Dual phase equal-atomic NbTaTiZr high-entropy alloy with ultra-ﬁne grain and excellent mechanical properties fabricated by spark plasma sintering

doi:10.1016/j.jmst.2021.03.024

Abstract

Abstract:

Super-high strength NbTaTiZr high-entropy alloys (NbTaTiZr HEAs) have been successfully fabricated by the mechanical alloying (MA) with spark plasma sintering (SPS) technology, which is 2-fold compared with that of NbTaTiZr HEAs prepared by vacuum arc melting (VAM). After the SPS process, the bulk NbTaTiZr alloy samples are provided with dual-phase body-centered cubic (BCC) structure and nanoscale grain size about 500 nm that is obviously smaller than that of NbTaTiZr HEA fabricated by VAM. When the sintering temperature is 800 °C, the compressive fracture strength is the highest reaching at 2511 ± 78 MPa. When the sintering temperature is 1000 °C, the fracture strain is the highest reaching at 12.8%, and compressive fracture strength and yield strength also reach at 2274 ± 91 MPa and 2172 ± 47 MPa, respectively. The excellent mechanical properties of bulk NbTaTiZr alloy samples are attributed to the merits of MA and SPS, and the collaboration effect of ultra-fine grains strengthening, solid solution strengthening and interstitial solid solution strengthening.

Key words: NbTaTiZr high-entropy alloys, Mechanical alloying, Ultra-fine grain, Spark plasma sintering, Mechanical properties, Corrosion behavior

Tao Xiang, Zeyun Cai, Peng Du, Kun Li, Zongwei Zhang, Guoqiang Xie. Dual phase equal-atomic NbTaTiZr high-entropy alloy with ultra-ﬁne grain and excellent mechanical properties fabricated by spark plasma sintering[J]. J. Mater. Sci. Technol., 2021, 90: 150-158.

Figures/Tables 13

Table 1 Composition of Hanks’ solution with pH 7.4.

Reagent	Amount
Deionized water	1000 mL
NaCl	8.00 g
KCl	0.40 g
NaHCO₃	0.35 g
CaCl∙2H₂O	0.19 g
Na₂HPO₄∙7H₂O	0.09 g
MgSO₄∙7H₂O	0.20 g
KH₂PO₄	0.06 g
Glucose	1.00 g

Fig. 1. (a) XRD patterns, (b) grain size of the alloy powders at different ball-milling time, (c) morphology and size distribution of alloy powders at ball-milling time of 40 h.

Table 2 EDS and ICP results of the mechanically alloyed powders at 40 h (at.%).

Type	Ti	Zr	Nb	Ta
Design atomic ratio	1	1	1	1
EDS results	1	0.889	0.937	0.904
ICP results	1	0.884	0.977	0.974

Fig. 2. XRD patterns (a) of the alloy powders sintered at different temperatures, and (b) deconvoluted (211) peak using Lorentz function.

Fig. 3. Back scattered-electron images of the alloy powders sintered at different temperatures: (a) low-magnified image at 700 °C and (b-f) high-magnified images at (b) 700 °C, (c) 800 °C (d) 900 °C, (e) 1000 °C, (f) 1100 °C, respectively.

Fig. 4. (a) TEM image of the NbTaTiZr HEA sintered at 1000 °C and (b, c, e, f) corresponding EDS maps of Ti, Zr, Nb and Ta, respectively; (d) element contents of two different contrast regions in black and white.

Fig. 5. TEM images of the NbTaTiZr HEA sintered at 1000 °C: (a) bright-field image and (b) its partially enlarged image; (c) SAED patterns of the Ta (Nb)-rich region and Ti (Zr)-rich region, respectively.

Fig. 6. Vickers micro-hardness of the bulk NbTaTiZr HEAs at different sintering temperatures.

Fig. 7. (a) Compressive engineering stress-strain curves of the NbTaTiZr HEAs at room temperature, (b) yield strength and plastic strain of the NbTaTiZr HEAs compared with previous reports [14,16,24,39,[42], [43], [44]].

Table 3 Several fundamental mechanical properties of the NbTaTiZr HEAs sintered at different temperatures.

Sintering temperature (°C)	σ_y (MPa)	σ_max (MPa)	ε (%)	ρ (g/cm³)	E (GPa)
700	-	2477 ± 67	-	8.79	142
800	-	2511 ± 78	-	8.84	143
900	2378 ± 67	2410 ± 48	2.84	8.87	141
1000	2182 ± 47	2274 ± 91	12.80	8.88	138
1100	1838 ± 27	2165 ± 25	10.65	8.87	137

Fig. 8. SEM images taken from fracture surfaces of NbTaTiZr HEAs subjected to compressive failure: (a, b) sintered at 700 °C; (c, d) sintered at 1000 °C.

Fig. 9. Potentiodynamic polarization curves of NbTaTiZr HEAs sintered at different sintering temperatures, as well as Ti-6Al-4 V alloy and pure Ti for comparison in Hanks’ solution at 37 °C.

Table 4 Electrochemical properties measured in Hanks' solution at 37 °C for NbTaTiZr HEAs, Ti-6Al-4 V alloy and commercial pure Ti.

Alloy	E_corr (V_SCE)	I_p (µA/cm²)	E_pit (V_SCE)
700 °C	-0.520	0.139	0.671
800 °C	-0.500	0.146	0.537
900 °C	-0.446	0.369	1.110
1000 °C	-0.478	0.185	-
1100 °C	-0.498	0.240	-
CPTi	-0.600	0.273	-
Ti-6Al-4V	-0.640	0.146	-

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