Thermodynamics of α′(Fe-Rich bcc) + α″(Cr-Rich bcc) → α(bcc) and αpara → αferro Transformations in Fe-20 wt pct Cr Alloy: Drop Calorimetry Study and Elucidation of Magnetic Contribution to Phase Stability

There is considerable uncertainty among diverse assessments of phase equilibrium in Fe-Cr alloys, especially around ( α′ + α″) / α miscibility gap region. This is largely due to the difficulty associated with the rigorous incorporation of the interplay between magnetic and chemical contribution to phase stability, in particular its composition and temperature dependencies through theory, in the absence of reliable experimental data. Toward this cause, accurate enthalpy measurements have been made on homogenized Fe-20 wt pct Cr alloy using inverse drop calorimetry, in the temperature range 298 K to 1473 K (25 °C to 1200 °C). The experiments revealed two distinct phase transformations: (i) at 720 ± 10 K (447 ± 10 °C), the Fe-20Cr alloy transformed from α ′(Fe-rich) + α″ (Cr-rich) two-phase microstructure to α single phase and (ii) at 925 ± 10 K (652 ± 10 °C), the ferromagnetic single-phase α transformed to paramagnetic state. Both these transformations are clearly attested by the measured enthalpy increment variation with temperature. The enthalpy data obtained in this study have been combined with available literature information to forge an integrated theoretical assessment of the energetic aspects of α′ + α″ → α, and α ferro → α para transformations. In addition, a comprehensive evaluation of enthalpy and heat capacity data for Fe-20Cr alloy in the temperature range 0 K to 1473 K (−273 °C to 1200 °C), with explicit incorporation of magnetic contribution has also been made.