Ultra-High Mixing Entropy Alloys with Single bcc, hcp, or fcc Structure in Co–Cr–V–Fe–X (X = Al, Ru, or Ni) Systems Designed with Structure-Dependent Mixing Entropy and Mixing Enthalpy of Constituent Binary Equiatomic Alloys

Abstract

Non-equiatomic high-entropy alloys (HEAs) for which the mixing (S mix ), configuration (S config ), and equivalent ideal (S ideal ) entropies satisfy S mix > S config = S ideal were reported for CoCrVFe(Al, Ru, or Ni) systems. Three Co 20 Cr 20 Fe 20 V 10 X 30 (X = Al, Ru, or Ni) alloys (referred to as Al 30 , Ru 30 , and Ni 30 alloys) were studied here using conventional arc melting and subsequent annealing. The X-ray diffraction profiles revealed that the Al 30 , Ru 30 , and Ni 30 alloys annealed at 1600 K for 1 h exhibited B2 ordered, hcp, and fcc structures, respectively. A single structure was verified by scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy. Thermodynamic calculations of S mix normalized by the gas constant (S mix /R) revealed that Al 30 , Ru 30 , and Ni 30 alloys at 1600 K had S mix /R = 0.833, 1.640, and 1.618, respectively, where the latter two alloys exceeded S config /R = 1.557. A compositionally optimized Alcontaining HEA for S mix with a single bcc structure was computationally predicted and verified experimentally for the Al 6 Co 27 Cr 34 Fe 19 V 14 alloy (Al 6 alloy). The non-equiatomic Al 6 alloy with S config /R = 1.480 exhibited S mix /R of 1.703 at 1600 K, surpassing S config /R = ln 5 = 1.609 for the exact equiatomic (EE) quinary alloy. The bcc Al 6 , hcp Ru 30 , and fcc Ni 30 alloys were regarded as ultra-high mixing entropy alloys (UMHEAs) according to S mix > S config . Structure-dependent S mix and the mixing enthalpy of constituent binary EE alloys are useful for future UHMEAs as a subset of HEAs.

Extended Valence Electron Concentration Analysis for Designing Single-Phase High-Entropy Alloys

“…Another example is achieved by applying high-pressure torsion 8) to a Co 20 Cr 26 Fe 20 Mn 20 Ni 14 alloy that transforms from an fcc to an hcp structure at 77 K. Excluding these exceptions, the constituent elements of hcp-HEAs are mainly limited to 4dor 5d-TMs or lanthanides (Lns), which are necessary elements for obtaining single-phase hcp-HEAs. [9][10][11][12][13][14][15] Currently, the minimum amount of 4d-or 5d-TMs or Lns in hcp-HEAs is as large as 30 at% (Co 20 Cr 20 Fe 20 Ru 30 V 10 ) 11) comprising (3d-TM) 70 Ru 30 .…”

Stabilization of Equiatomic Solutions Due to High-Entropy Effect

“…Based on phase diagrams, the existence of a single solid-solution phase region is prevented by the formation of i) intermetallic compounds, ii) miscibility gaps, or iii) other solid-solution phases. Although Takeuchi et al 8,[11][12][13] examined the stability of solution phases based on the valence electron concentration (VEC), the stability of solution phases with respect to intermetallic compounds and miscibility gaps has not yet been discussed. Therefore, the purpose of this study was to examine the relative stability of single-phase HEAs with FCC or BCC structures in the formation of compounds and miscibility gaps based on the CALPHAD (CALculations of PHAse Diagrams) technique 14) .…”