高密度纳米孪晶Ni的可控制备与力学变形行为研究

In the classical methods for strengthening materials, the method of twin boundary strenghening has heavy development potentiality, especially for the metal Ni with high stacking fault energy，and how to realize regularly arranged controllable preparation for the high density of nano-scale twins structure is the hotspot and difficulty in the field of materials research. Based on the previous work, the study on the preparation of Ni with high density of nano-scale twins was proposed in this work. First, Columnar twins Ni was prepared through electrodeposition; Second, for Cu/Ni multilayers with the special growth texture and the specific grain size were prepared through electrodeposition, Nickel with high density of nano-scale twins was prepared through rolling technology and selective etching. This research mainly focused on the research of the deformation mechanisms and mechanical property of new materials in order to explored the intrinsic reciprocity for the mechanical properties and thermal stability with the parameters, such as the lamellar size, twin boundary migration, and influence between the twin boundaries and defects. The study developed a theoretical model of mechanics in order to elucidate the mechanisms of strength and ductility. The study did not enrich both microstructure design and strengthening mechanism, but also had great academic value and application significance.

在金属材料的强化方法中，孪晶界强化法具有非常大的发展潜力，尤其对于具有较高层错能的金属Ni，如何实现高密度孪晶结构规则排列的可控制备是材料研究领域的热点与难点。基于前期研究基础，本项目提出两种制备高密度孪晶结构Ni的研究思路：（一）通过电沉积技术制备出柱状孪晶结构Ni材料；（二）基于Cu基体制备具有特定生长织构和晶粒尺寸的Cu/Ni多层薄膜材料，通过轧制工艺，以及选择性刻蚀掉Cu层，设计制备出高密度形变孪晶规则排列的Ni材料。重点研究这种新型金属材料的变形机理与力学性能特征，探索孪晶片层尺寸，孪晶界迁移，孪晶界与缺陷之间的相互作用等参数对相关力学性能和热稳定性的影响及其内在的相互关系，建立相关基础理论模型，阐明高密度孪晶Ni的强韧化机理。本项目的研究可以丰富材料微观组织结构设计与材料强韧化理论，具有重要的学术价值与应用意义。

本项目完全按照计划进行，采用电沉积技术制备了具有特定生长取向的镍材料及尺度均匀的Cu/Ni微米多层膜。对Cu/Ni多层膜进行轧制获得纳米孪晶规则排列的镍层薄膜，获得了将近6GPa的维氏硬度值，利用纳米压痕测试方法研究了纳米孪晶镍的强韧化机制；对具有特殊晶体学取向的Ni材料，研究其循环形变力学性能与微结构变化，提出形变孪晶的产生机制并指出形变孪晶产生的晶粒尺寸效应，阐明该材料的形变强化与软化机制。. 晶粒层状排列Ni材料的形变微观组织表明：在室温低应变速率下，该材料产生形变孪晶，形变孪晶界迁移和普通晶界迁移共同协调该材料的塑性变形，获得较好的延伸率；循环形变力学性能出现形变强化与软化现象，开始阶段，强化效应是由于位错网络形成，而其后的软化效应，除了克服位错网络阻力外，还与形变孪晶或层错的出现有关，形变孪晶的产生主要源于由晶界发出的肖克莱不全位错迁移；原位拉伸断口显示，单晶镍线形变由形变孪晶或层错主导，源于样品表面发出的肖克莱不全位错，大量位错塞积处即应力集中区同样可以促进形变孪晶的产生。这不符合相关研究者分子动力学模拟结果，主要因为模拟边界条件选择与实际形变过程中的特殊环境不同，并且形变孪晶具有晶粒尺度效应。对柱状晶Ni，在动态拉伸过程中，产生形变孪晶的晶粒尺寸大约为1m左右；对层状晶Ni，在常规拉伸过程中，产生形变孪晶的晶粒尺寸范围大约为200～500nm，主要为300nm左右。