Grain Distribution for Optimal Strength in Homogeneous and Gradient Nano-Grained Coppers

The distribution of grains plays a crucial role in determining the strength of polycrystalline copper when the grain size is constant. Herein, the mechanical properties of homogeneous nano-grained (HNG) and gradient nano-grained (GNG) coppers with different grain distributions are examined using molecular dynamics (MD) simulations. The HNG-ordered structure has all triple junctions (TJs), whereas the random structure contains many quadruple and quintuple junctions. When grain size is below the critical size in the inverse Hall–Petch relationship, the high-density TJs in the HNG-ordered structure effectively inhibit grain boundary (GB) softening compared with the random structure, leading to higher strength. However, when grain size is above the critical size, subgrains are produced inside the large grains due to dislocation slip. In addition, disordered atoms in HNG-random structure are stacked in the quadruple and quintuple junctions, resulting in thicker GBs. This triggers grain boundary migration, and forms more subgrains at GBs. Subsequently, grain boundary sliding and grain rotation of subgrains induce partial recrystallization in the structure. This consecutively triggered deformation mechanism leads to extra strengthening in the random structure. Further research indicates that combining small-grained ordered and large-grained random structures can be a new approach to effectively strengthen GNG materials.