Thermal vacancy formation and D03 ordering in nanocrystalline intermetallic (Fe3Si)95Nb5

Abstract

The correlation between thermal vacancy characteristics and D03 ordering in ball-milled nanocrystalline (Fe3Si)95Nb5 was studied by combined measurements of x-ray diffraction and positron annihilation. Structural stabilization due to grain boundary segregation of Nb enables high-temperature positron lifetime measurements up to 1023 K from which vacancy formation parameters identical to those in single-crystalline D03 Fe3Si are deduced. Measurements of coincidence Doppler broadening show that prior to the onset of thermal defect formation in the nanocrystallites positrons are annihilated in Nb-enriched grain boundaries. The D03 ordering of the initially disordered Fe3Si nanocrystaltites is discussed on the basis of the thermal vacancy characteristics and self-diffusion behavior.