Composition Dependence on the Evolution of Nanoeutectic in CoCrFeNiNb_x (0.45 ≤ x ≤ 0.65) High Entropy Alloys

Abstract

The effect of Nb addition in arc-melted CoCrFeNiNb x (0.45 x 0.65) high entropy alloys (HEAs) on the phase evolution, stability, refinement of the microstructure, and mechanical properties are investigated. Minor fluctuation of Nb modifies the microstructure from hypoeutectic (x ¼ 0.45) to eutectic (x ¼ 0.5) and hypereutectic (x ¼ 0.55) containing 134-200 nm thin nanolamellar FCC γ-Ni and HCP Fe 2 Nb-type Laves phases. The nano-eutectic CoCrFeNiNb 0.5 HEA shows high yield strength (2060 AE 5 MPa) and strain hardening up to 2200 AE 10 MPa with 17.0 AE 0.5% compressive plasticity. Transmission electron microscopic studies of partially deformed specimen has been revealed that the activity of dislocations is present in the eutectic FCC/Laves lamellae and at their interface. The stability of the phases in CoCrFeNiNb x and other eutectic HEAs as reported in the literature, has been assessed by estimating mixing entropy (ΔS mix ), mixing enthalpy (ΔH mix ), atomic size differences (δ), valence electron concentration, Pauling electronegativity (Δχ P ), and Allen electronegativity (Δχ A ) to predict the evolution and coexistence of eutectic phases.

Phase Engineering of High‐Entropy Alloys

“…Chanda and Das indicated that ΔH mix was the driving force to form eutectic phases. [86] The mixing enthalpy of CoCrFeNi HEA became more negative after adding Nb with a larger negative mixing enthalpy. The phase structure changed from a single-phase FCC to eutectic FCC and HCP phases.…”

“…Generally, the addition of high mixing‐enthalpy elements into HEAs can make phase composition fluctuate. Chanda and Das indicated that ∆ H mix was the driving force to form eutectic phases 86. The mixing enthalpy of CoCrFeNi HEA became more negative after adding Nb with a larger negative mixing enthalpy.…”

“…T m is the average melting point of all constituent elements. The ratio Omega (Ω) of T m ∆ S mix and |∆ H mix | is often used to evaluate the solid‐solution formation ability, which is more accurate than the individual entropy or enthalpy 86,87…”

“…Specifically, the electronegativity difference (Δχ) is related not only with the lattice type but also with the composition of the HEA. Δχ is defined as follow86 where c i is the atomic percentage and χ i refers to the Pauling electronegativity for the i th component.…”

“…The ratio Omega (Ω) of T m ΔS mix and |ΔH mix | is often used to evaluate the solid-solution formation ability, which is more accurate than the individual entropy or enthalpy. [86,87] The high value of ΔH mix restricts the formation of solidsolution phase. Generally, the addition of high mixing-enthalpy elements into HEAs can make phase composition fluctuate.…”