Tensile Properties and Deformation Behavior of Several Cast Ni-Based Superalloys Fabricated by Different Solidification Ways

doi:10.1007/s40195-016-0499-6

Abstract

Abstract:

The tensile properties and deformation behavior of several cast Ni-based superalloys, respectively, in the equiaxed, columnar-crystal and single-crystal styles are comparatively studied. The effects of solidification way, heat treatment and strain rate on the tensile properties are discussed in detail. It is found that the reduction of grain boundaries by the feasible solidification ways offers cast Ni-based superalloys the potential capability of improving the mechanical properties, the ultimate achievement of which is also confirmed to lie on the appropriate modifications of chemical composition and heat treatment. The prolongation of solid solution facilitates the precipitation of fine secondary γ′ phase, whereas the extension of high-temperature aging leads to the coarsening of secondary γ′ phase. The combination of these two aspects has a crucial influence on the tensile properties. Under tensile applied stress, the surface grains of DZ-A alloy deform slightly, while the inner grains deform heavily. This deformation inhomogeneity is ascribed to the occurrence of cracks or oblique grains near the surface of specimens and the sliding or decohesion of grain boundaries between the surface and inner grains. Regardless of strain rate, the ILTDM (intermediate-low-temperature ductility minimum) phenomenon always happens in the temperature range from 400 to 600 °C in all the investigated alloys, the occurrence of which is closely related to the strong strain-hardening behavior in the deformation process. Finally, the interaction of slip bands which are the main deformation mode below 600 °C is established to be the essential reason for the strain hardening.

Key words: Ni-based superalloy, Tensile property, Intermediate-low-temperature ductility minimum (ILTDM), Solidification way, Strain rate, Deformation inhomogeneity

Hai-Tao Li, Yong-Chun Liang, Wan-Li Zhong, Xue-Zhi Qin, Guo ?Jian-Ting, Lan-Zhang Zhou, Wei-Li Ren. Tensile Properties and Deformation Behavior of Several Cast Ni-Based Superalloys Fabricated by Different Solidification Ways[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 280-288.

Figures/Tables 12

Table 1 Chemical compositions of the investigated alloys (wt%)

Alloy	C	Cr	Co	W	Mo	Al	Ti	Ta	B	Zr	Ni
K-A	0.09	13.9	10.0	4.3	1.5	4.0	2.7	4.7	0.03	0.03	Bal.
DZ-A DD-A	0.07	13.2	10.0	4.6	1.7	4.0	2.4	4.8	0.02	-	Bal.

Fig. 1 Strength a and elongation b variations for K-A and DZ-A alloys under the V1 mode at different temperatures

Fig. 2 Strength a and elongation b variations for DZ-A and DD-A alloys under the V2 mode at different temperatures

Fig. 3 Dendritic microstructures of DZ-A a and DD-A b alloys

Fig. 4 Strength a and elongation b variations for DZ-A alloys under different strain rate modes at different temperatures

Fig. 5 Stress-strain curves of DZ-A alloys: aV1 mode; bV2 mode

Fig. 6 Stress-strain curves of DD-A alloy under the V2 mode

Fig. 7 Surface morphologies of DD-A specimens tensile ruptured at different temperatures: a, bRT; c 400 °C; d 500 °C; e 700 °C; f 800 °C. b Magnification of the white pane interior ina. Machining mark was produced when the specimens for tensile testing were machined

Table 2 Tensile properties of DZ-A alloy under different heat treatment conditions

Temperature (°C)	SHT			PHT
Temperature (°C)	UTS (MPa)	YS (MPa)	δ(%)	UTS (MPa)	YS (MPa)	δ(%)
RT	1255	1025	10.5	1260	970	11.2
500	1125	948	5.5	1100	900	8
900	815	600	34.5	795	580	34.5

Fig. 8 Distributions of γ′ phase along the applied stress in the heat-treated a and tensile-ruptured DZ-A alloys at RT b, 600 °C c, 900 °C d

Fig. 9 Longitudinal microstructures of DZ-A alloy tensile ruptured at different temperatures: a,b 700 °C; c 600 °C (“A” represents the slightly deformed area, and “B” represents the heavily deformed area)

Fig. 10 Surface or near-surface cracks of DZ-A alloy tensile ruptured at 800 °C: a low-magnification image; b cracking of carbides; c cracks on the boundaries between the oblique and regular grains. The bidirectional arrow in a represents the applied stress direction during tension and the growth direction of the DZ-A alloy during solidification

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