纳米晶镍中应力诱导晶界迁移行为的研究

Nanostructured materials offer great applications as a new generation of structural materials due to its superior mechanical properties. Stress-driven grain boundary migration has been widely accepted as a novel deformation mechanism for nanostructured materials, as an addition to dislocation slip and deformation twinning in its coarse-grained counterpart. The grain boundary migration can facilitate the nano-grains to accommodate the plastic strain. Moreover, the grain growth as a consequence of stress-driven grain boundary migration provides adequate rooms for dislocation storage during plastic deformation, which can improve the work hardening of nanostructured materials. Therefore, the stress-driven grain boundary migration and related grain growth offer a unique pathway to optimize the strength and ductility of nanostructured materials by tailoring grain boundary behaviors, and has also been recognized as a key factor for the long term performance of nanostructured materials. The proposed study will be exploring the stress-driven grain boundary migration and related grain growth in electrodeposited nanocrystalline Nickel subjected to various applied stress and time period. The study outlined in this proposal will measure the grain boundary migration velocity, and establish the relationship of stress-driven grain boundary migration with applied stress, grain orientations and grain boundary misorientations, which will be further explained in terms of the underlying mechanism of grain boundary migration. The analyses on the dependence of grain boundary migration with stress, grain orientations and grain boundary misorientations will provide an opportunity to identify the mechanism of stress-driven grain growth in terms of atomic diffusion, dislocation slip and so on. The results from this study will also provide a baseline for accessing the life performance of nanostructured materials under multiple stress states, and offer a new pathway to tailor its mechanical performance by means of optimizing the grain and/or grain boundary structures.

由于其优异的力学性能，纳米结构材料被认为是一种具有广泛工程应用前景的新型结构材料。在变形机制的研究中发现，除了粗晶材料中常见的位错滑移和变形孪生机制外，应力诱导的晶界迁移和晶粒长大也是一种十分重要的协调变形机制。应力诱导的晶界迁移和晶粒长大行为，不仅可以调整纳米结构材料的强塑性匹配，同时也是影响其长时间使役行为的关键因素之一。本项目将采用电沉积方法制备的纳米晶纯镍作为研究对象，通过研究纳米晶镍在不同外加应力、时间条件下的应力诱导的晶界迁移行为，确定应力诱导的纳米晶界迁移速率，揭示纳米晶界迁移与应力、晶粒和晶界取向之间的关联规律及影响机制，从而进一步阐明应力诱导晶界迁移的物理机制。研究结果对于全面认识纳米晶界迁移的物理机制以及纳米晶材料的使役行为有着十分重要的科学意义，同时也为优化设计纳米晶材料的晶粒、晶界结构实现变形机制调控及力学性能优化提供理论基础。

由于其优异的力学性能，纳米结构材料被认为是一种具有广泛工程应用前景的新型结构材料。在变形机制研究中发现，纳米结构材料中的晶界行为扮演着十分重要的角色，纳米晶界迁移及晶粒长大是其中一种十分重要的协调变形机制。应力诱导的晶界迁移和晶粒长大行为，不仅可以调整纳米结构材料强塑性匹配，同时也是影响其长时间使役行为的关键因素之一。在本研究中利用电沉积法制备纳米晶镍，研究应力诱导下的晶界迁移行为。通过对纳米晶镍施加准静态拉伸，应力释放，高速气炮等变形方式，研究不同应变速率以及不同应力作用的晶界迁移行为。研究结果表明，应力诱导的晶粒长大与晶粒间取向差之间没有必然的依赖关系，这也是应力诱导晶粒长大与热诱导晶粒长大之间的一个明显区别。同时发现当相邻晶粒的位错滑移系错配度因子越小，晶界两侧的晶粒位向越复杂，晶界迁移的倾向性越低，揭示了纳米晶镍中应力诱导的晶界迁移与晶界性质的内在联系及影响机制。基于前期的研究成果，本研究又针对如何抑制纳米晶在应力诱导下出现晶粒长大现象这一问题，提出了全新的解决方案。成功在纳米晶镍中引入二维材料石墨烯，制备的镍/石墨烯复合材料抗拉强度高达1400 MPa，同时均匀延伸率可以达到3.3%。研究结果表明石墨烯可以有效钉扎晶界，抑制晶粒长大，同时激发形变孪生机制，以孪生的方式协调变形，使得制备的镍/石墨烯复合材料不仅具有极高的强度，同时具有良好的塑性。