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Solidification behavior and microstructure evolution of Nb-Si-Mo alloy in ultrasonic field

https://doi.org/10.1016/j.ijrmhm.2022.105933Get rights and content

Highlights

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    The ultrasonic vibration is successfully introduced into ultra-high temperature Nb-Si-Mo melt.

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    The nucleation and growth of β-Nb5Si3 phase in ultrasonic field are explored.

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    The standing wave causes the specific energy distribution, making the silicide in linear arrangement.

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    The ultrasonic wavelength and the introducing efficiency are calculated.

Abstract

The ultrasonic irradiation is successfully introduced into Nb-Si-Mo melt. The coarse silicide has been refined significantly. Ultrasonic irradiation can increase the solid solubility of Si in Nbss phase. The vibration attenuates during propagation in transition region, leading to the silicide with specific shape in linear arrangement. The wavelength and introducing efficiency of the standing wave are calculated.

Introduction

Nbsingle bondSi alloys have received numerous interest for fabricating high temperature components because of their outstanding mechanical properties [1]. However, many investigations illustrated that the poor fracture toughness is caused by the oversized silicide. To improve the mechanical properties, Guo et al. [2] and Bewlay et al. [3] have investigated directional solidification to refine the silicide. However, the directionally solidified microstructure shows obvious anisotropy. Consequently, it is necessary to find a novel method for isotropic microstructure of Nbsingle bondSi alloys. Recently, ultrasonic treatment (UST) has attracted extensive attentions for its cavitation and acoustic streaming effects, which can refine grains significantly [4]. According to the previous studies of Zheng [5], UST was successfully introduced into high chemical reactivity TiAl melt. On this basis, we improve the equipment for the Nbsingle bondSi melt above 1800 °C. This work is to describe the solidification behavior in ultrasonic field, furthermore, to illustrate the influence of ultrasonic irradiation on the microstructural evolution.

Nb-20Si-5Mo (at.%) alloys were fabricated by arc melting with 650A arc current under argon gas protection. The purity of raw material used in this study is 99.95% or higher. All the ingots were melted five times for the uniformity of elements. The UST is introduced indirectly by a JY-V 9.6 generator with 20 μm amplitude and the vibration sustained for 100 s with the ultrasonic power of 2000 W and 20.5 kHz. The detailed experimental procedures are referred to author's previous study [6]. The height and diameter of the ingots are about 13 mm and 25 mm, respectively. In this paper, the ultrasonic vibration region typically about half the height of the ingots under the experimental parameters mentioned above. And we focus on the solidification behavior and microstructure evolution in this region. The ingots are cut along the longitudinal direction, ground and polished for observation. A Quanta 200F (SEM) was used to observe microstructure and EDS was used to analyze the chemical composition. The micron scale silicide and the phase orientation are observed by Talos F200X (TEM).

Section snippets

Results and discussion

The microstructures of the Nb-20Si-5Mo alloys are illustrated in Fig. 1, which consist of white Niobium solid solution (Nbss) phase and gray primary silicide phase. From Fig. 1 a, c, e of as-cast specimen, the primary β-Nb5Si3 has sharp corners and an average size of about 25 μm. In the top area, the heat is input into melt by arc, which is the farthest from water-cooled crucible, making the cooling rate the lowest. Therefore, the primary β-Nb5Si3 phase has enough time to grow. The blocky

Conclusions

In conclusion, ultrasonic vibration was successfully introduced into the elevated temperature melt, and the microstructure of the Nb-20Si-5Mo alloys transform from coarse silicide and Nbss/β-Nb5Si3 eutectic to the full eutectic without primary silicide, which means UST has a significant refinement effect in Nbsingle bondSi based melt. And UST can increase the solid solubility of Si element in Nbss, which decreases the Si atoms for forming silicide phase. Furthermore, the change of melt state caused by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge support from the National Natural Science Foundation of China (51825401) and the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (P2021-004).

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