Grain boundary effect on nanoindentation: A multiscale discrete dislocation dynamics model

TL;DR

This study uses a multiscale discrete dislocation dynamics and finite element model to analyze how grain boundaries influence nanoindentation behavior in aluminum bicrystals, revealing dislocation interactions and hardness variations.

Contribution

It introduces a coupled 3D DDD-FEM model to simulate dislocation-GB interactions during nanoindentation, providing new insights into the underlying mechanisms.

Findings

Confirmed pop-in events during nanoindentation.

Showed hardness increases as GB-indenter distance decreases.

Developed a quantitative model linking hardness to dislocation pile-up and GB constraints.

Abstract

Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GB) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a three-dimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.