Microstructure, Mechanical Properties, and Corrosion Behavior of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi High-Entropy Alloys

doi:10.1007/s40195-019-00935-x

Abstract

Abstract:

The development of high-entropy alloys (HEAs) has stimulated an ever-increasing interest from both academia and industries. In this work, three novel MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi HEAs containing low thermal neutron absorption cross section elements were prepared by vacuum arc melting. The microstructure, mechanical properties, and corrosion behaviors were investigated. A dominant body-centered cubic (BCC) phase was present in all these three HEAs. In addition, an ordered Laves phase was found to be another major phase in both MoNbFeCrV and MoNbFeCrTi alloys, whereas an ordered BCC (B2) phase was observed in the MoNbFeVTi alloy. The phase formation in these three alloys was discussed. It is found that the formation of the secondary phase in these alloys is mainly ascribed to the large atomic size difference and electronegativity difference. All the three HEAs show high hardness, high yield strength but limited plasticity. Moreover, the MoNbFeCrV, MoNbFeCrTi and MoNbFeVTi alloys exhibit excellent corrosion resistance in both deaerated 1 mol/L NaCl and 0.5 mol/L H₂SO₄ solutions at room temperature. However, further composition adjustment and/or thermomechanical processing is required to enhance the mechanical properties of the three alloys.

Key words: High-entropy alloy, Crystal structure, Microstructure, CALPHAD, Mechanical property, Corrosion

Chao Xiang, Zhi-Ming Zhang, Hua-Meng Fu, En-Hou Han, Jian-Qiu Wang, Hai-Feng Zhang, Guo-Dong Hu. Microstructure, Mechanical Properties, and Corrosion Behavior of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi High-Entropy Alloys[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(9): 1053-1064.

Figures/Tables 18

Fig. 1 X-ray diffraction patterns of a MoNbFeCrV, b MoNbFeCrTi, c MoNbFeVTi high-entropy alloys

Fig. 2 a-c SEM backscatter electron images of the as-cast MoNbFeCrV alloy, d-h elemental mappings of Mo, Nb, Fe, Cr, and V

Table 1 Chemical compositions (at.%) of the as-cast MoNbFeCrV alloy determined by EDS

Region (phase)	Mo	Nb	Fe	Cr	V
Overall	19.9	21.0	20.5	18.7	19.9
BCC	31.0	21.3	11.0	16.0	20.7
Laves	11.2	24.1	30.2	19.8	14.7
Transition layer	26.1	17.8	13.7	18.8	23.6
Sigma	17.5	8.7	21.8	21.7	30.3

Fig. 3 a Equilibrium phase diagram for MoNbFeCrV alloy, b non-equilibrium solidification curve for MoNbFeCrV alloy using Scheil-Gulliver models, c composition of liquid phase, dcomposition of BCC phase, e composition of Laves phase, f composition of Sigma phase

Table 2 Predicted composition for MoNbFeCrV alloy at 800 °C and 1000 °C using TTNI8 database

Temperature (°C)	Phase	Mo	Nb	Fe	Cr	V
800	BCC#1	43.6	29.8	1.1	7.6	17.8
	Laves	5.9	27.4	39.5	27.2	0
	Sigma	6.5	0	33.6	15.8	44.1
	BCC#2	6.7	2.3	5.5	44.3	41.2
1000	BCC#1	31.0	21.9	5.0	16.7	25.4
	Laves	6.5	26.8	39.8	26.9	0
	Sigma	7.2	0	34.7	18.2	39.8

Fig. 4 a-c SEM backscatter electron images of the as-cast MoNbFeCrTi alloy, d-h elemental mappings of Mo, Nb, Fe, Cr, and Ti

Table 3 Chemical compositions (at.%) of the as-cast MoNbFeCrTi alloy determined by EDS

Region (phase)	Mo	Nb	Fe	Cr	Ti
Overall	19.9	20.7	20.1	19.3	20.0
White regions (BCC)	34.9	26.2	8.5	14.4	16.0
Gray regions (Laves)	8.6	18.1	29.6	28.0	15.7
Dark regions (B2)	3.8	6.2	28.8	11.7	49.5

