Segregation induced order-disorder transition in Cu(Au) surface alloys

Using in-situ transmission electron microscopy and atomistic simulations, we report atomic-scale observations of segregation-induced structure changes in the surface and subsurface region of a Cu(Au) solid solution in both reductive and oxidative environments. In a H-2 atmosphere, Au segregation induces the formation of a two-atomic-layer thick ordered surface alloy with an L1(0) terminated surface layer. By switching to an O-2 atmosphere, the outermost surface develops into an Au-missing row reconstruction and simultaneously the second layer experiences an order-disorder transition via intralayer atomic exchanges. The chemical disordering then propagates to the outermost surface, driven by oxygen adsorption induced Cu surface segregation. This transforms the L1(0) missing-row reconstruction into a non-reconstructed, oxygenated surface. These observations provide a mechanistic detail regarding the evolution of the surface and subsurface of this alloy in response to environmental stimuli, and are relevant to a wide range of technologically relevant processes. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.