Influence of surface relaxation on solute atoms positioning within atom probe tomography reconstructions

C Oberdorfer and T Withrow and LJ Yu and K Fisher and EA Marquis and W Windl, MATERIALS CHARACTERIZATION, 146, 324-335 (2018).

DOI: 10.1016/j.matchar.2018.05.014

While thermal or field-induced surface migration has been invoked often in past literature to rationalize the experimental observations of solute and impurity atoms segregation to specific poles and zones lines in atom probe tomography (APT) reconstrations, it does not satisfactorily explain all experimental results. Using a combined finite-element and molecular dynamics simulation approach that models the field evaporation process from mechanically relaxed emitter structures, we show that an additional mechanism, athermal relaxation, can also drive significant solute migration. The mechanism is illustrated for Cu in solid solution in Al, which exhibits segregation of Cu to < 111 > poles and along 110 zone lines in reconstructed APT data. The higher bonding energy of Cu atoms on the surface, confirmed using density-functional theory calculations, causes longer retention on the surface and preferential evaporation of neighboring Al atoms with subsequent athermal relaxation of Cu into higher-neighbor-count sites left vacant by evaporating Al neighbors. This in turn results in barrier-free long-range migration of the Cu atoms by a series of relaxation events that follow the receding terrace ledges towards high- neighbor count poles and zone lines. This mechanism fundamentally limits spatial resolution in the limit of zero temperature.

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