PST TiAl单晶高温长时组织稳定性研究

Polysynthetic twinned (PST) TiAl single crystals achieve a superior combination of ductility and strength, as well as resistance to creep. The transition temperature of strength degradation is raised from approximately 650°C to higher than 950°C. Furthermore, minimum creep rate and creep rupture life at 900°C increase by one to two orders of magnitude compared with existing American TiAl-4822 alloys. All of these give PST TiAl single crystals significant application prospects. However, the aero-engine blades need to service at elevated temperature for long time (lifetime＞10000h), which makes the existing high-temperature short-time tensile and creep properties far from adequate. So at least the thermal structural stability of PST TiAl single crystals needs to be investigated that whether it fully satisfies the strict requirements of long time high-temperature environment service for aero-engine blades. This proposal will focus on the systematic study of the microstructure evolution and stability limit of PST TiAl single crystals at a long time high-temperature condition, revealing how and why their microstructure lose stability and cause degeneration after long time thermal exposure. Our efforts will be focused on illuminating the impact of microstructural instability and degeneration on mechanical properties, judging the possibility of applying PST TiAl single crystals in aero-engine blades, which will lay the foundation for further research.

高温PST TiAl单晶实现了强度、塑性和蠕变抗力的优异结合与跨越性提升，强度开始下降温度由约650℃提高到950℃以上，900℃最小蠕变速率和持久寿命优于美国4822合金1~2个数量级，具有重大应用前景。但航空发动机叶片须在高温环境下长时服役（寿命＞10000h），仅有高温短时拉伸、蠕变性能还远远不够，至少还应考察其组织稳定性是否能够满足叶片高温长时服役的要求。本项目旨在探明PST TiAl单晶在高温长时条件下组织的演变规律和稳定性极限，揭示单晶组织发生失稳退化的方式及类别，阐明其对力学性能的影响规律，评判高温PST TiAl单晶用于航空发动机叶片的可能性，为下一步研究奠定基础。

PST TiAl单晶实现了强度、塑性和蠕变抗力的优异结合与跨越提升，具有重大应用前景，但其实际应用于航空发动机叶片，仅有高温短时拉伸、蠕变性能还远远不够，至少还应探明其组织稳定性是否能够满足叶片高温耐久性要求。本项目系统研究了PST TiAl单晶不同高温长时条件下的组织演化过程，首次揭示了PST TiAl单晶高温长时组织退化形式为α2片层溶解和120°旋转有序畴界迁移，阐明了PST TiAl单晶高温长时组织稳定性显著优于TiAl多晶合金的本质原因在于，不存在多晶里常见的平行分解、B2(ω)相析出、新的γ相析出、不连续粗化等现象。揭示了PST TiAl单晶高温长时组织退化机制为位移-扩散型相变和原子短程扩散，其中α2片层溶解属于位移-扩散型相变过程，通过界面断开上的肖克莱分位错沿(0001)α2面运动完成相界面迁移，同时伴随着原子的短程扩散以实现成分转变；120°旋转有序畴界迁移属于原子短程扩散过程，界面位错附近原子在界面能或化学势驱动下发生短程扩散，导致界面形成拉链状缺陷进而形成界面凸起，界面凸起不断长大使120°旋转有序畴界迁移至相邻界面上消失。阐明了PST TiAl单晶高温长时组织演化-力学性能关系，以强度下降比例和片层增厚比例为依据建立了组织退化至失稳的判据，初步探明了PST TiAl单晶高温长时组织稳定的时间极限和温度极限。研究成果为PST TiAl单晶实际应用于航空发动机叶片提供了重要设计依据，同时也为其他片层材料界面稳定性提升及组织稳定性评判标准的建立提供了理论指导。