| Researchers at Johns Hopkins University have created a form of pure copper that is six times stronger than normal with no loss in ductility. They have achieved this by using a traditional metalworking techniques and nanotechnology. The results of their work are reported in the October 31 issue of the journal Nature. The findings are significant as no one has ever reported such strength increases without sacrificing ductility in pure metals such as copper. By maintaining the ductility, the metal is less likely to fracture when stretched. Furthermore, the researchers have successfully been able to increase the strength of pure copper to levels only ever seen before in copper alloys. To obtain the unique properties, extreme cold and mechanical manipulation were used. The process involved taking a 25mm cube of copper and cooling it to –196°C in liquid nitrogen and then rolling it down to a thickness of 1mm, with cooling between each rolling pass. Rolling induced a high dislocation density, interrupting the uniform nature of the crystal lattices. The low temperature prevented the defects from returning to their original alignments. The rolled material was then recrystallised at 200°C. This process eliminated the dislocations, and fine dislocation-free grains were formed. By inducing a high degree of dislocations during rolling, a larger number of smaller grains were formed during recrystallisation. In this case, the grains were only a few hundred nanometres in size, several hundred times smaller than the original grains. The reduction in grain size is responsible for the increase in strength, with an increasing number of grain boundaries hindering the movement of dislocations. Ductility was introduced into the material by inducing abnormal or non-uniform grain growth, which resulted in approximately 20-25% of the grains growing to a larger size, producing a bimodal grain size distribution. The combination of very small and large grains gives the material its strength and ductility properties, which is important in the forming and processing of high strength copper components. Potential applications for the technology may include microelectronics and biomedical devices. |