Unravelling the metal borides evolution in the transient liquid phase bonding of Ni-based alloys via high-throughput transmission electron microscopy and first-principles thermo-kinetic calculations

doi:10.1016/j.jmst.2020.12.066

Abstract

Abstract:

The understanding of temperature and time-dependent metal borides precipitation/dissolution is crucial for the design of the transient liquid phase (TLP) bonding process of Ni-based alloys. It however remains elusive despite substantial research efforts for many years mainly owing to the uncertainty on the precipitated metal borides and the complex thermo-kinetics in the process. In this paper, we have unambiguously constructed the micro/nanoscale map of the precipitated metal borides in the TLP bonded Ni-based Inconel 718 superalloy via a high-throughput transmission electron microscopy (TEM) analysis. Five types of metal borides were found to precipitate in the diffusion affected zone (DAZ) when the isothermal solidification is completed. They are the M₅B₃ with stacking faults, Ti-rich M₃B₄, and Nb-rich M₃B₄ at the grain boundaries as well as single-crystalline M₅B₃ and M₃B₂ inside the grains. Notably, the crystal structure of the faulted M₅B₃ was rationalized by a hybrid modelling approach integrating first-principles calculation and TEM experiments. The sublattice model, with the optimized thermodynamic model parameters, was used to reproduce the metal borides precipitation map in the TLP process using DICTRA software. Coupling with multiscale simulation and experimental data, the present work built a modified thermo-kinetic model, which enables the design of the TLP bonding process of Ni-based alloys that often involves complicated time-temperature schedules and the precipitation/dissolution of a variety of different phases. The strategy can be applied to the design of the brazing process of other alloys or hybrid materials such as ceramics and metal.

Key words: Ni-based superalloy, Transmission electron microscopyTransient liquid bonding, Precipitation, Kinetics

Kewu Bai, Ming Lin. Unravelling the metal borides evolution in the transient liquid phase bonding of Ni-based alloys via high-throughput transmission electron microscopy and first-principles thermo-kinetic calculations[J]. J. Mater. Sci. Technol., 2021, 85: 118-128.

Figures/Tables 12

Fig. 1. (a) SEM image of the half cross-section of the joint brazed at 1090 °C for 600 s. The microstructure of the brazing joint is composed of ISZ, DAZ, and base metal Inconel 718 (from right to left). The red line denotes the ISZ/DAZ interface. The orange bars in the figure represent the region-of-interest cut by FIB, which are specially selected to cover the major DAZ with a length of approximately 45 μm from the ISZ/DAZ interface. (b) Pt coating shows the position of three TEM samples in the DAZ. (c?e) SEM images of FIB lamellae 1?3.

Fig. 2. Structural and compositional analysis of the M3B2 phase. (a) TEM image of M3B2 at the grain interior of base metal. (b) EDX spectrum taken from the M3B2 phase; (c) and (d) selected area diffraction patterns recorded along [001] and [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]] zone axis respectively.

Fig. 3. Structural and compositional analysis of the single-crystalline M5B3 phase inside the grain of the base metal: (a) TEM image of M5B3 inside the grain; (b) EDX spectrum; (c) and (d) diffraction patterns viewed along [010] and [-331] zone axis respectively showing a single-crystal structure of the grain.

Fig. 4. Structural and compositional analysis of the M5B3 phase containing a high density of stacking faults: (a) TEM image. The inset exhibiting a high density of stacking faults in the grain; (b) EDX spectrum; (c) diffraction pattern recorded along [010] zone axis, inset showing the weak spots from possible formation of superstructures; and (d) high-resolution TEM image showing the stacking faults in the M5B3.

Fig. 5. Structural and compositional analysis of two types of M3B4 phases at the grain boundary of the base metal Inconel 718: (a1) TEM image of the Nb-rich M3B4; (a2) EDX spectrum of the Nb-rich M3B4; (a3) and (a4) the diffraction patterns of the Nb-rich M3B4; (b1) TEM image of the Ti-rich M3B4; (b2) EDX spectrum of the Ti-rich M3B4; (b3) and (b4) the diffraction patterns of the Ti-rich M3B4.

Table 1 List of boride precipitates after the brazing process. The composition of metals is measured by EDX, and the lattice constants are derived from TEM analysis.

