基于α/γ相变调控的高强纳米晶铁基合金增塑研究

Nanocrystalline (NC) metallic materials generally exhibit high strength, but their tensile plasticity becomes disappointingly poor, which significantly limits their practical application. It is demonstrated that the implement of heterogeneous microstructure is effective in improving the tensile plasticity. To this end, the prevalent approaches generally focus on single-phase microstructures by engineering the architectures of grains and interfaces, thus involving no phase transformations. Therefore, the corresponding processing protocols cannot be universally accessible to multi-component/multi-phase Fe-based alloys involving phase transformations. As for NC Fe alloys, however, phase transformations are conventionally considered as triggering factors for inducing the loss of nanocrystallity (e.g. phase instability), thus underestimating their significance in designing and fabricating NC alloys. This project is devoted to tailoring microstructure with phase transformation. Using the NC Fe-Mn-C alloy as the model system, we will experimentally and theoretically investigate the size and alloying effects on the ferrite (α)/austenite (γ) phase transformation in the NC Fe alloys. Then, the α/γ phase transformation will be used to introduce the γ with different length scales into the α nano-matrix, forming heterogeneous nanostructures. Further, the mechanical properties and the underlying deformation mechanisms will be explored. On this basis, some principles in designing NC Fe alloys with high strength and good tensile plasticity by α/γ phase transformation, will be proposed. This project therefore is highly important for the design of NC Fe alloys with a good combination of strength and tensile plasticity, and the development of advanced high strength steels.

纳米晶金属材料强度高但拉伸塑性低，严重制约了此类材料的应用与发展。引入非均质结构是提高纳米晶金属材料拉伸塑性的有效途径；然而，当前非均质结构往往针对单相内晶粒/界面结构的构筑实现而不涉及相变，因此相应的工艺无法普适地应用于涉及多种相变的多元多相铁基合金。对于纳米晶铁基合金而言，相变往往被视为均质纳米结构失稳的诱因，而其在纳米结构材料设计中的作用并未得到应有重视。本项目立足“相变调控组织”，以纳米晶Fe-Mn-C合金为模型体系，通过实验研究以及理论建模，系统探究晶粒尺寸和合金化效应对纳米晶铁基合金α/γ相变的影响，利用α/γ相变将不同尺度奥氏体引入到纳米晶铁素体基底中形成非均质组织材料，进一步探究力学性能和变形机理，最终总结基于α/γ相变调控纳米结构进行高强高塑纳米晶铁基合金设计的一般性规律。本项目的研究对设计高强高塑纳米晶铁基合金及发展先进高强钢具有重要理论指导意义。

利用固态相变设计高强高韧纳米结构材料具有重要意义。本项目选择纳米晶铁基合金铁素体(α)/奥氏体(γ)相变，着眼于相变机制、广义稳定性概念与纳米结构材料增塑。主要结论如下：(1) 针对界面控制型α/γ相变，发现了越小越慢现象，揭示了晶界对相变抑制作用；(2) 从热力学和动力学协同出发，提出关联相变与塑性变形的广义稳定性概念，据此给出设计高性能材料的热-动力学判据：为获得优异的强度-塑性组合需设计大驱动力-大广义稳定性的相变/变形；(3) 将广义稳定性用于基于相变调控的纳米结构Fe合金，在多种强化和变形机制控制下，合金的屈服强度和最大抗压强度达~2.61 GPa和~3.32 GPa，塑性达~35%。理论分析和微观结构表征表明，大驱动力-大广义稳定性的热-动力学关系可以从相变贯穿至塑性变形过程，最终获得良好的力学性能。基于上述成果，本项目加深了纳米晶相变动力学研究，提出的金属材料设计热-动力学判据展示了相变和变形的热-动力学关联，有助于加速筛选相变以获得优异的力学性能，对高强高韧纳米晶铁基材料设计极具意义。