Effect of post-bond heat treatment on the microstructure and high temperature mechanical property of a TLP bonded γ′-strengthened co-based single crystal superalloy

doi:10.1016/j.jmst.2020.05.078

Abstract

Abstract:

Post-bond heat treatment (PBHT) applied to a transient liquid phase (TLP) bonding joint is an effective approach to remove the brittle borides and improve its properties. Herein, we proposed two types of PBHT strategies to obtain a TLP bonded γ′-strengthened Co-based single crystal superalloy, and the microstructural characteristics and tensile properties of the two heat treated joints were compared to identify the optimal PBHT strategy. The evolution of the brittle boride in the joint after the PBHT was studied by using in-situ microscopy. The experimental results allowed to provide a theoretic model to quantitatively evaluate the distribution of the brittle phase after the optimal PBHT and analyze the joint fractures to understand the failure mechanisms. The obtained results revealed that a post-bond solid solution treatment performed to the joint at a high temperature (over 1275 °C) could decrease the area fraction of the boride from 7.2 % to 1.4 % and increase the elongation from 1.9 % to 7.8 %. This work emphasizes the relevance of solid solution temperature when a PBHT strategy is applied.

Key words: Transient liquid phase (TLP), γ′-strengthened co-based single crystal superalloy, Heat treatment sequence, Brittle boride, Tensile property

S.Y. Wang, Y. Sun, C.Y. Cui, X.F. Sun, Y.Z. Zhou, Y.M. Ma, H.L. An. Effect of post-bond heat treatment on the microstructure and high temperature mechanical property of a TLP bonded γ′-strengthened co-based single crystal superalloy[J]. J. Mater. Sci. Technol., 2021, 80: 244-258.

Figures/Tables 18

Table 1 Compositions of the Co-based superalloy and B-Ni76CrWB powder (wt.%).

Material	Ni	Cr	W	Al	Ta	Co	B
Co-base superalloy	13-15	3-5	13-15	4-6	7-9	Bal.	<0.1
B-Ni76CrWB	Bal.	13-15	6-8	<0.1	<0.1	<0.1	3-5

Fig. 1. The applied heat treatments for the TLP joint. (a) HT2, (b) HT3.

Fig. 2. (a) Specimen configuration for tensile test. (b) Schematic showing the measuring method for the area fraction (AF) of the DAZ-boride.

Fig. 3. SEM images showing the different areas of the TLP joints of the as-bond joint and the two post-bond heat treated joints. (a), (d) and (g) HT1; (b), (e) and (h) HT2; (c), (f) and (i) HT3.

Fig. 4. Comparison of the size, area fractions and shape parameters of the γ′ phase in different zones of the TLP joint before and after the PBHTs. (a) Area fraction. (b) Average radius of γ′ phase. (c) Shape parameters of the γ′ phase in different bonding samples. Herein, the shape parameter of the γ′ phase in DAZ is not counted, as it is affected greatly by the boride.

Fig. 5. Comparison of the chemical composition of the γ′ phase in each bonding samples.

Fig. 6. Comparison of the experimental data with the LSW and TIDC theoretical values. (a) HT2; (b) HT3.

Fig. 7. SEM and TEM images showing the morphology and type of the DAZ-borides in HT2 and HT3. (a), (b) HT2; (c), (d) HT3.

Fig. 8. Area fractions of DAZ-borides. (a) AFs of borides in the whole DAZs of HT1, HT2 and HT3. (b) AF versus distance: comparison of partial area fraction of DAZ-borides at different positions of HT2 and HT3. The interface of ISZ/DAZ is set as origin.

Fig. 9. Confocal scanning laser microscope images showing the evolution of DAZ-borides at different temperature. (a)-(f) Heating process; (g)-(l) Cooling process.

Fig. 10. Microstructure of the DAZ-borides under different PBHT parameters. (a) 1100 °C /100 h. (b) 1100 °C /500 h. (c)-(d) 1260 °C /10 h. (e) 1280 °C /2 h. (f) Morphology of the boride away from the ISZ.

Fig. 11. Schematic illustrating the evolution of DAZ-borides during the high temperature (1280 °C) post-bond heat treatment. (a) Heating process ( T<1270 °C). (b) Heating process (T = 1270 °C). (c) Holding process ( T>1270 °C). (d) Cooling process (T<1270 °C).

Fig. 12. Distribution of boron concentration: comparison of theoretical models with experimental results. (a) Schematic showing the theoretical model. (b) Theoretical models versus experimental results.

Fig. 13. Tensile properties of the TLP joints. (a) Strength curves. (b) Elongations.

Fig. 14. SEM images illustrating the longitudinal-section fractures of the TLP joints after the high temperature tensile tests. (a) HT1; (b) HT2; (c)-(f) HT3.

Fig. 15. Fracture surfaces of the joints after high temperature tensile tests. (a), (b) HT1; (c), (d) HT2; (e), (f) HT3.

Fig. 16. SEM images illustrating the ISZ-SBs of different joints, and TEM image showing the pile-up of dislocations. (a) HT1; (b) HT3; (c) HT2; (d) TEM images showing the deformation microstructure of the SB in HT2.

Fig. 17. Schematics illustrating the fracture mechanisms of different joints. (a) HT1, coalescence of cracks coming from the SB and borides; (b) HT2, coalescence of BICs; (c) HT3, movement of dislocations at high temperature promotes the formation of microvoids, which leads to the coalescence of SBCs.

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