Mechanical behavior of (Ni, Fe)Cr₂O₄ spinel grain boundaries studied by molecular dynamics simulations

Using molecular dynamics simulations, we study the fracture initiation of grain boundaries in NiCr₂O₄ and FeCr₂O₄ of spinel structure. These compounds are representative of corrosion layers of nickel-chromium-iron austenitic stainless alloys. Uniaxial deformation is applied to several symmetric tilt-, twist-, and random-grain boundaries until complete decohesion is reached, in order to measure the critical cleavage stresses. We find that this mechanical quantity depends on the chemical composition and the structure of the grain boundaries. A correlation between the critical stress and the misorientation angle of the grain boundaries can be established. It is also found than the twist- and the random-grain boundaries exhibit the weakest resistance. Furthermore, these simulations show a localization of the elastic response in the vicinity of the grain boundary plane. Therefore, we were able to compare our simulation results with those provided by the theoretical model based on the universal binding energy relation that relates the critical stress to the cleavage grain boundary energy. It shows that this model provides similar critical stresses than the molecular dynamics simulations if the localization distance parameter is defined as the distance related to the deformation of the grain boundary thickness during the deformation.