EVOLUTION OF LONG-RANGE ORDER AND COMPOSITION FOR RADIATION-INDUCED PRECIPITATE DISSOLUTION

Disordering and dissolution of L${1}_{2}$ ordered \ensuremath{\gamma}\ensuremath{'} precipitates under irradiation at temperatures between room temperature and 623 K are investigated by means of transmission electron microscopy and field-ion microscopy with atom probe. The combination of both experimental techniques allows us to follow the disordering process as well as chemical decomposition of the precipitates with atomic resolution. During room-temperature irradiation and for increasing irradiation fluence, the concentration profiles across the precipitates show a broadening of the \ensuremath{\gamma}/\ensuremath{\gamma}\ensuremath{'} interface. The experimentally obtained depth profiles can be interpreted assuming a dissolution process of the concentration inhomogeneities due to ballistic transport only. A correlation analysis of the experimental data yields a mixing diffusion coefficient of ${\mathit{D}}_{\mathrm{mix}}$/K=(0.${75}_{\mathrm{\ensuremath{-}}0.4}^{+0.2}$) ${\mathrm{nm}}^{2}$ ${\mathrm{dpa}}^{\mathrm{\ensuremath{-}}1}$. Depending on irradiation temperature, two dissolution regimes are observed. For a displacement rate of ${10}^{\mathrm{\ensuremath{-}}3}$ dpa ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, the precipitates first disorder and then dissolve in a disordered state at temperatures below 540 K, while disordering and dissolution occur simultaneously at temperatures between 540 and 623 K. These results demonstrate that disordering of the precipitates is not necessarily required for the dissolution. The results are in accordance with recent theoretical predictions for the dissolution mechanism of ordered precipitates under irradiation. \textcopyright{} 1996 The American Physical Society.