Abstract
Part II of this paper concerns the resistance changes observed on the isothermal ageing of solution-treated Nimonic 80A. Ageings were carried out in the temperature range 800 K to 1040 K. The nucleation of γ' is initially controlled by the supersaturation of quenched-in vacancies; this transient rapidly decays to leave precipitation controlled by thermally activated vacancies. The activation energy of the initial ageing rate, of 1.10 ± 0.10 eV, is associated with the migration of Al and Ti into Ni-rich regions and possibly with a relaxation process on nucleation. The time for the resistance change to reach a maximum is strongly temperature-dependent in 80A. The activation energy for this process, 2.9 ± 0.3 eV, agrees with the accepted values for the isothermal diffusion activation energy
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