Influence of Al substitution for Sc on thermodynamic properties of HCP high entropy alloy Hf0.25Ti0.25Zr0.25Sc0.25-xAlx from first-principles investigation

HfTiZrSc is a novel hexagonal close-packed refractory high entropy alloys, addition of Al as typical additives significantly enhances the strength and ductility. To reveal the effect of Al addition on thermal properties, we combine the EMTO-CPA ab initio calculations with quasi-harmonic Debye-Gruneisen model to studied the thermodynamic properties of the Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x <= 15 at.%) HEAs. With Al substitution for Sc, the strength of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x <= 15 at.%) is enhanced significantly, despite essentially similar thermal softening trend. Thermal expansion coefficient of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx increase with increasing Al content. Thermodynamic entropy of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x <= 15 at.%) increases dramatically with temperature, the trend is to some degree suppressed with increase of Al content, which is discussed from vibrational, electronic and configuration entropy contribution. The influences of Al content on Debye-temperature and Gruneisen parameter are also further studied. The present investigation is valuable for optimal designing of overall performance of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx alloys.