| 其他摘要 | With high strength, high modulus, good oxidation and creep resistance properties and excellent fatigue properties, γ-TiAl alloys are becoming a preferred choice for structural material of the advanced aero-engine system. However, the low toughness at room temperature of γ-TiAl alloys prepared by various processes greatly restricted their application. The introduction of directional solidification technology to control the lamellar microstructure of γ-TiAl alloys can enhance the room temperature ductility to beyond 5% in a certain orientation. In order to achieve this goal, this paper surveys and investigated the principle, technology and properties of the directional solidification of TiAl alloys, with the aim to identifying a way to producing useful directionally solidified materials.
The experiment is mainly carried out in an optical floating zone furnace with high temperature gradience (200℃/cm). The structure of the furnace and the experimental methods has been described in detail. The four basic techniques, general directional solidification, seeding DS growth, seeding DS growth on small seeds and single crystal growth, were investigated. In order to meet the requirements of the experiment, three kinds of methods were used to prepare the feed rods of Ti-43Al-3Si, Ti-Al-Nb and binary TiAl alloys. Among them, the suction casting method III can efficiently prepare the feed rods with high purity, high uniformity and high-precision alloy composition to meet the needs of DS experiments of a variety of multi-component TiAl alloys.
The seeding directional solidification technology is an effective way to control the lamellar microstructure of TiAl alloy. The Ti-43Al-3Si alloy was taken as an example to study in detail the preparation of the seeds including polycrystalline seeds and seeding DS growth. The preferred DS ingot, PST crystal and the target DS ingots with full lamellae parallel to the growth direction were obtained at a growth rate of 5 mm/h. The mechanism of seeding DS growth, effect of seed shape and alloy composition on the seeding course was discussed in detail. The basic principle of seeding directional solidification was demonstrated in the case of lamellar TiAl. The way to adjust and accurately control the alloy composition and to prepare the seed in a controlled manner was identified. The preferred lamellar texture of Ti-43Al-3Si ingot that is expected to be seeds was obtained.
The DS technology to control lamellar structure without seeding is more feasible and was investigated comprehensively. The Ti-49Al and Ti-51Al PST crystal and the Ti-50Al bi-crystal was grown in the directional solidification without seeding. Then, an operational seedless DS technology by alloying is investigated in order to obtain the DS ingots with lamellar interface parallel to the growth direction. A Ti-44Al-Nb ingot designed in this work achieved the objective. Not only the lamellar texture with 35°～47° angle to growth direction but also the target structure accounted for more than 70% of rod with lamellar texture parallel to growth direction were obtained in the seedless directional solidification of the Ti-44Al-Nb ingot. More importantly, the alloy has a strong "convergence" and is expected to control the lamellar structure parallel to the growth direction with ultra-fast growth rate of 600 ~ 6000mm/h. This is of great significance to solve the shell mould problem.
In order to verify the forecast results of Ti-44Al-Nb alloy and analyze the microstructure and properties of various TiAl DS ingots, the optical floating zone furnace was modified and improved. Through the development of high-purity argon purification systems, suction casting systems for high-purity active materials, as well as to upgrade the speed control system of the optical floating zone furnace were set up, allowing highly efficient and advanced directional solidification applicable to high-activity, high purity metal materials. At the same time, the homemade DS shell mould of TiAl alloys was applied in the directional solidification of Ti-43Al-3Si alloys in a ram-type furnace. To lay a solid foundation for the solution to the well-known shell mould problem and application of TiAl DS growth, the method to make thermally stable shell mould of TiAl alloys was also studied. |
修改评论