Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy

doi:10.1007/s40195-019-00936-w

Abstract

Abstract:

The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied. The microstructure of the alloy without Co (K4750 alloy) and the alloy containing Co (K4750-Co alloy) were analyzed. Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation of η phase during long-term aging treatment. In K4750-Co alloy, the morphology of MC carbide at the grain boundary (GB) remained dispersed blocky shape and no η phase was observed after aging at 750 °C for 3000 h. However, in K4750 alloy, almost all the MC carbides at GBs broke down into granular M₂₃C₆ carbide and needle-like η phase. The addition of cobalt could delay the decomposition of MC carbides, which accordingly restricted the elemental supply for the formation of η phase. The stress rupture tests were conducted on two alloys at 750 °C/430 MPa. When Co is substituted for Fe in K4750 alloy, the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment. This was mainly attributed to increased concentration of Al, Ti and Nb in γ′ phase in K4750-Co alloy, which further enhanced the strengthening effect of γ′ phase. After aging at 750 °C for 3000 h, substitution of Co for Fe can also cause the stress rupture life at 750 °C/430 MPa to increase from 48.72 to 208.18 h. The reason was mainly because MC carbide degradation and η phase precipitation in K4750 alloy, which promoted the initiation and propagation of micro-crack during stress rupture testing.

Key words: Nickel-based superalloy, Co addition, MC degradation, Stress rupture property

Xiao-Xiao Li, Mei-Qiong Ou, Min Wang, Xian-Chao Hao, Ying-Che Ma, Kui Liu. Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(12): 1501-1510.

Figures/Tables 13

Table 1 Chemical compositions of K4750 and K4750-Co alloys (wt%)

Alloy	C	Cr	W + Mo	Ti + Al + Nb	B	Ni	Fe	Co
K4750	0.12	20.18	4.75	5.50	0.0093	Bal.	4.33	0
K4750-Co	0.12	19.36	4.53	5.60	0.0070	Bal.	0	3.99

Fig. 1 Microstructures of K4750 alloy a-c, K4750-Co alloy d-f after standard heat treatment: a, dMC carbide; b, eM23C6 carbide; c, fγ′ phase

Fig. 2 Microstructures of K4750 alloy a-c, K4750-Co alloy d-f after long-term aging at 750 °C for 3000 h: a, d decomposed MC; b, e needle-like phase; c, fγ′ phase

Fig. 3 EPMA analysis of degenerated MC carbide in K4750 alloy after long-term aging at 750 °C for 3000 h: a secondary electron image; b Ni; c Fe; d C; e Ti; f Nb; g Cr; h Mo; i W

Fig. 4 TEM analysis of K4750 alloy after long-term aging at 750 °C for 3000 h: a, b decomposition zone images; selected area diffraction pattern of cMC, dM23C6 and eη phase; f EDS spectrum of η phase

Fig. 5 Distribution of C in MC carbide of K4750 alloy after long-term aging at 750 °C for 3000 h

Fig. 6 Schematic diagrams of MC carbide degradation in K4750 alloy during long-term aging

Fig. 7 EPMA analysis of MC carbides in K4750-Co alloy after long-term aging at 750 °C for 3000 h: a secondary electron image; b Ni; c Fe; d C; e Ti; f Nb; g Cr; h Mo; i W

Fig. 8 a TEM morphology of K4750-Co alloy after long-term aging at 750 °C for 3000 h, selected area diffraction pattern of bMC, cM23C6, d EDS spectrum of M23C6

Table 2 Stress rupture properties of K4750 and K4750-Co specimens in SHT and SHT + 750 °C/3000 h conditions

Alloy	Heat treatment	Life (h)	Elongation (%)	Heat treatment	Life (h)	Elongation (%)
K4750	SHT	164.10	10.4	SHT?+ 750 °C/3000 h	48.72	16.0
K4750-Co	SHT	264.67	9.7	SHT?+ 750 °C/3000 h	208.18	7.4

Fig. 9 Concentration variation of a Al, b Ti,c Nb at γ/γ′ interface of K4750 and K4750-Co alloy after standard heat treatment

Fig. 10 Longitudinal microstructures of stress rupture specimens in long-term aging condition: a, b K4750 alloy; c, d K4750-Co alloy

Fig. 11 Fracture surfaces and longitudinal microstructures of stress ruptured specimens in long-term aging condition: a, c K4750 alloy; b, d K4750-Co alloy

