Effect of Nb Content on Microstructure and Mechanical Properties of Mo0.25V0.25Ti1.5Zr0.5Nbx High-Entropy Alloys

doi:10.1007/s40195-022-01399-2

Abstract

Abstract:

High-entropy alloys (HEAs) have significant application prospects as promising candidate materials for nuclear industry due to their excellent mechanical properties, corrosion resistance and irradiation resistance. In this work, the Mo_0.25V_0.25Ti_1.5Zr_0.5Nb_x (x = 0, 0.25, 0.5, 0.75 and 1.0) HEAs were designed and fabricated. The alloys were prepared by vacuum arc melting, and all the ingots were annealed at 1200 °C for 24 h. The microstructures, crystal structures, hardness and mechanical properties of Mo_0.25V_0.25Ti_1.5Zr_0.5Nb_x HEAs were investigated. The as-cast alloys are composed of single BCC phase. Moreover, the single BCC phase is retained after annealing at 1200 °C for 24 h. The compressive and microhardness tests show that the strength and hardness of the alloys decrease gradually with the increase of Nb content, while the plasticity increases and the fracture mode of the alloy changes from brittle fracture to ductile fracture, which is mainly owing to the decrease of grain size. The addition of Nb significantly improves the plasticity of the Mo_0.25V_0.25Ti_1.5Zr_0.5Nb_x alloys. In particular, Nb1.0 alloy can reach 28.32 % strain without fracture, which exhibits promising potential in industrial application.

Key words: High-entropy alloy, CALPHAD, Microstructure, Hardness, Mechanical properties

Fengqi Zhang, Chao Xiang, En-Hou Han, Zijian Zhang. Effect of Nb Content on Microstructure and Mechanical Properties of Mo_0.25V_0.25Ti_1.5Zr_0.5Nb_x High-Entropy Alloys[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(10): 1641-1652.

Figures/Tables 17

Table 1 Atomic radius (r), melting point (${T}_{\mathrm{m}}$), VEC, thermal neutron absorption cross-section (${\sigma }_{\mathrm{A}}$) and Vickers hardness (${H}_{\mathrm{v}}$) of each element

Element	r (nm)	${T}_{\mathrm{m}}$ (°C )	VEC	${\sigma }_{\mathrm{A}}$(barn)	${H}_{\mathrm{v}}$ (MPa)
Mo	0.136	2620	6	2.48	1530
V	0.132	1890	5	5.08	628
Ti	0.146	1668	4	6.09	970
Zr	0.16	1852	4	0.185	903
Nb	0.143	2468	5	1.15	1320

Table 2 Calculated empirical parameters ${\Delta H}_{\mathrm{mix}}$, $\Delta {S}_{\mathrm{mix}}$, Ω, δ and VEC of Mo0.25V0.25Ti1.5Zr0.5Nbx high-entropy alloys

Alloys	${\Delta H}_{\mathrm{mix}}$ (kJ/mol)	${\Delta S}_{\mathrm{mix}}$ (J/mol·K)	Ω	δ (%)	VEC
Nb0	- 2.24	9.05	8.47	5.66	4.3
Nb0.25	- 1.42	10.76	16.31	5.45	4.36
Nb0.5	- 0.83	11.29	29.85	5.26	4.42
Nb0.75	- 0.40	11.46	63.90	5.09	4.46
Nb1.0	- 0.08	11.44	319.51	4.93	4.5

Fig. 1 a ${\Delta H}_{\mathrm{mix}}$-x, b $\Delta {S}_{\mathrm{mix}}$-x, c Ω-x, d δ-x and e VEC-x plots for the Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75, 1.0) alloy systems

Fig. 2 a Non-equilibrium solidification curves for the Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75, 1.0) alloys. Calculated equilibrium phase diagrams for the b Nb0 alloy, c Nb0.25 alloy, d Nb0.5 alloy, e Nb0.75 alloy and f Nb1.0 alloy

Table 3 Liquidus temperature (Tliq), solidus temperature (Tsol) and decomposition temperature (Tdec), the ratio ((Tsol - Tdec)/Tdec) value, equilibrium phase constitution and volume fraction at 500 °C of Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75 and 1.0) alloys

