高强韧纳米孪晶钢微观结构调控及其动态变形机理研究

Materials can be dramatically strengthened by inducing high density of stable nanoscale twins without sacrificing the ductility, physical properties and processability. As an important metallic structural material, steel can acquire the nanotwinned microstructure to simultaneously achieve high strength and high ductility by methods of magnetron sputtering deposition, reverse transformation, severe plastic deformation et al. However, these methods need special technological equipment as well as partly restrict the size of the products at present, so large-scale industrial production cannot use these technologies. In this project, large nanotwinned steel sheet will be manufactured from twinning-induced plasticity steel through the routines of cold rolling followed by recovery with normal industrial equipment. The effect of processing parameters including cold rolling reduction, recovery temperature and time on the critical microparameters such as dislocation density and twin volume fraction will be investigated in order to control the microstructure and optimize the mechanical properties. Materials are usually under dynamic loading condition with high strain rate during the process of manufacturing and service. However, the plastic deformation of nanotwinned steel under dynamic loading has not be studied adequately at current. In present project, the dynamic mechanical properties and the related microstructures will be tested and observed. The deformation mechanism of nanotwinned steel under high strain rate will be explored. Through the investigation of this project, the theory for manufacture and deformation of nanotwinned steel will be improved. Further fundamental understanding of nanotwinned materials for large-scale industrial production and engineering application will be achieved after the research of present project.

通过形成稳定的纳米孪晶结构能够显著提高材料强度，并且不危害塑性、物理性能以及加工性能。钢铁作为重要的金属结构材料，可通过磁控溅射沉积、逆相变、剧烈塑性变形等方法形成高强度高韧性的纳米孪晶钢。然而，目前以上加工方法对工艺设备有特殊需求，并且一定程度上限制了材料的尺寸，不适合大规模工业生产。本项目采用常见的工业设备对孪晶诱导塑性钢进行冷轧-回复处理以制备大尺寸纳米孪晶钢板材。通过研究冷轧量、回复温度与时间等工艺参数对位错密度、孪晶体积分数等关键微观参数的影响，以实现调控微观结构，进而优化力学性能。加工与服役过程中，材料将受到动态（高应变速率）载荷作用，然而目前对纳米孪晶钢的动态塑性变形的研究很少。本项目将通过观测动态力学性能与相应微观结构，探索纳米孪晶钢在高应变速率下的变形机理。通过本项目能够完善纳米孪晶钢的制备与变形理论，并为其大规模工业化生产与工程应用提供理论基础。

伴随现代社会经济发展，对结构材料的强度、韧性和塑性等力学性能有着更高的要求。在材料中引入高密度纳米孪晶结构能够显著提升材料的强度且不影响塑性，是未来结构材料的重要发展方向之一。作为重要的金属结构材料，钢铁可以通过磁控溅射沉积、逆相变、剧烈塑性变形等方法形成纳米孪晶结构，获得良好的综合力学性能。但是由于特殊设备的需求、产品尺寸的限制、生产效率的制约导致以上加工方法无法应用于工业规模化生产。本项目以低层错能的孪晶诱导塑性钢为基础原料，采用常规工业设备，进行以冷轧-热处理为主的高效热机械处理，成功制备出具有优异力学性能的大尺寸纳米孪晶钢板材。..为进一步优化纳米孪晶钢的力学性能，本项目对冷轧、热处理等阶段的微观结构进行系统观测，掌握了热机械过程中位错、纳米孪晶、晶界、珠光体等关键微观结构的演化规律以及对力学性能的影响机制。冷轧过程的研究结果说明50%冷轧量能够生成大量高密度纳米级孪晶。在短时间热处理过程中，回复是微观结构变化的主要机制，位错密度从冷轧态将近4.0×10^5 m-2可降低至回复后的约0.5×10^5 m-2，而晶界、孪晶界则保持稳定。回复过程未显著降低纳米孪晶钢的强度，同时提升其延伸率和加工硬化率。热处理过程中，亚稳态奥氏体在塑性变形之后，通过长时间合金元素扩散能够在高能晶界处形成由铁素体和渗碳体组成的珠光体组织，珠光体对纳米孪晶钢的塑性有着不利影响同时会引起再结晶发生。再结晶发展过程迅速不可控，引起纳米孪晶钢的强度的显著下降。在高应变速率条件下的塑性变形过程中，纳米孪晶钢具有较高的结构稳定性，以位错增殖为主要变形机制并伴随剪切带、二次孪晶的形成，因此相较于准静态变形过程展现出较高的强度和加工硬化能力。..通过本项目研究系统掌握了冷轧-热处理过程中纳米孪晶钢微观结构的变化规律，获得了应变速率与力学性能之间的关系，实现了结构性能的科学预测和人工调控，对于高性能纳米孪晶钢的工业化生产和应用具有重要指导意义。