1.7.5.3 Ordered alloys

This chapter discusses production of atomic defects in concentrated alloys and ordered alloys. The uncertainties in the interpretation of resistivity damage rates in terms of defect production found in dilute alloys are increased in concentrated alloys. Resistivity changes due to atomic rearrangement may even exceed those from defect production. Low temperature damage rates in concentrated alloys are tabulated. At low temperatures, when atomic defects are immobile, resistivity changes in disordered alloys result exclusively from the production of atomic defects, while in ordered alloys also irradiation induced disordering (mixing) contributes. The damage rates in ordered alloys are always higher than in the corresponding pure metals or in the disordered alloys. Changes of the long range order parameters S are mostly ascribed to replacement collision sequences. In monoatomic metals or in disordered alloys, the effect of replacement sequences is mainly to separate the interstitial from its nascent vacancy during the displacement process, while in ordered alloys the exchange of atoms along the sequence may also change the order. S can be determined by X-ray (XD) or electron (ED) diffraction, or less directly by resistance (ERM), magnetization (MAG) or infrared emissivity (IRM) measurements. Replacement to displacement ratios N_r/N_d in metals under irradiation are provided. N_d is derived from experimental damage rates. For determining the disordering coefficient k, the resistivity contribution of atomic defects can be taken into account by combining initial damage rate measurements on ordered and disordered alloys. Saturation values of the long range ordering parameter under irradiation at temperature T and flux φ are tabulated.