Investigation of Creep Behavior and Microstructure in Nanocrystalline Materials

技术：本提案描述了对大块纳米晶（NC）材料变形行为的基本研究，其特征是晶粒尺寸在1-100纳米范围内。了解nc材料变形过程的起源和性质，特别是在蠕变条件下是必不可少的。一般来说，变形过程的识别或发展需要从综合实验蠕变测量中获得的指导性信息。研究发现，蠕变特性，包括应力指数及其随应力的变化、激活体积和蠕变速率的大小，不能用当前的变形过程来解释。该项目将设计新的实验并进行仔细的分析，旨在系统地表征nc材料的变形行为。特别是，这些实验和分析旨在解决以下几个问题：(a) nc材料变形的激活体积是恒定的还是取决于诸如晶粒尺寸和温度等变量；(b)蠕变过程中测量的应变是否主要来自边界滑动；(c)变形过程中是否存在位错活动；(d) NC-Ni是否可以在保持几乎相同强度的情况下提高延展性；(e)在NC-Ni中报告的蠕变特性是否代表其他nc -材料（如NC-Cu）的行为；(f) NC-Cu是否表现出从传统的Hall-Petch强化到纳米级软化的转变。为了寻求上述问题的答案，将采用新的方法。例如，在研究边界滑动时，将采用最近用于研究超细颗粒材料滑动的原子力显微镜技术。此外，为了解决位错活动的问题，将利用以下新发现：作为UCI初步研究工作的结果，有可能引入NC-Ni相干西格玛-3退火孪晶的微观结构，该孪晶可以作为位错运动的障碍。这些孪晶在NC-Ni中的存在将被用作检查蠕变过程中是否发生位错活动的标记。此外，将尝试结合退火处理、力学测试和显微组织检查来确定在何种条件下，相干西格玛-3退火孪晶可以在保持几乎相同强度的同时提高延展性，并避免显著的晶粒生长。在该计划的初始阶段，将使用脉冲电流电沉积制备的NC-Ni和NC-Cu。以后的研究将包括镍钨合金。非技术：数控材料为许多结构应用提供了有趣的可能性。为了探索其中的一些可能性，了解nc材料变形过程的起源和性质，特别是在蠕变条件下是必不可少的。一般来说，变形过程的识别或发展需要从综合实验蠕变测量中获得的指导性信息。除了提高对nc材料变形的基本理解，并确定控制这些材料蠕变强度和延展性的因素外，该计划还将有助于研究生和本科生的教育和职业生涯。

TECHNICAL: This proposal describes a fundamental investigation into the deformation behavior of bulk nanocrystalline (NC) materials, which are characterized by a grain size in the range 1-100 nm. An understanding of the origin and nature of deformation processes in NC-materials especially under creep conditions is essential. In general, the identification or development of deformation processes requires guiding information that can be obtained from comprehensive experimental creep measurements. It has been found that the creep characteristics including the stress exponent and its variation with stress, the activation volume, and the magnitude of the creep rates cannot be accounted for by current deformation processes. This project will devise new experiments and perform careful analysis that aim at systematically characterizing the deformation behavior of NC-materials. In particular, these experiments and analysis are designed to address several questions including the following: (a) whether the activation volume for deformation in NC-material is constant or depends on variables such as grain size and temperature; (b) whether the strain measured during creep mostly arises from boundary sliding; (c) whether dislocations activity exists during deformation; (d) whether ductility can be improved in NC-Ni while maintaining nearly the same strength; (e) whether the creep characteristics reported in NC-Ni are representative for the behavior of other NC-materials such NC-Cu; and (f) whether NC-Cu exhibits a transition from conventional Hall-Petch strengthening to nano-scale softening, In seeking answers for the aforementioned questions, novel approaches will be adopted. For example, in investigating boundary sliding, the technique of atomic force microscopy that was very recently used to study sliding in ultra-fine-grained materials will be adopted. Also, for addressing the question of dislocation activity, the following new discovery will be utilized: As a result of preliminary research efforts at UCI, it is possible to introduce in the microstructure of NC-Ni coherent Sigma-3 annealing twins that can serve as barriers to dislocation motion. The presence these twins in NC-Ni will be used as a marker to check whether dislocation activity occurs during creep. Furthermore, combinations of annealing treatment, mechanical testing, and microstructural examination will be attempted to define the conditions under which the presence of coherent Sigma-3 annealing twins can enhance ductility while maintaining nearly the same strength and avoiding significant grain growth. In the initial phase of the program, NC-Ni and NC-Cu prepared by pulsed current electro-deposition will be used. Later studies will include Ni-W alloys. NON-TECHNICAL: NC materials offer interesting possibilities related to many structural applications. In order to explore some of these possibilities, an understanding of the origin and nature of deformation processes in NC-materials especially under creep conditions is essential. In general, the identification or development of deformation processes requires guiding information that can be obtained from comprehensive experimental creep measurements. In addition to providing improved fundamental understanding of deformation in NC-materials and identifying factors that control creep strength and ductility in these materials, the program will contribute to the education and professional careers of graduate and undergraduate students.