Enthalpies of formation of high entropy and multicomponent alloys using oxide melt solution calorimetry

Understanding the thermodynamics of multi-principal element alloys (MPEA), also known as high entropy alloys (HEA) is crucial to addressing their synthesis, properties, stability, materials compatibility, and technological applications. While the thermodynamics of binary and ternary alloys are relatively well investigated both experimentally and theoretically in the last five decades, this is not the case for multicomponent (>4 elements) alloys. The classical calorimetric methods (e.g., liquid metal bath, direct reaction, and flux calorimeter) used for binary and trinary alloys will often encounter difficulties for alloys with more than three elements. Thus, a more flexible and general calorimetric approach is needed. The present work introduces a new methodology for measuring the enthalpy of formation from elements of metals based on high-temperature oxidative calorimetry. This technique is based on the well-developed oxide melt solution calorimetry (OMSC). Using this method, the heats of formation of four binary (AlNi, AlFe, AlCo, and AlFe3), four quaternary CrFeCoNi, AlCrCoNi, AlFeCoNi, AlCrFeCo and four quinary, AlCrFeCoNi, Al0.3CrFeCoNi, CrMnFeCoNi and AlTiVNbTa alloys were obtained. The values in kJ/(mol atom) are: AlNi, 60.11 +/- 3.94; AlFe, 23.46 +/- 2.39; AlCo, -51.80 +/- 1.89; AlFe, -13.72 +/- 2.39; CrFeCoNi, -3.56 +/- 1.01; AlCrCoNi, -15.30 +/- 2.69; AlFeCoNi, -28.82 +/- 1.32; AlCrFeCo, -14.27 +/- 2.17; AlCrFeCoNi, -20.97 +/- 4.17; Al0.3CrFeCoNi, -8.30 +/- 2.56; CrMnFeCoNi, -3.27 +/- 2.97; AlTiVNbTa, -11.93 +/- 4.80. The results are compared with predicted values from the Miedema model and with available experimental data. The methodology is thus established to be generally useful for determining the thermodynamic properties of MPEAs.