Understanding and Modeling the Creep Behavior of Lamellar TiA1 Based Alloys

全层状两相TiAl基金属间合金提供了非常有吸引力的机械性能组合，被认为是取代镍基高温合金的强有力的候选者，在涉及温度高达900℃的几种结构应用中，这些合金的蠕变性能是主要关注的。不幸的是，我们对包括TiAl在内的许多先进材料中控制高温变形的过程的理解目前相当有限。在这些高级合金中潜在的蠕变机制往往与纯金属中完全不同；稳态蠕变的影响比纯金属小得多，瞬态变形过程(即：原生蠕变和原生蠕变主导了蠕变行为。在这些情况下，Dorn对幂律蠕变的描述不再有效，试图用激活能和应力指数来描述蠕变行为，从最小蠕变速率中得出，收效甚微。这对预测蠕变性能具有深远的影响，因为用于蠕变分析的FEM代码需要输入表征材料蠕变行为的蠕变规律。只要有可能，我们都希望这些定律是建立在物理变形机制的基础上的。广泛引用的多恩对幂律蠕变的描述是基于导致稳态蠕变的恢复过程中的扩散辅助爬升。然而，正如PI的RIA相关研究所显示的那样，在大多数金属间合金中，包括TiAl，导致纯金属稳态蠕变的扩散辅助恢复过程被变形微观结构的逐渐演变所取代。因此，Dorn方程不能用于模拟这类合金的蠕变，有必要为TiAl基层状合金建立一套基于机理的蠕变关系。这项工作的主要目标是推导出一套基本的蠕变定律，这些定律是基于微观结构演变作为蠕变应变函数的观察。这将需要机械和材料的紧密结合，并将涉及三个特定长度尺度的工作：i)微观变形机制将通过蠕变应变达到不同程度的全片层多晶试样的透射电镜观察来确定和表征；ii)晶粒尺寸、片层间距和片层取向的介观影响将通过单晶和微样品蠕变试验来分离和表征。iii)这些合金的宏观蠕变行为将用基于微观和细观测量的本构关系来建模。PI在蠕变测试、TEM和TiAl方面的经验与共同PI在开发多相材料连续模型方面的专业知识相结合，以确保在本研究中力学和材料问题之间建立桥梁

*** 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 In these cases, the Dorn description of power-law creep is no longer valid and attempts to characterize the creep behavior with activation energies and stress exponents, derived from minimum creep rates, have met with very limited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.***