Chapter 10-Interfacial Segregation and Precipitation During Irradiation

Abstract

The microstructural evolution in alloy systems during irradiation at elevated temperatures is determined by, among other factors, a non-equilibrium segregation process termed radiation-induced segregation (RIS), which can occur as a consequence of: 1. The strong interaction between solutes and the point defects (vacancies and interstitial atoms) generated during irradiation, resulting in coupled transport of the solute atoms by the point-defect fluxes to and away from sinks, such as grain boundaries, free surfaces, dislocations loops, void surfaces, etc. The magnitude of the solute-point defect binding energy determines whether the solute flow is towards or away from the sinks. In general, undersize solutes such as silicon and phosphorus in α-iron bind strongly to the interstitials in a mixed dumbbell configuration, resulting in a marked enrichment at sinks. Conversely, the oversize solutes (Cr and Mo in α-iron) are weakly bound to vacancies and are depleted at sinks. These processes have been modeled [1-3] using a simplified analytical method similar to that for thermally induced non-equilibrium segregation [4,5]; however, these approaches apply essentially to dilute alloys containing < 1 at% solute, and their accuracy is limited in many cases by the lack of precise knowledge of the binding and migration energies of the solute-point defect complexes. 2. The Inverse Kirkendall Effect, whereby the faster-diffusing species exchange more often with the irradiation-induced vacancies migrating to sinks than slow-diffusing species. The fast-diffusing solutes are therefore depleted at sinks while the concentrations of the slow-diffusing species increase. This Inverse Kirkendall Effect due to vacancies has been modeled with some success for both dilute and concentrated alloys using rate theory in which the elemental distribution is obtained by the simultaneous solution of a series of partial differential equations defining the fluxes of all atomic species [6-12]. An Inverse Kirkendall Effect due to interstitials, analogous to the effect of vacancies, may also be expected [13]; the atom fluxes are in the same direction as the defect fluxes in this case, but the contribution of this effect to the RIS process is uncertain at present.