Understanding and Modeling the Creep Behavior of Lamellar TiA1 Based Alloys

了解层状 TiA1 基合金的蠕变行为并对其进行建模

• 批准号: 9713731
• 负责人: Kevin Hemker
• 金额: $ 26.99万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-09-15 至 2000-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9713731&HistoricalAwards=false
• 关键词: Understanding; Modeling; Creep; Behavior; Lamellar

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 In these cases, the Dorn description of power-law creep is no longer valid and attempts to characterize the creep behavior with activation energies and stress exponents, derived from minimum creep rates, have met with very limited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

*** 9713731 Hemker完全层状基准合金提供了机械性能的非常有吸引力的混合物，被认为是在多种结构性应用中更换镍基本超合金的强大候选物，涉及多达900'的温度。不幸的是，我们对控制许多高级材料（包括TIAL）的高温变形的过程的理解目前相当有限。 这些高级合金中的潜在蠕变机制通常与纯金属中的蠕变机制差异很大。 稳态蠕变的影响要比纯属金属小得多，并且已经发现瞬态变形过程（即一级和三级蠕变）占主导地位的蠕变行为。在这些情况下，对幂律蠕变的多元描述不再有效，并且试图用激活能量和应力指数来表征蠕变行为，并从最低蠕变率衍生出来，取得了非常有限的成功。 这对蠕变性能的预测产生了深远的影响，因为用于蠕变分析的FEM代码需要蠕变定律的输入，以表征材料的蠕变行为。在可能的情况下，希望根据物理变形机制获得这些定律。广泛参考的幂律蠕变的多子描述基于导致稳态蠕变的恢复过程中的扩散辅助攀登。但是，正如PI的RIA相关研究所示，在包括TIAL在内的大多数金属合金中，扩散辅助恢复过程导致纯金属中稳态蠕变被变形微结构的逐渐演变所取代。因此，Dorn方程不能用于在这组合金中模拟蠕变，并且有必要为基于TIAL的层状合金开发一组替代基于机制的蠕变关系。 这项工作的主要目标是得出基于对微观结构进化的观察结果的基本蠕变定律，这是蠕变菌株的函数。 这将需要紧密整合力学和材料，并将涉及三个特定长度尺度的工作：i）微观变形机制将被识别，并以对完全层状多晶样本的观察，这些样品的观察到已被捕捉到各种蠕变的各种蠕变的晶状体尺寸，ii）单一尺寸和lamellar sprient，lamellar sprient and lam space and lam spacept，lam sprient，lam sprient and lam sparn and lam sprient and lam sprient，lam lam sprient of limallar的表现和Microsample蠕变测试，以及iii）这些合金的宏观蠕变行为将以基于微观和介绍测量的本构关系进行建模。 PI在蠕变测试，TEM和TIAL方面的经验与Co-Pi在开发多相材料的连续模型方面的专业知识合作，以确保本研究中的力学和材料问题之间的桥梁。***