Transient Liquid Phase Bonding of IN738LC/MBF-15/IN738LC: Solidification Behavior and Mechanical Properties

doi:10.1016/j.jmst.2016.07.017

Abstract

Abstract:

Transient liquid phase (TLP) bonding of IN738LC superalloy was carried out using a rapidly solidified MBF-15 Ni-based foil. The effects of bonding temperature (1130-1170 °C) and time (5-120 min) as well as foil thickness (35-140 µm) were studied on the microstructure of joint region and its mechanical properties. The solidification sequence in the joint region was found to be (i) formation of γ solid solution in the isothermally solidified zone, followed by (ii) ternary eutectic of γ + Ni₃B + CrB, and finally (iii) binary eutectic of γ + Ni₃Si in the athermally solidified zone. Fine Ni₃Si particles were also formed via a solid state transformation within the γ matrix in the vicinity of eutectic products. A deviation of isothermal solidification kinetics from the standard parabolic TLP model was observed by increasing the bonding temperature to 1170 °C, which resulted in the formation of eutectic constituents at the joint centerline. The analysis of mechanical and fractographic test results revealed that the samples with complete isothermal solidification exhibit the highest shear strength, whereas the hard eutectic constituents act as preferential failure sites and lead to a significant reduction in the joint shear strength in samples with incomplete isothermal solidification.

Key words: Transient liquid phase bonding, IN738LC superalloy, Isothermal solidification, Amorphous brazing foil, Mechanical properties

Binesh B.,Jazayeri Gharehbagh A.. Transient Liquid Phase Bonding of IN738LC/MBF-15/IN738LC: Solidification Behavior and Mechanical Properties[J]. J. Mater. Sci. Technol., 2016, 32(11): 1137-1151.

Figures/Tables 23

Table 1. Chemical composition of cast IN738LC superalloy and MBF-15 interlayer (wt%)

Alloy	Ni	Cr	Co	Al	Ti	Mo	Si	Ta	W	B	Fe	Zr	Nb	C
IN738LC	Bal.	16.1	8.3	3.5	3.3	1.7	0.05	1.6	2.9	0.013	0.09	0.05	0.8	0.11
MBF-15	Bal.	13.0	1.0	-	-	-	4.5	-	-	2.8	4.2	-	-	0.03

Fig. 1. Schematic illustration of the shear test fixture.

Fig. 2. Optical micrographs of the TLP bonded area, brazed at 1130 °C for 30 min using a 70 µm thick interlayer.

Fig. 3. SEM (a) secondary and (b) backscattered electron images of the TLP bonded area, (c), (d) and (e) magnified secondary electron images of the ASZ zone in (a), brazed at 1130 °C for 30 min using a 70 µm thick interlayer.

Table 2. SEM/EDS analysis results of the micro-zones marked in Fig. 3

Micro-zones	Chemical composition (at.%) -----------------------------------------------
Micro-zones	Ni	Cr	Al	Si	Ti	Fe	Co	Nb	Mo	W
Zone 1 in Fig. 3(a)	67.22	14.25	2.66	5.60	1.11	4.09	2.26	1.47	0.74	0.60
Zone 2 in Fig. 3(c)	81.03	3.27	0.56	1.61	5.53	0.96	2.26	4.22	0.56	0.00
Zone 3 in Fig. 3(c)	6.58	78.24	1.20	0.88	0.37	1.95	0.98	1.43	6.83	1.55
Zone 4 in Fig. 3(c)	5.04	81.02	2.37	0.66	1.10	1.04	0.55	1.56	5.21	1.46
Zone 5 in Fig. 3(c)	72.13	2.65	0.82	18.61	1.11	1.85	1.56	1.13	0.13	0.00
Zone 6 in Fig. 3(d)	4.08	85.72	1.29	0.35	0.31	1.47	0.93	1.43	2.61	1.83
Zone 7 in Fig. 3(d)	77.50	3.51	1.77	0.74	6.33	1.02	2.64	3.71	1.53	1.26
Zone 8 in Fig. 3(d)	69.19	8.97	2.04	9.09	1.79	2.99	2.42	1.84	0.78	0.89
Zone 9 in Fig. 3(e)	70.31	7.93	1.84	10.23	2.14	1.91	2.36	1.37	0.94	0.96
Zone 10 in Fig. 3(e)	71.47	1.94	1.41	17.86	1.48	2.11	1.80	1.38	0.55	0.00

Fig. 4. (a) SEM image, (b) Cr, (c) Ni, (d) Si, (e) Mo, (f) Nb, (g) Ti, (h) W, and (i) Fe X-ray image analysis of the ASZ in Fig. 3.

