超高强度钢纳米相复合析出及强化机理

Ferritic steels strengthened by nano-scale precipitates have outstanding mechanical and metallurgical properties, including high strength and toughness, good corrosion resistance and desirable weldability. Thus, these steels have a great potential for structural use in advanced engineering systems and energy conversion power plants. The superior properties are derived from the co-precipitation of multiple particles of nano-clusters, nano-intermetallic phases and nano-carbides. The complex nano-particles produced by both alloy-composition control and thermo-mechanical treatment possess superior thermal stability and prominent hardening effects as compared with those single nano-precipitates.The objective of this proposal is to understand the basic mechanisms controlling the complex precipitation processes of the three nano-particles as well as the strengthening mechanisms of the nano-precipitates. Furthermore, our efforts will be focused on the enhancement of the stability of coherent interphase boundaries via control of boundary energy and chemistry. In order to achieve these goals, analytical tools of high-resolution transmission electron microscope (HRTEM), 3-dimenstional atom probe tomography (3D-APT) and small-angle neutron scattering (SANS) will be utilized to characterize microstructural features of these clusters and particles at the nanoscale. Also, computer-aided calculations based on Thermo-Calc and JMatPro software will be applied to determine the transformation kinetics, grain size, and phase formation in multicomponent ferritic steels. All these efforts will lead to the design of a new class of ultra-high strength steels with superior mechanical properties for structural applications.

超高强度纳米钢兼具高强度、高塑性和优异的耐腐蚀性与焊接性能，具有广阔的应用前景。已有的研究表明，纳米相结构对钢的力学性能有直接和重要的影响。具有复杂结构的多相同位共沉淀纳米相较单一纳米相具有更高的热稳定性和更显著的强韧化效果，而纳米相结构及其热稳定性又与钢的成分与热加工工艺密切相关。本项目提出富铜纳米团簇与纳米碳化物、纳米金属间化合物三者同位共沉淀形成复杂结构纳米相强化低碳低合金钢的新思路，利用HRTEM、3D-APT和SANS从局部原子尺度和统计分析相结合的方法对纳米相结构和成分分布进行表征，系统研究合金成分与时效处理对同位共沉淀复杂纳米相结构和异相界面结构状态的影响，探明纳米相形成、演化规律及其影响因素，深入理解复杂结构纳米相形核长大热、动力学机制，揭示纳米析出强化钢的强、韧化机理，为有效控制纳米相结构、进一步提高钢的综合性能、设计新型超高强度钢提供理论指导。

传统超高强度钢存在高碳或高合金含量引起的韧塑性、焊接性差及成本高等问题。本项目提出了富铜纳米团簇与纳米碳化物、纳米金属间化合物三者同位共沉淀形成复杂壳核结构纳米相强化低碳低合金钢的新思路，探明了合金成分、时效处理对同位共沉淀复杂纳米相结构、异相界面结构状态的影响规律。发现复杂结构纳米相在析出和长大过程中发生由B2结构的GP区到BCC结构、9R结构和FCC结构的演变规律，确定了结构转变的临界尺寸。调控合金成分和元素化学混合焓可以获得热力学稳定的同位共沉淀复杂壳核结构纳米相。揭示了纳米相形成的热力学机制，建立了纳米相长大动力学方程。获得了纳米相结构和界面状态对力学性能的影响规律，发现位错切过是纳米相强化的主要机制。通过复杂壳核结构纳米相强化，低碳超高强度钢屈服强度达到1600MPa以上，并保持10%的伸长率，比CR260IF汽车用高强度冷轧钢板的屈服强度320MPa提高了4倍。通过纳米相复合析出强化和逆变奥氏体韧化，超高强度马氏体钢抗拉强度达到2200MPa以上，兼具10%以上的伸长率。调控共格纳米κ碳化物析出，Fe-Mn-Al-C系低密度钢抗拉强度达到1200MPa，伸长率超过20%。为提高低碳钢耐海水腐蚀性能，还研究了金属间化合物耐腐蚀渗层。研究成果不但具有重要的实用价值，而且对纳米相强化金属材料研究有重要的科学意义。

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