Interfacial Structure and Processes in Lamellar Titanium Aluminide Alloys

0072721 Aindow这项研究发展了一个基本的理解的界面结构的TiAl基合金中的层状微观结构的演变和随后的合金，表现出这种微观结构的机械行为的影响。 层状γ/α 2界面已经通过常规的透射电子显微镜（TEM）和高分辨率电子显微镜（HREM）进行了非常详细的研究。在最近的HREM低失配层状合金的研究中，这位研究人员已经表明，这些界面上的步骤可以表现出更大的多样性的高度和伯格斯矢量比以前报道的。此外，他还发现台阶所表现出的伯格斯矢量随台阶的高度和方向而变化。在每种情况下，这些伯格斯矢量是一致的步骤是完美的界面断开所描述的庞德的拓扑理论的界面缺陷，而不是肖克利部分位错，如前所述。这些观察结果对此类合金中任何缺陷介导的界面过程具有广泛的影响，因为人们会期望完全断开的行为与相同界面中的肖克利偏分的行为完全不同。在这项研究中，缺陷介导的层状TiAl基合金的界面过程中进行了研究，通过测量作为热机械历史的函数的界面缺陷配置的变化。缺陷的性质是通过使用界面缺陷的拓扑理论来解释详细的TEM和/或HREM观测，并使用相关的通量分析来确定控制线缺陷运动的因素。这种方法解决了四个关键问题：（1）γ层状结构的形成机制;（2）界面台阶缺陷的相互作用;（3）通过γ/α 2界面的滑移传输;以及（4）在γ/α 2界面处引入失配位错。预计通过阐明TiAl金属间化合物合金中缺陷介导的界面过程的机制，该计划将对高温结构应用产生广泛的技术影响，特别是在航空航天领域。 这种理解是至关重要的，在确定合金的发展战略，为下一代层状TiAl基合金。该程序是拓扑理论用于解释工程合金的详细TEM/HREM数据的第一个实例。因此，预计这项工作将产生更广泛的科学影响，因为它将构成一项示范研究，以证明这一方法的效力。该计划为两名研究生提供综合培训，并将晶体学分析的基本严谨性与可转移的实验技能相结合。它还为学生提供了与英国科学家进行国际合作的机会。***

0072721AindowThis research develops a fundamental understanding of the effects of interfacial structure on the evolution of the lamellar microstructure in TiAl-based alloys and on the subsequent mechanical behavior of alloys that exhibit this microstructure. The lamellar gamma/alpha2 interfaces have been studied in great detail both by conventional transmission electron microscopy (TEM) and by high-resolution electron microscopy (HREM). In a recent HREM study of a low-misfit lamellar alloy, this investigator has shown that the steps on these interfaces can exhibit a greater diversity of heights and Burgers vectors than has been reported previously. Moreover, he found that the Burgers vectors exhibited by the steps varied with both the height and sense of the step. In each case these Burgers vectors were consistent with the steps being perfect interfacial disconnections as described by Pond's topological theory of interfacial defects, not Shockley partial dislocations as proposed previously. These observations have wide-ranging consequences for any defect-mediated interfacial processes in such alloys since one would expect the behavior of perfect disconnections to be radically different from that of Shockley partials in the same interface. In this research, defect-mediated interfacial processes in lamellar TiAl-based alloys are investigated by measuring changes in the interfacial defect configurations as a function of thermomechanical history. The character of the defects are determined by using the topological theory of interfacial defects to interpret detailed TEM and/or HREM observations, and the associated flux analysis are used to identify the factors controlling line defect motion. This approach addresses four key topics: (1) The mechanism for the formation of gamma lamellae; (2) The interaction of interfacial step defects; (3) Slip transmission through gamma/alpha2 interfaces; and (4) Introduction of misfit dislocations at gamma/alpha2 interfaces.%%%It is anticipated that by elucidating the mechanisms for the defect-mediated interfacial processes in TiAl intermetallic alloys the program will have broad technological impact for high temperature structural applications, particularly in the aerospace sector. This understanding is essential in determining alloy development strategy for the next generation of lamellar TiAl-based alloys. This program is the first instance where the topological theory is used to interpret detailed TEM/HREM data from engineering alloys. Thus, it is anticipated that the work will have broader scientific impact in that it will constitute a model study to demonstrate the efficacy of the approach. This program provides integrated training for two graduate students and combines the fundamental rigor of crystallographic analyses with transferable experimental skills. It also provides the students with the opportunity to participate in an international collaboration with scientists in England. ***