Phase Selection and Microhardness of Directionally Solidified AlCoCrFeNi2.1 Eutectic High-Entropy Alloy

doi:10.1007/s40195-021-01367-2

Abstract

Abstract:

In the present work, the solidification behaviors and microhardness of directionally solidified AlCoCrFeNi_2.1 eutectic high-entropy alloy (EHEA) obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi_2.1 EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures, indicating that the actual composition of the alloy slightly deviates from the eutectic point. However, it is interesting to observe that the full lamellar structure of this alloy is obtained through directional solidification. In order to explain this phenomenon, the maximum interface temperature criterion and the interface response function (IRF) theory are applied to calculate the velocity range of the transition from the primary phase to the eutectic, which is 1.2–2 × 10⁴ μm/s. Furthermore, microhardness is one of the important parameters to measure the mechanical properties of materials. Therefore, the microhardness test is performed, and the test result indicates that the microhardness (HV) increased with increasing growth velocity (V) or decreased with increasing lamellar spacing (λ). The dependences of λ and HV on V are determined by using a linear regression analysis. The relationships between the λ, V and HV are given as: $\lambda = 11.62V^{ - 0.48}$, HV = 305.5V 0.02and HV = 328.1λ^-^0.04, respectively. The microhardness of the AlCoCrFeNi_2.1 EHEA increases from 312.38 HV to 329.54 HV with the increase in growth velocity (5–200 μm/s). Thus, directional solidification is an effective method to improve the mechanical properties of alloys.

Key words: Directional solidification, Eutectic high-entropy alloy, Phase selection, Microstructure, Microhardness

Peng Peng, Shengyuan Li, Weiqi Chen, Yuanli Xu, Xudong Zhang, Zhikun Ma, Jiatai Wang. Phase Selection and Microhardness of Directionally Solidified AlCoCrFeNi_2.1 Eutectic High-Entropy Alloy[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(8): 1281-1290.

Figures/Tables 12

Fig. 1 OM micrograph of a sample after Vickers hardness test

Fig. 2 SEM images of microstructures of as-cast AlCoCrFeNi2.1 EHEA: a Bulky dendrites region; b Lamellar structures region; c Enlarged image marked in (b); d Enlarged image marked in b

Fig. 3 EDS mapping of as-cast AlCoCrFeNi2.1 EHEA: a Bulky dendrites region, elemental mapping for Al, Cr, Fe, Co and Ni, respectively; b Lamellar structures region, elemental mapping for Al, Cr, Fe, Co and Ni, respectively

Fig. 4 XRD patterns of as-cast and directionally solidified AlCoCrFeNi2.1 EHEA

Fig. 5 Dark-field TEM images of AlCoCrFeNi2.1 EHEA and corresponding selected-area electron diffraction and EDS mappings: a Bright field TEM image of as-cast AlCoCrFeNi2.1 EHEA; b SAED pattern of CoCrFe-rich phase (marked A by red color in Fig. 5(a)); c SAED pattern of NiAl-rich phase (marked B by yellow color in Fig. 5(a)); d EDS mapping of AlCoCrFeNi2.1 EHEA

Fig. 6 Solid-liquid interface morphologies of AlCoCrFeNi2.1 EHEA at different growth velocities: a 5 μm/s, b 10 μm/s, c 50 μm/s, d 100 μm/s, e 200 μm/s

Fig. 7 Steady-state longitudinal microstructure of directionally solidified AlCoCrFeNi2.1 EHAE grown at different growth velocities: a 5 μm/s, b 10 μm/s, c 50 μm/s, d 100 μm/s, e 200 μm/s

Table 1 Relevant physical parameters

Parameters	Eutectic	NiAl phase	Ref.
T_f (K)	1673	1911	[37]
C₀ (at.%)	48.53	15.24	[37]
ΔS_f (J mol^-1 K^-1)	8.12	8.12	[33]
a₀ (m)	2 × 10^-9	2 × 10^-9	[31]
$\varGamma$₍_mK₎	1.7 × 10^-7	3.8 × 10^-7	[49]
D_l (m² s^-1)	7.84 × 10^-9	6.44 × 10^-9	[49]
V₀ (m s^-1)	500	2338	[33]
R_g (J mol^-1 K^-1)	8.314	8.314

Fig. 8 Determination of the leading phase during directional solidification of AlCoCrFeNi2.1 EHAE through the comparison between the growth interface temperature of the eutectic and the primary phase by the maximum interface temperature criterion

Fig. 9 Interlamellar spacing at different growth velocities

Fig. 10 Microhardness of the AlCoCrFeNi2.1 EHEA: dependences of the HV on a Growth velocity and b Lamellar spacing for directionally solidified AlCoCrFeNi2.1 EHEA

Table 2 Values of lamellar spacings (λ), growth velocity (V) and microhardness (HV) for AlCoCrFeNi2.1 EHEAs

Growth velocity (μm/s)	Lamellar spacing (λ)	Microhardness values (HV)
5	5.31 ± 0.22	312.38 ± 1.0
10	3.79 ± 0.22	315.25 ± 1.2
50	1.73 ± 0.21	320.81 ± 2.0
100	1.24 ± 0.20	325.64 ± 1.0
200	0.88 ± 0.18	329.54 ± 1.8

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