Microstructure and mechanical properties of transient liquid phase bonding DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy

doi:10.1016/j.jmst.2021.04.024

Abstract

Abstract:

This study focuses on the transient liquid phase (TLP) bonding of DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy (MEA) using a BNi-2 filler alloy. The microstructure and mechanical properties of the TLP-bonded DD5/MEA joint were evaluated, and the microstructural evolution mechanism was investigated. The formation of the isothermal solidification zone (ISZ) depended on the diffusion of the melting-point depressants (Si and B elements) from the liquid filler into the DD5 and MEA substrates, as well as the dissolution of the substrates. Boron diffused along the γ channel of DD5 and reacted to form M₅B₃ boride, herein referred to as the diffusion-affected zone (DAZ I). Similarly, the Cr₅B₃ boride precipitated in the Ni-rich MEA matrix adjacent to the MEA substrate (i.e., DAZ II). Additionally, a coherent orientation of [0 1 1]_FCC// [2 4 0]_BCT and (2 0 0)_FCC//(0 0 2)_BCT was detected between M₅B₃ boride with a body-centered tetragonal (BCT) structure and the face-centered cubic (FCC) matrix. The performance of the joint was dominated by the properties of the bonding seam. As the bonding time increased from 20 to 80 min, the athermal solidification zone (including eutectic microstructure) was gradually replaced by the ISZ exhibiting excellent plastic deformation capability, and the shear strength of the joint was improved. The maximum shear strength (752 MPa) was achieved when the eutectic-free joint was bonded at 1050 °C for 80 min. The fracture morphology revealed a mixture mode, indicating the initiation of cracks in the DAZ II, mainly propagating in the ISZ, and passing through the DAZ I.

Key words: Microstructure, Mechanical properties, Transient liquid phase bonding, Single-crystal superalloy, Medium-entropy alloy

Shiwei Li, Jinglong Li, Junmiao Shi, Yu Peng, Xuan Peng, Xianjun Sun, Feng Jin, Jiangtao Xiong, Fusheng Zhang. Microstructure and mechanical properties of transient liquid phase bonding DD5 single-crystal superalloy to CrCoNi-based medium-entropy alloy[J]. J. Mater. Sci. Technol., 2022, 96: 140-150.

Figures/Tables 18

Table 1 Chemical composition (wt.%) of the (CrCoNi)94Al3Ti3 MEA and DD5 single-crystal superalloy.

Materials	Al	Co	Cr	Ti	Ta	Hf	Mo	W	Re	B	C	Ni
(CrCoNi)₉₄Al₃Ti₃	1.46	33.30	29.34	2.60	-	-	-	-	-	-	-	33.30
DD5	6.10	7.38	6.96	-	6.42	0.14	1.42	5.02	3.02	0.003	0.052	Bal.

Fig. 1. Etched microstructure: (a) dendritic trunk, and (b) interdendritic area of the DD5 single-crystal superalloy; (c) macro-profile, and (d) magnified morphology of the (CrCoNi)94Al3Ti3 MEA.

Fig. 2. Sketch of (a) the assembled sample for TLP bonding, and (b) shear testing set up for the TLP-bonded joint.

Fig. 3. Microstructure of the TLP-bonded DD5/MEA joint achieved at 1050 °C for 50 min: (a) interfacial microstructure and magnified morphologies of (b) ASZ, (c) DAZ I, and (d) DAZ II.

Table 2 Chemical composition (at.%) of the spots marked in Fig. 3.

Spots	Al	Ti	Si	Cr	Co	Ni	Mo	W	Re	Others	Possible phase
A₁	2.23	0.71	7.14	6.25	6.47	70.32	-	-	-	6.88	Ni(s.s)
A₂	2.16	0.63	2.81	7.29	8.66	77.95	-	-	-	0.50	Primary γ
A₃	0.12	-	0.13	6.86	7.56	83.66	-	-	-	1.67	Eutectic γ
B	0.73	-	20.21	1.58	6.73	70.20	-	-	-	0.55	Ni₃Si
C	2.90	-	-	47.06	4.70	17.56	7.74	11.86	4.13	4.05	Cr-Mo-W boride
D	0.42	0.95	0.20	80.02	6.81	3.62	-	-	-	7.98	Cr-rich boride

Fig. 4. TEM characterization of DAZ I marked with dashed square in Fig. 3(c): (a) microstructure in HADDF-STEM mode, (b) SAED pattern of region E, (c) HRTEM image and corresponding FFT pattern of the G/E interface zone, and (d) HADDF-HRTEM image of the phase interface in (c).

