Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni-Mo-Cr Superalloy

doi:10.1007/s40195-015-0277-x

Abstract

Abstract:

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni-Mo-Cr superalloys with different silicon contents (0 and 0.46 wt%) was investigated. Experimental results showed granular M ₂C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M ₂C carbides will transform into plate-like M ₆C carbides as exposure time increases. For the Si-containing alloys, only the granular M ₆C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.

Key words: Ni-Cr-Mo, Silicon, Carbides, Mechanical properties

Zhou-Feng Xu, Jia-Sheng Dong, Li Jiang, Zhi-Jun Li, Xing-Tai Zhou. Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni-Mo-Cr Superalloy[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 951-957.

Figures/Tables 10

Table 1 Chemical compositions of the experimental alloys (in wt%)

Alloy	Ni	Mo	Cr	Fe	Mn	Si	C
Standard alloy	71.88	16.6	7.09	3.83	0.52	0.46	0.04
Non-silicon alloy	71.6	16.7	7.35	4.19	0.44	0.00	0.053

Fig. 1 BSE micrographs showing the microstructure of Ni-Cr-Mo superalloys after solution treatment: a standard alloy; b non-silicon alloy

Fig. 2 Morphologies of the intergranular precipitates in the standard alloy after exposure at 700 °C for 100 h a, 500 h b, 2000 h c, 5000 h d

Fig. 3 TEM analysis of the intergranular carbides in standard alloy: a microstructure observation, aging at 700 °C for 100 h; b EDS analysis result and the corresponding diffraction pattern of a carbide in a; c microstructure observation, aging at 700 °C for 2000 h; d EDS analysis result and the corresponding diffraction pattern of a carbide in c

Fig. 4 Morphologies of the intergranular precipitates in the non-silicon alloy after exposure at 700 °C for 100 h a, 500 h b, 2000 h c, 5000 h d

Fig. 5 TEM analysis of the intergranular carbides in non-silicon alloy: a microstructure observation, aging at 700 °C for 100 h; b EDS analysis result and the corresponding diffraction pattern of a carbide in a; c microstructure observation, aging at 700 °C for 2000 h; d EDS analysis result and the corresponding diffraction pattern of a carbide in c

Fig. 6 Size of the intergranular carbides in the standard alloy and the non-silicon alloy after exposure for different time

Fig. 7 Effect of aging time on the ultimate strength of the two alloys at 700 °C

Fig. 8 Effect of aging time on the elongation of the two alloys at 700 °C

Fig. 9 SEM images of the fracture surfaces of the experimental alloys: a non-silicon alloy, aged at 700 °C for 100 h; b standard alloy, aged at 700 °C for 100 h; c non-silicon alloy, aged at 700 °C for 5000 h; d standard alloy, aged at 700 °C for 5000 h

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