Understanding microstructure and mechanical properties of (AlTa0.76)xCoCrFeNi2.1 eutectic high entropy alloys via thermo-physical parameters

doi:10.1016/j.jmst.2020.03.045

Abstract

Abstract:

(AlTa_0.76)_xCoCrFeNi_2.1 (x values in molar ratio, x = 0.1, 0.3, 0.5, 0.7, 1.0, and 1.5) alloys were designed to investigate the microstructure and mechanical properties of the eutectic high entropy alloys (EHEAs) consisting of FCC, B2, and Laves phases. Depending on the compositional variatio, clear microstructural variation was observed, as follows: (1) Group 1: FCC dendrite + Laves interdendrite (x = 0.1), (2) Group 2: FCC dendrite + fine-eutectic structure consisting of FCC and Laves phases (x = 0.3, 0.5 and 0.7), (3) Group 3: B2 dendrite + bimodal eutectic structure [FCC/B2 +Laves] (x = 1.0), (4) Group 4: Laves dendrite + eutectic structure consisting of B2 and Laves phases (x = 1.5). As the fraction of Laves or B2 phases increases, yield stress increases from 293 to 2336 MPa, while the plastic strain decreases from 50 % to 2%. Thermo-physical parameters, such as mixing entropy (ΔS_mix), mixing enthalpy (ΔH_mix), valence electron concentration (VEC), and atomic size difference (δ_r), were calculated to understand the microstructural variation. Two criteria (δ_r - VEC and δ_r - ΔH_mix) were utilized to elucidate the formation of the eutectic structures in the present EHEAs, revealing the usefulness of the thermo-physical parameters in the development of EHEAs.

Key words: High-entropy alloy, Eutectic high-entropy alloy, Microstructure, Mechanical properties, Thermo-physical parameter

Min Jung Kim, Gyeol Chan Kang, Sung Hwan Hong, Hae Jin Park, Sang Chul Mun, Gian Song, Ki Buem Kim. Understanding microstructure and mechanical properties of (AlTa_0.76)_xCoCrFeNi_2.1 eutectic high entropy alloys via thermo-physical parameters[J]. J. Mater. Sci. Technol., 2020, 57: 131-137.

Figures/Tables 11

Fig. 1. XRD patterns of as-cast (AlTa0.76)xCoCrFeNi2.1 (x = 0.1, 0.3, 0.5, 0.7, 1.0, and 1.5) alloys.

Fig. 2. Back-scattered electron (BSE) SEM images of as-cast (AlTa0.76)xCoCrFeNi2.1 alloys with x = 0.1 (a), 0.3 (b), 0.5 (c), 0.7 (d), 1.0 (e), and 1.5 (f). Insets of (b), (c) and (d) show secondary-electron (SE) SEM images, and (e) and (f) are BSE SEM images at a higher magnification. The dendrite and interdendrite region are marked in DR and ID, respectively.

Table 1 Chemical composition in dendrite region of as-cast (AlTa0.76)xCoCrFeNi2.1 (x = 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5) alloys (at.%), measured by EDS.

Alloys	Al	Ta	Co	Cr	Fe	Ni
Al_0.1	8.7	6.3	14.2	16.2	16.6	35.7
Al_0.3	5	5	18	17.5	18	36.5
Al_0.5	8.1	6	17	17	17.3	34.6
Al_0.7	13	5	15.9	15.5	16.4	34.2
Al_1.0	25.5	7.4	11.9	8.2	10.2	36.8
Al_1.5	4.6	29.4	15.9	17.7	18.1	14.4

Fig. 3. (a) Bright-field image of the dendrite region and fine-eutectic structure, and (b) coarse-eutectic structure of Al1.0. SAED patterns corresponding to the B2 (c) and FCC (d), and Laves phases (e), respectively.

Table 2 Chemical compositions of the phases in Al1.0 alloy (at.%).

Phase and microstructure		Al	Ta	Co	Cr	Fe	Ni
Dendrite	B2	25.5	7.4	11.9	8.2	10.2	36.8
Fine lamellar	FCC	6.9	3.4	15.7	16.8	20.4	36.8
Fine lamellar	Laves	1.8	26	18.9	16.2	16.4	20.7
Coarse lamellar	B2	25.7	2.8	10.2	3.6	12.7	45
Coarse lamellar	Laves	2.6	33.8	18.2	15.9	14.4	15.1

Table 3 Microstructural attributes and constitutive phases for (AlTa0.76)xCoCrFeNi2.1 alloys.

Alloys	Dendrite phase	Eutectic phases	Alloy groups
Al_0.1	FCC	-	1
Al_0.3	FCC	FCC + Laves	2
Al_0.5	FCC	FCC + Laves	2
Al_0.7	FCC	FCC + Laves	2
Al_1.0	B2	[B2 + Laves] + [FCC + Laves]	3
Al_1.5	Laves	B2 + Laves	4

Fig. 4. Compressive engineering stress-strain curves of as-cast (AlTa0.76)xCoCrFeNi2.1 (x = 0.1, 0.3, 0.5, 0.7, 1.0, and 1.5) alloys at room temperature (at a strain rate: 1 × 10-3 s-1). Note that the Al0.1 and Al0.3 did not fail under the compression strain up to 50 %.

Table 4 Mechanical properties of as-cast (AlTa0.76)xCoCrFeNi2.1 (x = 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5) alloys.

Alloys	σ_0.2 (MPa)	σ_max (MPa)	ε_p (%)
Al_0.1	293	No fracture
Al_0.3	503	No fracture
Al_0.5	974	2646	30.6
Al_0.7	1533	2604	15
Al_1.0	2336	2566	2
Al_1.5	2208	2342	2

Fig. 5. The Relationships between (a) the mixing enthalpy (ΔHmix) and atomic size difference (δr), (b) the valence electron concentration (VEC) and atomic size difference (δr) for the (AlTa0.76)xCoCrFeNi2.1 (x = 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5) alloys, and (c) the table representing typically- reported HEAs with different compositions and microstructures, which are marked by various symbols in (a) and (b). Black circles represent the Alx alloys developed in the present study. Blue square areas represent ranges to form eutectic structures from previous EHEAs, and bright and dark yellow ellipses indicate regimes to form BCC or FCC single-phase HEAs, respectively [32].

Table 5 Calculated values of ΔSmix, ΔHmix, VEC and δr for (AlTa0.76)xCoCrFeNi2.1 alloys according to the Eqs. (1)-(4).

Alloys	ΔS_mix (J k ^-1 mol^-1)	ΔH_mix (kJ mol^-1)	VEC	δ_r (%)
Al_0.1	12.05	-6.02	8.465	2.4
Al_0.3	13.09	-9.63	8.183	3.7
Al_0.5	13.69	-12.6	7.93	4.57
Al_0.7	14.05	-14.66	7.71	5.02
Al_1.0	14.37	-17.61	7.406	5.86
Al_1.5	14.54	-20.67	7.01	6.71

Table 6 Values of VEC and atomic radius among the alloying elements and mixing enthalpies for atomic pairs between elements used in the present study [32,33].

		Al	Ta	Co	Cr	Fe	Ni
VEC		3	5	9	6	8	10
Atomic radius (pm)		118	146	125	128	126	124
ΔH_mix (kJ mol^-1)	Al	?	-19	-19	-10	-11	-22
	Ta	?	?	-24	-7	-16	-29
	Co	?	?	?	-4	-1	0
	Cr	?	?	?	?	-1	-5
	Fe	?	?	?	?	?	-2
	Ni	?	?	?	?	?	?

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