PNNL

Abstract

Transmission electron microscopy (TEM) discs of PM2000 were coated on one side with a ~4 µm thick NiAl film and irradiated in HFIR to a neutron displacement damage dose of 21.2 dpa at 500?C. The dpa are primarily produced by fast neutrons, while the in situ He injection (ISHI) to 1230 appm derives from the thermal neutron two-step 59Ni(n,?) reaction. A uniform He injection profile extends up to about 8 µm below the NiAl coating, hence the remaining material experiences only dpa damage (21.2 dpa at 500?C). This report summarizes analytical TEM analyses to compare the two different regions, i.e., simultaneous He-injection/displacement damage versus displacement damage only, or high and very low He/dpa ratios, respectively. TEM analyses of the He implanted region reveals a high density of small ({100} and ½{111} dislocation loops and a low density of line dislocations, whereas the un-implanted side possesses predominantly line dislocations with a lower density of dislocation loops. In both regions, nanoscale fcc precipitates exhibiting a cube-on-cube orientation relationship with the bcc matrix were observed. Chemical analysis using energy dispersive x-ray spectroscopy (EDS) indicates this phase is rich in Al and Ti. It was also noted that substantial Cr segregation occurs around the dislocation loops, concurrent with Fe depletion. The clusters of fcc precipitates that do not appear to be associated with the existing loops or line dislocations. The amorphous Y-Al-O particles contain a high Al concentration (>20 at%) and Cr segregation/Fe-depletion at the periphery.