Kinetics of radiation-induced segregation in Ni-12.7 at.% Si

Abstract

The kinetics of radiation-induced segregation in Ni-12.7 at.% Si alloys was investigated using in situ, simultaneous Rutherford-backscattering spectrometry. It was observed that a precipitate layer of ${\ensuremath{\gamma}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{Ni}}_{3}\mathrm{Si}$ grew at the specimen surface during 2.0-MeV He and 2.75-MeV Li irradiations. The thickness of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ layer was measured as a function of dose, dose rate, and temperature. For all of the irradiation conditions the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ layer thickness grew proportionately to the square root of dose. The proportionality constant, or growth-rate constant, was dependent on temperature and dose rate. Below \ensuremath{\sim}570\ifmmode^\circ\else\textdegree\fi{}C the growth-rate constant displayed Arrhenius behavior with an apparent activation enthalpy of 0.30\ifmmode\pm\else\textpm\fi{}0.04 eV, and it depended approximately on the $\ensuremath{-}\frac{1}{4}\mathrm{th}$ power of the dose rate. Above 590\ifmmode^\circ\else\textdegree\fi{}C the growth-rate constant also displayed Arrhenius behavior but with an apparent activation enthalpy of -0.75\ifmmode\pm\else\textpm\fi{}0.15 eV; moreover, it was independent of dose rate in this temperature regime. A simple model for radiation-induced segregation is described which relates the segregation results to high-temperature point-defect properties in alloys. It shows that the apparent activation enthalpy of the growth-rate constant at low temperatures is equal to $\frac{1}{4}$ the enthalpy of vacancy migration, and that at high temperatures it is equal to $\ensuremath{-}\frac{1}{2}$ the enthalpy of vacancy formation. The model also correctly predicts the observed dose-rate dependences of the growth-rate constant in the two temperature regimes.