金属同材结构层状化强韧化途径与机理研究

How to keep good ductility of metallic materials as improving their strength is one of the most difficult long-term scientific problems needed to be solved in the current materials science field. From the physical reasons for strain hardening behavior and strain localization-induced fracture failure of the metals, a novel method to construct a single material laminated with microstructures, i.e. microstructure-layered material, is proposed in this project. Pure metals are selected as model materials. The microstructure-layered material will be prepared by intruducing 'ultrfine-grained or nano-grained' hard layers and "fine-grained or coarse-grained" soft layers alternatively using accumulated roll bonding or cold-rolled bonding techniques. Experiments and theoretical calculations of strain hardening and fracture behaviors wil be conducted systematically. The coulpling effects of strain hardening between the constituent layers with different scales and the mechansims will be clarified. The constrained effects of layer interfaces and the mechanisms of the strain localization initiation and development in the "hard" and "soft" layers during the delayed-fracture process will be elucidated.The theoretical models for the delayed-fracture and the strain hardenign ability improved by the strain hardening coupling effects will be proposed. The effective way to strengthen and toughen materials through the "microstructure-layered" method and the corresponding mechanism will be obtained. The results of the project will provide the theoretical basis and the design principles for the strengthening-toughening of high-performance enginnering structural materials.

如何在提高金属材料强度的同时又不失其良好的塑性已成为当今材料科学领域中长期存在且需亟待解决的重要科学难题之一。本项目从金属材料的应变硬化及应变局部化断裂失稳的物理本质出发，提出将同种材料中的晶粒尺度进行层状化组装（"同材结构层状化"）的新思想。项目拟以纯金属为模型材料，采用累积叠轧及冷轧焊合的方法在同种材料中交替引入具有"超细晶或纳米晶"结构的"硬层"和"细晶或粗晶"结构的"软层"，系统地开展结构尺度层状化金属的应变硬化与断裂行为的研究。澄清材料加工硬化过程中具有不同结构尺度单元层间的应变硬化耦合机理；阐明层状化材料断裂延迟过程中层间界面的约束作用及"软"/"硬"层中应变局部化萌生与发展的微观机制；提出层状化材料中层间应变硬化耦合效应提高应变硬化能力及延迟断裂的理论模型；探索"同材结构层状化"实现材料强韧化的有效途径及强韧化机理，为高性能工程结构材料的强韧化提供理论依据与设计原理。

本项目设计制备了一系列超细尺度层状金属材料，开展了由不同组元层尺度、种类及界面组合的一系列超细尺度层状金属材料的强度、韧性、断裂与失稳、疲劳行为等方面的实验研究工作；结合理论计算，探究了超细尺度层状金属材料的强化与韧化机理、断裂失稳控制机制以及纳米尺度金属疲劳损伤机理。项目获得如下主要创新结果：.超细尺度Cu/Ni层状材料拉伸行为研究结果表明：材料拉伸强度与塑性均随拉伸应变速率的增加而增加，层状材料中界面处的应力梯度随应变速率增加而增加，提高了材料加工硬化能力；具有1:20铜/镍厚度比的Cu/Ni层状材料同时具有较好的强度与韧性，证明了适当的组元层厚度比能够使层状金属材料获得最佳的强韧性匹配；.发现了随着Cu/Ni厚度比的减小，层状材料由Ni层的显著挠度变形及铜层颈缩被延迟的韧性断裂向铜层颈缩受阻并伴随Cu/Ni界面脱粘的脆性断裂转变规律。建立的悬臂梁挠度模型可具有高强度和高断裂应变的层状材料最优临界厚度比；揭示了超细尺度层状材料中高密度层界面通过周期性的应变梯度和分布在界面附近的几何必需位错可以有效延迟厚度方向上层状材料塑性变形失稳的规律。.发现了超细尺度Cu/Ni层状材料中超薄Cu层能够通过裂纹尖端应力集中释放，裂纹偏折和Cu/Ni界面脱粘等裂纹尖端钝化机制阻碍了疲劳主裂纹的扩展和次级裂纹的萌生，层状材料不仅具有高强度，且具有较高的疲劳极限与抗拉强度的比值。超薄Cu层的引入导致疲劳裂纹在界面附近扩展时发生了局部界面脱粘和裂纹偏折交替进行的行为，从而形成离散台阶/碎片状断裂形貌，最大化地实现界面对裂纹扩展的阻碍作用。.发现了Au薄膜厚度从930 nm减小到20 nm，其疲劳性能逐渐升高的规律。当薄膜厚度h90 nm时，薄膜疲劳裂纹沿异常长大晶粒中形成的表面侵入/挤出处萌生和扩展；当薄膜厚度减小到几十纳米时，疲劳加载下的相邻晶粒通过相互的孪晶形成以及晶界的分解，逐渐合并为一个晶粒，发生有限晶粒长大，其消耗了部分循环塑性应变，一定程度上提高了疲劳性能。基于20 nm厚的Au薄膜室温疲劳行为，提出了一个循环加载下“孪晶辅助纳米尺度金薄膜晶粒长大”的全新物理机制。