Microstructural changes and their effect on hardening in neutron irradiated Fe-Cr alloys

A series of Fe-3 to 18 at.% Cr binary ferritic alloys were neutron irradiated side by side, in the Advanced Test Reactor (ATR) at a temperature of similar to 320 degrees C to a dose of similar to 1.8 dpa. Three types of features that form under irradiation are relevant: (i) solute clusters; (ii) alpha' precipitates; and, (iii) dislocation loops. The size and number density of the precipitates and loops were measured by transmission electron microscopy (TEM) and compared to previous atom probe tomography (APT) observations. The loop density systematically decreases with increasing Cr, while the number of alpha' precipitates increase at 9Cr and above. A major objective of this work is to estimate the dispersed barrier obstacle strength factors (a(j)) for loops, clusters and alpha' precipitates, based on the combined microstructural observations and corresponding irradiation hardening measurements. Standard dispersed barrier-hardening models and computationally derived superposition rules were least square fit to determine the a(j). The optimized hardening predictions are in very good agreement with experiment if mixed linear sum and root square sum superposition rules are used. Five increments of 168 h isochronal anneals of the 6Cr alloy between 300 and 400 degrees C coarsened the loops, while 300 h anneals of the 18Cr at 500 and 600 degrees C coarsened and dissolved the alpha' precipitates, respectively, consistent with the Fe-Cr phase diagram. (c) 2019 Elsevier B.V. All rights reserved.