残余奥氏体相变及变形对Q&P钢损伤断裂行为的影响

本项目研究残余奥氏体相变及变形对Q&P钢本征断裂韧性参数的影响，及损伤形核和扩展的微介观机制，重点分析残奥的稳定性和形貌对断裂韧性及微介观机制的影响规律。内容包括1.采用拉伸及断裂实验并结合有限元计算，获得不同温度下Q&P钢的基于粘聚力单元法的本征断裂韧性参数，及其对残奥稳定性和形貌的依赖规律。2.采用SEM下的原位拉伸和μ-DIC方法观察块状残奥周围的损伤形核和扩展，分析损伤形核部位的介观应变分布特征，阐明块状残奥相变对裂纹形核和扩展的介观影响机制。3.采用TEM下的原位拉伸实验，观察纳米层片状残奥周围的损伤形核和扩展，对比分析形貌不同的残奥对损伤形核和扩展影响的差异。4.在TEM下精细表征残奥周围损伤的位错结构以及裂纹扩展尖端的位错组态，揭示残奥相变及变形对损伤形核和扩展影响的微观位错机制。本征断裂韧性参数测量以及微介观机制研究期望为提高Q&P钢的断裂韧性提供组织调控的理论参考。

This project will study the effects of deformation and phase transformation of retained austenite (RA) on damage and fracture behavior of quenching and partitioning (Q&P) steels, and the micro/meso-mechanisms of damage nucleation and propagation, focusing on how these mechanisms and fracture toughness are influenced by the stability and morphology of RA. Firstly, obtain intrinsic fracture toughness for Q&P steels under different temperatures based on cohesive elements method, and its dependence on the stability and morphology of RA, by combing tensile testing and fracture experiments and finite element analysis. Secondly, analyze the characteristics of meso-strain distribution around damage nucleation sites, and elucidate the influence of phase transformation of bulk RA on the meso-mechanisms of damage nucleation and propagation, by observing damage nucleation and propagation around bulk RA using in situ SEM tensile testing and μ-DIC analysis. Thirdly, observe the damage nucleation and propagation around nanolamellar RA by using in situ TEM tensile testing, and comparatively analyze the difference in the damage nucleation and propagation influenced by the bulk and nanolamellar RA. Lastly, characterize the dislocation structure of damage sites around RA and dislocation patterns around crack tip, and finally reveal the dislocation based mechanisms of how phase transformation and deformation of RA affects the damage nucleation and propagation. These study on the intrinsic fracture toughness and damage/fracture mechanisms at micro/meso-scale will help to consolidate the scientific foundation for fracture toughness improvement of Q&P steels.