Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys (x = 0, 0.5, 1.0) fabricated by the powder metallurgy process

doi:10.1016/j.jmst.2020.07.012

Abstract

Abstract:

Light and strong AlxCrNbVMo (x = 0, 0.5, and 1.0) refractory high-entropy alloys (RHEAs) were designed and fabricated via a the powder metallurgical process. The microstructure of the AlxCrNbVMo alloys consisted of a single BCC crystalline structure with a sub-micron grain size of 2-3 μm, and small amounts (< 4 vol.%) of fine oxide dispersoids. This homogeneous microstructure, without chemical segregation or micropores was achieved via high-energy ball milling and spark-plasma sintering. The alloys exhibited superior mechanical properties at 25 and 1000℃ compared to those of other RHEAs. Here, CrNbVMo alloy showed a yield strength of 2743 MPa at room temperature. Surprisingly, the yield strength of the CrNbVMo alloy at 1000℃ was 1513 MPa. The specific yield strength of the CrNbVMo alloy was increased by 27 % and 87 % at 25 and 1000℃, respectively, compared to the AlMo_0.5NbTa_0.5TiZr RHEA, which exhibited so far the highest specific yield strength among the cast RHEAs. The addition of Al to CrNbVMo alloy was advantageous in reducing its reduce density to below 8.0 g/cm³, while the elastic modulus decreased due to the much lower elastic modulus of Al compared to that of the CrNbVMo alloy. Quantitative analysis of the strengthening contributions, showed that the solid solution strengthening, arising from a large misfit effect due to the size and modulus, and the high shear modulus of matrix, was revealed to predominant strengthening mechanism, accounting for over 50 % of the yield strength of the AlxCrNbVMo RHEAs.

Key words: High-entropy alloy, Powder metallurgy, Refractory, Strengthening mechanisms, Oxide dispersoids

Byungchul Kang, Taeyeong Kong, Ho Jin Ryu, Soon Hyung Hong. Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys (x = 0, 0.5, 1.0) fabricated by the powder metallurgy process[J]. J. Mater. Sci. Technol., 2021, 69: 32-41.

Figures/Tables 14

Fig. 1. Equilibrium phase fraction-temperature diagrams of (a) Al0, (b) Al0.5, and (c) Al1.0.

Fig. 2. (a) XRD patterns of the sintered alloys, and magnified patterns of (b) Al0, (c) Al0.5, (d) Al1.0 at 2θ range from 25.0° to 47.5°.

Fig. 3. SEM-BSE image of (a) Al0, (b) Al0.5, (c) Al1.0, STEM image of (d) Al0, (e) Al0.5, (f) Al1.0 and EBSD inverse pole figure mapping images of (g) Al0, (h) Al0.5, (i) Al1.0.

Table 1 EDS analysis results of the matrix and inclusions on the microstructure of the Al0, Al0.5 and Al1.0 RHEAs.

		Al	Cr	Nb	V	Mo	O
Al₀	Nominal	-	25.0	25.0	25.0	25.0	-
	Matrix	-	25.6	24.8	25.4	24.2	-
	Inclusions	-	2.0	39.1	13.0	1.5	44.4
Al_0.5	Nominal	11.2	22.2	22.2	22.2	22.2	-
	Matrix	10.0	22.9	24.8	22.8	19.5	-
	Inclusions	36.5	1.8	4.6	2.7	3.3	51.1
Al_1.0	Nominal	20.0	20.0	20.0	20.0	20.0	-
	Matrix	19.9	20.5	19.3	22.0	18.3	-
	Inclusions	35.8	1.4	3.1	1.9	2.5	55.3

Table 2 Measured density, grain size of BCC matrix, volume fraction and mean size of oxide inclusions in Al0, Al0.5 and Al1.0 RHEAs.

	Density (g/cm³)	Grain size (μm)	Volume fraction of inclusion (%)	Mean size (nm)
Al₀	8.03 (100.2 %)	2.07 ± 1.48	1.71	333 ± 167
Al_0.5	7.53 (100.9 %)	2.09 ± 0.82	3.71	204 ± 123
Al_1.0	7.05 (101.4 %)	2.95 ± 1.33	3.79	169 ± 114

Fig. 4. The TEM-BF images and corresponding SAED and FFT of (a) Al0, (b) Al0.5 and (c) Al1.0.

Fig. 5. Compressive engineering stress and strain curves of the Al0, Al0.5 and Al1.0 at (a) 25℃ and (b) 1000℃.

Fig. 6. Temperature dependence of specific yield strength of the typical BCC RHEAs.

Table 3 Compressive yield strength, specific yield strength and fracture strain of the Al0, Al0.5 and Al1.0 RHEAs at 25℃ and 1000℃.

Temperature (℃℃)		Yield Strength (MPa)	Specific Yield Strength (MPa cm³/g)	Fracture Strain (%)
25	Al₀	2743	341.6	9.9
	Al_0.5	2497	331.6	13.5
	Al_1.0	2326	329.9	18.1
1000	Al₀	1513	188.4	16.4
	Al_0.5	1178	156.4	27.4
	Al_1.0	1085	153.9	> 30

Fig. 7. (a) The STEM image of the Al1.0 after the deformation at 1000℃. (b) FFT of the second phase marked as a yellow circle. (c) EDS mapping at the area marked as red square.

Fig. 8. Bright-field TEM images of Al0.5 RHEA.

Table 4 Density, shear modulus, total misfit effect, solid solution strengthening and phases of Al0, Al0.5 and Al1.0 togethered with typical BCC RHEAs. Shear modulus (G) of the typical BCC RHEAs was estimated by rule of mixture.

Alloys	Density (g/cm³)	G * (GPa)	$\left(\sum_{i} \varepsilon_{i}^{2} c_{i}\right)^{2 / 3}$	Δσ_ss (MPa)	Phases	Ref.
CrNbVMo	8.0	77.5	0.953	1642	BCC	This work
Al_0.5CrNbVMo	7.5	74.3	1.006	1661
Al_1.0CrNbVMo	7.1	71.3	1.026	1625
WNbMoTaV	12.4	87.9	0.620	1212	BCC	[5]
HfNbTaTiZr	9.9	48.7	0.682	738	BCC	[28]
AlCrMoNbTi	6.6	70.0	1.016	1581	BCC	[59]
TiZrNbV	6.5	41.2	1.103	1010	BCC	[60]
Al_0.3NbTa_0.8Ti_1.4V_0.2Zr_1.3	7.7	43.9	0.737	719	BCC	[30]
AlMo_0.5NbTa_0.5TiZr	7.4	48.3	0.690	741	BCC + B2	[30]
CrNbTiZr	6.5	58.4	1.610	2088	BCC + Laves	[61]

Fig. 9. The strengthening contributions to the yield strength of the Al0, Al0.5 and Al1.0.

Table 5 Strengthening contributions to the yield strength of the Al0, Al0.5 and Al1.0 RHEAs.

Alloys	Δσ_gb (MPa)	Δσ_dis (MPa)	Δσ_or (MPa)	Δσ_ss (MPa)	Predicted σ_y by quadratic summation (MPa)	Experimental σ_y (MPa)
Al₀	564	720	42	1642	2558	2743
Al_0.5	561	579	97	1661	2473	2497
Al_1.0	472	360	111	1625	2229	2326

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