Irradiation of materials with energetic particles drives them from equilibrium, and in alloys, this becomes manifest in a number of ways. One of them concerns nonequilibrium segregation. The creation of large supersaturations of point defects leads to persistent defect fluxes to sinks. In many cases, these point defect fluxes couple with solutes, resulting in either the enrichment or depletion of solutes at these sinks. This effect was first discovered by using in situ electron
irradiations in a high voltage electron microscope,27 and it has been systematically investigated subse­quently using ion irradiations,28 as the surface sink provides a convenient location to measure composi­tion changes. Unlike neutron irradiation, moreover, the damage created by ions is generally inhomoge­neous, reaching a peak level at some depth in the sample. As a consequence, point defect fluxes ema­nate from these regions. An example of this effect is shown in Figure 15 where a Ni—12.7at.% Si alloy was irradiated with protons. As the alloy is supersat­urated with Si prior to irradiation, Ni3Si precipitates

form in the sample. At the location of peak damage, the concentration of interstitials is the highest, and hence these defects flow outward from this region. These interstitials form interstitial-solute complexes with Si, resulting in a Si flux out of this area as well, depleting the region of Si. As a consequence, a region depleted of Ni3Si precipitates is observed at the peak damage depth. Note too that the surface sink for interstitials leads to enrichment of Si, resulting in a surface layer of Ni3Si. The region just below the surface accordingly becomes depleted of Si, leaving a zone depleted of Ni3Si precipitates.

While irradiation induced segregation can lead to nonequilibrium segregation and precipitation in sin­gle phase alloys, irradiation can also lead to dissolu­tion of precipitates in nominally two-phase alloys. An interesting example of this behavior concerns Ni-12 at.% Al alloys irradiated with 300 keV Ni ions.29 These alloys were first annealed at high tem­peratures to develop a two-phase structure of Ni3Al (g0) and Ni-10.5 at.% Al (g). The initial precipitate size, depending on the annealing time was 2.5 or 4.6 nm. As shown in Figure 16, the precipitates dis­order during irradiation at room temperature, owing to atomic mixing in cascades. The rate of disordering depends on the size of the precipitates, being slowest for homogeneous Ni3Al sample and fastest in the alloy with the smallest precipitates. The authors

suggest that the reason for this dependence on pre­cipitate size is that atomic mixing reduces the concentration of Al in the precipitates, which thereby accelerates the disordering. When the same irradiation is performed at higher temperatures, and radiation-enhanced diffusion takes place, the system does not completely disorder, but rather remains partially ordered, owing to a competition between disordering in the displacement cascades and reor­dering by radiation-enhanced diffusion. Noteworthy, however, is the size of the precipitate, as shown in Figure 16(c), where it is observed that the precipi­tates initially shrink in size, but then reach a steady state radius. Therefore, unlike in thermal aging, pre­cipitates in irradiated alloys can reach a stable steady state size that is a function of irradiation intensity and temperature. Similar behavior has been observed in two-phase immiscible alloys in which case a steady state size of precipitates is formed.3 This so-called ‘patterning’ phenomenon has been explained on the basis of a competition between disordering by atomic mixing in energetic collision events and reordering during thermally activated diffusion. For patterning, however, it is required that the atomic relocation distances during collisional mixing are significantly larger than the nearest neighbor distance. An inter­esting consequence of this requirement in regard to the present discussion of using ion irradiation to simulate neutron damage is that electron and proton irradiations, which do not produce energetic cascades or long relocation distances, should not induce com­positional patterning, but heavy ions and fast neutron irradiation, which do produce cascades, will cause patterning. Further details can be found in Enrique31 and Enrique et al31