Microstructure and precipitation behavior of advanced RAFM steels for high-temperature applications on fusion reactors

The composition of new eigth 9% Cr reduced activation ferritic/martensitioc steels (RAFMs) has been finely tuned in order to increase the amount of fine MX precipitates and reduce coarse M${}_{23}$C${}_6$ carbides through the application of thermomechanical treatments. The microstructural investigations by TEM/STEM and EELS have shown M${}_{23}$C${}_6$, M${}_2$X (Cr${}_2$N), and MX (Ta, V, and Ti-rich) precipitates after tempering at 750 °C/2 h. Higher N contens (0.04–0.03 wt%) seems to favored M${}_2$X precipitation over MX with V contents round 0.2 wt%. MX-Ti rich presents sizes larger than MX (Ta or V rich). EELS anlysis have shown that the MX (Ta, V and Ti rich) precipitated after tempering at 750 °C/2 h are mainly carbo-nitrides. Composition, size, number density and carbon and nitrogen content on MX and M${}_2$X is discussed in terms of the composition of each Alloy.