Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification

doi:10.1016/j.jmst.2017.03.017

Abstract

Abstract:

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.

Key words: Surface eutectic, Re addition, Single crystal, Superalloy

Cao Liang, Zhou Yizhou, Jin Tao, Sun Xiaofeng. Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification[J]. J. Mater. Sci. Technol., 2017, 33(11): 1308-1313.

Figures/Tables 8

Table 1 Nominal chemical compositions of the investigated Ni-base SC superalloys (wt%).

Alloy	Re	Cr	Mo	Co	W	Ta	Al	Hf	C	Ni
Re-free Alloy	0	7.15	1.50	7.80	5.05	6.60	6.30	0.15	0.07	bal
3 wt% Re alloy	3.00	7.15	1.50	7.80	5.05	6.60	6.30	0.15	0.07	bal
6 wt% Re alloy	6.00	7.15	1.50	7.80	5.05	6.60	6.30	0.15	0.07	bal

Fig. 1. Schematic diagram showing measurement of areal fraction: (a) distribution map of measurement zones in transverse section of single crystal bar, (b) size of one measurement zone.

Fig. 2. Micrographs of surface eutectics in samples with different Re contents: (a) OM image of Re-free alloy, (b) OM image of 6 wt% Re alloy, (c) SEM image of Re-free alloy, (d) SEM image of 6 wt% Re alloy.

Fig. 3. Quantitative analysis showing increasing volume fraction of eutectics as Re content increases.

Fig. 4. EPMA elemental distribution of representative surface eutectic in 6 wt% Re alloy.

Table 2 Segregation of Al and Ta in dendrite cores and eutectics.

	Al			Ta
Alloy	cAlD	cAlE	kAl	cTaD	cTaE	kTa
Re-free Alloy	5.95	7.01	0.85	4.05	6.11	0.66
3 wt% Re alloy	5.66	7.63	0.74	3.60	8.09	0.44
6 wt% Re alloy	5.24	8.14	0.64	3.61	10.76	0.34

Fig. 5. DTA results for heating processes in Re-free and 6 wt% Re alloys.

Table 3 Solidification characteristic temperatures of Re-free and 6 wt% Re alloys (TL: liquidus temperature, TS: solidus temperature, TL-TS: freezing range) (°C).

Alloy	T_L	T_S	T_L-T_S
Re-free Alloy	1382	1340	42
6 wt% Re alloy	1388	1338	50

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