Heat capacity of Fe-Al intermetallics: B2-FeAl, FeAl2, Fe2Al5 and Fe4Al13

The heat capacity of three Fe-rich bcc-based Fe-Al alloys (35, 40 and 45 at.% Al) and of the three Al-rich intermetallics ζ (FeAl2), η (Fe2Al5) and θ (Fe4Al13) were measured by DSC between 120 K and their respective melting points. For the Fe-rich bcc-based alloys, the heat capacity was measured in the temperature range of the A2/B2 transition which reaches a maximum value of 54.7 and 86.3J/K⋅mol for the samples with compositions of 35 and 40 at.% Al close to the order/disorder transition temperature of 1456 and 1555 K, respectively. The Debye temperature and the standard entropy values of the Al-rich intermetallic phases ζ (ΘD=455K, S298.15=24.45J/K⋅mol), η (ΘD=484K, S298.15=23.00J/K⋅mol) and θ (ΘD=469K, S298.15=23.65J/K⋅mol) were estimated based on low temperature cp values. All three intermetallics and the 45 at.% Al sample show a non-linear increase in cp by approaching the melting point which probably originates from the creation of thermal vacancies in the respective intermetallic phase. The enthalpy of formation of vacancies within the three phases ζ (Hf=156±4kJ/mol), η (Hf=181±3kJ/mol) and θ (Hf=95±2kJ/mol) was estimated based on the results of the high temperature cp measurements.