The Minor Precipitation at the Final Stage of U720Li Solidification

doi:10.1007/s40195-016-0414-1

Abstract

Abstract:

The minor precipitations caused by B and Zr which are the normal constituents of U720Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M₃B₂ and Ni₅Zr phases were observed in the U720Li alloy in as-cast state, which were usually accompanying with each other together with η-Ni₃Ti phase at the edge of eutectic (γ + γ′). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: γ matrix, eutectic (γ + γ′), η-Ni₃Ti, M₃B₂ and Ni₅Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of γ matrix increased the Ti concentration in the residual liquids and resulted in the eutectic (γ + γ′) formation; the (γ + γ′) formation increased the Ti/Al radio in the liquids and the η-Ni₃Ti was formed in front of the eutectic (γ + γ′); the η-Ni₃Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M₃B₂ boride is formed; the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni₅Zr is formed finally. The melting points are in the ranges of 1130-1140 °C for Ni₅Zr phase, 1180-1190 °C for M₃B₂ boride and 1190-1200 °C for η-Ni₃Ti phase.

Key words: Superalloy, U720Li, Solidification, M3B2 boride, Ni5Zr phase

Guang-Di Zhao, Lian-Xu Yu, Feng Qi, Fang Liu, Wen-Ru Sun, Zhuang-Qi Hu. The Minor Precipitation at the Final Stage of U720Li Solidification[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(6): 518-526.

Figures/Tables 12

Table 1 Chemical composition of U720Li alloy (wt%)

Cr	Co	W	Mo	Al	Ti	B	Zr	C	Ni
15.66	14.99	1.24	3.09	2.42	5.06	0.014	0.035	0.010	Bal.

Fig. 1 As-cast microstructures of U720Li alloy: a dendritic structure; b eutectic (γ + γ′) precipitation; cγ′ precipitation; dMC-carbide

Fig. 2 Precipitation of the minor phases in the interdendritic regions of the as-cast U720Li alloy

Table 2 Composition of the phases precipitated in the interdendritic region (EPMA results, at.%)

Element	W	Zr	B	Al	Mo	Ti	Cr	Co	Ni
γ′ particle in eutectic (γ + γ′)	0.05	0.10	0.00	6.97	0.32	16.34	3.39	10.45	62.37
Phase 1	0.09	1.67	0.00	3.37	0.52	18.08	3.33	11.30	61.63
Phase 2	1.34	0.00	27.94	0.03	25.52	4.70	33.05	3.16	4.26
Phase 3	0.01	15.85	0.00	1.20	0.70	5.06	4.69	12.26	60.24

Fig. 3 TEM images and the corresponding selected area diffraction patterns of the precipitates in the as-cast U720Li alloy: aη phase at [100] zone axis; bM3B2 boride at [] zone axis; c Ni5Zr phase at [011] zone axis

Fig. 4 Elemental distribution in the interdendritic region of the as-cast U720Li alloy

Table 3 The distribution and the partition coefficients of the elements in the as-cast U720Li alloy (EPMA results, wt%)

Element	Co	Al	Cr	Ti	Mo	W
Interdendritic region	13.82	2.83	12.74	8.08	2.56	0.74
Dendrite core	16.04	2.58	15.74	3.86	2.62	1.45
k	1.16	0.91	1.24	0.48	1.03	1.98

Fig. 5 Cooling DTA curve of U720Li alloy; the cooling rate is 10 °C/min

Fig. 6 Microstructures of the as-cast U720Li alloy soaked at 1130 °C for 10 min and then quenched in water: a the metallographic graph; b Ni5Zr phase and η phase not melted; cM3B2 boride not melted

Fig. 7 Microstructures of the as-cast U720Li alloy soaked at 1140 °C for 10 min and then quenched in water: a melted pools in front of the eutectic (γ + γ′); b morphology of a Zr-rich phase in a melted pool; c EDS spectrum of the Zr-rich phase

Fig. 8 Microstructures of as-cast U720Li alloy soaked at 1190 °C for 10 min and then quenched in water: a melted pools in front of the eutectic (γ + γ′); b morphology of a melted pool; c EDS spectrum of the Mo-Cr-rich boride; d SEM secondary electron image of a η phase not melted

Fig. 9 Microstructures of as-cast U720Li alloy soaked at 1200 °C for 10 min and then quenched in water: a melted pools in front of the eutectic (γ + γ′); b morphology of a melted pool

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