Atomistic simulations of the effects of segregated elements on grain-boundary fracture in body-centered-cubic Fe

We studied the detailed fracture behavior of a Sigma = 5 symmetrical-tilt grain boundary at low temperatures in Fe, using empirical interatomic potentials. For loadings just above the Griffith value, the crack propagates along the boundary for a distance of about 5 nm and then deflects toward the grains. When the boundary is loaded well above the Griffith criterion in pure bcc Fe, the crack deflects and propagates in an intragranular manner. Lattice trapping effects were observed in the initial stages, as the crack propagates along the grain boundary in a brittle manner with a periodicity given by the structural unit of the grain boundary. The effects of impurities on crack propagation along the grain boundary were simulated with various amounts of substitutional (Cr and Ni) and interstitial (H and C) impurities. The H impurities result in a strong embrittlement of the grain boundary, and no deflection of the fracture to the inside of the grains is observed. The element C has the opposite effect, inducing the deflection of the fracture to the interior of the grains from the beginning of the simulation. For the substitutional Ni and Cr impurities, the effects on grain-boundary fracture behavior are less dramatic, with Cr decreasing the resistance to grain-boundary fracture, if present in high concentrations. These effects agree with expectations based on the relative energies of segregation of the impurities to the grain boundary and free surface.