相变诱导高强度高塑性纳米超细晶钢及变形机理研究

There is a continued and significant interest to develop new generation of advanced steels with a high strength and high ductility combination for light-weight applications. Grain refinement is considered a potential approach to improve strength and ductility of engineering steels. Despite the excellent efforts, the understanding of inverse Hall-Petch relationship and deformation mechanism in nanograined structure continues to be unclear. In the context of obtaining high strength-high ductility combination, a novel processing route of developing nanograined/ultrafine-grained (NG/UFG) structure in metastable Fe-Cr-Ni alloy involving controlled phase reversion annealing of the cold deformed austenite has been developed in this proposal research. In this approach, severe deformation of metastable austenite at room temperature leads to strain-induced transformation of austenite to martensite. On annealing this severely deformed strain-induced martensite reverts back to austenite either via a martensitic shear or diffusional reversion mechanism. The phase reversion annealing sequence resulted in NG/UFG stainless steel that was characterized by a combination of ultrahigh yield strength and excellent elongation of 800-1000 MPa and 30%-40%. Utilizing the concept of phase reversion, the objective is to fundamentally understand grain size dependence on deformation mechanisms from NG to CG regime in a austenitic steel. The mechanistic contribution of twinning on strain hardening response and strain rate-sensitivity as a function of grain size, and the effectiveness of twinning in enhancing ductility of NG materials will also be studied in this proposed research. The proposed research provides a unique experience to researchers where physical metallurgy concepts will be introduced to develop NG materials, impacts the metals-related nanotechnology research being pursued.

高强度高塑性是钢铁材料的重要发展方向，采用晶粒细化的方法是获得这种性能的有效途径。目前对纳米晶钢铁材料的制备和机理已经开展了深入的研究，然而纳米尺度条件下逆Hall-Petch关系和变形机制等关键性的科学问题仍有待于解决。本项目以Fe-Cr-Ni合金或中高锰钢为研究材料，提出采用形变诱导马氏体相变-退火奥氏体逆转变的创新工艺，通过对变形程度、退火温度和时间等参数的控制，制备出具有高强度高塑性的纳米晶钢铁材料（屈服强度800-1000MPa，伸长率30%-40%）。通过对纳米到亚微米系列晶粒尺寸材料的微观变形行为和孪生现象的研究，分析不同尺寸晶粒中位错属性与孪生、晶粒尺寸与孪晶体积分数之间的关系，揭示纳米晶变形过程不全位错的特性并论述孪晶对位错增殖的作用，阐明变形孪晶形核促进纳米晶材料应变速率敏感性的本质以及促进纳米晶材料塑性提高的变形机理。本项目的研究，对丰富纳米晶钢铁材料的强塑性理论具有重要的意义

钢铁材料不断向高强度高韧性方向发展，细晶强化作为一种最重要的强化机制，成为先进高强钢研究的重要方向。但是，当晶粒细化至纳米甚至亚微米级别时，晶内的位错增殖及运动受到严重抑制，恶化了材料的塑性变形能力，限制了纳米/超细晶钢铁材料的工程应用。如何增强纳米/超细晶钢的塑性变形能力，提高纳米/超细晶钢的综合力学性能是本项目的研究重点。.本项目以Fe-Cr-Ni奥氏体不锈钢为材料，探索通过“冷变形+逆相变退火”的方式制备具有高强度高塑性的纳米/超细晶奥氏体不锈钢的新工艺，成功制备出具有高强度高塑性的纳米/超细晶奥氏体不锈钢；研究了冷变形及逆相变退火过程中组织转变过程，得出冷变形组织、退火温度以及马氏体逆相变机制对晶粒细化效果的影响；结合TEM及EBSD等手段，研究了纳米/超细晶钢拉伸变形组织演变过程，揭示了纳米/超细晶钢中形变孪晶形成机制，分析了孪生变形及形变诱导马氏体相变对纳米/超细晶钢加工硬化行为的影响；研究了纳米/超细晶钢的疲劳变形行为，揭示了晶粒尺寸对疲劳强度及断裂机制的影响。分析了疲劳变形主要变形机制，以及不同晶粒尺寸的奥氏体不锈钢中应变诱发马氏体的形成机制。此外，研究发现超细晶/微米晶复合非均质奥氏体钢在变形过程中超细晶与微米晶之间的变形不协调产生极强的背应力强化，显著提高强度，同时变形中由于奥氏体晶粒尺寸不同导致的多阶段加工硬化可显著提高塑性，实现高强度高塑性组合。.项目相关成果共发表学术论文16篇，SCI收录论文12篇。该项目形成的高强度高塑性纳米/超细晶奥氏体不锈钢制备技术，可以在不改变化学成分的基础上显著提高纳米/超细晶钢的塑性变形能力，提高其综合力学性能。该技术亦可应用至其他亚稳奥氏体材料，具有重要的科学意义。