Surface segregation of Ni-Cu binary alloys studied by an atom-probe

A focusing-type time-of-flight atom probe (field-ion microscope) was applied for a detailed investigation of surface segregation in Ni-Cu binary alloys over the entire composition range. Strong enrichment of copper was observed in the Ni-rich alloys, in agreement with previous reports. The surface enrichment factor and heat of Cu segregation were determined experimentally and were found to agree with various theoretical results. Cu segregation was limited to the topmost surface layer in almost all of the cases. The adjacent subsurface region exhibited small oscillations in the composition. A surprising finding was that nickel segregated to the surface in the Ni solute region (less than 16 at.?Ni). The Ni segregation extended over several layers in the near-surface region, a marked difference compared to the case of the Cu segregation.