Study Of Strengthening And Toughening,and Physically Based Modelling Of Martensite-based High-strength Steels

This project focused on the topic of processing-microstructure-property relationships of martensite-based high-strength steels was carried out.The work was guided by Integrated Computational Materials Engineering(ICME)approach,which combines various advanced characterization techniques and analyzing methods,theories and models for microstructural strengthening and toughening,theories and models for phase transformations,CALPHAD thermodynamic and kinetic databases,etc.This project aims to reveal the critical microstructure unit for strength and toughness of martensite,to interpret the coordination mechanisms of bainitic transformation and martensitic transformation,and to excavate the microstructural constituents and heat treatment technology for the optimal combination of strength and toughness of martensite-based high-strength steels.Finally,the goal is to bridge the heat treatment-microstructure-property relationships by physically based models,working for the predictions of microstructure and property for specific chemical composition and heat treatment technology,before the optimization and design of heat treatment and alloying of similar materials.The details and main conclusions are summaried as follows:(1)The relationship of the propagation of main crack and the hierarchical structure of martensite under loading was studied by Electron Back Scatter Diffraction(EBSD)technique,results suggesting that martensitic packet holds more resistance for crack propagation than martensitic block.Plus,the coordination of bainite-martensite phase transformations was studied by in-situ laser confocal microscopy,finding that the first-formed lath bainte can effectively refine the size of subsequent lath martensite,which increases the density of large-angle grain boundary and decreases the stress within microstructure,retards the nucleation and propagation of microcrack during loading,and enhances the low-temperature toughness of materials.(2)The influence of various heat treatment technologies on the combination of strength and toughness was studied.The results indicate that the dual-pahse microstructure of lath martensite and intercritical ferrite with characteristics of martensite holds better combination of strength and toughness than single-phase martensitic microstructure.In addition,the microsture of lath bainite + lath martensite achieved by controlling cooling rate of austenite,and the microstructure of refined lath martensite by double quenching technology can improve toughness,without losing strength.(3)Advanced quantitative Analytical Electron Microscopy(AEM)and precipitation modelling by Langer-Schwartz-Kampmann-Wagner(LSKW)approach within CALPHAD framework was comibined to characterize and model the quantitative information of Cu precipitation during tempering of maraging stainless steel 15-5 PH,including structure,size,number density,voume fraction,etc.There is a good argreement between the characterization and modelling,and relationship of heat treatment with precipitation is sucessfully bridged by physically based models.(4)The evolutions of the critical micro structural parameters for individual strengthening mechanisms of fresh martensite and tempered martensite were quantitatively characterized by EBSD,Transmission Kikuchi Diffraction(TKD),Electron Channeling Contrast Imaging(ECCI),X-ray Diffraction(XRD),etc.These quantitative data,together with strengthening models and mechanical testing were used to evaluate the strength contribution from Cu precipitation.In addition,the Cu precipitation strengthening was modelled by combining models for shearing and Orowan bypass mechanisms of precipitation strengthening and the LSKW-based precipitation modelling.This modelling results of precipitation strengthening agree fairly well with the evaluated results based on experiments.Thus,the heat treatment-microstructure-property(strength)relationships of martensite is bridged by physically based models.