Effects of Nb content in Ti-Ni-Nb brazing alloys on the microstructure and mechanical properties of Ti-22Al-25Nb alloy brazed joints

doi:10.1016/j.jmst.2017.03.021

Abstract

Abstract:

Vacuum brazing was successfully used to join Ti-22Al-25Nb alloy using Ti-Ni-Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti-Ni-Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, τ₃, and Ti₂Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached 359 MPa when the joints were brazed with eutectic Ti₄₀Ni₄₀Nb₂₀ alloy at 1180 °C for 20 min, and it was 321, 308 and 256 MPa at 500, 650 and 800 °C, respectively. Cracks primarily initiated and propagated in τ₃ compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.

Key words: Ti-22Al-25Nb alloy, Ti-Ni-Nb brazing alloy, Nb content, Microstructure, Mechanical properties

Wang Y., Cai X.Q., Yang Z.W., Wang D.P., Liu X.G., Liu Y.C.. Effects of Nb content in Ti-Ni-Nb brazing alloys on the microstructure and mechanical properties of Ti-22Al-25Nb alloy brazed joints[J]. J. Mater. Sci. Technol., 2017, 33(7): 682-689.

Figures/Tables 12

Fig. 1. Backscattered electron image (BEI) of (a) hypo-eutectic Ti45Ni45Nb10 alloy, (b) eutectic Ti40Ni40Nb20 alloy, and (c) hyper-eutectic Ti37Ni37Nb26 alloy.

Fig. 2. XRD pattern of hypo-eutectic Ti45Ni45Nb10 alloy, eutectic Ti40Ni40Nb20 alloy, and hyper-eutectic Ti37Ni37Nb26 alloy.

Fig. 3. Microstructure and corresponding elemental distribution of Ti-22Al-25Nb alloy joint brazed with eutectic Ti40Ni40Nb20 brazing alloy at 1180 °C for 20 min: (a) microstructure and (b-e) elements’ area distribution images.

Fig. 4. Magnified microstructures of Ti-22Al-25Nb alloy brazed joint: (a) zone I and (b) zone II.

Table 1 EDS results of chemical compositions at each location in Fig. 4 (at.%).

Locations	Al	Nb	Ti	Ni	Possible phases
A	16.73	32.19	46.83	4.25	B2+O
B	19.38	13.50	42.46	24.66	τ₃
C	16.40	33.07	46.92	3.61	B2+O
D	8.50	3.33	58.68	29.49	Ti₂Ni

Fig. 5. TEM results of the reaction phases in the joint: (a) morphology of reaction phases, (b) the SAED pattern was obtained from the solid circle in (a), and (c) was obtained from the dotted circle in (a).

Fig. 6. Effect of Nb content on the microstructure of Ti-22Al-25Nb brazed joints: (a) hypo-eutectic Ti45Ni45Nb10 alloy, (b) eutectic Ti40Ni40Nb20 alloy, and (c) hyper-eutectic Ti37Ni37Nb26 alloy.

Fig. 7. Effect of (a) Nb content and (b) test temperature on the shear strength of Ti-22Al-25Nb brazed joints.

Fig. 8. Typical fracture paths of Ti-22Al-25Nb joints brazed with eutectic Ti40Ni40Nb20 brazing alloy: (a) low magnification SEM image, (b) and (c) high magnification SEM image.

Fig. 9. Fractographs of Ti-22Al-25Nb joints brazed with eutectic Ti40Ni40Nb20 brazing alloy after shear test at (a, b) RT, (c, d) 500 °C, (e, f) 650 °C and (g, h) 800 °C.

Table 2 EDS results of chemical compositions at each location in Fig. 9 (at.%).

Locations	Al	Nb	Ti	Ni	Possible phases
A	18.88	14.48	42.31	24.33	τ₃
B	17.12	33.00	45.56	04.32	B2+O
C	17.58	12.61	44.41	25.40	τ₃
D	14.28	23.55	50.90	11.27	B2+O
E	20.50	14.50	42.04	22.96	τ₃
F	15.22	27.51	48.49	08.78	B2+O
G	15.38	11.16	45.84	27.62	τ₃
H	17.63	23.77	53.25	05.35	B2+O

Fig. 10. XRD analysis of the fracture surface of Ti-22Al-25Nb joints brazed with eutectic Ti40Ni40Nb20 brazing alloy after shear test at: (a) RT, (b) 500 °C, (c) 650 °C and (d) 800 °C.

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