Atomistic simulation of the irreversibility of surface cyclic slip in fcc metals
Résumé
Fatigue is one of the major damage mechanisms of metals [Suresh, 1998]. Cyclic deformation induces partial irreversible surface slips which are mainly responsible for crack initiation [Mughrabi, 2012]. Careful AFM measurements show that the fraction of irreversible surface steps can reach 80% in air.The objective of this study is to analyze the behavior of cyclic surface steps at the atomic level, in order to understand their irreversibility and evaluate the slip irreversibility factor. The considered metals are Al, Ni, Cu and Ag.MD simulations have been carried out at room temperature using EAM/FS potentials [Hardouin Duparc et al, 2005]. Edge dislocations are introduced in the simulation box and glide under the applied stress.As an edge dislocation reaches a free surface, two different kinds of surface steps are observed -If the angle between the free surface and the step is obtuse, then no reconstruction is observed after a long relaxation. An opposite stress of amplitude 1GPa is required for erasing this step which is much higher than the one applied during fatigue tests. -Otherwise, a reconstruction occurs during relaxation. Atoms move in order to increase the number of atomic bounds and minimize energy. In this case, an opposite stress of 5GPa is not enough to erase the reconstructed step. The introduction of dislocations of opposite sign can make the reversibility easier. But a reconstructed step is only partially reversible even after the opposite-sign dislocation reaches the free surface. After a half cycle, it leaves behind a permanent surface defect which could not be erased. Other mechanisms have been investigated involving larger steps and other dislocations introduced repeatedly. Results will be compared to experimental measurements [Weidner et al, 2011].Ab-initio calculations will be carried out to study the reactions between a fresh surface step and oxygen or water molecules which probably lead to higher irreversibility.