Effects of friction stir welding and post-weld annealing on nanostructured ferritic alloy

Abstract

Nanostructured ferritic alloys (NFAs) show great promise for use in high temperature energy systems, especially advanced fission and future fusion reactors. NFAs must be joined by solid state methods, such as friction stir welding (FSW), which is the focus of the present research. We address the key question on the effect of FSW on the ultrahigh density of nanoscale features (NFs) that imbue NFAs with their outstanding properties, including remarkable resistance to radiation damage. Atom probe tomography (APT) measurements suggest that the number densities N of the NFs are qualitatively similar before and after FSW, but they appear to be redistributed, resulting in a high degree of alignment along boundary and dislocation structures. The possibility of reprecipitating NFs during 1150°C–3 h anneals was also explored. However, the data show that both N and microhardness are lower after annealing the FSW alloy. Several limitations of the APT characterisation method are also discussed.