Understanding and Modeling the Creep Behavior of Lamellar TiA1 Based Alloys

* 9713731 Hemker 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，并且被认为是在涉及高达900 ℃的温度的若干结构应用中替代镍基超耐热合金的强有力的候选者。在这些温度下，这些合金的蠕变性能是主要关注的问题。不幸的是，我们对许多先进材料（包括TiAl）中控制高温变形的工艺的理解目前相当有限。 这些先进合金中潜在的蠕变机制通常与纯金属中的蠕变机制大不相同;稳态蠕变的影响比纯金属中的蠕变小得多，并且瞬态变形过程（即， 第一和第三蠕变）主导蠕变行为。在这些情况下，幂律蠕变的多恩描述不再有效，并且尝试用从最小蠕变速率导出的激活能和应力指数来表征蠕变行为，已经取得了非常有限的成功。 这对蠕变性能的预测具有深远的影响，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定律。只要有可能，就希望这些定律建立在物理变形机制的基础上。广泛引用的幂律蠕变的多恩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相关研究所示，在大多数金属间化合物合金中，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。出于这个原因，多恩方程不能用于模拟这组合金的蠕变，并且有必要为TiAl基片状合金开发一组替代的基于机制的蠕变关系。 这项工作的主要目标将是获得一套基本的蠕变规律，是基于观察的微观结构的演变作为蠕变应变的函数。 这将需要机械和材料的密切结合，并将涉及三个具体长度尺度的工作：i）微观变形机制将通过对已经蠕变到各种蠕变应变量的全层状多晶样品的TEM观察来识别和表征，ii）晶粒尺寸、片层间距、和层状取向将被分离，并通过单晶和微样品蠕变测试表征，和iii）这些合金的宏观蠕变行为将通过基于微观和介观测量的本构关系建模。PI在蠕变测试、TEM和TiAl方面的经验与co-PI在开发多相材料连续模型方面的专业知识相结合，以确保本研究中力学和材料问题之间的桥梁。

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 In these cases, the Dorn description of power-law creep is no longer valid and attempts to characterize the creep behavior with activation energies and stress exponents, derived from minimum creep rates, have met with very limited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