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J. Mater. Sci. Technol.  2019, Vol. 35 Issue (11): 2600-2607    DOI: 10.1016/j.jmst.2019.07.013
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Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering
Liu Qing, Wang Guofeng*(), Sui Xiaochong, Liu Yongkang, Li Xiao, Yang Jianlei
National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150006, China
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The MoNbTaTiV refractory high-entropy alloy (RHEA) with ultra-fine grains and homogeneous microstructure was successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The microstructural evolutions, mechanical properties and strengthening mechanisms of the alloys were systematically investigated. The nanocrystalline mechanically alloyed powders with simple body-centered cubic (BCC) phase were obtained after 40 h MA process. Afterward, the powders were sintered using SPS in the temperature range from 1500 °C to 1700 °C. The bulk alloys were consisted of submicron scale BCC matrix and face-centered cubic (FCC) precipitation phases. The bulk alloy sintered at 1600 °C had an average grain size of 0.58 μm and an FCC precipitation phase of 0.18 μm, exhibiting outstanding micro-hardness of 542 HV, compressive yield strength of 2208 MPa, fracture strength of 3238 MPa and acceptable plastic strain of 24.9% at room temperature. The enhanced mechanical properties of the MoNbTaTiV RHEA fabricated by MA and SPS were mainly attributed to the grain boundary strengthening and the interstitial solid solution strengthening. It is expectable that the MA and SPS processes are the promising methods to synthesize ultra-fine grains and homogenous microstructural RHEA with excellent mechanical properties.

Key words:  Refractory high-entropy alloy      Ultra-fine grain      Mechanical alloying      Spark plasma sintering      Mechanical properties     
Received:  08 April 2019     
Corresponding Authors:  Wang Guofeng     E-mail:
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1The authors equally contributed to this work.

Cite this article: 

Liu Qing, Wang Guofeng, Sui Xiaochong, Liu Yongkang, Li Xiao, Yang Jianlei. Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering. J. Mater. Sci. Technol., 2019, 35(11): 2600-2607.

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Property Mo Nb Ta Ti V
Purity (%) 99.9 99.9 99.9 99.5 99.9
Powder size (μm) <2 <48 <48 <74 <48
Table 1  Characteristics of the original elemental powders.
Fig. 1.  (a) XRD patterns and (b) morphology and size distribution of the MoNbTaTiV RHEA powders.
Alloying time (h) Mo Nb Ta Ti V
0 20a 20a 20a 20a 20a
40 19.08 22.19 20.11 19.76 18.86
Table 2  EDS results of the mechanically alloyed powders at 40 h (at.%).
Fig. 2.  XRD patterns of the bulk MoNbTaTiV RHEAs sintered at different temperatures.
Fig. 3.  Back scattered-electron images of the MoNbTaTiV RHEAs sintered at different temperatures: (a) 1500 °C; (b) 1600 °C; (c) 1700 °C.
Fig. 4.  TEM bright field images of the MoNbTaTiV RHEAs sintered temperatures of (a) 1500 °C, (b) 1600 °C, (c) 1700 °C, (d) SAED pattern of the matrix phase and (e) SAED pattern of the precipitation phase.
Fig. 5.  Microstructures of the MoNbTaTiV RHEAs: (a) HAADF image; (b-h) corresponding EDS maps of Mo, Nb, Ta, Ti, V, O and N.
Sintering temperature (°C) Average grain size (μm) Volume fraction (%) Element content (at.%)
matrix precipitation matrix precipitation O N
1500 0.42 0.15 95.06 4.94 2.93 1.42
1600 0.58 0.18 95.67 4.33 3.04 1.42
1700 1.33 0.28 96.12 3.88 3.15 1.54
Table 3  Average sizes and volume fractions of the matrix and precipitation phases, and the contents of O and N in the bulk MoNbTaTiV HEAs sintered at different temperatures.
Fig. 6.  Vickers micro-hardness of the MoNbTaTiV RHEAs at room temperature.
Fig. 7.  Compressive engineering stress-strain curves of the MoNbTaTiV RHEAs at room temperature.
Sintering temperature (°C) σy (MPa) σmax (MPa) ε (%) ρ (g cm-3)
1500 1877 2812 21.1 9.22
1600 2208 3238 24.9 9.45
1700 2179 3125 23.6 9.46
Table 4  Yield strengths (σy), fracture strengths (σmax), plastic strains (ε) and densities (ρ) of the MoNbTaTiV RHEAs at room temperature.
Fig. 8.  Compressive fracture morphologies of the MoNbTaTiV RHEAs at room temperatures after sintered at (a) 1500 °C, (b) 1600 °C, (c) 1700 °C and (d-f) corresponding magnified images of (a-c).
Alloy Process σy (MPa) σmax (MPa) ε (%) Refs.
MoNbTaTiV MA + SPS 2208 3238 24.9 This work
MoNbTaTiV Arc-melting 1400 2450 30 [9]
MoNbTaVW MA + SPS 2612 3472 8.8 [23]
MoNbTaVW Arc-melting 1246 1270 1.7 [3]
MoNbTaW Arc-melting 1058 1211 2.1 [3]
NbTaTiVW Arc-melting 1420 ≈1800 20 [10]
NbTaVW Arc-melting 1530 ≈1700 12 [10]
MoNbTaV Arc-melting 1525 2400 21 [27]
HfMoTaTiZr Arc-melting 1600 1743 4 [28]
HfMoNbTaTiZr Arc-melting 1512 1828 12 [28]
HfMoNbTiZr Arc-melting 1719 1803 10.1 [29]
AlNbTiV Arc-melting 1020 1318 5 [30]
CrMo0.5NbTa0.5TiZr Arc-melting 1595 2046 5 [31]
Table 5  Mechanical properties of the MoNbTaTiV RHEA sintered at 1600 °C and some typical RHEAs at room temperature.
Fig. 9.  Compressive yield strengths and plastic strains of the typical RHEAs at room temperature.
Sintering temperature (°C) O and N contents in matrix (at.%) Δσor (MPa) Δσiss (MPa) Δσgr (MPa) σ0 + Δσss (MPa)
1500 0.28 119 1376
1600 0.97 91 426 315 1376
1700 1.58 51 543 209 1376
Table 6  O and N contents in matrix and calculated contributions of the strengthening mechanisms: Orowan strengthening (Δσor), interstitial solid solution strengthening (Δσiss), grain boundary strengthening (Δσgr), intrinsic strength and substitution solid solution strengthening (σ0 + Δσss).
Fig. 10.  Relationship between the compressive yield strength and grain size.
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