Ti-Al-Ru合金凝固组织的多尺度演变与强塑性调控

TiAl-based alloys are considered as the light-weight high-temperature structural materials in the next-generation, becoming a research hotspot in the field of advanced aerospace materials. How to further improve the service temperature of TiAl-based alloys (> 800 °C) and develop high temperature Ti-Al alloys, has attracted close attentions in recent years. The Ru element (belong to Platinum Group Metals, PGMs) possesses the advantages of high melting point, excellent oxidation resistance, extremely high corrosion resistance and low cost. As the alloying element, it has been successfully applied in some metallic materials such as superalloy, titanium alloys, steel, which can significantly improve the high-temperature performance of these materials.. Therefore, this aim of the current project is to inducing the element of Ru into TiAl-based alloys. A potential way will be founded to guide the development of new generation Ti-Al alloys, by means of investigating several key issues involving with the solidification microstructure and its relationship to mechanical properties in the Ti-Al-Ru alloy. The project will be carried out in the following four aspects: (1) the microstructure evolution and phase transition path in Ti-Al-Ru alloys under equilibrium solidification; (2) the microstructure formation and its multi-scale characteristics under temperature-controlled solidification/cooling (over a broad range of parameters); (3) the precipitation and control of the G phase during aging and its effects on the mechanical properties; (4) the way and its mechanism of the plasticity improvement for the (α2 + γ ) Ti-Al-Ru alloy. On that basis, the influence rules and the mechanisms of Ru element on the solidification microstructure of TiAl alloy will be clarified. Also, the relationship between the microstructure and the mechanical properties (strength and plasticity) of Ti-Al-Ru alloy will be revealed, followed by obtaining a pathway for the microstructure control. These results will provide theoretical support to the development of Ti-Al-Ru alloy.

Ti-Al系合金作为新一代轻质高温结构材料，是当前空天材料的研究热点。如何发展更高服役温度的Ti-Al系合金（> 800 ℃），近年来受到密切关注。Pt族金属中的Ru元素具有高熔点、抗氧化、极耐蚀、低成本的优点，可显著提升结构材料的高温性能。因此，本项目拟对Ti-Al进行Ru合金化，通过研究Ti-Al-Ru合金凝固组织及其力学关联中的几个关键问题，为发展高温Ti-Al系合金提供一条可能的途径。项目将围绕以下四部分开展：(1)平衡凝固下Ti-Al-Ru合金组织结构与相变路径；(2)宽参数凝固/冷却下组织形成规律及其多尺度特征；(3)时效过程中G相析出调控及其对力学性能的影响；(4)α2+γ双相Ti-Al-Ru合金的增塑途径及机理。在此基础上，将阐明Ru元素对Ti-Al合金凝固组织的作用规律及机制，并揭示合金组织结构-强塑性关联机理与调控方法，为发展Ti-Al-Ru合金提供理论支撑。

γ-TiAl合金具有高比强、高蠕变抗力及优良抗氧化等优异高温性能，被认为是在800~900℃范围内替代镍基高温合金，实现航空航天能动装备减重增效的唯一候选材料。贵金属Ru元素对TiAl合金的晶体结构、相变以及力学行为均能产生较大的影响。然而，一方面，关于Ti-Al-Ru合金相变路径的研究不够系统，无法有效地指导热处理组织调控工作；另一方面，Ru元素的强韧化机理大多未经实验证实，且主导机制也并不明确。本研究以三元Ti-(42, 45, 48)Al-(1.0, 2.0)Ru及多组元Ti-48Al-4Nb-2Cr-(0, 0.3, 1.0)Ru合金为研究对象，研究了含Ru-TiAl合金的相变与相变路径，分别发现了α→γ+τ1与β→γ+τ1两种共析分解反应，其产物形貌及晶体学取向关系与冷却速度密切相关。绘制了1000℃~1420℃之间的伪二元相图，发现Ru元素的β稳定能力与W接近，是最强的β稳定元素之一。发现了Ru元素能够促进γ-TiAl合金亚稳组织的析出。在此基础上，采用多步热处理工艺将Ti-48Al-4Nb-2Cr-0.3Ru合金调控为双态组织，室温抗拉强度与塑性分别提升至484MPa和1.51%。第一性原理计算表明，当Ru原子占据γ/γ真孪晶界面与∑5重位点阵界面处的Al位置时，体系自由能分别降低13%和11%，促进块状与羽毛状组织的析出。对Ti-48Al-4Nb-2Cr与Ti-48Al-4Nb-2Cr-1.0Ru合金进行850℃原位同步辐射压缩测试，结果显示后者的抗压强度较前者高110MPa。研究表明，弥散析出的纳米τ1相颗粒可有效地阻碍位错滑移，提升抗压强度。压缩变形后，各相均存在明显的丝织构。晶格应力分析表明Ru元素可降低γ相的层错能，诱导形变孪晶的形成，进而提升γ相的塑性。此外，本研究首次发现了τ1相的塑性变形行为，其变形机制为<110>系位错的滑移。