Transmission electron microscopy characterization of the nanofeatures in nanostructured ferritic alloy MA957

Bright field transmission electron microscopy (TEM), high-resolution transmission electron microscopy, scanning transmission electron microscopy and energy dispersive X-ray spectroscopy were combined to comprehensively characterize the Ti-Y-O enriched nanoscale features (NF) in a nanostructured ferritic alloy (NFA), MA957. The characterization included the NF number densities, size distributions, volume fractions, crystal structures and compositions. In-foil measurements were complemented by studies of NF on C extraction replica foils. The conditions examined included: two heats of extruded MA957, one of these heats following long-term thermal aging at 1000 degrees C for 19.53 kh, and two friction-stir-weld variants. The study revealed the presence of diffracting complex oxide phases that varied somewhat with the feature size. However, the smallest features, with diameters (d) < 5 nm, were found to be consistent with cubic pyrochlore Y2Ti2O7 complex oxides with average Y/Ti ratios of similar to 1. Long-term thermal aging coarsened the NF, while friction stir welding had relatively little effect on their number density and size distribution. The results presented here are briefly compared with those reported in other recent publications along with a short discussion of the reasons why the compositions measured by TEM differing from those found in atom probe tomography studies, and the implications of the comprehensive database developed here to the irradiation tolerance and mechanical behavior of NFA. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.