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A Multiscale Model of Rate Dependence of Nanocrystalline Thin Films
Fernando Stump
University of Illinois at Urbana-Champaign
407 W. White St., Apt 14., Champaign, 61820, US
Phone: 1 (217) 778-8291, Email: fstump2@illinois.edu
Nikhil Karanjgaokar
University of Illinois at Urbana Champaign, Urbana, IL
Philippe H. Geubelle
University of Illinois at Urbana Champaign, Urbana, IL
Ioannis Chasiotis
University of Illinois at Urbana Champaign, Urbana, IL
Abstract:
Experimental observations have shown that nano-crystalline materials can exhibit strong rate
dependency [1]. This phenomenon can be attributed to the fact the as the grain size decreases the ratio
between grain boundary area and grain volume increases, and so the deformation mechanisms mediated
by the grain boundary start to play a more important role. Simultaneously, the small free path between
two grain boundaries constraints the dislocation movement, thereby decreasing the ductility of the
material. The rate dependence can thus be explained by the competition between the two deformation
mechanisms, the grain boundary diffusion and dislocation movement inside the grains.
Motivated by this observation, a multiscale finite element formulation has been developed to simulate
thin film tensile and creep experiments. In a two-dimensional (plane stress) framework, a Voronoi
tessellation is used to create a multi-grain representative volume element (RVE). The RVE is
discretized in volumetric elements inside the grains and interfacial elements along the grain boundaries.
The inter-granular processes are modeled by cohesive (interfacial) elements that account for diffusionmediated
sliding between grains. The intra-granular behavior is captured by a rate-dependent singlecrystal
plasticity model that represents the evolution of the various slips systems present in each grain.
The key parameters entering the description of the grain interior and grain boundary models are
calibrated through comparison with high strain rate tensile tests and creep experiments, respectively.
The prediction of the viscoplastic response of gold thin films is validated against tensile test
measurements obtained over seven decades of strain rate. The relative contribution of the two
microstructural damage mechanisms is analyzed.
References:
[1] I. Chasiotis, C. Bateson, K. Timpano, A.S. McCarty, N.S. Barker, and J.R. Stanec. Strain rate
effects on the mechanical behavior of nanocrystalline au films. Thin Solid Films, 515(6):3183-3189,
2007.
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