Effect of microstructure on the high-temperature deformation behavior of Nb–Si alloys

Abstract

Deformation behavior of Nb–Si–Zr alloys is investigated at various temperatures ranging from R.T. to 1670 K. The master alloy ingots composed of Nb–18.1 at% Si–1.5 at% Zr doped with Mg are Ar-arc-melted. The ingots contain Nb rods (radius: 1 μm) in Nb₃Si matrix formed by eutectic reaction. Alloys are subjected to heat treatments at 1923 K for 4–100 h to obtain a large Nb network structure with small silicide (α-Nb₅Si₃) particles by decomposing Nb₃Si matrix into Nb and Nb₅Si₃ through a eutectoid reaction. Compression tests are conducted at room temperature in air and at elevated temperatures in Ar atmosphere. At 1471 K the maximum strength is 500 MPa and compressive ductility is higher than 10% with a strain rate of 1.0 × 10⁻⁴ s⁻¹, while at room temperature the maximum strength is over 1500 MPa and compressive ductility is about 1.5%. The high-temperature deformation obeys a power-law type equation. The stress exponent n is evaluated to be 4.8 and the apparent activation energy is 350 kJ/mol. The Vickers indentation at room temperature revealed that the crack propagation at room temperature is suppressed effectively by ductile Nb. This suggests that the Nb aggregate in the network structure acts as a large Nb grain containing fine Nb₅Si₃ particles, which might be beneficial for ductility at low temperatures.

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.

• Fine Nb rods in the same eutectic cell have the same crystallographic orientation.

• 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.