低晶界能高稳定性纳米晶Cu的力学性能及变形机制研究

Nanograined metals have received extensive research due to their high strength. However, nanograined metals are usually prone to strain localization early and failure with little plasticity due to their weak ability of strain hardening caused by the unique grain boundary mediated deformation mechanism. Previous research found that in nanograined Cu, Ni, etc. prepared by severe plastic deformation, as their grain sizes are below the certain sizes, their grain boundary energies become low and the nanograins exhibit high stability due to the effect of mechanically-induced grain boundary relaxation. Grain boundary activities are inhibited and partial dislocation activities become an important deformation mechanism in grains below that size, which provides a potential for strain hardening during deformation and give a new possible way to realize the synergy of strength and plasticity in nanograined metal. Therefore, based on previous researches, we will prepare high-stability nanograined Cu with low grain boundary energy by plastic deformation combined with thermal treatment in this study. The mechanical property of the high-stability nanograined Cu will be investigated through tensile test, and their deformation mechanism during tension will be analyzed. The structure-property relationship of the high-stability nanograined Cu will be revealed. We will explore the effectiveness of the strategy by suppressing the grain boundary mediated deformation mechanism and activating twinning or partial dislocation motion to accommodate plastic strain to improve the synergy of strength and plasticity in a high-stability nanograined metal with low grain boundary energy for the first time. In this study, we probe that improving mechanical property of materials by grain boundary modification based on the theory of “making materials plain”. Our findings will enrich the understanding of the mechanical behavior of nano-scale grains.

纳米金属材料因其高强度而受到广泛地研究，但由于其晶界运动协调变形的独特机制，使其加工硬化能力差，易出现应变局域化而表现出极差的塑性。前期研究发现在塑性变形法制备的梯度纳米晶Cu、Ni等中，当晶粒小于一定尺寸时，由于机械诱导晶界驰豫效应，纳米晶表现出低晶界能高稳定性的特征，且变形过程中晶界运动受到抑制，不全位错运动成为重要的变形机制，这有望提高纳米晶加工硬化能力，为优化其强塑性匹配提供新思路。因此，结合前期研究结果，本项目旨在采用塑性变形结合热处理等工艺制备低晶界能高稳定性纳米晶Cu，通过拉伸实验评价其力学性能，分析其变形机制，揭示其结构-力学性能关系。探索在低晶界能高稳定纳米晶金属中，实现通过抑制晶界运动，激活孪生或不全位错运动协调塑性变形，以优化纳米金属强塑性匹配的可行性。本项目基于“材料素化”思想，探索通过晶界调控的方式提升材料力学性能。相关研究结果将丰富对纳米尺度晶粒力学行为的认识。

纳米金属材料因其高强度而受到广泛地关注和研究，但由于其晶界运动协调变形的独特机制，使其加工硬化能力差，易出现应变局域化而表现出极差的塑性。前期研究发现在塑性变形法制备的梯度纳米晶Cu、Ni等中，当晶粒小于一定尺寸时，由于机械诱导晶界弛豫效应，纳米晶表现出低晶界能高稳定性的特征，且变形过程中晶界运动受到抑制，不全位错运动成为重要的变形机制，有望提高纳米晶加工硬化能力。因此，本项目采用塑性变形法制备低晶界能高稳定性纳米晶Cu，研究了不同纯度不同变形条件对晶界弛豫效应的影响，并对晶界弛豫的高热稳定纳米晶Cu进行了拉伸测试，分析了其力学性能和变形行为，揭示其结构-力学性能关系。本工作首先采用表面机械碾磨处理制备了系列不同纯度（99-99.9999 wt.%）的梯度纳米结构Cu试样，发现不同纯度纳米晶Cu均在接近70 nm发生晶界弛豫效应，但高纯度Cu纳米晶弛豫程度更高，热稳定性更好；分析表明晶界处杂质元素的存在使得晶界在变形过程中形核孪晶或层错更困难，一定程度上阻碍了晶界弛豫，同时杂质元素存在的晶界弛豫后能量相对较高，因此高纯金属晶界弛豫效应更显著。基于此，综合考虑加工制备的挑战，本工作采用99.999 wt.%纯Cu为研究对象，通过调整制备工艺参数制备得到平均晶粒尺寸为25 nm的均匀弛豫态纳米晶样品，并以表面机械碾磨制备的平均晶粒尺寸为60 nm的弛豫态纳米晶和105 nm的普通非弛豫纳米晶作为参照样品，对弛豫态纳米晶的拉伸力学性能和变形机制进行了研究。发现普通105 nm纳米晶Cu、60 nm和25 nm弛豫态纳米晶Cu均匀延伸率分别约为2.5-3%、3.5%、5.5%，屈服强度分别约450、500、650 MPa；微观结构分析表明普通纳米晶以晶界运动和位错运动主导，弛豫纳米晶主要以不全位错运动主导。尽管非弛豫与弛豫态纳米晶Cu在拉伸变形过程中均难以产生有效的加工硬化，但弛豫态纳米晶由于晶界运动被抑制，应变软化效应受到抑制，表现出同步增强的屈服强度和均匀延伸率。相关研究为认识晶界调控对纳米晶力学性能的影响提供了重要参考。