钢铁材料中马氏体相变强韧化机制的分子动力学模拟

Advanced high-strength steels can obtain high strength and high plasticity simultaneously, breaking through the strength-ductility trade-off dilemma in traditional steels. Previous studies indicate that high strength and high plasticity can be obtained by controlling the martensitic transformation in steels. Therefore, understanding the mechanisms of mechanically induced martensitic transformation and its influences on the mechanical properties of steels are the keys to guide the design of high-performance steels. However, there has been no scientific consensus regarding the martensitic transformation paths and variant selection mechanisms, which limits the design and further optimization of high-performance steels. Previous studies of the applicant indicated that tensile directions had significant effects on the martensitic transformation paths, and hence the deformation mechanism and mechanical properties of pure iron. Based on the previous studies, this project aims to investigate the following scientific problems in Fe and Fe-based alloys during tensile deformation: (1) The influences of deformation conditions, the original grain size, the addition of carbon on the microstructural evolution of materials; (2) The factors that affecting the martensitic transformation paths and variant selections; (3) The effects of the forming microstructures on the deformation mechanisms and mechanical properties of materials. It is expected that this research will further deepen our understanding on the deformation mechanism and reveal the strengthening and toughening mechanisms in steels, which can provide both theoretical basis and instructive guidance for the design, research and optimization of new types of high-performance steels.

先进高强钢同时兼备高强度和高塑性，突破了传统钢铁材料中强度与塑性不可兼得的瓶颈问题。研究表明，可通过控制马氏体相变来同时提高钢的强度和塑性。因此，确定钢在变形过程中的马氏体相变机制及其对力学性能的影响是指导高性能钢铁材料设计的关键。然而目前国内外对马氏体相变的路径和变体选择机制仍缺乏全面的认识，这制约了高性能钢铁材料的设计和进一步优化。申请人前期的研究发现，不同的拉伸变形方向将显著影响铁中的马氏体相变路径，继而影响材料的变形机制与力学性能。本项目拟在申请人前期的研究基础上，进一步深入探讨Fe及Fe基合金在拉伸变形过程中：1）变形条件、变形前初始晶粒尺寸、碳元素对材料显微组织演变的影响；2）马氏体相变路径和变体选择的影响规律；3）微观组织的演变对材料的变形机制和力学性能的影响。研究结果可望进一步完善对钢铁材料强韧化机制的认识，为高性能钢铁材料的设计、研发和优化提供理论基础与指导依据。

先进高强钢同时兼备高强度和高塑性，突破了传统钢铁材料中强度与塑性不可兼得的瓶颈问题。可通过控制马氏体相变来同时提高钢的强度和塑性。本项目通过分子动力学模拟方法系统研究了纯铁及不同碳含量的Fe-C合金在不同塑性变形条件下的微观组织演变以及动态加载情况下材料的力学性能响应，在此基础上，深入探讨了Fe及Fe-C合金的塑性变形机制。此外，通过形核热力学分析，首次提出了纯铁中有关马氏体相变的非经典形核理论。研究表明：1. 纯Fe中马氏体通过亚临界晶核合并以及分步形核的非经典形核方式，可避免经典形核理论中提出的高形核能垒；2. 外界施加应力可引起纯Fe中发生面心立方（FCC）到体心立方（BCC）结构的马氏体相变，不同的加载条件导致相变过程中发生变体选择现象，进而引起相邻的片状马氏体发生合并长大的现象，最终影响材料的晶粒尺寸及力学性能；3. Fe-C合金在塑性变形过程中可发生FCC、BCC及密排六方（HCP）结构之间的相互转变，新相优先从碳原子附近形核，之后新相通过界面的迁移长大，而碳原子可以钉扎界面的迁移，从而起到延缓相变的作用，最终引起材料内部晶粒的细化，对材料起到强化作用；4. 不同的拉伸变形方向影响Fe-C合金中碳原子在各相晶格间隙的占位，占据不同间隙位置的碳原子对材料中位错的钉扎能力不同，从而影响材料的强度和塑性等力学性能。项目可提高对钢铁材料中强韧化机制的认识，并为高性能钢铁材料的设计和发展起到指导作用。