Nanophase Copper

The hardness and strength of a metal are measured by studying its deformation in response to an applied force. A metal is deformed when its crystalline atomic planes slide over each other. An analogy may be the bump in a rug that can be pushed across the floor. In other words, a dislocation in a plane of atoms is moved until a barrier is reached, such as a grain boundary, where the micron-sized grains are differently oriented.

One interesting advance in metal technology is the ability to assemble nanometer-size clusters of atoms in grain sizes of less than 100 nanometers in diameter instead of having the micron-size grains found in a typical metal. A graph of hardness versus grain size is shown.

With grain sizes averaging about 10 nanometers, this nano phase copper metal has a hardness more than three times the hardness of normal copper metal. Why?

Answer

With smaller grain sizes, one would expect there to be many more grain boundaries in the nanophase copper metal than in normal copper. The extra grain boundaries would stop or impede any moving dislocation, thereby making the nanophase copper much harder. However, the surprise turned out to be that nanophase copper is mostly dislocation-free! Lacking large numbers of moving dislocations, these nanophase metals are much stronger.