Alloys	T_liq (°C)	T_sol (°C)	T_dec (°C)	(T_sol - T_dec) /T_sol	Phase constitution and volume fraction
Nb0	1664	1632	557	0.66	BCC#1 (0.70) + HCP (0.30)
Nb0.25	1708	1657	570	0.66	BCC#1(0.47) + BCC#2(0.10) + HCP (0.43)
Nb0.5	1756	1680	648	0.61	BCC#1 (0.35) + BCC#2 (0.18) + HCP (0.47)
Nb0.75	1801	1702	683	0.60	BCC#1 (0.28) + BCC#2 (0.24) + HCP (0.48)
Nb1.0	1841	1723	700	0.59	BCC#1 (0.22) + BCC#2 (0.29) + HCP (0.49)

Fig. 3 a X-ray diffraction patterns of the as-cast Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75 and 1.0) alloys, b the magnified (110) peaks of BCC phase in the as-cast alloys

Fig. 4 SEM back-scatter electron images of the as-cast a Nb0, b Nb0.25, c Nb0.5, d Nb0.75 and e Nb1.0 alloys

Fig. 5 SEM/EDS mapping of Mo, V, Ti, Zr and Nb in the as-cast a Nb0, b Nb0.5 and c Nb1.0 alloys

Table 4 Chemical composition of the as-cast Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75 and 1.0) alloys

Alloy	Region	Elements (at.%)
Alloy	Region	Mo	V	Ti	Zr	Nb
Nb0	Nominal	10.0	10.0	60.0	20.0
	Overall	11.1	10.1	58.5	20.3
	Dendrite	8.6	10.0	57. 5	23.9
	Interdendrite	11.5	9.3	60.7	18.5
Nb0.25	Nominal	9.1	9.1	54.5	18.2	9.1
	Overall	9.2	8.7	53.2	19.3	9.6
	Dendrite	7.0	9.2	51.7	24.5	7.6
	Interdendrite	9.7	8.0	54.5	17.7	10.1
Nb0.5	Nominal	8.3	8.3	50.0	16.7	16.7
	Overall	8.2	8.1	49.1	18.1	16.5
	Dendrite	9.2	7.2	49.1	15.7	18.8
	Interdendrite	8.4	7.7	49.9	18.6	15.4
Nb0.75	Nominal	7.7	7.7	46.1	15.4	23.1
	Overall	7.9	7.8	44.8	17.2	22.3
	Dendrite	9.9	6.1	43.5	14.8	25.7
	Interdendrite	6.4	7.4	46.1	21.3	18.8
Nb1.0	Nominal	7.1	7.1	42.9	14.3	28.6
	Overall	7.4	7.0	42.1	16.2	27.3
	Dendrite	9.1	5.8	40.7	13.6	30.8
	Interdendrite	6.1	7.5	43.3	20.5	22.6

Fig. 6 a X-ray diffraction patterns of Mo0.25V0.25Ti1.5Zr0.5Nbx (x = 0, 0.25, 0.5, 0.75 and 1.0) alloys after homogenization treatment at 1200 °C for 24 h, b the magnified (110) peaks of BCC phase in the annealed alloys

Fig. 7 SEM back-scatter electron images of the annealed a Nb0, b Nb0.25, c Nb0.5, d Nb0.75 and e Nb1.0 alloys

Fig. 8 a Bright field TEM images, b selected area electron diffraction pattern and c elemental mappings of Mo, V, Ti, Zr, and Nb of the annealed Nb0 alloy

Fig. 9 Vickers hardness curve of the annealed Mo0.25V0.25Ti1.5Zr0.5Nbx alloys as a function of Nb contents (x = 0, 0.25, 0.5, 0.75 and 1.0)

Table 5 Vickers hardness of the studied alloys in annealed states (HV)

Nb0	Nb0.25	Nb0.5	Nb0.75	Nb1.0
408.6 ± 6.7	387.2 ± 7.2	346.6 ± 7.3	354.8 ± 5.3	349.9 ± 6.0

Fig. 10 a Room-temperature compressive stress-strain curves, and b yield strength (σ0.2), compressive strength (σp), and fracture strain (εf) of the annealed Mo0.25V0.25Ti1.5Zr0.5Nbx alloys

Table 6 Yield strength (σ0.2), compressive strength (σp), and fracture strain (εf) of the annealed Mo0.25V0.25Ti1.5Zr0.5Nbx alloys

Alloy	σ_0.2 (MPa)	σ_p (MPa)	ε_f (%)
Nb0	1020.04	1244.25	9.88
Nb0.25	1099.05	1473.81	19.57
Nb0.5	1087.55	1470.75	22.23
Nb0.75	1084.84	1416.54	23.63
Nb1.0	905.05	1307.26	28.32

Fig. 11 Fracture morphologies of the annealed a, b Nb0, c, d Nb0.25, e, f Nb0.5, g, h Nb0.75 and i, j Nb1.0 alloys

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