HRTEM studies on particles coherency and strengthening mechanism in FeCrAl ODS model alloys with addition of Zr/Hf

Alloy compositions on oxides and the strengthening are vital to materials design and development, which deserve in-depth investigation. Here, detailed crystal structures and interface coherency of totally more than 1000 oxides in Fe-15Cr-3.3Al ODS model alloys with 0.51 wt% Zr and 0.46 wt% Hf addition (denoted as P1 and P2, respectively) were investigated. The oxides in P1 are mainly trigonal Y4Zr3O12 and Y4Al2O9 (YAM, yttrium aluminum monoclinic) while in P2, they are Y2Hf2O7 (pyrochlore) and YAM. The Y2Hf2O7 oxides occupy 74.5 % of oxides in P2 while Y4Zr3O12 oxides account for 65.9 % in P1, manifesting that 0.46 wt% Hf addition can greatly suppress the formation of YAM oxides than 0.51 wt% Zr addition. The mean diameter of oxides in P2 is smaller than P1 due to the formation of smaller Y2Hf2O7 and YAM oxides. Almost all Y4Zr3O12 and Y2Hf2O7 oxides are coherent with the matrix whereas some YAM oxides show a semi-coherency relationship. The semicoherent YAM oxides take a percentage of 5.0 % in P2, in contrast to that of 8.9 % in P1, indicating an improvement of oxide coherency by Hf addition. A size dependence of coherency of YAM oxides was observed in both specimens. The strengthening mechanism of ODS model alloys is analyzed by using dispersion barrier hardening (DBH) model. The calculated barrier strength factor, α for coherent oxides is 0.20, which is smaller than that for semi-coherent oxides, 0.36.