Atom-probe fim investigation of surface segregation in Ni-Cu, stainless steel 410 and Pt-Au alloys

Abstract

An absolute composition depth profile of a Ni-5%Cu alloy was obtained with a single atomic layer spacial resolution. The first layer Cu concentration is found to be 54.1 ± 4.7% on the (111) plane at 550 ± 50°C. The near surface layers are slightly depleted with Cu, and the Cu concentration returns to bulk value in about 5 atomic layers. A comparison with existing quasichemical theories of surface segregation is made. It is found that the regular solution model without surface relaxation does not give a close fit to our data. Although the ideal solution model gives a very good fit for the first layer Cu concentration, it does not account for the observed slight Cu depletion in the near surface layers. When surface relaxation of bond en thalpy is taken into consideration, a Cu depletion in the second atomic layer is predicted for an ideal solution model. Similarly a Cu depletion in the near surface layers is predicted for a regular solution model. For the ideal solution's case, a relaxation parameter δ = 0.022 gives the best fit while for the regular solution model, a δ = 0.17 gives the best fit. A comparison with results obtained from other macroscopic surface techniques on the same system are also made. The segregation of Cr in stainless steel 410 was also discussed. A binary regular solution model was found to fit the data better than an ideal solution model. However, it is also pointed out that a ternary alloy solution model is needed for a more accurate comparison. A preliminary study on Pt-5%Au indicates that Au segregates to the first atomic layer on the (111) plane.