A comprehensive study on the thermodynamics of the Al₁₃Fe₄ solid solution in the Al-Fe-Mn ternary system

This paper presents investigations on the thermodynamic stability of the Al₁₃Fe₄ solid solution in the Al-Fe-Mn ternary system as well as its crystal chemistry characterisation. In order to carry out this study, the isobaric heat capacity of the solution was measured at high temperature using a high precision three-dimensional probe. The heat capacity of the Al₁₃Fe₄ binary compound measured in this study between 600 K and 1223 K is Cp(T)=23.97+8.18.10⁻²T−7.63.10⁵T². This value is significantly different from other measurements reported in the literature. We show in this paper that our measurements are more accurate than those available in the literature thanks to the use of a more precise sensor (3D-Cp sensor versus planar DSC sensor). As a result, our measurements are more thermodynamically consistent with the enthalpy of formation values of the Al₁₃Fe₄ compound available in the literature, as well as with our own enthalpy of formation measurement performed independently of the heat capacity measurement presented in this paper. Measurements of the enthalpy of formation of the solid solution were also performed by in-situ synthesis in a DSC. These new measurements were complemented by DFT calculations of the enthalpy of formation at 0 K of the solid solution. Our calculations and measurements show that the substitution of Fe by Mn is responsible for a large increase in the solid solution enthalpy of formation. These new experimental and calculated data were used to develop a thermodynamic model of the solution. Finally, we utilized our model to calculate the Fe site occupation factors (sof) of the solid solution structure over its entire chemical composition range (simulating the complete substitution of all Fe atoms with Mn) as a function of temperature. The chemical ordering of the solid solution was thus quantified, revealing its ideal nature near its melting temperature. These results allowed the development of a new SL-model for the solution, both simpler and more reliable than those used in the literature.

Mots clés

Heat capacity measurement; Formation enthalpy measurement; DFT calculations; Crystal chemistry; Thermodynamic modelling; Aluminium alloys