Inverse Hall-Petch Effect in Atomistic Machining of Polycrystalline Copper Structures

In this study, a preliminary investigation about the grain size effect in machining of polycrystalline copper structures at atomistic scale is carried out using molecular dynamics simulation. Four copper structures with different grain sizes are chosen for simulation. The four structures consist of 16, 64, 128, and 256 grains, and the corresponding equivalent grain sizes are 13.6, 8.6, 6.8, and 5.4 nm, respectively. The results show that significant smaller forces are required to machine the copper workpiece in both the tangential and thrust directions as the grain size decreases. The magnitude of equivalent stress distribution also becomes smaller with the decrease of grain size. It disagrees with the commonly accepted strengthening effect (i.e., the Hall-Petch relation) for polycrystalline materials as a result of grain size reduction. This phenomenon can be explained by the inverse Hall-Petch relation proposed in literature in recent years. According to the new relation, the polycrystalline material strength decreases as the grain size decreases within a threshold value. This can be further attributed to the fact that the dominant deformation mode is changed from dislocation movement to other mechanisms such as grain boundary sliding with very fine nano-structured polycrystalline.