Microstructural Design of Two Phase Titanium Aluminides for High Temperature Applications

TiAl合金由γ-TiAl和α_2Ti_3Al相组成，被认为是高温结构应用的候选材料。为了满足工程应用的需要，需要进一步提高其高温力学性能。本研究主要研究了显微组织与高温力学性能之间的相关性，得到以下结果.当晶粒尺寸大于100 μ m时，TiAl合金的蠕变强度与晶粒尺寸无关。在晶粒尺寸小于100 μ m的范围内，蠕变强度随晶粒尺寸的减小而降低. TiAl合金的蠕变强度对组成相的体积分数，即铝浓度不敏感.全层TiAl合金的蠕变强度是通过细化其层间距来提高的。然而，在低应力下， ...更多信息 由于细片层显微组织的显著退化，细片层显微组织消失。蠕变过程中的动态再结晶和片层组织的不连续粗化是导致细片层组织退化和弱化的主要原因.通过高温退火可以稳定层状组织。稳定化处理防止了蠕变过程中的组织退化，即使在低应力下，层片细化强化也是有效的. PST晶体只有一个片层团，其蠕变强度强烈依赖于应力轴与片层片之间的夹角。硬取向晶体的片层板平行或垂直于应力轴排列，提供比随机取向的多晶TiAl合金高得多的蠕变强度。软取向晶体的应力轴与片层板成中间角，表现出与多晶体相似的蠕变强度。这一结果表明，多晶TiAl合金的蠕变强度可以通过控制其织构来提高。少

TiAl alloys consists of gammaTiAl and alpha_2Ti_3Al phases, and are considered to be a candidate for high temperature structural applications. A further improvement of their high temperature mechanical properties is required for their engineering applications, For the improvement, it is important to know how to optimize their microstructure. In this research, the correlation between the microstructures and high temperature mechanical properties were studied, and the following results were obtained.1. Creep strength of TiAl alloys is independent of grain size for grain sizes greater then 100mum. In the grain size range smaller than 100mum, their creep strength may decrease with decreasing grain size.2. Creep strength of TiAl alloys is insensitive to the volume fraction of constituent phases, in other words, aluminum concentration.3. Creep strength of fully lamellar TiAl alloys is improved by the refinement of their lamellar spacing. At low stresses, however, the high creep strength of f … More ine lamellar microstructure disappears due to the significant degradation of the fine lamellar microstructure. Dynamic recrystallization and discontinuous coarsening of lamellar microstructure during creep are responsible for the degradation, and the consequent weakening of the fine lamellar microstructure.4. The lamellar microstructure can be stabilized by annealing at a high temperature. The stabilization treatment prevent the microstructural degradation during creep, and the strengthening by lamellar refinement becomes effective even at low stresses.5. Creep strength of PST crystals, having only one lamellar colony, depends strongly on the angle between the stress axis and the lamellar plates. The hard oriented crystal, whose lamellar plates are aligned parallel or perpendicular to the stress axis, provides substantially higher creep strength than randomly oriented polycrystalline TiAl alloys. The soft oriented crystal, having an intermediate angle of stress axis to the lamellar plates, shows similar creep strength to the polycrystals. This result suggests that creep strength of polycrystalline TiAl alloys can be improved by the control of their texture. Less