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

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 threedimensional 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 2 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.

Parameterization of a Non-local Crystal Plasticity Model for Tempered Lath Martensite Using Nanoindentation and Inverse Method

“…But not only the pure number of blocks below the indenter is relevant, also the presence of geometrically necessary dislocation accumulation close to grain boundaries must be considered. This has been shown experimentally by Yang and Vehoff (2007) and, very recently, by discrete dislocation dynamic simulations of Lu et al (2019). Figure 12 shows a view through the indented model in our simulations.…”

Dislocation–grain boundary interactions: recent advances on the underlying mechanisms studied via nanoindentation testing

“…It was concluded that the difficulty in the slip transfer across GBs is pronounced at the GBs corresponding to poorly aligned slip systems and consequently, associated with the low values of m ′ . Focusing on the smaller scales than the continuum level, a discrete dislocation study by Lu et al [117] has recently investigated a bicrystal indented at the area of a GB. Figure 8 in [117] is reproduced here in Fig.…”

“…Focusing on the smaller scales than the continuum level, a discrete dislocation study by Lu et al [117] has recently investigated a bicrystal indented at the area of a GB. Figure 8 in [117] is reproduced here in Fig. 6 and shows two plateaus in the force data which are associated with displacement bursts at the order of 10% of burgers vector magnitude ( b = 0.25nm ) given in Table 1 in [117].…”

Nanoindentation and Microstructure in the Shear Band in a Near Beta Titanium Alloy Ti-5Al-5Mo-5V-1Cr-1Fe

“…Therefore, the deformation is easier for the BCC crystal structure. It is reported that the nanoindentation hardness inside the grains was lower than that near the grain boundaries due to the grain boundaries strengthening effect [18]. Due to the different scales of the β phase grain sizes and nanoindentaion, it is unclear whether the deformation area is inside the β phase grains or in the β phase grain boundaries.…”