The Effects of Applied Stress on the Irradiation Induced Microstructures of Dilute Nickel Alloys

Abstract

A detailed transmission electron microscopy (TEM) study has been performed on Ni-2.48% Si alloy specimens irradiated, in the stressed and unstressed condition, to a dose of 0.2 dpa at 623 K, with 3.5-MeV protons in the Proton Irradiation Creep Facility on the Harwell 6-MeV Van de Graaff accelerator. The objective of the work was to obtain detailed information on the size distributions of Frank loops, the primary irradiation produced microstructural feature in this material, which could be compared directly with the measured irradiation creep behavior. Low dose irradiations have been employed in order to minimize loop-loop interactions and maintain the microstructural record. Because of specimen to specimen temperature variations the main body of the analysis was performed on eight grains from a single TEM disk, cut from a creep specimen irradiated at 50 MPa. The effect of applied stress on loop size and planar loop densities was assessed by comparing data from randomly oriented grains with a single grain, which had equal resolved normal stresses on each of the four {111} planes. The Frank loop density is greatest on {111} planes with the highest normal stress. The applied stress influences the partitioning of loops between the different {111} planes but does not influence the overall loop density or the loop size. The present results support the stress-induced preferred nucleation (SIPN) or stress assisted rotation model of irradiation creep and no evidence has been found for either the stress-induced preferred absorption (SIPA) or stress-induced climb and glide (SICG) models. Frank loops account for less than 25% of the measured strain. The discrepancy probably arises from the unexpectedly high density and large size of unfaulted 110 loops. Additional information is required on vacancy loops and unfaulted loops in order to resolve the discrepancy between estimated and measured strains.