AES study of Cu and S surface segregation in a ternary Ni-Cu(S) alloy in combination with a linear heating method

Highlights: • Surface segregation of Cu and S in a Ni-Cu(S) ternary alloy were investigated. • AES in combination with a linear heating method were used. • Cu initially segregated to the surface and then desegregate with S segregation. • The modified Darken model was employed to obtain segregation parameters. • Segregation parameters for both Cu and S in Ni-Cu(S) ternary alloy were obtained. Surface segregation of Cu and S in a Ni-18.7 at.%Cu alloy with 0.0007 at.%S was studied by means of Auger electron spectroscopy. A linear temperature programmed heating method was employed to in-situ heat the sample in the temperature range from 423 K to 1121 K at a constant rate of 0.02 K/s. It was found that the Cu started to segregate at 560 K and reached the maximum surface coverage values of 41.4 at.% at 896 K. Once the Cu reached the maximum surface coverage, it started to desegregate until Cu was replaced by S. At that moment, S segregation reached the maximum surface coverage values of 17.1 at.%. The modified Darken model simulations for both the kinetic and equilibrium in a ternary system were compared with the measured surface segregation profile results yielding the segregation parameters. The segregation parameters for both Cu and S in Ni-Cu(S) ternary alloy, namely the segregation energy ($\Delta G^0$), the interaction parameter (Ω) and the diffusion parameters (pre-exponential factor (D_o) and the activation energy (Q)) were determined as: $\Delta G_{C\text{u}}^0$ = −36.0 kJ/mol, $\Delta G_S^0$ = −136.0 kJ/mol, Ω_Cu-Ni = 7.6 kJ/mol, Ω_S-Ni = 28.1 kJ/mol and Ω_Cu-S = −10.3 kJ/mol, D_{Cu in Ni} = 8.6 × 10⁻¹⁴exp(-145.2 kJ/RT) m²/s and D_{S in Ni} = 9.2 × 10⁻²exp(-224.0 kJ/RT) m²/s.