Optimizing mechanical and magnetic properties of AlCoCrFeNi high-entropy alloy via FCC to BCC phase transformation

doi:10.1016/j.jmst.2020.12.080

Abstract

Abstract:

FCC, BCC and B2 phases, the most common phases in high-entropy alloys (HEAs), are widely investigated to tailor their mechanical and magnetic performances. The detailed investigation of FCC to BCC/B2 phase transformation of AlCoCrFeNi HEA in this paper reveals its evolution and structure-properties relations in terms of both temperature and holding duration. With increasing heat treatment temperature and duration, such transition will progress simultaneously at both the dendric core (DC) region and inter-dendric (ID) region and the volume of phase transformation from FCC to BCC phases is greater than FCC to B2 phases, resulting in increased yield strength and saturation magnetization. The obvious phase transition of the AlCoCrFeNi HEA at 1200 °C can enhance its yield strength and saturation magnetization as a sacrifice of its fracture strain. However, an excellent combination of mechanical-magnetic properties was achieved when heat-treated at 1100 °C for 50 h by optimizing both the transformation and the size of B2 phases. Our present study could pave ways to design the HEAs or other alloys with an optimum combination of mechanical and magnetic properties for application-oriented viewpoints.

Key words: High-entropy alloy, Phase transformation, Mechanical property, Magnetic property

Chendong Zhao, Jinshan Li, Yudong Liu, Xiao Ma, Yujie Jin, William Yi Wang, Hongchao Kou, Jun Wang. Optimizing mechanical and magnetic properties of AlCoCrFeNi high-entropy alloy via FCC to BCC phase transformation[J]. J. Mater. Sci. Technol., 2021, 86: 117-126.

Figures/Tables 13

Fig. 1. Back-scattered electron image illustrating (a) the microstructures of as-cast AlCoCrFeNi HEA. (b) and (c) are the ID and DC region at higher magnification, respectively. (d) EBSD phase map shows the distribution of FCC, BCC and σ phases in the ID region.

Fig. 2. XRD patterns of AlCoCrFeNi HEA at the as-cast state and different heat treatment conditions.

Fig. 3. Back-scattered electron image (a-e) and EBSD phase maps (a1-e1) illustrating the microstructures and phase distributions of AlCoCrFeNi HEA under different heat treatment conditions: (a, a1) 1000 °C - 10 h, (b, b1) 1100 °C - 10 h, (c, c1) 1200 °C - 10 h, (d, d1) 1100 °C - 50 h, (e, e1) 1200 °C - 50 h. The green and red colour in EBSD phase maps denote FCC and BCC phases, respectively.

Fig. 4. Back-scattered electron image illustrating the microstructures of AlCoCrFeNi HEA at high magnification under different heat treatment conditions: (a) 1100 °C - 10 h, (b) 1200 °C - 10 h, (c) 1100 °C - 50 h, (d) 1200 °C - 50 h.

Fig. 5. EBSD analyses of AlCoCrFeNi HEA heat-treated at 1200 °C for 10 h. (a) macro EBSD phase map and (b) its corresponding SEM image. The magnification SEM image of (c): region Ⅰ in (b) and (d): region Ⅱ in (b), the inset shows their corresponding EBSD phase maps.

Fig. 6. (a) Back-scattered electron image of DC region at microscale of the AlCoCrFeNi HEA heat-treated at 1200 °C for 10 h. The TEM lamella (marked by the red dotted line) was then lifted out by a manipulator and thinned to become electron-transparent shown in (b).

Fig. 7. (a) HADDF-STEM image and EDS mappings of the TEM lamella. (b) HADDF-STEM image of region Ⅰ (marked as the orange square in (a)), the inset in the HADDF image shows the SADP along [011] zone axis in FCC phase.

Table 1 Compositions of the FCC and BCC phases (except B2 phases) in atomic percent from TEM at 1200 °C-10 h condition.

Region	Al/%	Co/%	Cr/%	Fe/%	Ni/%
FCC	7.45	24.08	25.50	25.91	17.06
BCC-1	2.38	20.98	41.79	28.98	5.87
BCC-2	2.49	20.91	40.43	29.65	6.52

Fig. 8. (a) Micro-diffraction pattern from [001]BCC ZA. (b) Micro-diffraction pattern from [001]B2 ZA. (c) Dark field image made from the {110} superlattice spot highlighted by the yellow circle in (b) showing the lighted-up B2 precipitates in region Ⅱ (marked by the red square in Fig. 7(a)).

Fig. 9. Engineering compressive stress-strain curves of AlCoCrFeNi HEA at different heat treatment conditions compressed under room temperature.

Table 2 Mechanical and magnetic properties of AlCoCrFeNi HEA at the as-cast state and different heat treatment conditions.

Alloy state	Compressive yield strength (MPa)	Ultimate compressive strength (MPa)	Fracture strain (%)	Magnetization at 2 T and 298 K (emu/g)
1000 °C-10h	910 ± 13	3015 ± 25	38 ± 2	19.64
1100 °C-10h	931 ± 5	2940 ± 53	37 ± 1	29.42
1100 °C-50 h	977 ± 13	2982 ± 62	36 ± 2	34.24
1200 °C-10h	1148 ± 21	2606 ± 24	28 ± 2	48.56
1200 °C-50 h	1203 ± 21	2400 ± 45	22 ± 1	51.28

Fig. 10. AFM height images (measured at 298 K) of the AlCoCrFeNi HEA heat-treated at 1200 °C for 10 h: (a) ID region. (b) DC region. The corresponding phase images (magnetic domains) of the ID region and DC region are shown in (c) and (d), respectively.

Fig. 11. M-H curves (range from -20,000 Oe to 20,000 Oe, measured at 298 K) of AlCoCrFeNi HEA at different heat treatment conditions.

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