Atomic Analysis of Cleavage Fracture in Nickel Crystal under Tension with Transverse Constraint.

Abstract

The criterion for cleavage fracture in nickel crystals was elucidated by the atomic analysis in order to determine the evaluation method of strength of micro-materials. The analysis was based on a molecular dynamics simulation using the EAM (embedded atom method) potential. First, a simulation was conducted on a nickel single crystal with several size of the simulation cell under tension along the [001] direction where the transverse deformation was constraint. As the number of atoms in a cluster, n, increases, the magnitude of strain at fracture decreases monotonically. In n=4, the cleavage fracture takes place at the tensile strain, ε=0.263, which is smaller than the critical strain of lattice instability, ε=0.304, calculated on the basis of the Born's analysis. Since the displacement of each lattice have fluctuation, the maximum local strain reaches the critical strain of lattice instability when the global strain is 0.263. In order words, the freedom of mobility brings about the reduction of fracture strain. In 32≤n≤256, the cleavage does not take place due to the transformation which is out of scope in this study. In n≥500, the cleavage initiates from the local lattice where the shear deformation on (111) planes meet when the local strain exceeds the lattice instability. A tensile simulation was also carried out for a nickel with Σ5 twist grain boudaries under traverse constraint. Cleavage fracture at a grain bouudary is observed at ε=0.130 with shear deformation in a grain. It is revealed from the detail observation that the local strain at grain boundaries are much higher than that of grains and the cleavage fracture at the grain boundary takes place when it reaches the lattice instability. Thus, the criterion for cleavage fracture is quantitatively estimated by applying the lattice instability of Born's analysis to the local strain.