Understanding and Modeling the Creep Behavior of Lamellar TiA1 Based Alloys

*9713731 HEMKER全层状两相TiAl基金属间化合物合金具有非常吸引人的机械性能组合，被认为是在温度高达900℃的几种结构应用中取代镍基高温合金的有力候选者。在这些温度下，这些合金的蠕变性能是首要问题。不幸的是，我们对包括TiAl在内的许多先进材料的高温变形控制过程的了解目前相当有限。这些先进合金的基本蠕变机制往往与纯金属的蠕变机制有很大不同；稳态蠕变的影响比纯金属小得多，而瞬时变形过程(即，...一次蠕变和三次蠕变)已被发现主导蠕变行为。在这些情况下，Dorn对幂定律蠕变的描述不再有效，用最小蠕变速率得出的激活能和应力指数来表征蠕变行为的尝试取得了非常有限的成功。这对蠕变性能的预测有着深远的影响，因为用于蠕变分析的有限元程序要求输入表征材料蠕变行为的蠕变定律。只要有可能，这些定律最好是以物理变形机制为基础的。被广泛引用的幂定律蠕变的Dorn描述是基于导致稳态蠕变的恢复过程中的扩散辅助攀移。然而，正如PI的RIA相关研究表明，在包括TiAl在内的大多数金属间化合物合金中，导致纯金属稳态蠕变的扩散辅助恢复过程被变形组织的逐渐演变所取代。因此，Dorn方程不能用来模拟这组合金的蠕变，因此有必要为TiAl基片层合金发展一套基于机制的蠕变关系。这项工作的主要目标将是推导出一套基本的蠕变定律，这些定律是基于对作为蠕变应变函数的微观结构演变的观察。这将需要力学和材料的紧密结合，并将涉及三个特定长度尺度的工作：i)将通过对蠕变到不同蠕变应变的全片层多晶样品的透射电子显微镜观察来识别和表征微观变形机制；ii)将通过单晶和微样品蠕变试验来分离和表征晶粒度、片层间距和片层取向的介观效应；以及iii)将使用基于微观和细观测量的本构关系来模拟这些合金的宏观蠕变行为。PI在蠕变测试、透射电子显微镜和TiAl方面的经验与共同PI在开发多相材料的连续介质模型方面的专业知识相结合，以确保在本研究中的力学和材料问题之间架起一座桥梁。

*** 9713731 Hemker Fully lamellar two phase TiAl based intermetallic alloys offer a very attractive mix of mechanical properties and are considered to be strong candidates for replacing nickel base superalloys in several structural applications involving temperatures of up to 900' C. At these temperatures, the creep performance of these alloys is of primary concern. Unfortunately, our understanding of the processes that control high temperature deformation in many advanced materials, including TiAl, is currently rather limited. The underlying creep mechanisms in these advanced alloys are often quite different from that in pure metals; the influence of steady-state creep is much smaller than it is in pure metals, and transient deformation processes (i.e.. primary and tertiary creep) have been found to dominate the creep behavior. In these cases, the Dorn description of power-law creep is no longer valid and attempts to characterize the creep behavior with activation energies and stress exponents, derived from minimum creep rates, have met with very limited success. This has profound consequences for the prediction of creep performance, because the FEM codes used for creep analysis require the input of creep laws that characterize the creep behavior of the material. Wherever possible it is desirable to have these laws based on the physical deformation mechanisms. The widely referenced Dorn description of power-law creep is based on diffusion assisted climb in recovery processes that lead to steady state creep. However, as is shown in the PI's RIA related research, in most intermetallic alloys, including TiAl , the diffusion-assisted recovery processes which lead to steady state creep in pure metals are replaced by a gradual evolution of the deformation microstructure. For this reason, the Dorn equation cannot be used to model creep in this set of alloys and it is necessary to develop an alternative set of mechanism-based creep relations for TiAl based lamellar alloys. The primary goal of this work will be to derive a fundamental set of creep laws that are based on observations of microstructural evolution as a function of creep strain. This will require a close integration of mechanics and materials and will involve work at three specific length scales: i) the microscopic deformation mechanisms will be identified and characterized by TEM observations of fully lamellar polycrystalline specimens that have been crept to various amounts of creep strain, ii) the mesoscopic effects of grain size, lamellar spacing, and lamellar orientation will be separated and characterized with single crystal and microsample creep tests, and iii) the macroscopic creep behavior of these alloys will be modeled with constitutive relations that are based on the micro-and mesoscopic measurements. The PI's experience with creep testing, TEM, and TiAl has been teamed with the co-PI's expertise in developing continuum models of multiphase materials in order to assure a bridge between the mechanics and materials issues in this study.***