High Rate
Deformation
Group

Grain Boundary Strength

Nano-indentation experiments are being used to determine the strength of different types of boundaries in copper.

In these experiments, the hardness of different types of boundaries is characterized using a Digital Instruments Multimode AFM equipped with a Triboscope nano-indention system from Hysitron. We are interested in determining what factors influence boundary hardness (boundary orientation, local dislocation density, etc) and correlating this to dislocation behavior observed during in-situ TEM deformation experiments. Particular attention is being given to the behavior of twin boundaries during these experiments.

Initially, the boundary hardness of several different copper samples is being studied. These include: three different bi-crystal copper samples, as-received and annealed polycrystalline copper, and a copper sample with engineered grain boundaries. The bi-crystal sample was produced from single crystal copper that was pulled in tension at 4.2 K and then rolled to 74% thickness reduction. This process produces a lamellar structure consisting of bands of deformation twins and matrix. Two samples of this material were subjected to heat treatment where one was partially recovered and the other fully recovered. A third sample was not recovered. The bi-crystal material was supplied by our collaborator Marek Niewczas at McMaster University. The engineered grain boundary material was supplied by our collaborator Mukul Kumar at Lawrence Livermore National Lab.

Experimental

During the nano-indentation experiments, force is applied to the sample using a Berkovich diamond indenter tip. The tip is loaded to a peak force of ~500 煮 under a trapezoidal loading profile shown in Figure 1 below.

Figure 1. Loading profile used for nano-indentation experiments

The loading results in a force vs. displacement curve. A typical curve is shown below in Figure 2. From this, the software extrapolates hardness, elastic modulus and contact depth from the unloading portion curve (shown in green in Figure 2).

Figure 2. Data output from the loading showing local hardness, elastic modulus and contact depth

The software is also equipped with a tool for performing cross-sectional analysis. This allows for the determination of the indent diameter as well as the diameter of the elastic-plastic transition zone.

Results

Well Annealed Polycrystalline Copper

Future Work

Examine how the hardness changes across the grain boundary in the polycrystalline copper samples.

Use Focused Ion Beam (FIB) milling to prepare TEM specimens from the indented regions.

Characterize the behavior of dislocations in the vicinity of the of the grain boundary.