Microstructures and Residual Stress in Polycrystalline Materials: Their Nondestructive Characterization and Effects on Mechanical Properties

提案：DMS-0406004PI：志成人类研究所：肯塔基大学题目：多晶材料中的微结构和残余应力：其无损表征及其对机械性能的影响许多材料(例如岩石和金属薄板)是多晶的：它们是不同取向、大小和形状的微小微晶的聚集体。多晶材料的介观结构，包括晶体的立体学和取向，强烈影响其宏观力学性能(例如，金属板材的成形性)。多晶材料也可以施加预应力。例如，通过表面强化处理，在飞机发动机的关键部件上人为地施加一层薄薄的次表层压缩残余应力，以改善其高周疲劳行为和材料损伤容限。细观结构对多晶固体各种宏观性质的影响往往通过一些粗略定义的微观结构变量表现出来。识别这些微观结构变量并描述它们对多晶材料力学行为的影响是材料设计和无损表征的前提。这个项目的目标有三个：(1)通过只在水平面上实现均匀化，并使应力和纹理本质上是深度的有界函数，推导出沿纹理多晶半空间表面沿不同方向传播的瑞利波的数学公式，该半空间带有不均匀的次表层残余应力。(2)推导了以取向分布函数和二阶组构张量为自变量的板材塑性势的显式表达式。(3)发展了通过测量超声衰减各向异性来确定颗粒形状的数学理论。航空发动机制造商非常有兴趣将表面强化处理的有益影响纳入发动机关键部件的寿命预测中。这只能通过开发能够验证和量化残余应力水平的无损测量方法来实现。本项目的第一部分提供了一种可能的超声方法的理论基础，该方法可以无损地检测由低塑性抛光引起的亚表面残余应力层，这是一种新兴的表面增强技术。第二部分和第三部分是由于连铸铝合金板材在质量改进和在线质量控制方面存在的一些技术问题，与传统的直接冷铸铸锭相比，节能25%以上，经济节约14%以上，但成形性往往较差。第二部分致力于数学工作，以补充PI和他在肯塔基大学化学和材料工程的同事之间的联合研究项目；联邦铝业康卡斯特公司(CACI)是该联合项目的工业合作伙伴。第三部分的最终目标是开发一种用于在线监测板材晶形的超声波技术。

Proposal: DMS-0406004PI: Chi-Sing ManInstitution: University of KentuckyTitle: Microstructures and Residual Stress in Polycrystalline Materials:Their Nondestructive Characterization and Effects on Mechanical PropertiesABSTRACTMany materials (e.g., rocks and sheet metals) are polycrystalline: they are aggregates of tiny crystallites of various orientations, sizes, and shapes. The mesoscale structure of polycrystalline materials, which includes the stereology and orientations of the crystallites, strongly affect their macroscopic mechanical properties (e.g., the formability of sheet metals). A polycrystalline material may also be prestressed. For instance, through surface-enhancement treatments a thin subsurface layer of compressive residual stress is artificially imparted on critical components of aircraft engines to improve their high-cycle fatigue behavior and material damage tolerance. The effects of mesoscale structure on various macroscopic properties of polycrystalline solids are often manifested through some coarsely defined microstructural variables. Identifying such microstructural variables and delineating their effects on mechanical behavior of polycrystalline materials are prerequisites for material design and nondestructive characterization. The objectives of this project are threefold: (1) By implementing homogenization only over horizontal planes and letting stress and texture be essentially bounded functions of depth, derive mathematical formulae for the dispersion of Rayleigh waves propagating in various directions along the surface of a textured polycrystalline half-space which carries an inhomogeneous subsurface layer of residual stress. (2) Derive explicit formulae of plastic potentials of sheet metals that include the orientation distribution function (which defines crystallographic texture or the volume fraction of crystallites in each orientation) and the second-order fabric tensor (which gives a rough description of grain size and shape) as independent variables. (3) Develop a mathematical theory for determination of grain shape from measurements of anisotropy in ultrasonic attenuation.Manufacturers of aircraft engines are very interested in incorporating the beneficial effects of surface-enhancement treatments into life predictions of critical engine components. This can be accomplished only through development of nondestructive measurement methods capable of verifying and quantifying residual stress levels. Part 1 of this project provides the theoretical basis of a possible ultrasound method for nondestructive inspection of the subsurface residual stress layer induced by low plasticity burnishing, an emerging surface-enhancement technique. Parts 2 and 3 are motivated by some technical problems in quality improvement and on-line quality control of continuous cast aluminum alloy sheets, which, as compared with their counterparts made from conventional direct chill cast ingots, provide energy savings of over 25% and economic savings of over 14% but often have inferior formability. Part 2 is devoted to mathematical work that supplements a joint research project between the PI and his colleagues at Chemical and Materials Engineering, University of Kentucky; Commonwealth Aluminum Concast, Inc. (CACI) is the industrial partner of the joint project. Part 3 has its ultimate goal to develop an ultrasound technique for on-line monitoring of grain shape in sheet metals.