Formation and significance of topologically close-packed Laves phases in refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) inherently have a high potential to form topologically close-packed (TCP) Laves phases. Phase engineering to control the amount and type of TCP-Laves phases plays an important role in physical and mechanical properties of RHEAs. The present investigation addressed key parameters to TCP-Laves phase formation in RHEAs by considering thermodynamic calculations and empirical relations. Two novel TiVCrZrCo and TiVCrZrFe RHEAs were designed and fabricated as alloy models with high melting points (>2000 K) and relatively low density (∼6.5 g.cm−3) with multiphase microstructure including BCC matrix together with C14 and C15 Laves phases. Existence of atoms such as V, Co, Cr and Fe together with Zr, Ti, Nb and Hf refractory elements promote TCP-Laves phase formation. Very negative values for mixing enthalpy of atom pairs (ΔHmix) and remarkable differences between atomic sizes of the constitutive elements (δ) are key factors in the Laves phase formation in multicomponent alloys. The parameters including ΔHmix and δ must be in the defined range of −20 ≤ ΔHmix ≤ −3 kJ.mol−1 and 4 <δ < 10 to appear Laves phase in the microstructures of HEAs. TCP-Laves phase formation led to increase and decrease in hardness and fracture toughness, respectively. The understanding of key factors in RHEAs design can lead to phase engineering and the fabrication alloys with a desirable amount of TCP-Laves phase, which are suited for hydrogen storage applications or for applications in harsh operating environments exposed to high temperatures, irradiations, wear and erosion.