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

Disordering add dissolution pf L1(2) ordered gamma' precipitates under irradiation at temperatures between room temperature and 623 Ii are i nvestigated by means' of transmission electron microscopy and field-io n micros copy with atom probe. The combination of both experimental te chniques allows us to follow the disordering process as well as chemic al decomposition of the precipitates with atomic resolution. During ro om-temperature irradiation and Tor increasing irradiation fluence, the concentration profiles across the precipitates show a broadening of t he gamma/gamma' interface; The experimentally obtained depth profiles can be interpreted assuming a dissolution process of the concentration inhomogeneities due to ballistic transport only. A correlation analys is of the experimental data yields a mixing diffusion coefficient of D -mix/K=(0.75(-0.4)(+0.2)) nm(2) dpa(-1). Depending on irradiation temp erature, two dissolution regimes are observed. For a displacement rate of 10(-3) dpa s(-1), the precipitates first disorder and then dissolv e 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 ac cordance with recent theoretical predictions for the dissolution mecha nism of ordered precipitates under irradiation.