Effect of microstructure on the high-temperature deformation behavior of Nb–Si alloys
Introduction
As energy consumption is an imminent problem in mass production, development of a more efficient combustion system is anticipated as the solution and a new class of heat-resisting materials are required. Although the Ni-based superalloys have been the superior heat-resistant materials, the melting point does not reach 1700 K which is the combustion temperature of the current high-performance jet engine. The melting point of Nb is about 1000 K higher than that of Ni, while its density is lower than that of Ni and any other refractory metals such as Mo, W and Ta. So, Nb-based material is one of the most promising materials. Since the 1960s, various Nb-based alloys have been developed for high-temperature applications [1], but further improvement of both strength and oxidation resistance at elevated temperatures were required. Since the mid-1980s, Nb–Nb3Al two-phase alloys have been investigated [2], and also a broad research on Nb-Nb5Si3 two-phase alloys has been conducted because the Nb5Si3 intermetallic compound shows high strength at high temperature, which is suitable for the dispersoid in Nb solid solutions [3], [4], [5], [6]. However, the maximum solid solubility of Si in Nb terminal solid solution is as small as 3.5 at.% which provides a very limited fraction of α-Nb5Si3 precipitate phase in the Nb matrix through any heat treatment. So far, it is quite a natural consequence to utilize the eutectic reaction to obtain metal-silicide in situ composite [4]. On the other hand, the authors have paid attention to combine a eutectic reaction at 2188 K (L − >Nb + Nb3Si) and a eutectoid reaction at 2043 K (Nb3Si − >Nb5Si3 + Nb) to obtain a microstructure with a high-volume fraction of fine Nb5Si3 dispersed in Nb grains through a sequence depicted in Fig. 1 [7], [8], [9]. Keys for the microstructure development are as follows.
- (1)
Fine Nb rods in the same eutectic cell have the same crystallographic orientation.
- (2)
The formation of Nb plates through the eutectoid reaction starts at Nb-rod/Nb3Si–matrix interface. In the process Nb plates have the same crystallographic orientation with that of Nb-rods. Consequently, a eutectic microstructure turns into a Nb network without Nb grain boundaries.
For this microstructure control, Zr and Mg addition were found to be inevitable. Zr accelerates the eutectoid decomposition, and Mg accelerates the spheroidization of Nb5Si3 (Fig. 1) [7], [8], [9], [10]. Although a similar microstructure was also reported by Kim et al. obtained by extremely high temperature heat treatment of Nb–Mo–Si alloy, the alloys contain no Nb3Si phase and the principles of the microstructure evolution might be different.
With the present microstructure composed of small Nb5Si3 dispersoids embedded in ductile Nb solid solution, the propagation of cracks initiated in the brittle Nb5Si3 during the deformation at room temperature is expected to be suppressed, and the small dispersoids act as effective obstacles for dislocation motion at high-temperature region. The purpose of this study is to understand the relationship between the newly developed microstructure and mechanical properties such as strength at high temperatures and toughness at R.T.
Section snippets
Experimental procedure
Recently, it was found that the microstructure which resembles a binary eutectic microstructure can be obtained in Nb–Si–1.5 at.% Zr alloys with Si composition around 18.1 at.% [11]. Several alloy ingots with a fixed composition of Nb–18.1Si–1.5Zr + 100ppm Mg are arc-melted. Heat treatments of alloy ingots wrapped by Ta foils were conducted under a high-purity Ar-flow atmosphere at 1923 K for 4–100 h. Microstructure observations were conducted on specimen surfaces carefully polished with colloidal SiO2
Microstructure
The microstructures of as-cast and heat-treated specimens are shown in Fig. 2. The constituent phases are examined by EPMA. In Fig. 2(a), bright and gray areas are Nb and Nb3Si, respectively, and the areas surrounded by circles are eutectic cells. In Fig. 2(b), bright and dark areas are Nb and Nb5Si3, respectively. The bright Nb phase has a tendency to connect each other, forming Nb networks. It was also confirmed by using EBSD analysis that each Nb area forming a network has an identical
Concluding remarks
Microstructure and mechanical properties of Nb-based alloy containing an intermetallic compound Nb5Si3 are investigated. The EBSD analysis of heat-treated specimens revealed that eutectic Nb and eutectoid Nb have an identical crystallographic orientation in each of eutectic cells and they form a Nb network with embedded Nb5Si3. The network had a size almost the same of that of eutectic cells in as-cast specimens.
Compression tests revealed that the strength at 1471 K is comparable to that of Ni
Acknowledgement
This study was supported by Grant-in-Aid for Scientific Research, Ministry of Education, Culture, Sports, Science and Technology, Japan, No. 19206078.
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Present address: Hitachi Ltd., Yokohama, Japan.