Abstract
Interaction between dislocations and grain boundaries (GBs) in the forms of dislocation absorption, emission, and slip transmission at GBs significantly affects size-dependent plasticity in fine-grained polycrystals. Thus, it is vital to consider those GB mechanisms in continuum plasticity theories. In the present paper, a new GB model is proposed by considering slip transmission at GBs within the framework of gradient polycrystal plasticity. The GB model consists of the GB kinematic relations and governing equations for slip transmission, by which the influence of geometric factors including the misorientation between the incoming and outgoing slip systems and GB orientation, GB defects, and stress state at GBs are captured. The model is numerically implemented to study a benchmark problem of a bicrystal thin film under plane constrained shear. It is found that GB parameters, grain size, grain misorientation, and GB orientation significantly affect slip transmission and plastic behaviors in fine-grained polycrystals. Model prediction qualitatively agrees with experimental observations and results of discrete dislocation dynamics simulations.
Highlights
Fine-grained polycrystalline materials with grain sizes ranging from hundreds of nanometers to tens of microns can be widely used as structural or functional materials in small-scaled engineering such as microdevices and microelectromechanical systems
The grain boundaries (GBs) model consisting of the GB kinematic relations and microscopic force balance equations for slip transmission captures the influence of geometric factors including the misorientation between the incoming and outgoing slip systems and GB orientation, GB defects, and stress state at GBs
With the decrease of grain thickness, the plastic slip decreases in the middle of grains but increases near the GB. This is due to the fact that in the middle, the plastic slip is governed by dislocation interaction which is stronger if the grain thickness is smaller, while, near the GB, the plastic slip is dominated by slip transmission which is easier to occur in the film with a smaller grain thickness
Summary
Fine-grained polycrystalline materials with grain sizes ranging from hundreds of nanometers to tens of microns can be widely used as structural or functional materials in small-scaled engineering such as microdevices and microelectromechanical systems. Conventional plasticity theories fail to capture dislocation-GB interactions and the influence of GNDs and are incapable of predicting size-dependent plasticity in fine-grained polycrystals. It is vital to properly model the above microscopic dislocation-relevant mechanisms, especially dislocation-GB interaction mechanisms at the continuum level, so as to construct non-classical plasticity theories aimed at the accurate characterization of size-dependent plasticity in fine-grained polycrystals. Additional microscopic boundary conditions at GBs/surfaces are required for the completeness of the mathematical framework This provides the possibility to incorporate the GB/surface-dislocation interactions into those continuum theories. The GB model consisting of the GB kinematic relations and microscopic force balance equations for slip transmission captures the influence of geometric factors including the misorientation between the incoming and outgoing slip systems and GB orientation, GB defects, and stress state at GBs. The model is applied to study the plastic behavior of a bicrystal thin film under plane constrained shear. Results predicted by the model qualitatively agrees with those from experimental observations and DDD simulations
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