Molecular dynamics simulation of surface step reconstruction and irreversibility under cyclic loading

Abstract : The mechanical behaviour of surface steps naturally created by the glide of dislocations subjected to cyclic loading is examined using molecular dynamics simulations. Three face centred cubic metals, Al, Cu and Ag are analysed. An atomistic reconstruction phenomenon is observed at these surface steps which can induce strong irreversibility. Three different mechanisms of reconstruction are defined. They induce different reconstruction rates and various relief evolutions depending on the temperature. Surface slip irreversibility under cyclic loading is analysed. All surface steps are intrinsically irreversible under usual fatigue laboratory loading amplitude without the arrival of opposite sign dislocations. A surface step is reversible only when an opposite sign dislocation subsequently glides on a nearby atomic plane. Steps created by opposite sign dislocation glides on non neighbouring planes are irreversible. The irreversibility cumulates cycle by cycle and a micro-notch is produced whose depth increases cyclically. A rough estimation of surface irreversibility is carried out for pure edge dislocations in persistent slip bands in wavy materials. This gives an irreversibility fraction between 0.5 and 0.75 in copper. An analysis coupling surface mechanisms with the classical bulk slip irreversibility model proposed by Differt, Essmann and Mughrabi in 1986 and applied to pure screw dislocations gives an irreversibility fraction of 0.62 in copper. Similar estimations in nickel give irreversibility fractions around 0.6 and 0.8 for pure edge and screw dislocations respectively. These values are in agreement with recent atomic force microscopy measurements.