Disorder–order and structural phase transformations in nanocrystalline Fe 3 − xMn xGe alloys synthesized by mechanical alloying

Abstract

We studied the effect of Mn addition to the binary Fe 3Ge alloy to understand the site substitution, chemical ordering, and phase transformation behavior in the nanocrystalline state. The as-milled Fe 3 − x Mn x Ge ( x=0.0, 0.3, 0.45, 0.6 and 0.75) alloys synthesized in the nanocrystalline state by mechanical alloying were initially formed in a disordered bcc phase (α) although this was not the equilibrium phase. Heat treatments were carried out at 350 and 520 °C to observe the structural evolution to the equilibrium phases by Mössbauer spectroscopic and X-ray diffraction measurements. For the x=0.0 as well as Mn substituted alloys the phase formed was α 1(DO 3) after heat treatment at 350 °C. However, for the Mn alloys there was a segregation of Mn atoms when the ordered phase developed in the nanocrystalline system in contrast to bulk Fe 3X alloys where Mn substitutes at preferential sites in the DO 3 structure up to a large concentration. The α 1(DO 3) → ε′(L1 2) phase transformation in Mn substituted alloys was not only governed by surface energy considerations as observed previously for x=0.0 alloy composition but kinetic effects were also present.