Nitrogen-implantation-induced transformation of iron to crystalline Fe16N2 in epitaxial iron films

Abstract

Crystalline phase Fe16 N2 has been found in nitrogen-implanted epitaxial iron films on mirror-polished (100) MgO substrates. Implantations of N+2 at an acceleration voltage of 140 keV transformed the bcc iron into a partially ordered nitrogen martensite with a body-centered-tetragonal (bct) structure. The amount of the ordered part was increased to some extent by an annealing treatment in vacuum at 150 °C for 2 hr. An implantation with a dose of 4×1016 N2/cm2 transformed 60% iron in a film into the bct martensite. The constituent of the ordered phase (Fe16 N2 ) in the bct martensite was 16 wt. %. After the annealing, the constituent increased to 24%. It is proposed that a formation of lower temperature and metastable compound such as Fe16 N2 can be made available through energetic ion implantation.