钛铝熔体遗传性影响的定向片层结构及其变形机理研究

Due to it exhibits properties of low density, high specific strength, excellent burning and oxidizing resistance at elevated temperature from 700℃ to 900℃, γ-TiAl alloy has been receiving considerable attention as one of the promising structural materials for application in aviation, automotive and power generation industries. However, the intrinsic brittleness of TiAl alloys as an intermetallic compound has seriously restricted its applications. Through controlling the composed γ/α2 lamellar boundaries aligned to the tensile direction by unidirectional solidification technique, the performance of TiAl based alloy can be improve, especially a good combination of room temperature toughness, ductility and high temperature strength. Owing to the peritectic nature of TiAl, the complex rules of competitive crystals growth and the changes in solidification path often lead to difficulties in the control of directional solidification microstructure and precipitation in terms of the shape, content and orientation. Particularly for those engineering TiAl, the diversity in chemical composition, differences in melting process and inter-mediate alloy composition often lead to unstable mechanical behaviour, especially the ductility at room temperature. Therefore, hereditary study on the TiAl alloy melt structure and lamellar structure may lead to the solutions for these problems to be delivered. In the present research, the rapid quenching and resistivity measurements methods were combined to study the structure transformation behaviour of TiAl alloy melt structure and lamellar hereditary. The effects of melt structure on the solidification behaviour, lamellar orientation and deformation mechanism, during the directional solidification process in the cold crucible, have been investigated. The theoretical basis has been developed for the materials optimization of TiAl alloy, performance improvement and the prospect engineering applications.

TiAl是最佳的下一代轻质耐热结构选材，通过减重促进航空、航天和运输等领域的高效和减排。TiAl的本征脆性可通过具有规则片层取向排列的定向凝固组织来改善。但TiAl属包晶合金体系，复杂的晶体竞争生长规则和多变的凝固路径走向常常导致对定向凝固组织和析出相，如形态、含量和取向的控制变得困难。特别是可工程化的TiAl由于其组分多样，熔炼工艺与中间合金成分的不同，常造成性能的不稳定，尤其是对室温塑性的影响很大。因此，是否可通过对TiAl合金熔体结构与片层组织之间的遗传性研究，找到克服上述困难的方法。本研究项目拟采用液态和电固/固相结合的方法，对TiAl合金熔体结构转变行为及片层组织遗传性进行研究，阐明冷坩埚定向凝固过程中，熔体结构对凝固行为、片层组织取向分布的作用机理，揭示定向凝固TiAl合金片层取向对变形过程的影响机制。为TiAl基合金构件的材料优化、性能提高以及具备工程化应用前景奠定理论基础。

TiAl合金具有密度低，比强度高和高温抗氧化及抗蠕变性能好的特点，是下一代航空、航天和运输等领域轻质耐热结构的最佳选材。为了进一步解决TiAl合金本征脆性大的难题，本项目通过系统的实验研究和理论分析，将熔体悬浮过热技术应用于冷坩埚定向凝固TiAl 坯锭制备过程，揭示了TiAl合金熔体淬火过程中的传热机制以及所制备快速胞晶的组织特征和微观力学性能，阐明了不同合金元素对TiAl合金定向凝固过程的影响机制，实现了对TiAl合金定向凝固组织及片层结构取向化的精确控制，在此基础上，系统研究并掌握了TiAl合金定向片层组织的变形机理和强韧化机制，为TiAl进入实际应用阶段奠定理论基础。. 具体成果：(1) 通过有限元数值模拟，发现凸台悬浮定向凝固冷坩埚可以产生更大的电磁悬浮推力，从而有效实现合金熔体的过热。(2)采用熔体淬火的方法制备出了一种非常细小均匀的TiAl合金快速胞晶组织，它主要由α2相组成，其胞晶间距为0.68~3.6 μm左右。(3) 快速胞晶组织是由于Ga-In合金液对TiAl合金熔体最外层极快速冷却形成的，其生长形貌主要受传热过程的影响。快速胞晶在生长过程中的冷却速率和生长速率分别为2.1×10^6~10^5 K/s和105~5 mm/s。(4) 快速胞晶组织具有优异的显微力学性能(纳米硬度为8.457±0.336 GPa)，比一般的组织形态提高了15~60%。(5) 添加0.2 at.%Er、0.2 at.%C和0.5 at.%C合金化元素能够显著细化定向凝固Ti-47Al-2Nb-2Cr合金的柱状晶尺寸。(6) Er细化定向凝固TiAl合金片层间距的微观机制是通过提高γ相的形核率，C则通过提高γ相形核和抑制γ相生长的双重作用来细化定向凝固TiAl合金片层间距。(7) 随着生长速率的增加，含有Er、C、Mn的定向凝固TiAl合金的初生相会由单一β相转变为β和α两相。(8) 添加0.2 at.%Er、0.2 at.%C和1 at.%Mn元素能显著提高定向凝固Ti-47Al-2Nb-2Cr合金的综合力学性能。(9) 位错滑移是定向凝固TiAl合金塑性变形的基本机制，片层界面位错环的释放是塑性变形过程中的位错增殖机制。(10) 通过引入高密度变形纳米孪晶可以同时提高富铬TiAl基合金的强度和塑性，有望进一步实现TiAl合金的强韧化。