Influence of the coexistence of Hf and Ti on phases stability in Fe_(35)Cr_(35)V_(20)Hf_(5)Ti_(5) high entropy alloy

Abstract

A new high-entropy alloy, Cr₃₅Fe₃₅V₂₀Hf₅Ti₅, was designed by applying several theoretical models. The alloy was produced by low-pressure arc-melting, and its microstructure and thermophysical, mechanical, and magnetic properties were characterized. The as-cast microstructure exhibited a body-centered cubic (BCC) phase forming dendrites (D), while the interdendritic (ID) phase exhibited a eutectic-like pattern with a substoichiometric λ-Fe₂Hf structure rich in Fe, Hf, and Ti. Aging and quenching treatments were performed to study the thermal stability of the alloy, revealing decomposition of the λ phase. After aging at 960 °C, the λ phase transforms into Fe₂Ti, accompanied by inhomogeneous Ti segregation at the ID/D interfaces and a limited number of Ti-rich precipitates embedded in the BCC phase. A new cubic FeHf₂ phase was identified after quenching the samples in water from 1000 °C, along with an increase in the precipitation density of Ti-rich particles in the BCC phase together with the formation of small HfC precipitates. The decrease in Vickers hardness of the alloy with thermal treatment, particularly the decrease in nanohardness of dendrites as assessed by nanoindentation maps, demonstrates that alloy strengthening depends more on the presence of oversized atoms (Hf and Ti) in a solid solution than on intermetallic hard precipitates. The as-cast alloy exhibits weak, unsaturated magnetization, and the new phases formed after aging decrease the magnetization and coercive field of the alloy. The material aged at 960 °C presents a thermal conductivity of 11.8 W/K⋅m at RT and thermal expansion coefficient of 11.0 ×10−6 K−1 at 100 °C.