Mechanics associated with grain-boundary diffusion and sliding in polycrystals and its application to nanocrystals

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.

[1] Wei, Bower, Gao, JMPS, 2007,
in press, doi:10.1016/j.jmps.2007.08.007.

[2] Wei, Bower,  Gao, Scrip. Mat., 2007;57:933.

[3] Wei and   Gao, Mat. Sci. Eng. A  2007, in press, doi:10.1016/j.msea.2007.05.054.

[4] Wei, Bower,  Gao, Acta Mater, accepted.

[5] Rajagopalan, Han, Saif, Science 2007;315:1831.

[6] Schwaiger,
Moser, Dao, Chollacoop, Suresh, Acta Mater. 2003;51:5159.

[7] Fu, Benson, Meyers, Acta Mater. 2004;52:4413.

[8] Wei, Su,
Anand, Acta Mater. 2006;54:3177.

[9] Wei, Anand, JMPS  2004;52:2587.

[10] Warner, Sansoz, Molinari, Int  J of Plasticity 2006;22:754.

[11] Zhu,
Asaro, Krysl, Bailey, Acta Mater. 2005;53:4825.

[12] Jérusalem,
Stainier, Radovitzky, Phil. Mag. 2007;87:2541.