Preparation of high-purity Ti–Si alloys by vacuum directional solidification

Highlights

• An efficient approach to eliminate the main impurities in Ti–Si alloys.

• No waste slag, acid, and gas were discharged in the process of purification or separation.

• Vacuum directional solidification was newly employed to prepare high-purity Ti–Si alloy.

• TiSi₂ was prepared directly through the separation of Ti–Si alloys.

Abstract

To prepare high-purity Ti–Si alloys, we newly used vacuum directional solidification technology to purify and separate Ti–Si alloys, which have different compositions (Ti-8.45 wt%Si, Ti-63.7 wt%Si, Ti-71.6 wt%Si, and Ti-84.1%wt%Si). The pulling-down rates in the directional solidification were to be 3 μm/s, 5 μm/s, and 7 μm/s, respectively, to investigate their effects on the purification. The results showed that vacuum directional solidification can purify Ti–Si alloys efficiently without discharging waste acid, slag and gas, and the slower pulling-down rate could result in better purification results. The impurities, Fe, Al, V, and Ni, could be removed due to their segregation behavior on the boundary of the solid and liquid phases. However, the impurities Ca and Mg could be removed using the vacuum technology because their vapor pressures were significantly higher than those of Si and Ti. TiSi₂ could be precipitated and separated from the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys. The precipitated TiSi₂ could be agglomerated at the bottom of the Ti–Si alloys, under the effect of gravity. The phases in the Ti–Si alloys, after vacuum directional solidification, were observed and determined using an electron probe micro-analyzer (EPMA). The results showed that the eutectic Ti-8.45 wt%Si alloy consisted of Ti₅Si₃ and Ti, the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys consisted of TiSi₂ and eutectic Si–Ti alloy at the Si-rich side, and the Ti-84.1 wt%Si alloy consisted of Si particles and eutectic Si–Ti alloy at the Si-rich side. The study shows that vacuum directional solidification is an efficient and clean technology for the purification of Ti–Si alloys, and for the preparation of TiSi₂ through the separation of Ti–Si alloys.

Introduction

As a new type of cast titanium alloy, eutectic Ti–Si alloy (Ti-8.45 wt%Si) is different from traditional Ti alloys in terms of the strengthening mechanism. For example, the Ti matrix can be strengthened by the ceramic Ti₅Si₃ phase and the eutectic structure of β-Ti [1]. The eutectic Ti–Si alloy has excellent performance not only due to its low density, high specific strength, and good corrosion resistance, similar to those of traditional cast titanium alloy, but also due to the narrow solidification interval and the superior casting [2,3]. Therefore, the eutectic Ti–Si alloy is a new cast titanium alloy that exhibits excellent performance under high-temperature, is strong, wear-resistant, and low-cost. It, therefore, meets the requirements needed for aero-engine compressor parts, aircraft fasteners, and large-scale thin-wall complex titanium alloy castings [4,5]. In addition, as TiSi₂ has a low-density, high-temperature oxidation resistance, high-temperature stability, and high-temperature strength, it can be employed as high temperature structural material [6,7]. TiSi₂ is also employed as gate electrode wiring, as interconnect lines, and as Schottky diode and ohmic contact materials because of its low resistivity and favorable field-emission properties. Therefore, TiSi₂ plays an important role in the technology for integrated circuit contacts and bonding [[8], [9], [10]].

Recently, methods for the direct preparation of Ti–Si alloys using molten salt electrolytic metal oxides have been proposed. For example, Li et al. [11] prepared a Ti–Si–Fe alloy, which contains Mg, Ca, and Al impurities, at 900 °C, in CaCl₂ molten salt with a sintered TiO₂ and ilmenite mixture (Ti: Fe = 1:1 atomic ratio) as the cathode, and a graphite rod as the anode. Zou et al. [12] prepared Ti–Si alloy using the molten salt electrolysis multi-component Ti/Si metal oxide. However, the Ti–Si alloy thus prepared also contained Ca, Mg, Al, and Fe impurities. Therefore, it is clear that Ti–Si alloys prepared by molten salt electrolysis contain some impurities. Few studies have been carried out for purification of Ti–Si alloys, but there are some methods for purifying metallurgical-grade Si (MG-Si), including acid leaching [13,14], solvent refining [[15], [16], [17]], vacuum refining [18,19], directional solidification [[20], [21], [22]], etc. The methods of acid leaching and solvent refining will produce waste acid solution, especially HF was employed in process of Si purification. Vacuum refining and directional solidification are physical methods which do not discharge waste acid, slag and gas. The directional solidification technology has been employed to purify Si and to separate Si crystals from Si alloys, such as Si-Al [20,23,24], Si-Sn [21,[25], [26], [27]], Si–Cu [22,28] and Si–Ni [29].

Therefore, in this study, we propose the use of the directional solidification technology with a vacuum system (vacuum directional solidification) to purify the Ti–Si alloy. Vacuum directional solidification is a clean technology to purify Ti–Si alloy, because the main impurities are eliminated by physical phenomenon such as segregation behavior at the solid-liquid interface, and volatile behavior from the molten melt to the vacuum. The vacuum system, equipped with the directional solidification furnace, is used to eliminate impurities such as Ca and Mg, which can volatilize easily, making the directional solidification technology more efficient for the purification of Ti–Si alloys. In addition, the directional solidification technology is also employed to separate TiSi₂ from the Ti–Si alloys. Finally, the mechanisms for purifying and separating Ti–Si alloys, using vacuum directional solidification, are also discussed.