Micro-alloying effects of Ta and B on nano-oxides and grain boundaries in 13CrWTi-ODS ferritic alloys

We present a combined electron microscopy and first-principles calculation study on the dominant formation of ultra-fine Y-Ti-Ta-O nano-oxides in Ta+B micro-alloyed 13CrWTi-ODS alloys fabricated by mechanical alloying, HIPing and annealing. These Y-Ti-Ta-O nano-oxides are suggested to be mostly Y2(Ti,Ta)2O7, with a mean size of -7 nm and a number density of -6.8 x 1023 m- 3. They are most likely formed as a consequence of Ta substitution in early-precipitated Y2Ti2O7 nano-oxides. Very a few low nm-sized yttrium borides can be also induced. Besides, excess B can be strongly attractive to both the Fe/Y2Ti2O7 interface and grain boundaries (GBs), which may slightly enhance the adhesion of some low-sigma GBs but slightly weaken the Fe/Y2Ti2O7 interface. Excess Ta can enhance the Fe/Y2Ti2O7 interface, but can be more attractive to GBs, causing serious degradation and even loss of adhesion. Therefore, although Ta+B micro-alloying shows great benefits in refining both grains and nanooxides, their alloying amounts must be restricted to the minimum necessary level.