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

9981023 Vitek在本研究计划中，详细研究了钛铝化物（TiAl）的变形，以了解位错和孪晶的协调生成和运动如何发生，片层边界一侧的滑移发展如何影响另一侧的滑移，片层边界中的界面剪切涉及到什么程度，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，相互之间的滑移模式和层状界面的相互作用进行了研究，看看如何建立在TiAl沟道流动变形行为。键序势，包括在TiAl键合的共价分量。 这使得一个更完整的，定量的位错核心结构的图片。非化学计量比的影响，这是已知的TiAl中发挥非常重要的作用，以及进行了研究。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。实验涉及变形的Ag/Cr多层膜的平面中的膜，同时测量横向应变，看看是否在这种材料中的通道流的结果。理论程序分析了两种材料的滑移模式。 由于一个是BCC而另一个是FCC，因此在界面的两侧涉及不同的滑移系统。这项研究建立在以前的NSF支持的计划上，该计划是关于TiAl的延展性以及变形如何穿过多合成孪晶TiAl中的各种界面。 在许多情况下，在这种材料中占主导地位的变形模式不是具有最大分辨剪切应力的变形模式，而是事实上似乎是与层间边界具有最有利相互作用的变形模式，即，边界不仅响应于块体中的变形模式，它们实际上在令人惊讶的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“沟道流”，因为它是由滑动矢量携带的，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，即使单组超级位错也可以产生相同的变形。TiAl中位错核的模拟表明，这两种通常以如此不同的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?***