Evaluation of the suitability of Fe₄₀Co₃₀Ni₃₀ as a precursor for Fe-rich FeCoNi-based high-entropy semi-hard magnets

Highlights

• Successfully fabricated Fe₄₀Co₃₀Ni₃₀.

• Alloy powders were micron-sized constituting nanocrystalline γ phase.

• As-fabricated and thermally-treated alloys exhibited semi-hard magnetic behavior.

• Adopted Weighted Euclidean distance-approach to rank Fe-rich FeCoNi precursors.

• Fe₄₀Co₃₀Ni₃₀ is a favorable precursor for fabricating high-entropy semi-hard magnets.

Abstract

We evaluated the suitability of Fe₄₀Co₃₀Ni₃₀ as a precursor for Fe-rich FeCoNi-based high-entropy semi-hard magnets. Mechanically alloying the metal powders 40 at.% Fe, 30 at.% Co, and 30 at.% Ni for 12 h effected micron-size Fe₄₀Co₃₀Ni₃₀ powder comprised of nanocrystalline γ phase. Fe₄₀Co₃₀Ni₃₀ evinced semi-hard magnetic properties at room-temperature—saturation magnetization (M_S) ~146 Am²/kg and intrinsic coercivity (H_CI) ~4.30 kA/m—as well as at below-room-temperatures. The M_S at 0 K, μ_H, and H_CI at 0 K were estimated as ~155 Am²/kg, ~1.60 μ_B, and ~7 kA/m, respectively. Alloy powders after thermal-treatment exhibited tunable semi-hard magnetic behavior at below-room temperatures and room temperature; their magnetic properties were comparable to certain commercial semi-hard magnets. The weighted Euclidean distance-based approach of decision-making proposed the Fe-rich FeCoNi medium-entropy alloy—Fe₄₀Co₃₀Ni₃₀—as a favorable precursor, having an optimal combination of properties, for designing and fabricating novel FeCoNi-based high-entropy semi-hard magnets.

Introduction

The multiple-principle-elements alloys present a remarkable combination of functional, structural, and chemical properties that broaden the scope for developing high-performance magnetic alloys [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. The multiple-principle-elements magnetic alloys are typically FeCoNi-based: prominently have equiatomic ratios of ferromagnetic Fe, Co, and Ni and one or more of the non-ferromagnetic Si, Mn, Cr, and so forth [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. The constituent elements and fabrication techniques significantly impact the structure and magnetics [3,4,[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. The nanostructured multiple-principle-elements magnetic alloys exhibit tunable magnetic properties [7,13,18,[21], [22], [23]]. Mechanical alloying, a far-from-equilibrium and top-down fabrication process, depends on interdiffusion of constituents in the solid-state induced by severe cold working, and it is a widely adopted technique to fabricate nanostructured multiple-principle-elements magnetic alloys [7,[27], [28], [29]].

The multiple-principle-elements alloys are classified based on configurational entropy (ΔS_conf) as low-entropy alloys (ΔS_conf < 8.32 J/mol K), medium-entropy alloys (8.32 J/mol K ≤ ΔS_conf < 12.47 J/mol K), and high-entropy alloys (ΔS_conf ≥ 12.47 J/mol K). ΔS_conf is expressed as [[1], [2], [3], [4], [5], [6]]:

ΔS_conf = - R Σ X_i ln (X_i)

Here R is the ideal gas constant (~8.32 J/mol K), and X_i is the mole fraction of the ith alloying element [[1], [2], [3], [4], [5], [6]]. The FeCoNi-based multiple-principle-elements magnetic alloys constituting non-equiatomic ferromagnetic elements are currently gaining attention [2]. The equiatomic FeCoNi alloy's ΔS_conf is ~9.15 J/mol K, and further alloying increases the ΔS_conf. It is consequential that the ΔS_conf of the alloys constituting non-equiatomic ferromagnetic elements is not only higher than 8.32 J/mol K but also close to 9.15 J/mol K. The Ni-rich, Co-rich, and Fe-rich non-equiatomic FeCoNi compositions Ni₄₀Fe₃₀Co₃₀, Co₄₀Fe₃₀Ni₃₀, and Fe₄₀Co₃₀Ni₃₀, respectively, each have ΔS_conf = 9.08 J/mol K and satisfy the above criteria: 8.32 J/mol K ≤ ΔS_conf ≤ 9.15 J/mol K and ΔS_conf → 9.15 J/mol K [30,31]. In the past [[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]], the structure and magnetism of a multitude of Fe-rich ternary FeCoNi alloys—Fe_xCo_yNi_z (x ≥ y ≥ z)—fabricated by a myriad of techniques (chemical, electrochemical, mechanical, and physical) have been evaluated (Table 1). However, the structure and the magnetics of Fe₄₀Co₃₀Ni₃₀, which has the highest ΔS_conf (9.08 J/mol K) among the Fe-rich FeCoNi alloys, is notably missing.

We investigated the structure and magnetic properties of mechanically alloyed Fe₄₀Co₃₀Ni₃₀. We evaluated the suitability of Fe₄₀Co₃₀Ni₃₀ as a precursor for Fe-rich FeCoNi-based high-entropy semi-hard magnets: (i) Fabricated the Fe₄₀Co₃₀Ni₃₀ powder by mechanically alloying the constituent metal powders Fe-40 at.%, Co-30 at.%, and Ni-30 at.%., (ii) Explored the phase evolution and structure upon mechanical alloying, (iii) Examined the stability of the phase(s) and magnetics of the nanocrystalline alloy—from below-room- to above-room-temperatures, and (iv) Compared the properties of Fe₄₀Co₃₀Ni₃₀ with other Fe-rich FeCoNi medium-entropy alloys. The weighted Euclidean distance-based approach identified Fe₄₀Co₃₀Ni₃₀ as a favorable precursor, among the Fe-rich FeCoNi medium-entropy alloys, having an optimal combination of properties, for designing and fabricating novel FeCoNi-based semi-hard magnetic high-entropy alloys.