Extraordinary performance of the stable, nanocrystalline Cu-Ta system under extreme dynamic mechanical load

The influence of shock loading on nanocrystalline materials has heretofore not been examined in large part due to the instability of nanocrystalline structures under extreme mechanical and thermal loads. However, a recently developed binary alloy of nanocrystalline Cu and Ta has proven to remain nanocrystalline up to temperatures of 800 °C and, under uniaxial stress, up to strain rates of 10⁴ s^-1. Here, the performance of a nanocrystalline Cu-Ta alloy is investigated as both temperature and strain rate increase to the extremes under shock compression up to approximately 15 GPa. Post deformed micrographs reveal a shift from full dislocation to twin dominated deformation mechanisms above 10³ s^-1. In the shock regime, however, no significant microstructure changes are observed between the unshocked and shock recovered samples. Further, despite this apparent lack of change under shock loading, spall strength of the material is only slightly improved over polycrystalline Cu. While the refined grain size appears to contribute to arresting nucleated spall cracks, the plethora of grain boundaries and Ta clusters may act as potential void nucleation sites for spall crack initiation reducing the benefits of grain boundary strengthening.