Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy

doi:10.1016/j.jmst.2022.05.029

Abstract

Abstract:

Various cooling scenarios (water, oil, air and furnace) were employed to study the impacts of the solution cooling rate (SCR) on the microstructure and creep behavior of a novel single-crystal (SX) superalloy. The results showed that the cubic degree and size of the γ′ phases were inversely proportional to the SCR. The creep life first increased and then dropped dramatically with a reduction in the SCR. The creep life of the sample cooled with air cooling (AC) was the highest, up to 144.90 h at 800 °C/750 MPa and 160.15 h at 1100 °C/137 MPa. During creep at 800 °C/750 MPa, the improved creep life of the AC sample was mainly attributed to the fine cubic γ′ phases, which decreased the rate of γ′-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations. The AC helped the γ′ phases become rich in Al, Ti and Ta while depleted in Co and Cr, which enhanced its stacking fault energy, thus promoting the formation of dislocation locks. Meanwhile, the largest negative lattice misfit caused by AC induced denser γ/γ′ interface dislocation networks at 1100 °C/137 MPa, which efficiently reduced the minimum creep rate. The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample, proving its greatest creep resistance. Interestingly, the size of the secondary γ′ phases first decreased and then increased sharply with decreasing SCR during creep at 1100 °C/137 MPa, when fine secondary γ′ phases had a positive role in the blockage of dislocation movement in the matrix. Eventually, the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys.

Key words: Solution cooling rate, Microstructure, Creep property, Deformation mechanism, Single crystal superalloy

Xipeng Tao, Yunling Du, Xinguang Wang, Jie Meng, Yizhou Zhou, Jinguo Li, Xiaofeng Sun. Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy[J]. J. Mater. Sci. Technol., 2022, 131: 14-29.

Figures/Tables 20

Table 1. Chemical compositions (wt.%) of the experimental alloys.

	Cr	Co	Al	W	Ta+Mo+Ti	Ni
Nominal	8	8	5.5	8	8.5	Bal.
Measured	8.06	8.06	5.42	8.02	8.47	Bal.

Fig. 1. Schematic illustration of main experimental procedures in preparation, heat treatment, mechining, testing and analysis of samples (not to scale).

Fig. 2. As-solutionized morphologies of the experimental samples with different SCRs: (a) WC, (b) OC, (c) AC, (d) FC.

Table 2. Primary γ? morphology, volume fraction, size, and lattice misfit as well as residual stress in four as-solutionized samples.

Cooling rate	γʹ morphology	γ′ volume (%)	Average γʹ size (nm)	Lattice misfit (%)	Residual stress
WC	Small spherical	53	216	-0.36	Very high
OC	Small spherical +butterfly	55	253	-0.33	High
AC	butterfly	58	284	-0.31	Normal
FC	Large butterfly	62	320	-0.25	Low

Fig. 3. Heat-treated morphologies of the experimental samples with different SCRs: (a) WC, (b) OC, (c) AC, (d) FC.

Table 3. The γ′ volume fraction, γ/γ′ lattice misfit, dimensions of the primary γ′ phases and the channel widths of samples after various SCRs.

Cooling rate	Cube length (nm)	Channel width (nm)	Latticemisfit (%)	Morphology	γ′ volume (%)
WC	325 ± 164	67 ± 34	-0.08	spherical	63
OC	387 ± 138	71 ± 23	-0.11	spherical+cubic	65
AC	468 ± 185	85 ± 63	-0.23	cubic	66
FC	975 ± 201	312 ± 56	-0.14	cubic	64

Fig. 4. Typical γ/γ′ partitioning ratio of alloying elements in the experimental samples with various SCRs.

Fig. 5. Typical creep curves of samples at various temperatures and stresses. (a) 800 °C/750 MPa; (b) 1100 °C/137 MPa; (c) comparison of creep life of samples; (d) change of minimum creep rate of samples.

Fig. 6. Comparison of creep stress-rupture behavior of various superalloys in terms of LMP.

Fig. 7. Deformation features in crept samples at 800 °C/750 MPa. (a) Schematic of the ruptured sample after creep test [26]; (b-e) morphologies of slipping traces; (f-i) cross-sectional microstructures 5 mm away from the fracture surfaces; (b, f) WC sample; (c, g) OC sample; (d, h) AC sample; (e, i) FC sample.

Fig. 8. Dislocation configuration of the sample crept up to fracture at 800 °C/750 MPa: (a) WC; (b) OC; (c) AC; (d) FC.

Fig. 9. Longitudinal-section microstructures 5 mm away from the fracture surfaces of samples: (a) WC; (b) OC; (c) AC; (d) FC; (e) changes of the linked number NA in the crept samples; (f, g) changes of the γ channel width and the γ? phase thickness in samples.

Fig. 10. Configuration and density of superdislocations in the samples after creep rupture at 1100 °C/137 MPa: (a) WC, (b) OC, (c) AC, (d) FC; (e-l): STEM image and the corresponding element mappings of the typical secondary γ′ phases.

Fig. 11. Bright-field images of interfacial dislocation networks and secondary γ′ phases in the γ matrix: (a) WC sample, (b) OC sample, (c) AC sample, (d) FC sample.

Fig. 12. Cross-sectional microstructures interaction between dislocation and secondary γ′ particle in AC sample.

Table 4. Stacking fault energy of γ′ phase in various alloys at 800 °C.

Samples	Stacking fault energy (mJ/m²)
WC	233.6
OC	154.2
AC	136.7
FC	190.2

Fig. 13. Magnification images of various types of stacking fault locks and schematic diagrams of formation mechanism in AC sample: (a, c) I-type; (b, d) II-type.

Table 5. Visibility and invisibility of dislocation segments.

Beam	g	Partial Dislocation-I	Partial Dislocation-II	b
[001]	$\bar{2}00$	Visible	Visible	$\pm \frac{a}{2}\left[ 1\bar{1}0 \right]$
[001]	$\bar{2}\bar{2}0$	Non-visible	Part-visible	$\pm \frac{a}{2}\left[ 1\bar{1}0 \right]$
[013]	$3\bar{1}0$	Part-visible	Non-visible	$\pm \frac{a}{2}\left[ 101 \right]$
[013]	$\bar{1}3\bar{1}$	Visible	Non-visible	$\pm \frac{a}{2}\left[ 101 \right]$

Fig. 14. TEM images of the interfacial dislocation networks after 1000 h isothermal aging at 1100 °C in: (a) WC; (b) OC; (c) AC; (d) FC. (e) Plot of the lattice misfit in the four samples determined from high-temperature equilibrium and post-crept dislocation network spacing at the corresponding lattice misfit.

Fig. 15. Schematic illustration of the evolution of secondary γ′ precipitates in the various samples.

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