Correlation of radiation-induced changes in mechanical properties and microstructural development of Alloy 718 irradiated with mixed spectra of high-energy protons and spallation neutrons

Alloy 718 is a gamma ' (Ni-3(Al,Ti))-gamma " (Ni3Nb) hardenable superalloy with attractive strength, and corrosion resistance. This alloy is a candida te material for use in accelerator production of tritium (APT) target and b lanket applications, where it would have to withstand low-temperature irrad iation by high-energy protons and spallation neutrons. The existing data ba se, relevant to such irradiation conditions, is very limited. Alloy 718 has therefore been exposed to a particle flux and spectrum at the Los Alamos N eutron Science Center (LANSCE), closely matching those expected in the A-PT target and blanket applications. The yield stress of Alloy 718 increases w ith increasing dose up to similar to0.5 dpa, and then decreases with furthe r increase in dose. The uniform elongation, however, drastically decreases with increasing dose at very low doses (<0.5 dpa), and does not recover whe n the alloy later softens somewhat. Transmission electron microscopy (TEM) investigation of Alloy 718 shows that superlattice spots corresponding to t he age-hardening precipitate phases gamma ' and gamma " are lost from the d iffraction patterns for Alloy 718 by only 0.6 dpa, the lowest proton-induce d dose level achieved in this experiment. Examination of samples that were neutron irradiated to doses of only similar to0.1 dpa showed that precipita tes are faintly visible in diffraction patterns but are rapidly becoming in visible. It is proposed that the gamma ' and gamma " first become disordere d (by <0.6 dpa), but remain as solute-rich aggregates that still contribute to the hardness at relatively low dpa levels, and then are gradually dispe rsed at higher doses. Published by Elsevier Science B.V.