Strength of nanoscale polycrystalline copper under shear

The strengths and shear moduli of several polycrystalline copper systems have been calculated with the molecular dynamics method and effective-medium potential. The grain size varied between 2 and 10 nm, and systems were sheared beyond the yield point at room temperature. An inverse Hall-Fetch behaviour was seen: the strength decreased with decreasing grain size. Similar behaviour was seen for the shear modulus. These were caused by the grain boundaries, which allow grain-boundary sliding and have low elastic constants. Results were compared with the tensile strength, and the Von Mises failure criterion did hold quite well even on these very small scales. Details of the potential model are discussed in the appendix.