As mentioned above, one of the most spectacular consequences of RIS is that it can completely modify the stability ofprecipitates and the precipitate micro- structure.47 When the local solute concentration in the vicinity of a point defect sink reaches the solubil­ity limit, RIP can occur in an overall undersaturated alloy. RIP of the y’-Ni3Si phase is observed, for exam­ple, in Ni—Si alloys28 at concentrations well below the solubility limit (Ni3Si is an ordered L12 structure and can be easily observed in dark-field image in transmis­sion electron microscopy (TEM)). In this case, it is believed that RIS is due to the preferential occupation of interstitials by undersized Si atoms.28 The y’-phase

can be observed on the preexisting dislocation network, at dislocation loops formed by self-interstitial cluster­ing,28 at free surfaces45 or grain boundaries.48 The fact that the y’-phase dissolves when irradiation is stopped clearly reveals the nonequilibrium nature of the pre­cipitation. This is also shown by the toroidal contrast of dislocation loops (Figure 4(a)): the y’-phase is observed only at the border of the loop on the disloca­tion line where self-interstitials are annihilated; when the loop grows, the ordered phase dissolves at the center of the loop, which is a perfect crystalline region where no flux of Si sustains the segregation.

In supersaturated alloys, the irradiation can com­pletely modify the precipitation microstructure. It can dissolve precipitates located in the vicinity of sinks when RIS produces a solute depletion. For example, in Ni—Al alloys,49 dissolution of y’-precipitates is observed around the growing dislocation loops due to the Al depletion induced by irradiation (Figure 5), and in supersaturated Ni—Si alloys, Si segregation towards the interstitial sinks produces dissolution of the homo­geneous precipitate microstructure in the bulk, to the benefit of the precipitate layers on the surfaces28 (Figure 6) and grain boundaries.50

In the previous examples, RIS was observed to produce a heterogeneous precipitation at point defect sinks. But homogeneous RIP of coherent pre­cipitates has also been observed, for example, in Al-Zn alloys.51 Cauvin and Martin52 have proposed a mechanism that explains such a decomposition. A solid solution contains fluctuations of composition. In case of attractive vacancy-solute and interstitial — solute interactions, a solute-enriched fluctuation tends to trap both vacancies and interstitials, thereby favoring mutual recombination. The point defect concentrations then decrease, producing a flux of new defects toward the fluctuation. If the coupling with solute flux is positive, additional solute atoms

arrive on the enriched fluctuations, and so it con­tinues, till the solubility limit is reached.