Atomistic simulation of surface cyclic slip irreversibility in FCC metals

Abstract : The mechanical behaviour of surface steps created by the emergence at the free surface of gliding dislocations, subjected to cyclic loading is examined using molecular dynamics simulations. Different face centred cubic metals, Al, Cu, Ag and Ni are analysed. An atomistic reconstruction phenomenon is observed at these surface steps which can induce strong irreversibility. The irreversibility cumulates and a micronotch is produced whose depth increases cyclically. A rough estimation of surface irreversibility for pure edge dislocations gives an irreversibility fraction between 0.5 and 0.75 in copper. An analysis coupling surface mechanisms with the classical EGM bulk slip irreversibility model gives an irreversibility fraction of 0.62 in copper for pure screw dislocations, contrary to many sketches proposed in the past. It seems that oxygen molecules cannot lead to higher irreversibility as they have no major influence on different mechanisms linked to surface relief evolution.