Experimental And Computational Studies Of Thermodynamics And Diffusion Kinetics In Systems Related To Lightweight High Strength Steels

To meet the growing demand for automotive lightweighting and safety improvements,lightweight steels with high specific strength have attracted much attention of materials scientists.Knowledge of both thermodynamics and diffusion kinetics of lightweight steels is of critical importance in rationally designing alloy composition and exploring the effect of heat treatment parameters on material performance.The present work focuses on the fundamental investigation of thermodynamics and diffusion kinetics on the lightweight steels within the framework of CALPHAD approach.Then a quantitative relationship between composition/processing-microstructure-properties has been preliminarily established.According to the studies of the lightweight steels alloy systems and the anlysis of practical problems,the major contents of the present work can be summarized as follows:(1)Based on the reliable experimental data on phase diagram and thermochemical properties and the ab intio data,the present work has assessed all sub-systems of lightweight steels and then optimized the Mn-Si,Fe-Mn-Al,Fe-Mn-Si,Fe-Al-C,Fe-Si-C,Mn-Al-C and Mn-Si-C systems.Finally,the thermodynamic database for the lightweight steels has been constructed.The present thermodynamic description can satisfactorily reproduce the homogeneity range of the ?-carbide and its phase equilibria.Moreover,the constructed thermodynamic database can successfully predict the solidification behavior and the solid-state phase transformation of the lightweight steels.(2)Based on the reliable experimental diffusion data,the present work has assessed the diffusion mobilities for the BCC and FCC phases in all sub-systems and then preformed the diffusion couple experiments in FCC Fe-Si,BCC Fe-Mn-Al,FCC Fe-Mn-Si and BCC Fe-Al-Si alloys.After the ciritcal review,the present work has optimized the diffuison mobilities in BCC Fe-Al,Fe-Mn-Al,Fe-Mn-Si and Fe-Al-Si alloys,and FCC Fe-Al,Fe-Si,Al-Si,Fe-Mn-Si,Fe-Al-C and Fe-Si-C alloys.Finally,the mobilitiy database for the lightweight steels has been constructed.(3)Based on the constructed thermodynamic and kinetic databases,the precipitation behavior of the ?-carbide in the Fe-Mn-Al-Si-C lightweight steels during aging has been simulated using the TC-PRISMA software in the present work.The volume fraction and mean particle redius of the K-carbide can be successfully calculated.By means of the model for the order strengthening mechanism,the precipitation strengthening contribution to the yield strength has been predicted.The integrated computational materials engineering technique for lightweight stees has been preliminarily developed in the present work.