Element Segregation and Solidification Behavior of a Nb, Ti, Al Co-Strengthened Superalloy ЭК151

doi:10.1007/s40195-019-00894-3

Abstract

Abstract:

The as-cast microstructure, element segregation and solidification behavior of a multi-alloyed superalloy ЭК151 have been investigated. The results show that the severe element segregation leads to the complicated precipitations at the inter-dendritic region, including η-Ni₃(Ti, Nb), eutectic (γ + γ′) and Laves, which shows the characteristics of both Ti, Al-strengthened and Nb-strengthened alloys. Differential thermal analysis, heating and quenching tests reveal the solidification sequence as follows: Liquids → γ matrix → (Nb, Ti)C → η-Ni₃(Ti, Nb) → eutectic (γ + γ′) → Laves. The melting points are between 1250 and 1260 °C for (Nb, Ti)C, between 1200 and 1210 °C for η phase, between 1180 and 1190 °C for eutectic (γ + γ′) and Laves. γ′ initially precipitates from matrix at 1150 °C and achieves the maximum precipitation at 1130 °C. According to the microstructure evolution captured during solidification and composition analysis by an energy dispersive spectrometer and electron probe microanalyzer, (Nb, Ti)/Al ratio is put forward to explain the formation of η-Ni₃(Ti, Nb) and eutectic (γ + γ′). The solidification of γ matrix increased the Nb, Ti concentration in the residual liquids, so the high (Nb, Ti)/Al ratio would result in the formation of η-Ni₃(Ti, Nb); the precipitation of the phase consumed Nb and Ti and decreased the (Nb, Ti)/Al ratio in the liquid, which led to the precipitation of eutectic (γ + γ′). Laves formed by the sides of η-Ni₃(Ti, Nb) and in front of the eutectic (γ + γ′) after Al, Ti were further depleted by the two phases and Cr, Co, Mo were rejected to liquids.

Key words: Superalloy, ЭК151 Solidification, Eutectic (γ + γ′), η-Ni₃(Ti;Nb), Laves

Yuan-GuoTan, Fang Liu, An-Wen Zhang, Da-Wei Han, Xiao-Yu Yao, Wei-Wei Zhang, Wen-Ru Sun. Element Segregation and Solidification Behavior of a Nb, Ti, Al Co-Strengthened Superalloy ЭК151[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(10): 1298-1308.

Figures/Tables 13

Table 1 Chemistry (wt%) of ЭК151 alloy

C	Cr	Co	W	Mo	Al	Nb	Ti	V	B	Ni
0.06	10.86	14.93	2.93	4.41	3.62	3.26	2.73	0.59	0.003	Bal.

Fig. 1 As-cast microstructures of ЭК151 alloy at the edge a, half radius b, center c of the ingot

Fig. 2 As-cast microstructures of ЭК151 alloy: a phases precipitated in inter-dendritic region; bγ′ phases precipitated in inter-dendritic region

Table 2 Compositions of the matrix and precipitates precipitated in the inter-dendritic region measured by EPMA (at.%)

Element	V	Cr	Co	Ni	W	Nb	Mo	Ti	Al
Matrix	0.68	12.36	15.28	54.15	0.41	2.79	2.78	4.60	6.97
Phase A	0.45	4.65	13.38	58.39	0.34	7.30	1.11	8.61	6.76
Phase B	0.43	6.37	12.30	58.89	0.26	3.28	1.38	6.84	9.30
Phase C	0.60	20.94	19.67	33.40	0.68	9.92	8.20	3.21	3.07
Phase D	1.02	1.02	0.97	3.49	1.11	59.65	3.88	28.89	0.53

Fig. 3 TEM images and the corresponding selected area diffraction patterns of the precipitates in the as-cast ЭК151 alloy: aη phase at [2ˉ42ˉ32ˉ42ˉ3] zone axis; b eutectic (γ + γ′) at [112] zone axis; c Laves phase at [0001] zone axis; dMC-carbide at [112] zone axis

Table 3 Distribution and partition coefficients of the elements in the as-cast ЭК151 alloy (EPMA results, wt%)

Element	V	Cr	Co	Ni	W	Nb	Mo	Ti	Al
Inter-dendrites	0.64	11.22	15.56	55.69	1.54	3.63	4.38	3.67	3.54
Dendrites	0.77	12.06	17.04	56.79	2.86	1.30	3.67	1.95	3.55
k	1.20	1.07	1.10	1.02	1.86	0.36	0.84	0.53	1.00

Fig. 4 Element distribution in the inter-dendritic region of the as-cast ЭК151 alloy

Fig. 5 Heating DTA curve of ЭК151 alloy

Fig. 6 Microstructures of as-cast ЭК151 superalloy soaked at 1110 °C a, 1130 °C b, 1150 °C c, 1170 °C d

Fig. 7 Images and the corresponding SAED patterns of the white particles in the alloy soaked at 1130 °C: a SEM image of white particles; b TEM image of white particles; c corresponding SAED patterns at [011] axis zone of particles; d TEM-EDS of the particles

Fig. 8 Microstructures of η phase and chilled morphology of eutectic Laves a and chilled morphology of eutectic (γ + γ′) b, c EDS spectrum of the eutectic Laves, d EDS spectrum of eutectic (γ + γ′)

Fig. 9 Microstructures of η phase and chilled morphology of eutectic Laves soaked at 1210 °C a and chilled morphology of η phase soaked at 1200 °C b, c EDS spectrum of the η phase soaked at 1210 °C, d EDS spectrum of η phase at 1200 °C

Fig. 10 Microstructures of as-cast ЭК151 superalloy soaked at 1250 °C for 10 min and then quenched in water for melting pools a and MC-carbide b, c EDS spectrum of the MC-carbide

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