at low strain rates to plasticity dominated flow in
nanocrystals with larger grain size deformed at higher strain rates (See [3] and [4] ).

Our
model is similar in some respects to several mesoscopic models developed to
study the deformation of nanostructured materials. In these models, the grain
boundaries were either approximated as layers of finite thickness which undergo
plastic deformation but have dissimilar properties to the bulk of the grain (Schwaiger
et al. [6] , Fu et al. [7] , Wei et al. [8] ); or sharp interface models to account for the effects of grain-boundary
sliding and separation (Wei and Anand [9] , Warner et al. [10] , Zhu et al. [11] , Jérusalem et al. [12] ).
The
new ingredient in our recent work is that grain-boundary diffusion and viscous
grain-boundary sliding are directly modeled; strain-rate sensitive deformation induced
by grain-boundary diffusion and grain-boundary sliding are automatically taken
care of.