Effects of Homogenization Treatment on the Microsegregation of a Ni-Co Based Superalloy Produced by Directional Solidification

doi:10.1007/s40195-021-01192-7

Abstract

Abstract:

To reduce microsegregation, a series of homogenization treatments were carried out on a Ni-Co based superalloy prepared through directional solidification (DS). The element segregation characteristics and microstructural evolution were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The results show that the elements are non-uniformly distributed in the solidified superalloy, in which W and Ti have the greatest tendency of microsegregation. Furthermore, severe microsegregation leads to complicated precipitations, including η-Ni₃Ti and eutectic (γ + γ′). EPMA results show that Al and Mo are uniformly distributed between the eutectic (γ + γ′) and γ matrix, whereas Ti is segregated in the eutectic (γ + γ′) and η phases. The positive segregation element Ti, which is continuously rejected into the remaining liquid during γ matrix solidification, promotes the formation of eutectic (γ + γ′) and the transformation of the η phase. According to the homogenization effect, the optimal single-stage homogenization process of this alloy is 1180 °C for 2 h because of the sufficient diffusion segregation of the elements. In the present study, a kinetic diffusion model was built to reflect the degree of element segregation during homogenization, and the diffusion coefficients of W and Ti were estimated.

Key words: Ni-Co based superalloy, Element segregation, Homogenization, Diffusion kinetics

Zijian Zhou, Rui Zhang, Chuanyong Cui, Yizhou Zhou, Xiaofeng Sun. Effects of Homogenization Treatment on the Microsegregation of a Ni-Co Based Superalloy Produced by Directional Solidification[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(7): 943-954.

Figures/Tables 14

Table 1 Chemical compositions of the Ni-Co based alloy (wt%)

Ni	Co	Cr	Al + Ti	Mo	W	C + B + Zr
Bal.	20-25	13-15	7.4-8.4	2.4-2.8	1.1-1.5	0.04-0.11

Fig. 1 a Macrograph of the longitudinal section of the as-cast DS specimen, b enlarged region corresponding to figure a

Fig. 2 SEM micrographs of a longitudinal cross sections of the DS specimen, b irregular γ′ in the interdendritic region and c fine γ′ in the dendrite core region

Fig. 3 SEM micrographs of main precipitates in the new Ni-Co based alloy: a (γ + γ′) eutectic; cη phase; b, d EDS point analyses from the eutectic and η phase constituent; e TEM morphology of ηphase (the inset is the SAED pattern of η phase)

Table 2 Compositions of the different microstructures in the interdendritic region (wt%)

	Ni	Co	Cr	Al	Ti	Mo	W
Matrix	51.15	22.16	11.17	3.44	8.59	2.45	1.04
(γ + γ′) eutectic	60.91	17.63	3.84	5.73	11.40	0.38	0.11
η phase	58.42	16.93	3.23	2.51	18.15	0.67	0.09

Fig. 4 EPMA mappings of the elemental distribution taken from the interdendritic region

Table 3 Element distribution in different regions (wt%) and segregation coefficient

	Co	Cr	Al	Ti	Mo	W
Dendritic core	25.70	15.72	2.53	3.86	2.90	2.36
Interdendritic region	23.22	12.11	2.64	7.82	2.74	1.43
k	1.11	1.30	0.96	0.49	1.06	1.65

Fig. 5 Dendritic microstructure evolution of the ingot after homogenizing for 2 h at different temperatures: a 1100 °C; b 1120 °C; c 1140 °C; d 1160 °C; e 1180 °C, f 1200 °C

Fig. 6 Microstructure evolutions of eutectic and η phase in the ingot after homogenizing for 2 h at different temperatures: a 1100 °C; b 1120 °C; c 1140 °C; d 1160 °C; e 1180 °C, f 1200 °C

Fig. 7 Dendritic microstructure evolutions of the ingot after homogenizing at 1180 °C for different hours: a 1 h; b 1.5 h; c 2 h; d 4 h

Fig. 8 Variation of segregation coefficient of different elements with homogenization time at 1180 °

Table 4 Variations in W and Ti contents of the as-cast and homogenized alloys across the EDS point analyses

Temperature	Time (h)	W (wt%)			Ti (wt%)
Temperature	Time (h)	C_M	C_m	$\delta$	C_M	C_m	$\delta$
As-cast	0	2.49	1.17	1	7.94	3.86	1
1180 °C	1	2.37	1.52	0.64	6.86	4.42	0.63
1180 °C	1.5	2.28	1.58	0.53	6.79	4.70	0.51
1180 °C	2	2.15	1.61	0.41	6.41	4.88	0.37
1180 °C	4	2.02	1.62	0.32	6.11	4.89	0.29

Fig. 9 Linear relationship between ln $\delta$ and t for a W, b Ti, c Mo, d Cr, e Co, f Al at 1180 °C

Table 5 Diffusion coefficients of W, Ti, Mo, Cr, Co and Al at 1180 °C (× 10-15 m2 s-1)

W	Ti	Mo	Cr	Co	Al
11.02	11.97	12.76	19.97	20.82	23.25

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