The formation of structural disorder in FeNi-based alloys – Theoretical approach

Abstract

• Positive values of enthalpy of formation of amorphous phase in Fe-Ni system confirmed on the basis of Miedema’s model. • Insignificant influence of Co doping on the enthalpies of formation in FeNi system. • Promotion of amorphous phase and solid solution formation ability depending on Cu substitution. • Cu as promising candidate when aiming destabilization of FeNi crystalline structure. FeNi-based alloys are known to crystallize in bcc or fcc-structures depending on their composition. In many slowly cooled objects, like meteorites, L 1 0 -type phase (tetrataenite) with its characteristic layered Fe/Ni structure has been found. Nevertheless, the synthesis of L 1 0 FeNi phase in industrial amount in a bulk form has not been possible, mainly due to sluggish diffusion and low order/disorder temperature of this phase. We present results of the calculations of formation enthalpies for solid solution and amorphous phase in Fe-Ni system. Additionally, we performed calculations for Co and Cu substitutions (found in meteoritic matter). Our calculations confirm poor glass forming ability of FeNi system and limited influence of Co in this aspect. In contrast, Cu is a promising candidate when aiming at destabilization of FeNi solid solution, as a starting point in L 1 0 phase formation. Formation enthalpy for some Cu-substituted compositions reaches highly negative values. It is expected that at least local chemical segregation should be facilitated by the presence of Cu atoms (attributed mainly to different values of Cu-Co and Cu-Ni interfacial enthalpies).