Fig. 5. TEM-EDS results of DAZ I: (a) HADDF image and corresponding elemental distribution maps of (b) Al, (c) Co, (d) Ni, (e) Cr, (f) Mo, (g) W, and (h) Re.

Table 3 Chemical composition (at.%) of the regions marked in Fig. 4.

Regions	Al	Ta	Si	Cr	Co	Ni	Mo	W	Re	Others	Possible phase
E	8.02	2.95	6.12	4.62	8.19	63.35	0.42	0.41	0.69	5.23	γ
F	10.74	4.03	5.10	1.60	4.93	64.23	0.22	0.54	0.32	8.29	γʹ
G	4.00	4.30	1.84	42.25	4.50	11.76	7.64	10.25	6.10	7.36	Cr-Mo-W-Re boride

Fig. 6. TEM characterization of DAZ II marked with dashed square in Fig. 3(d): (a) microstructure (HADDF image), (b) SAED pattern of region H, (c) and (d) HRTEM images and corresponding FFT patterns of regions H and I respectively.

Fig. 7. Elemental distribution maps corresponding to Fig. 6(a): (a) Al, (b) Co, (c) Cr, (d) Ti, (E) Ni, and (f) Si.

Table 4 Chemical composition (at.%) of the regions marked in Fig. 6.

Regions	Al	Ti	Si	Cr	Co	Ni	Others	Possible phase
H	4.09	1.63	5.50	12.71	18.54	57.54	-	Ni-rich MEA
I	1.77	0.60	0.26	80.79	8.80	1.80	6.48	Cr₅B₃

Fig. 8. Microstructure of the DD5/MEA joint bonded for different times: (a) interfacial microstructure for 20 min and magnified images of (b) DAZ I and (c) DAZ II; (d) interfacial microstructure for 80 min and magnified images of (e) DAZ I and (f) DAZ II.

Table 5 Width of the DAZ and the proportion of ISZ in SZ obtained with different bonding times.

Bonding time (min)	Width of DAZ Ⅰ (μm)	Width of DAZ II (μm)	Proportion of ISZ in SZ
20	15.7	57.7	9%
50	39.2	113.4	70.5%
80	51.0	120.6	100%

Fig. 9. Microstructural evolution model of the DD5/MEA joint that is TLP-bonded at 1050 °C for 50 min: (a) formation of liquid BNi-2 interlayer, (b) precipitation of M5B3-type borides, (c) isothermal solidification at 1050 °C, and (d) complete solidification during the cooling process.

Fig. 10. Nanoindentation results on the DD5/MEA joint TLP-bonded for 50 min: (a) distribution of the elastic modulus and hardness across the bonding interface, (b) typical load-depth plots.

Fig. 11. Shear strengths of the DD5/MEA joints TLP-bonded for different bonding times.

Fig. 12. Macrofracture and magnified morphologies of the DD5/MEA joints that were TLP-bonded at 1050 °C for (a, b) 20 min, (c, d) 50 min, and (e, f) 80 min.

Table 6 Chemical compositions (at.%) of the spots marked in Fig. 12.

Spots	Al	Si	Co	Cr	Ni	Others	Possible phase
A	3.29	9.25	5.02	3.34	75.23	3.87	Ni-Si eutectic
B	3.41	6.61	4.96	8.53	70.29	6.20	Ni(s.s)
C	3.10	3.02	17.29	32.80	41.21	2.58	Cr₅B₃ + Ni-rich MEA
D	14.88	7.04	4.83	6.30	64.43	2.52	γʹ

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