Mechanical Grain Growth in Nanocrystalline Copper

Nanograined materials have some unusual properties. To maintain the small size of the grains, grain growth should be avoided. But recently grain growth has been observed under an indenter at liquid-nitrogen temperatures. Such grain growth has never been reported before. How can this happen and how can it be prevented? These questions are answered here using a simple tilt boundary. It is found that high purity and nonequilibrium structure are necessary conditions for mechanical grain growth. The material must be pure enough so that free dislocations are available to move out of the boundary. But the boundary should not be in the lowest-energy state so that extra dislocations are available to be emitted by stress. Based on these conditions, methods can be devised to avoid low temperature grain growth.

Figure 1

The effect of grain size on the impurity segregation at the grain boundaries.

Figure 2

Stress required to remove one or two free dislocations from wall showing it is easier to remove 2 than 1 [unit of stress: $\mu b/2\pi h(1-\nu )$].

Figure 3

Stress required to remove 3 free dislocations from wall, showing 4 different paths. [Unit of stress: $\mu b/2\pi h(1-\nu )$.]

Figure 4

The four different paths of removing 3 free dislocations in wall, showing one special path of $x_0=x$.

Figure 5

Stress required to remove an extra dislocation in wall, showing a much lower stress than to remove a free dislocation [unit of stress: $\mu b/2\pi h(1-\nu )$].

Figure 6

Stress required to remove 2 extra dislocations from wall, showing that the stress is less than that required to remove only one. [Unit of stress: $\mu b/2\pi h(1-\nu )$.]