A correlative multidimensional study of γ′ precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

doi:10.1016/j.jmst.2020.10.025

Abstract

Abstract:

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum (Ta) content (2Ta, 5Ta and 8Ta in wt %) was investigated. Ta addition significantly affected the γ′ precipitate morphology, γ/γ′ lattice misfit and microstructural stability during long-term aging. Results showed that the partitioning behaviors of solutes were enhanced by Ta addition, meanwhile, the reversal partitioning behavior of W was triggered which partitioned from γ′ precipitate to γ matrix. The elemental concentration redistribution caused variations in lattice misfit from positive to negative, the values of lattice misfit were measured to be 0.16 % for 2Ta alloy, then decreased to -0.07 % for 5Ta alloy and negatively increased to -0.23 % for 8Ta alloy. These variations in the lattice misfit were reflected on the transition of γ′ morphology from round-cornered cuboidal shape to cuboidal with sharp corners, accomplished with increasing shape parameter ratio η. Consequently, the optimal γ′ shape could be obtained at lattice misfit of approximately 0.3 %. The γ′ coarsening investigation at 900 °C (up to 2000 h) indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy, accompanied by the enhanced capability of resisting γ′ coalescence. By incorporating the calculated interfacial energy, computational modeling, Thermo-Calc and PrecipiCalc, were employed to elucidate the γ′ kinetic pathways, the simulation results agreed with experiments, indicating that the model and parameters were reasonable. Additionally, it was found that there was no overlap between γ′ nucleation and coarsening when the γ/γ′ interfacial energy increased to a critical value.

Key words: Tantalum, Ni-based superalloys, Atom probe tomography, Lattice misfit, Coarsening kinetics

Jiachen Zhang, Taiwen Huang, Kaili Cao, Jia Chen, Huajing Zong, Dong Wang, Jian Zhang, Jun Zhang, Lin Liu. A correlative multidimensional study of γ′ precipitates with Ta addition in Re-containing Ni-based single crystal superalloys[J]. J. Mater. Sci. Technol., 2021, 75: 68-77.

Figures/Tables 16

Fig. 1. Shape parameter ratio (η) to quantitatively assess γ′ shapes.

Fig. 2. SEM images of microstructure after standard heat treatment with measured shape parameter ratios in (a) 2Ta alloy, (b) 5Ta alloy, (c)8Ta alloy and (d)DSC heating curves of three investigated alloys.

Table 1 γ′ volume fraction (f %), γ′ size (d, nm) and phase transformation temperatures of three alloys (°C). Characteristic temperatures were obtained by DSC.

Alloy	f	d /nm	T_L /°C	T_S /°C	T_γ_′/°C
2Ta	52.69 ± 1.64	469.24 ± 46	1391	1343	1171
5Ta	57.64 ± 1.17	449.13 ± 31	1379	1339	1198
8Ta	63.91 ± 4.45	411.51 ± 40	1360	1313	1211

Fig. 3. APT reconstructed atom distribution maps and ROIs of the three alloys.

Table 2 Concentration distribution of elements in γ/γ′ phases of various alloys (atomic fraction, at. %).

		Cr	Co	Mo	Re	Al	Ti	Ta	W	Ni
2Ta	γ	21.43	13.25	0.68	1.68	3.38	0.59	0.09	1.73	57.57
	γ′	2.87	3.93	0.33	0.16	15.14	4.43	0.86	2.09	70.24
	K_i^γ'/γ	0.13	0.3	0.49	0.1	4.48	7.51	9.56	1.21	1.22
5Ta	γ	24.47	14.15	0.86	1.8	2.94	0.63	0.21	2.03	52.91
	γ′	2.42	4.12	0.38	0.11	14.58	4.68	2.27	1.92	69.5
	K_i^γ'/γ	0.1	0.29	0.44	0.06	4.96	7.43	10.81	0.95	1.31
8Ta	γ	28.36	14.52	0.92	1.9	2.5	0.53	0.31	2.52	48.44
	γ′	1.99	4.25	0.25	0.08	14.22	4.76	3.93	1.32	69.2
	K_i^γ'/γ	0.07	0.29	0.27	0.04	5.59	8.98	12.68	0.52	1.43

Fig. 4. Elemental partitioning coefficients between the γ′ and γ phases.

Fig. 5. γ′ microstructure evolution in three alloys subjects to isothermal aging periods of 100, 200, 500, 1000 and 2000 h at 900 °C. The shape parameters were also labeled.

Fig. 6. The temporal evolution of the structural properties of the γ′ precipitates in three alloys aged at 900 °C. Three quantities were displayed as a function of aging time: (a) volume fraction, (b) equivalent diameter and (c) number density.

Fig. 7. X-ray diffraction peak (asymmetric (400) plane reflection) for three alloys with heat-treated: (a)2Ta, (b)5Ta and (8Ta) alloys, showing two overlapping peaks corresponding to the γ matrix and γ′ precipitate. The XRD data for 5Ta alloy was extracted from our previous work [6].

Fig. 8. X-ray diffraction peak (asymmetric (400) plane reflection) for three alloys after aging for 2000 h: (a)2Ta, (b)5Ta and 8Ta alloys, showing two overlapping peaks corresponding to the γ matrix and γ′ precipitate.

Table 3 Measured lattice misfits δ (%) and shape parameter η of experimental alloys by high resolution XRD. The X-ray diffraction peaks for 500 h were not shown here.

Alloy	Parameter	Heat-treated	500 h	2000 h
2Ta	δ	0.16	0.12	0.1
	η	0.45	0.22	0.15
5Ta	δ	-0.07	-0.05	0.03
	η	0.41	0.21	0.11
8Ta	δ	-0.23	-0.21	-0.19
	η	0.6	0.43	0.41

Fig. 9. Relationship between the lattice misfits and shape parameter η of experimental alloys, some experimental alloys as well as some typical commercial Ni-based superalloys: Alloy PX-2 [17], MC520 [31], Alloy D5 [17], SRR-99 [32], AM3 [33], CMSX-2 [34], CMSX-4 [32] and CMSX-10 [35].

Fig. 10. Plot showing the linear fit of average precipitate size (r3, in nm3) vs aging time (t, in s) during isothermal aging at 900 °C. The corresponding LSW rate constant (K, in nm3·s-1) and coefficient of determination (R2) were also labeled.

Fig. 11. (a) A proxigram showing the width of γ/γ′ interface of three alloys measured by the Cr concentration profile; (b) Interfacial energies for the three alloys as a function of temperatures, and marked experimental results.

Fig. 12. (a) γ′ volume fraction, (b) mean radius and (c) number density evolution measured experimentally and simulated by kinetic modeling for three alloys, the insert was the partial magnification of temporal number density evolution during aging at 900 °C for three alloys.

Fig. 13. Simulated γ′ precipitate size distribution at the maximum value of precipitate number density for three alloys. The corresponding critical radius were also marked. (a) 2Ta alloy, (b) 5Ta alloy and (c) 8Ta alloy.

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