Fig. 5 a Equilibrium phase diagram for MoNbFeCrTi alloy, b non-equilibrium solidification curve for MoNbFeCrTi alloy using Scheil-Gulliver models, c composition of BCC (B2) phase 3.2.3 MoNbFeVTi The MoNbFeVTi alloy (Fig. 6a-c) consists of three distinct phases with the chemical composition given in Table 4. This alloy also exhibits a dendritic microstructure. Elemental mappings, as shown in Fig. 6d-h, indicate that the bright dendrites are enriched with Mo and Nb, but depleted of Fe, V, and Ti. The gray phase in the interdendritic regions is rich in Fe, V, and Ti. Then it is seen that the minor dark phase (denoted as Ti-rich phase in Fig. 6b) is heavily enriched with Fe and Ti. The volume fractions of the dominant bright dendrites, the (Fe, V, Ti)-rich gray phase, and the minority dark (Fe, Ti)-rich phase are about 60.6%, 38.1%, and 1.3%, respectively.

Fig. 6 a-c SEM backscatter electron images of the as-cast MoNbFeVTi alloy, d-h elemental

Table 4 Chemical compositions (at.%) of the as-cast MoNbFeVTi alloy determined by EDS

Region (phase)	Mo	Nb	Fe	V	Ti
Overall	19.3	19.3	21.4	19.4	20.6
White regions (BCC)	31.3	24.5	9.0	18.9	16.3
Gray regions (B2)	4.8	16.8	39.9	18.6	19.9
Dark regions (B2)	2.7	6.7	36.5	12.0	42.1

Fig. 7 a Equilibrium phase diagram for MoNbFeVTi alloy, b non-equilibrium solidification curve for MoNbFeVTi alloy using Scheil-Gulliver models

Table 5 Calculated enthalpy of mixing (ΔHmix), the entropy of mixing (ΔSmix), atomic size difference (δ), Ω, γ, Λ, and electronegativity difference (Δχ, calculation based on Allen electronegativity)

Alloy	ΔH_mix (kJ/mol)	ΔS_mix (J/(mol K))	δ (%)	Ω	γ	Λ	Δχ_Allen (%)
MoNbFeCrV	-?6.72	13.38	5.36	4.71	1.167	0.47	8.84
MoNbFeCrTi	-?9.28	13.38	6.69	3.34	1.192	0.30	10.34
MoNbFeVTi	-?8.48	13.38	5.77	3.65	1.191	0.40	9.89

Fig. 8 a Compressive engineering stress-strain curves of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys and fracture surfaces of b MoNbFeCrV alloy, c MoNbFeCrTi alloy, dMoNbFeVTi alloy

Table 6 The 0.2% offset yield strength (σ0.2), compressive strength (σp), and fracture strain (εf) of the MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys

Alloy	Yield stress, σ_0.2 (MPa)	Compressive stress, σ_p (MPa)	Fracture strain, ε_f (%)
MoNbFeCrV	2663	2688	2.7
MoNbFeCrTi	1647	1811	0.9
MoNbFeVTi	1707	1729	1.7

Fig. 9 a Potentiodynamic polarization curves of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys in deaerated 1 mol/L NaCl solution and SEM images of b MoNbFeCrV, cMoNbFeCrTi, d MoNbFeVTi alloys after electrochemical test

Table 7 Electrochemical parameters of the MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys in deaerated 1 mol/L NaCl solution

Alloy	E_corr (mV_SCE)	i_corr (μA/cm²)	i_pass (μA/cm²)
MoNbFeCrV	-?671	0.07	5.01
MoNbFeCrTi	-?477	0.38	4.35
MoNbFeVTi	-?457	0.33	6.26

Fig. 10 a Potentiodynamic polarization curves of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys in deaerated 0.5 mol/L H2SO4 solution and SEM images of b MoNbFeCrV, cMoNbFeCrTi, d MoNbFeVTi alloys after the electrochemical test

Table 8 Electrochemical parameters of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi alloys in deaerated 0.5 mol/L H2SO4 solution

Alloy	E_corr (mV_SCE)	i_corr (μA/cm²)	i_pass (μA/cm²)	E_b (mV_SCE)
MoNbFeCrV	-?275	0.91	14.09	969
MoNbFeCrTi	-?258	1.35	7.86	-
MoNbFeVTi	-?285	0.70	12.77	-

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