纳米金属材料蠕变行为的跨尺度和多尺度效应及主控变形机制的研究

In present project, we try to investigate the creep deformation mechanism of nanocrystalline metals by quantitatively analyzing both activation parameters and creep related parameters. Focusing on the cross-scale effect corresponding to average grain size and multi-scale effect corresponding to grain size distribution,based on face-centered cubic metal system, investigate the intrinsic connections between microstructure, defect size and creep behavior in nanocrystalline metals, specifically concerning on the mechanism related to grain boundary and staking fault, establishing the material and mechanical models based on the effects of both cross- and multi-scales. By performing creep test at various temperature and mechanical fields, derive activation parameters, i.e., activation energy, activation volume and strain rate sensitivity, and mechanical parameters, i.e., stress exponent, creep rate etc, of creep deformation at near room temperature, quantitatively analysis the mechanical response of the creep behavior from microstructural evolution in nanocrystalline metals. Here we try to indicate the size effect at cross- and multi- scale and related deformation mechanisms, and try to clarify those long-lasting controversial observations in previous studies concerning the creep behavior of nanocrystalline metals. We expect the achievements derived from this project could also be used in corresponding bulk nanocrystalline metals with face-centered cubic lattice structure.

本项目拟结合蠕变热激活参量和蠕变力学参量的定量分析研究纳米金属材料的蠕变变形机制。着眼于晶粒大小对应的跨尺度变形机制转变及晶粒尺寸分布对应的多尺度变形机制共存的复杂体系，以面心立方金属为例，探讨微观结构、缺陷尺度与纳米晶蠕变行为的关联性和规律性，特别关注晶界和层错的作用机制，建立以晶粒跨尺度和分布多尺度为主要特征的纳米多晶金属蠕变的材料学及力学模型。采用力-热耦合场下的变温变载蠕变实验，在近室温及低温区间内获取激活能、激活体积、应变率敏感系数等热激活参量及应力指数、蠕变应变率等力学参量，量化分析微观结构特征尺寸对纳米多晶金属蠕变行为的力学响应。揭示蠕变性能与变形机制的纵向跨尺度与横向多尺度效应，尝试澄清纳米尺度晶粒中蠕变在实验表征、理论模型等方面出现的若干矛盾现象，为进一步理解和解释相应的蠕变内在物理机制提供实验和理论支撑。

本项目围绕纳米金属材料的蠕变变形行为，在系统表征纳米多晶、纳米晶体/晶体多层膜、纳米非晶/晶体多层膜等纳米尺度金属材料微观结构的基础之上，着眼于多尺度结构相互关联、作用导致的尺寸效应，主要研究了特征微结构、缺陷尺度与纳米金属材料蠕变行为及相关力学行为的内在关联和依赖性。特别是将研究对象由单一的纳米多晶金属材料拓展到纳米多层膜及其与金属玻璃复合的复杂材料体系，采用实验和计算机模拟相结合的方法，研究了激活能、激活体积、应变速率敏感性等热激活参量与硬度、应力指数及蠕变参量对微结构特征尺寸的依赖性。此外，对引入的金属玻璃体系，系统的研究了在弛豫、回复及微结构空间异质性的调控与力学性能的关联，给出了弛豫及回复过程随预应力、应力循环及循环热处理的变化趋势及内在物理机制，对非晶/晶体纳米复合材料的硬度、蠕变速率及剪切变形等行为开展了较为深入的研究。特别是对纳米多晶金属中的晶界弛豫对其蠕变变形的影响，晶体与非晶间互为限制层时各自的强化机制、蠕变机制及应变速率敏感性的变化，晶体/非晶多层膜的剪切行为及其对强度及塑性的影响等方面取得了较为突出的成果。