非均匀层片纳米结构TWIP钢塑性变形的微观机制研究

Bulk nanostructured (NS) metals are a typical kind of ultra-high-strength materials. Unfortunately NS metals usually exhibit rather low uniform tensile elongation (ductility) due to their limited strain-hardening rate upon tensile deformation, and this is the main issue for their structural applications. Recently, numerous efforts have been paid to enhance the ductility by deploying heterogeneous structure in grain size and spatial distribution for enhanced strain hardening rate to suppress plastic instability, attaining unexpected good ductility compared with their coarse-grained counterparts. In this project, firstly the applicant aims to design and synthesis the heterogeneous lamellar (HL) nanostructure by means of severe cold-rolling processes and subsequent critical annealing treatments. Secondly, various mechanical tests are conducted, including quasi-static uniaxial tensile, tensile loading-unloading-reloading, cyclic stress relaxation, and in-situ DIC and SEM tests, together with micro-hardness and nano-indentation examinations, for the investigations of plastic deformation behaviors and corresponding stress-strain responses, and their interactions and evolutions with tensile plastic strains. Thirdly, Microstructural characteristics and evolutions are invesitgated by TEM, including the formations and distributions of mobile dislocations, deformation twins and lamellar boundaries during the non-homogeneous plastic deformation. Special attention is paid to illuminate the relationship between the HL nanostructures and mechanical responses and to reveal the microscopic mechanisms of strain-hardening behavior in the HL nanostructured metals. Finally, The results of this project should provide insights for enhancing strength-ductility synergy and for industrial application of such HL metals.

块体纳米金属是一类典型超高强度材料。由应变硬化率过快降低导致的室温拉伸塑性不足是其结构应用的主要瓶颈问题。利用非均匀层片结构诱发额外应变硬化为解决上述问题提供了新思路。本项目拟以Fe-30Mn-3Si-3Al (wt.%)孪晶诱发塑性(TWIP)钢为研究对象，采用大应变量冷轧和后续部分再结晶退火技术，设计并可控制备非均匀层片纳米结构TWIP钢样品；通过准静态单轴拉伸、拉伸加卸载、循环应力松弛和显微硬度测试，结合原位拉伸条件下的数字图像相关(DIC)和扫描电镜观测，研究应变硬化行为特性和强韧化特点；利用电子显微技术等手段研究软/硬层片界面分布特征，以及层片内部位错、孪晶、晶界等晶体缺陷在拉伸变形过程中的形成、增殖和演化及其与宏观塑性行为的关联特性，重点研究位错、孪晶等晶体缺陷间的交互作用行为及其随应变的演化规律。研究结果将为探索纳米金属强韧化提供依据，并对此类材料的工业化应用提供参。

为突破均质纳米结构材料高屈服强度与高均匀伸长率不可兼备的结构应用瓶颈，本课题围绕非均匀变形诱发额外加工硬化效应开展研究，针对含TWIP效应的奥氏体钢、中熵合金等模型材料，以及TWIP钢-低碳钢非均匀复合层片结构钢等工业金属材料，通过系统的准静态/动态、极低温度条件下力学行为表征，以及精细微结构观测，在复相层片、非均匀复合层片、多级晶粒等三种非均匀结构中，研究了非均匀材料的准静态/动态力学性能响应、位错组态、形变孪晶及相变等微结构的演化规律，以及微结构与加工硬化率之间的关联特性。在学术贡献方面，得出以下主要结论：1)在具有不同层间屈服强度差异性的非均匀复合层片结构中，揭示了晶界影响区的存在导致应变梯度和背应力，诱发额外加工硬化和强化；在准静态和动态剪切条件下界面背应力硬化显著提高拉伸塑性和动态性能；2)提出一种适合于单相面心立方金属的跨尺度非均匀构筑策略，在准静态和动态变形条件下，实现了高强度与高韧塑性的优异匹配，揭示了非均匀晶粒之间的变形协调和应变分配能够促进加工硬化，在动态变形过程中发生晶粒细化，延缓剪切带的萌生，促进动态剪切塑性；3)在国际上率先开展了非均匀晶粒结构TWIP中熵合金夏比缺口冲击韧性(Aĸ)的相关研究，在4.2-77K温度域内，首次采集到该合金创纪录高的Aĸ值域，揭示非均匀晶粒结构TWIP材料在冲击载荷下，裂纹尖端迸发多重剪切带与纳米形变孪晶交互作用的一种独特韧化机制；4)针对晶粒尺寸非均匀分布的高层错能单相金属铁，研究并阐明近微米尺度的非均匀晶粒分布对屈服点效应、Lüders应变行为、以及Hall-Petch关系式系数等的影响规律，揭示了近微米尺度非均匀晶粒结构的屈服行为和加工硬化机理；5)针对异质化合金设计、非均匀微结构构筑、非均匀塑性变形行为、背应力及背应力硬化在拉伸变形过程中的动态演化行为，以及背应力定量/原位测定方法等进行总结和综述。研究成果为进一步深入研究非均匀纳米材料的组织与性能调控及优化提供参考。