Soft-Magnetic High-Entropy AlCoFeMnNi Alloys with Dual-Phase Microstructures Induced by Annealing

doi:10.1007/s40195-020-01086-0

摘要/Abstract

Abstract:

The simultaneous enhancement of magnetic and mechanical properties is desirable but challenging for soft-magnetic materials. A fabrication strategy to meet this requirement is therefore in high demand. Herein, bulk equiatomic dual-phase AlCoFeMnNi high-entropy alloys were fabricated via a magnetic levitation induction melting and casting process followed by annealing at 700-1000 °C, and their microstructures as well as mechanical and magnetic properties were investigated. The as-cast alloy possessed a single metastable B2-ordered solid solution that decomposed upon annealing into a dual-phase structure comprising an Al- and Ni-rich body-centered cubic (BCC) matrix and Fe- and Mn-rich face-centered cubic (FCC) precipitates both in the grain interior and along the grain boundaries. The magnetic and mechanical properties were closely related to the relative volume fraction of FCC in the alloy. The FCC volume fraction could be increased by increasing the annealing temperature, thereby offering tunable properties. The optimal annealing temperature for balanced magnetic and mechanical properties was found to be 800 °C. The alloy annealed at this temperature had an average BCC grain size of 12 ± 3 μm and FCC volume fraction of 41 ± 4%. Correspondingly, the saturation magnetization and coercivity reached 82.57 Am²/kg and 433 A/m, respectively. The compressive yield strength and fracture strength were 1022 and 2539 MPa, respectively, and the plasticity was 33%. Owing to its adjustable microstructure and properties, the AlCoFeMnNi alloy has potential for use as a multi-functional soft-magnetic material.

Key words: High-entropy alloy, Microstructure, Phase transformation, Soft-magnetic material, Precipitation

. [J]. 金属学报英文版, 2020, 33(8): 1124-1134.

Chengbo Yang, Jing Zhang, Meng Li, Xuejian Liu. Soft-Magnetic High-Entropy AlCoFeMnNi Alloys with Dual-Phase Microstructures Induced by Annealing[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(8): 1124-1134.

图/表 14

Fig. 1 Ingots prepared by magnetic levitation induction melting and casting

Fig. 2 XRD patterns of as-cast and annealed AlCoFeMnNi alloys

Fig. 3 TEM image of the as-cast AlCoFeMnNi alloy. The inset is the SAED pattern of the B2 phase recorded along the [001] zone

Fig. 4 TEM images of AlCoFeNiMn alloy annealed at 700 °C: a DFTEM image showing micron-sized grains and precipitates; b SAED pattern of the FCC precipitate and c SAED pattern of the BCC matrix

Fig. 5 TEM images of AlCoFeNiMn alloy annealed at 800 °C: a DFTEM image showing micron-sized grains and precipitates; b SAED pattern of the FCC precipitate and c SAED pattern of the BCC matrix

Fig. 6 STEM-HAADF image (top left) and STEM-EDS elemental maps of alloy annealed at 700 °C

Fig. 7 STEM-HAADF image (top left) and STEM-EDS elemental maps of alloy sample annealed at 800 °C

Fig. 8 HRTEM image showing an area of the matrix close to a precipitate (bottom right) in the alloy sample annealed at 700 °C. The insets show the fast Fourier transform (FFT) patterns of the BCC matrix and FCC precipitate, as well as an inverse FFT (IFFT) image of the matrix. The FFT pattern inset in the IFFT image shows B2 superlattice reflection spots

Fig. 9 SEM images of the AlCoFeMnNi alloys in a as-cast condition and after annealing at b 700 °C, c 800 °C, d 900 °C, e 1000 °C for 10 h. f Variation in the average grain size of the matrix and the volume fraction of the FCC precipitates with annealing temperature

Table 1 EDS analysis resultsa of the matrix and precipitates in the as-cast and annealed alloys

Sample	Phase	Fe (at.%)	Co (at.%)	Ni (at.%)	Al (at.%)	Mn (at.%)
As-cast	Matrix (B2)	20.72 ± 0.1	20.14 ± 0.2	20.07 ± 0.2	19.13 ± 0.3	19.94 ± 0.5
HEA-700	Matrix (BCC)	12.33 ± 0.3	21.25 ± 0.2	24.77 ± 0.2	24.26 ± 0.5	17.39 ± 0.8
HEA-700	Precipitates (FCC)	27.61 ± 0.3	19.11 ± 0.3	16.66 ± 0.2	9.97 ± 0.2	26.65 ± 0.3
HEA-800	Matrix (BCC)	12.33 ± 0.4	20.30 ± 0.4	23.71 ± 0.3	25.43 ± 0.1	18.23 ± 0.3
HEA-800	Precipitates (FCC)	31.92 ± 0.3	20.04 ± 0.3	13.36 ± 0.3	10.35 ± 0.3	24.33 ± 0.3
HEA-900	Matrix (BCC)	11.88 ± 0.1	20.19 ± 0.3	24.10 ± 0.5	25.65 ± 0.2	18.18 ± 0.3
HEA-900	Precipitates (FCC)	27.81 ± 0.4	21.03 ± 0.2	16.08 ± 0.3	9.65 ± 0.1	25.51 ± 0.1
HEA-1000	Matrix (BCC)	12.55 ± 0.3	21.17 ± 0.3	25.37 ± 0.5	24.18 ± 0.2	16.73 ± 0.1
HEA-1000	Precipitates (FCC)	36.15 ± 0.3	22.12 ± 0.3	10.03 ± 0.5	3.98 ± 0.2	27.72 ± 0.1

Fig. 10 Magnetization curves of the as-cast and annealed alloys; the inset shows an enlarged view of the curve within the dotted box

Table 2 Magnetic and compressive properties of AlCoFeMnNi alloys at 27 °C

Sample	Magnetic properties		Compressive properties
Sample	M ^a_s (Am²/kg)	H ^b_c (A/m)	σ ^c_y (MPa)	σ ^d_max (MPa)	? ^f_p (%)
As-cast	117.56	576	1365	1694	15
HEA-700	86.73	501	1186	2105	23
HEA-800	82.57	433	1022	2539	33
HEA-900	86.92	505	915	2528	33
HEA-1000	99.02	527	998	2321	31

Table 3 Comparison of the soft-magnetic properties of as-cast high-entropy alloys

Alloy	Structure	M ^a_s (Am²/kg)	H ^b_c (A/m)	References
FeCoNiMn_0.25Al_0.25	FCC	268	101	[28]
FeCoNiMnAl	BCC + B2	147.86	629	[27]
FeCoNiMnGa	FCC	80.43	915	[27]
FeCoNiMn	FCC	18.14	119	[27]
FeCoNiCr	FCC	13.95	1252.1	[37]
FeCoNiCrCuAl	FCC + BCC	46	3582	[38]
FeCoNiCrAl	BCC	64	1273	[33]
FeCoNiCrAl_1.25	BCC	43.05	1416	[34]
FeCoNiMnSn	FCC	80	3435	[27]
FeCoNiMnCr	FCC	1.39	10,804	[39]
FeCoNiAl_0.2Si_0.2	FCC	131	1508	[40]
FeCoNiAlCu	FCC	84	12,892	[19]
Fe₂₉Ni₂₉Co₂₈Cu₇Ti₇	FCC + BCC	111.54	266.65	[17]

Fig. 11 Compressive engineering stress (σ)-strain (ε) curves of the as-cast and annealed HEAs

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