Structural stability of high entropy alloys under pressure and temperature.

The stability of high-entropy alloys (HEAs) is a key issue before their selection for industrial applications. In this study, in-situ high-pressure and high-temperature synchrotron radiation X-ray diffraction experiments have been performed on three typical HEAs Ni₂₀Co₂₀Fe₂₀Mn₂₀Cr₂₀, Hf₂₅Nb₂₅Zr₂₅Ti₂₅, and Re₂₅Ru₂₅Co₂₅Fe₂₅ (at. %), having face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) crystal structures, respectively, up to the pressure of ~80 GPa and temperature of ~1262 K. Under the extreme conditions of the pressure and temperature, all three studied HEAs remain stable up to the maximum pressure and temperatures achieved. For these three types of studied HEAs, the pressure-dependence of the volume can be well described with the third order Birch-Murnaghan equation of state. The bulk modulus and its pressure derivative are found to be 88.3 GPa and 4 for bcc-Hf₂₅Nb₂₅Zr₂₅Ti₂₅, 193.9 GPa and 5.9 for fcc-Ni₂₀Co₂₀Fe₂₀Mn₂₀Cr₂₀, and 304.6 GPa and 3.8 for hcp-Re₂₅Ru₂₅Co₂₅Fe₂₅ HEAs, respectively. The thermal expansion coefficient for the three studied HEAs is found to be in the order as follows: fcc-Ni₂₀Co₂₀Fe₂₀Mn₂₀Cr₂₀>bcc-Hf₂₅Nb₂₅Zr₂₅Ti₂₅ ≈hcp-Re₂₅Ru₂₅Co₂₅Fe₂₅. [ABSTRACT FROM AUTHOR]