Interfacial Control of Deformation Modes in TiAl and Other Lamellar Materials: Experimental and Theoretical Study

在本研究项目中，我们将对钛铝合金TiAl中的变形进行详细的跟踪研究，以了解位错和孪晶是如何协同产生和运动的，在片层边界一侧的滑移发展如何影响另一侧的滑移，在多大程度上参与了片层边界的界面剪切，以及孪晶和普通位错运动的顺序是什么。在理论部分的努力，滑移模式的相互作用和与层状界面进行了研究，以了解如何建立通道流动变形行为的TiAl。使用了包含TiAl中键的共价成分的键序电位。这使得对位错核心结构有了更完整、更定量的了解。我们还研究了非化学计量学的作用，它在TiAl中起着非常重要的作用。研究的第二个方向是探索这种变形模式发生在其他层状材料中的可能性，例如具有相干或半相干界面的多层复合材料。实验涉及到银/铬多层膜在薄膜平面上的变形，同时测量横向应变，看看是否在这种材料中产生通道流动。理论程序分析了两种材料的滑移模式。由于一个是BCC，另一个是FCC，不同的滑移系统涉及到接口的两侧。这项研究建立在先前nsf支持的TiAl延展性项目的基础上，以及变形如何穿过复合孪晶TiAl的各种界面。在许多情况下，在这种材料中占主导地位的变形模式并不是那些具有最大剪切应力的变形模式，但事实上，似乎是那些与层间边界具有最有利相互作用的变形模式，即，边界不仅响应块体中的变形模式，它们实际上在很大程度上决定了主导变形模式是什么。这种变形模式被称为“通道流”，因为它是由滑移矢量携带的，其净和平行于边界本身。这种与界面的相互作用通常涉及共面普通位错和孪晶的复杂协调运动，即使一组超级位错可以产生相同的变形。TiAl中位错核的模拟表明，这两种通常以不同速率发生的变形模式如何在这种材料中紧密耦合，但许多问题仍未得到解答，其中最重要的是：这种变形模式有多普遍？* * *

9981023Vitek In this research program deformation in titanium aluminide, TiAl, is followed in detail in order to see how coordinated generation and motion of dislocations and twins occurs, how the development of slip on one side of a lamellar boundary affects slip on the other side, to what extent interfacial shear in the lamellar boundaries is involved, and what the sequence of the twinning and ordinary dislocation motion is. In the theoretical portion of the effort the interaction of the slip modes with each other and with the lamellar interfaces are investigated to see how channeled flow deformation behavior is established in TiAl. Bond order potentials that include the covalent component of the bonding in TiAl are used. This allows for a more complete, quantitative picture of the dislocation core structure. The effect of off-stoichiometry, which is known to play a very important role in TiAl, is investigated as well. A second direction of the research is to explore the possibility that this deformation mode occurs in other lamellar materials, such as in multilayer composites with coherent or semicoherent interfaces. The experiments involve deformation of Ag/Cr multilayers in the plane of the film while measuring lateral strains to see if channeled flow results in this material. The theoretical program analyzes the slip modes in the two materials. Since one is BCC and the other FCC, different slip systems are involved on either side of the interfaces. %%%This research builds on a previous NSF-supported program on the ductility of TiAl and how deformation crosses the various interfaces in polysynthetically twinned TiAl. In many cases, the deformation modes that dominate in this material are not those with the maximum resolved shear stress, but, in fact, appear to be the ones that have the most favorable interactions with the interlamellar boundaries, i.e., the boundaries do not just respond to the deformation modes in the bulk, they actually determine, to a surprisingly large extent, what the dominant deformation modes are. This deformation mode is called "channeled flow", because it is carried by slip vectors, the net sum of which is parallel to the boundaries themselves. This interaction with the interfaces commonly involves a complex coordinated motion of coplanar ordinary dislocations and twinning, even though the same deformation could be produced by a single set of super dislocations. The simulations of dislocation cores in TiAl have shown how these two deformation modes that normally occur at such different rates are closely coupled in this material, but many questions remain unanswered, not the least of which is: How general is this kind of deformation mode?***