	Composition	Standard structure	Unit cell (Å)	Wyckoff positions
	Composition	Standard structure	Unit cell (Å)	Atom	X	Y	Z
M₃B₂	Cr_0.30Nb_0.30Mo_0.30Fe_0.05Ni_0.03Ti_0.01	Tetragonal, P4/mbm (127)	a = 5.96, c = 3.24	Mo, Nb (4 h)	0.173	0.673	0.5
				Cr (2a)	0	0	0
				B (4 g)	0.388	0.888	0
M₅B₃	Cr_0.82Fe_0.06Mo_0.08	Tetragonal, I4/mcm (140)	a = 5.56, c = 10.57	M (16I)	0.166	0.666	0.15
M₅B₃	Cr_0.82Fe_0.06Mo_0.08	Tetragonal, I4/mcm (140)	a = 5.56, c = 10.57	M (4c)	0	0	0
M₅B₃ with stacking faults	Cr_0.52Nb_0.32Mo_0.12Fe_0.02Ti_0.02	Tetragonal, I4/mcm (140)	a = 5.30, c = 11.1	B (8 h)	0	0	0.25
M₅B₃ with stacking faults	Cr_0.52Nb_0.32Mo_0.12Fe_0.02Ti_0.02	Tetragonal, I4/mcm (140)	a = 5.30, c = 11.1	B (4a)	0.625	0.125	0.25
M₃B₄	Nb_0.86Ti_0.12	Orthorhombic, Immm (71)	a = 3.29, b = 14.07, c = 3.13	M (2c)	0.5	0.5	0
	Nb_0.86Ti_0.12	Orthorhombic, Immm (71)	a = 3.29, b = 14.07, c = 3.13	M (4 g)	0	0.1861	0
	Nb_0.3Ti_0.7	Orthorhombic, Immm (71)	a = 3.21, b = 13.61, c = 3.12	B (4 g)	0	0.3607	0
	Nb_0.3Ti_0.7	Orthorhombic, Immm (71)	a = 3.21, b = 13.61, c = 3.12	B (4 h)	0	0.4351	0.5

Fig. 6. SEM-EDX mapping of the half-cross section of the joint brazed at 1090 °C for 600 s, in which the DAZ/ISZ interface is delineated by the dash-line.

Fig. 7. Distribution map of the boride phase revealed by TEM analysis.

Fig. 8. The fully relaxed supercell structure of the SF-M5B3 with tetragonal structure, in which one-layer CrNb2B2.is sandwiched by five-layers of Cr5B3. The green balls, blue balls, and black balls represent the Nb, Cr, and B atoms respectively. Here, T and A denote the trigonal prisms and anti-square prisms in the atomic structure of the metal borides, respectively. T’ and A’ denote the rotated trigonal prisms and the rotated anti-square prisms compared with T and A along the [001] stacking direction.

Table 2 Thermodynamic models parameters of the metal borides in a format of SGTE [44], in which colons denote different sublattices of the metal borides. The elements between colons denote the atomic occupation site in respective sublattices. GHSERi denotes the Gibbs energy of the pure element i referred to the enthalpy of pure element i at 298.15 K in its standard element reference (SER) state.

Phases	Thermodynamic model parameters (J/mol)
SF-M₅B₃ 3.577778 0.044444 0.377778 CONSTITUENT :Cr,Mo,Nb:Nb: B:	G(SF-M₅B₃, Cr:Nb:B)	-39504.5-0.4840×T+0.577778×GHSERCR+.377778×GHSERBB+0.04444×GHSERNB
	G(SF-M₅B₃, Mo:Nb:B)	-36239.9-3.2061×T+0.577778×GHSERMO +0.377778×GHSERBB+0.04444×GHSERNB
	G(SF-M₅B₃, Nb:Nb:B;0)	-47591.5-13.0464×T+0.62222×GHSERNB+.377778×GHSERBB
	G(SF-M₅B₃, Cr,Mo:Nb:B;0)	-15000
	G(SF-M₅B₃, Cr,Nb:Nb:B;0)	-39500
	G(SF-M₅B₃, Mo,Nb:Nb:B;0)	-85000
M₃B₄ M₃B₄ 2 3 4 CONSTITUENT :Nb,Ti: B :	G(M₃B₄, Nb:B)	-558460-187.0986×T+14×T×LN(T) +3×GHSERNB+4×GHSERBB
	G(M₃B₄,Ti:B)	-657580-48.2735×T+3×GHSERTI+4×GHSERBB
	G(M₃B₄, Nb,Ti:B;0)	19200
	G(Nb3B₄,Nb,Ti:B;2)	-198500

Fig. 9. (a) Simulated diffusion profile of the joint brazed at 1090 °C for 600 s, in comparison with experimental data [9]. (b) The simulated phase distribution of the brazing joint. The ISZ and DAZ are delineated by the red dash lines. The ISZ and base metal Inconel 718 are delineated by the blue dash line. Element occupations at metal sublattice of the various precipitates (c) Ti-rich M3B4 and Nb-rich M3B4, (d) The SF-M5B3, (e) M5B3 and (f) M3B2 in the DAZ in comparison with experimental data by TEM-EDX.

Fig. 10. (a) The simulated liquid (ASZ) half-width of the brazing joint at different time; (b) The simulated phase distribution of the TLP joint brazed for 3600 s.

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