References

[1]	R.C. Reed, The Superalloys:Fundamentals and Applications (Cambridge University Press, New York, 2006)
[2]	C.T. Sims, W.C. Hagel, The Superalloys-Vital High Temperature Gas Turbine Materials for Aerospace and Industrial Power (Wiley, New York, 1972)
[3]	T.M. Pollock, S. Tin, J. Propuls.Power 22, 361 (2006)
[4]	X. Xin, W.H. Zhang, L.X. Yu, F. Liu, Mater. Sci. Forum 816, 613 (2015)
[5]	W.Z. Wang, T. Jin, J.L. Liu, X.F. Sun, H.R. Guan, Z.Q. Hu, Mater. Sci. Eng. A 479, 148 (2008)
[6]	Y. Yuan, Y.F. Gu, C.Y. Cui, T. Osada, Z.H. Zhong, T. Tetsui, T. Yokokawa, H. Harada, MRS Bull. 26, 2833(2011)
[7]	L.Z. Zhuang, G.L. Chen, J.L. Xu, Mater. Mech. Eng. 2, 11(1987)
[8]	M.Q. Ou, X.C. Hao, B.F. Wan, T. Liang, Y.C. Ma, K. Liu, J. Mater. Sci. Technol. 33, 1300(2017)
[9]	X.C. Hao, L. Zhang, X. Zhao, T. Liang, Y.C. Ma, K. Liu, Mater. Sci. Forum 816, 586 (2015)
[10]	M.Q. Ou, Y.C. Ma, H.L. Ge, B. Chen, S.J. Zheng, K. Liu, Mater. Sci. Eng. A 736, 76 (2018)
[11]	W.Z. Wang, T. Jin, J.L. Liu, Mater. Sci. Eng. A 479, 148 (2008)
[12]	M. Nathal, L. Ebert, Metall. Mater. Trans. A 16, 1863 (1985)
[13]	Q.Y. Shi, X.F. Ding, M.L. Wang, Metall. Mater. Trans. A 45, 1833 (2014)
[14]	G.L. Chen, L.Z. Zhuang, J.L. Xu, Acta Metall. Sin. 22, 453(1987). (in Chinese)
[15]	S. Ma, L. Carrol, T.M. Pollock, Acta Mater. 55, 5802(2007)
[16]	C.Y. Cui, Y.F. Gu, Y. Yuan, H. Harada, Scr. Mater. 64, 502(2011)
[17]	F. Xue, H.J. Zhou, Q. Feng, MATEC Web Conf. 14, 15002(2014)
[18]	C.M.F. Rae, R.C. Reed, Acta Mater. 49, 4113(2001)
[19]	G. Lvov, V.I. Levit, M.J. Kaufman, Metall. Mater. Trans. A 35, 1669 (2004)
[20]	W. Sun, X.Z. Qin, J.T. Guo, L.H. Lou, L.Z. Zhou, Acta Metall. Sin. 52, 455(2016). (in Chinese)
[21]	J.X. Yang, Q. Zheng, X.F. Sun, H.R. Guan, Z.Q. Hu, Mater. Sci. 41, 6476(2006)
[22]	J.X. Yang, Q. Zheng, X.F. Sun, H.R. Guan, Z.Q. Hu, Mater. Sci. Eng. A 429, 341 (2006)
[23]	S.M. Seo, I.S. Kim, J.H. Lee, C.Y. Jo, H. Miyahara, K. Ogi, Metall. Mater. Trans. A 38, 883 (2007)
[24]	J. Wang, Master thesis, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 2011 (in Chinese)
[25]	Y.L. Xu, L. Zhang, J. Li, X.S. Xiao, X.L. Cao, G.Q. Jia, Z. Shen, Mater. Sci. Eng. A 544, 48 (2012)
[26]	G.N. Maniar, J.E. Bridge, Metall. Trans. 2, 95(1971)
[27]	G.N. Maniar, J.E. Bridge, H.M. James, Metall. Trans. 2, 1484(1971)
[28]	J.X. Zhang, J.C. Wang, H. Harada, Y. Koizumi, Acta Mater. 53, 4623(2005)
[29]	T. Link, A. Epishin, B. Fedelich, Philos. Mag. 89, 1141(2009)
[30]	X.M. Dong, X.L. Zhang, K. Du, Y.Z. Zhou, T. Jin, H.Q. Ye, J. Mater. Sci. Technol. 28, 1031(2012)
[31]	X.Z. Qin, J.T. Guo, C. Yuan, C.L. Chen, J.S. Hou, H.Q. Ye, Mater. Sci. Eng. A 485, 74 (2008)
[32]	M.E. Kassner, T.A. Hayes, Int. J. Plast. 19, 1715(2003)
[33]	T. Krol, D. Baither, E. Nembach, Acta Mater. 52, 2095 (2004)
[34]	C.N. Wei, H.Y. Bor, L. Chang, J. Alloys Compd. 509, 5708(2011)
[35]	Y.S. Lim, D.J. Kim, S.S. Hwang, H.P. Kim, S.W. Kim, Mater. Charact. 96, 28(2014)
[36]	M.Q. Ou, X.C. Hao, Y.C. Ma, R.C. Liu, L. Zhang, K. Liu, J. Alloys Compd. 732, 107(2018)