Fig. 5. XRD pattern of the shear fracture surfaces of the TLP joints brazed at 1130 °C for (a) 5 min using a 35 µm thick interlayer and (b) 30 min using a 70 µm thick interlayer.

Fig. 6. Schematic illustration of the TLP bonding process: (a) MBF-15 interlayer melted, (b) substrate dissolution, (c) start of isothermal solidification, (d) middle step of isothermal solidification, (e) complete isothermal solidification, (f) incomplete isothermal solidification (TB: bonding temperature; Tl: liquidus temperature; Ts: solidus temperature; TE: eutectic temperature; Ci: initial composition of liquid; Cs,Cs′ and Cs″: instant solidus composition; ClCl,Cl′ and Cl″: instant liquidus composition; CE: eutectic composition).

Fig. 7. SEM images of DAZ in the TLP bonded sample brazed at (a) 1150 °C for 60 min; (b) 1130 °C for 5 min using a 35 µm thick interlayer and (c) magnified image of region B in (b).

Table 3. SEM/EDS analysis results of the needle-like precipitates and adjacent γ matrix (zone A) in Fig. 7(a)

Phase	Chemical composition (at.%) ----------------------------------------------
Phase	Ni	Cr	Al	Si	Ti	Fe	Co	Nb	Mo	W
Needle-like precipitates	40.51	28.68	8.11	1.89	3.42	0.11	5.14	1.66	5.01	5.46
γ matrix	62.66	11.50	8.63	1.55	3.57	0.43	8.24	1.76	0.12	1.54

Fig. 8. (a) SEM image of DAZ and (b) Cr, (c) Ni, (d) Mo, (e) Nb, and (f) Si X-ray elemental images in the TLP bonded sample brazed at 1130 °C for 5 min using a 35 µm thick interlayer.

Fig. 9. Schematic illustration of boron distribution across the TLP bonded zone.

Fig. 10. Optical micrographs of the joint brazed at 1130 °C for (a) 5, (b) 15, (c) 30, and (d) 60 min using a 35 µm thick interlayer.

Fig. 11. Optical micrographs of the joint brazed at 1150 °C for (a) 30 min, (b) 60 min, and at 1170 °C for (c) 30 min and (d) 60 min using a 35 µm thick interlayer.

Fig. 12. Kinetics of the isothermal solidification during TLP bonding of IN738LC samples at various bonding temperatures using a 35 µm thick interlayer.

Table 4. Parameters of the IN738LC/MBF-15/IN738LC bonding system used for the theoretical model

C₀	C_s^[32]	C_m	D₀ (m²/s)^[46]	Q (kJ/mol)^[46]	R (J/(mol K))
12.44	0.3	0.065	0.0144	211	8.314

Fig. 13. Comparison of predicted and experimentally bonding time for completion of isothermal solidification at various bonding temperatures.

Fig. 14. SEM micrographs showing γ-γ′ eutectic constituents in the sample brazed at 1170 °C for 60 min.

Fig. 15. Optical micrographs of the joint brazed at 1130 °C for 60 min with a foil thickness of (a) 35, (b) 70, (c) 105, and (d) 140 µm.

Fig. 16. (a) Optical micrograph showing hardness indenter traces in the TLP-bonded region and (b) the hardness profile of the sample brazed at 1130 °C for 60 min using a 140 µm thick interlayer.

Fig. 17. (a) Optical micrograph showing hardness indenter traces in the TLP-bonded region and (b) the hardness profile of the sample brazed at 1130 °C for 60 min using a 35 µm thick interlayer.

Fig. 18. Variations of the shear strength and ASZ width with (a) bonding time at 1130 °C and (b) bonding temperature for 30 min.

Fig. 19. (a) and (b) Optical micrographs of cross sections normal to shear fracture surfaces, and (c) and (d) SEM micrographs of the joint fracture surfaces (brazed at 1130 °C for (a) and (c) 15 min, and (b) and (d) for 60 min using a 35 µm thick interlayer).

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