Researchers from the Materials Science department of the Johns Hopkins University recently published a report on how they produced a pure form of copper that is six times stronger than natural copper, while it retains the ductility (ability to stretch) of the material. Doctoral student Yinmin Wang, with faculty advisor En Ma, research scientist Mingwei Chen, and post-doctoral fellow Fenghua Zhou, published their findings in October's issue of Nature magazine.
The importance of the achievement stems from the fact that the superior strength of the metal was achieved without the use of foreign substances.
Alloys may increase the strengths of metals more than 100 fold, but having produced a pure metal with such an increase in strength can open doors for industries with highly specialized needs, such as micro-engineering and the biomedical industries.
The technique the developers used was surprisingly simple; based on traditional methods of using temperature changes to strengthen metals, the researchers simply cooled the copper and shaped it, then heated it up again.
Starting with a small block of copper, the researchers cooled the block in liquid nitrogen to about negative 196 degrees Celsius and kept the block in that environment for three to five minutes.
They then rolled the cube flat to a thickness of about one millimeter, affecting the crystalline structure of the copper. The copper crystals had been arranged in certain patterns, which were severely deformed by the cold rolling process, the severe cold having kept the crystals immobile enough to keep their new, higher-energy, crystalline formations.
The next step was to place the copper in an oven, heated to 200 degrees Celsius, for three minutes. This allowed the deformed crystal patterns to reform into stable crystal lattices; the only parts of the copper, however, that reformed significantly were those that had been deformed by the original rolling process. This reformation and recrystallization created crystal patterns that were extremely small, due to the high density of deformed crystal formations after rolling.
The resulting patterns were in the range of hundreds of nanometers (several millionths of a meter), several hundred times smaller than the original crystal structures, giving the resulting copper immense strength.
The idea is that a crystal structure in itself is rather difficult to be shaped, but when many crystals are packed together, they not only are difficult to form, but are packed together, which means even more energy must be exerted to adjust the total shape of the crystal combinations.
The smallness of the post-procedural crystal patterns means that more individual crystal patterns make up the volume of the metal, so the metal's strength is increased.
This sounds as if the techniques administered on the metal would make the metal immutable, however, the scientists' heating methods allowed for abnormal grain growth, meaning that some of the crystals merged together and grew to a larger size, about 20 to 25 percent of the crystals, to be more precise. This combination of small and large crystal patterns allowed for both the gain of strength and retainment of flexibility and ductility.
The group will not be basking in its glory too long, and says that its next research will be in the application of these techniques on similar metals to explore the extent of their effectiveness.
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