Eutectic-reaction brazing of Al0.3CoCrFeNi high-entropy alloys using Ni/Nb/Ni interlayers

doi:10.1016/j.jmst.2021.12.066

Abstract

Abstract:

In this study, a novel eutectic-reaction brazing of Al_0.3CoCrFeNi high-entropy alloys (HEAs)¹ was investigated with a design of laminated Ni/Nb/Ni interlayers. The typical Al_0.3CoCrFeNi brazing seam consisted of proeutectic γ, the lamellar eutectic structure composed of face-centered cubic (FCC) phase (eutectic γ) and C14 Laves, as well as a few amounts of Nb based solid solution. A high density of nanoscale ordered L1₂ (γ′) phase was precipitated within γ phase matrix. With the brazing temperature raised from 1200 to 1320 °C, the dissolution volume of Al_0.3CoCrFeNi alloy into Ni-Nb liquid was increased. The microstructure of the brazing seam changed from hypereutectic (blocky Laves + γ phase) to eutectic (Laves + eutectic γ) to hypoeutectic structures (proeutectic γ + Laves/γ eutectic structure). The shear strength of joints was increased gradually due to the formation of eutectic lamellae and the cellular growth of proeutectic γ phase. The maximum shear strength of the joint brazed at 1320 °C for 10 min was up to 592 MPa, reaching 95% of the base metal. The joint mainly ruptured in the soft proeutectic γ phase in a ductile fracture mode.

Key words: High-entropy alloys, Brazing, Eutectic structure, Shear strength

Y. Lei, J. Sun, X.G. Song, M.X. Yang, T.L. Yang, J. Yin. Eutectic-reaction brazing of Al_0.3CoCrFeNi high-entropy alloys using Ni/Nb/Ni interlayers[J]. J. Mater. Sci. Technol., 2022, 121: 245-255.

Figures/Tables 16

Fig. 1. (a, b) microstructure and XRD pattern of Al0.3CoCrFeNi BM, (c-e) illustrations for assembly of specimens, sample cutting and shear test.

Fig. 2. Interfacial microstructure of the Al0.3CoCrFeNi joint brazed at 1240 °C for 10 min. (a, b) BSE images; (c) line scanning results of red line profile in Fig. 2(b).

Table 1. Chemical composition (at.%) of points A-D in Fig. 2(b).

Points	Al	Co	Cr	Fe	Ni	Nb	Possible phase
A	3.74	17.00	20.05	20.15	35.19	3.87	Ni(s, s)
B	4.83	15.13	17.19	17.05	40.33	5.47	FCC phase
C	1.22	18.90	13.90	14.37	29.39	22.21	Nb-rich IMC
D	1.20	4.76	7.10	6.57	8.20	72.17	Nb(s, s)

Fig. 3. TEM-EDS results of partial EZ. (a) HADDF image and the corresponding elemental distribution images of (b) Al, (c) Cr, (d) Fe, (e) Ni, (f) Nb, (g) Co, (h) Al, Co, Cr, Fe, Nb, Ni.

Fig. 4. TEM characterization of the EZ enclosed by a blue dashed line marked in Fig. 3(a). (a) BF image; (b, c) SAED patterns and HRTEM image of phase B; (d, e) SAED patterns and HRTEM of phase C; (f) HRTEM image of the phase B/phase C interface; (g) BF image of partial EZ; (h) HAADF image of the magnified region in (g); (i) EDS line results along the direction of black line in (h); (j) EDS mapping scan results of the region in (h).

Fig. 5. EBSD result of the joint brazed at 1240 °C for 10 min. (a) Auto Image Quality (IQ) map; (b) Phase map and (c) Inverse Pole Figure (IPF) map.

Fig. 6. Microstructures of the Al0.3CoCrFeNi joints brazed at different temperatures for 10 min. (a) 1180 °C, (b) 1200 °C, (c) 1220 °C, (d) 1240 °C, (e) 1260 °C, (f) 1280 °C, (g) 1300 °C, (h) 1320 °C.

Table 2. Chemical compositions (at.%) of the points E-K in Fig. 6.

Points	Al	Co	Cr	Fe	Ni	Nb	Possible phases
E	2.24	0.70	1.44	2.34	2.36	90.92	Nb(s, s)
F	0.07	0.53	0.43	1.04	51.22	46.71	Nb₇Ni₆
G	-	0.25	0.47	3.52	75.91	19.86	NbNi₃
H	2.11	0.19	3.09	0.91	89.81	3.90	Ni(s, s)
I	5.22	12.25	14.98	15.24	46.55	5.76	γ phase
J	-	16.14	13.78	14.58	33.78	21.71	Laves phase
K	-	-	-	-	9.34	90.66	Nb(s, s)

Fig. 7. Shear strength of the Al0.3CoCrFeNi joints brazed at different temperatures for 10 min.

Fig. 8. Nano-indentation results of the joint brazed at 1260 °C for 10 min: (a) distribution of the hardness and the elastic modulus across the brazing joint, (b) typical load-depth plots.

Fig. 9. Fracture morphologies of the joints brazed at different temperatures for 10 min. (a, b) 1180 °C; (c, d) 1200 °C; (e, f) 1260 °C; (g, h) 1300 °C; (i, j) 1320 °C; (k, l) Schematic diagram of fracture paths of the joints brazed at 1260 °C and 1300 °C for 10 min.

Fig. 10. XRD patterns of the fracture surfaces of joints brazed at different temperatures for 10 min.

Fig. 11. Schematic diagram of the microstructure evolution mechanism of Al0.3CoCrFeNi/Ni/Nb/Ni/Al0.3CoCrFeNi joint.

Fig. 12. The Nb-Ni binary phase diagram [17].

Fig. 13. DSC curve of laminated Nb/Ni/Al0.3CoCrFeNi specimen. The inset diagram is the specimen preparation.

Fig. 14. The pseudo binary phase diagram of CoCrFeNi-Nb alloy calculated from the Thermo-Calc in Ref [30].

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