| 1 | Kinetics and microstructural modeling of isothermal austenite-to-ferrite transformation in Fe-C-Mn-Si steels显示文摘During the multi-stage processing of advanced high-strength steels, the austenite-to-ferrite transformation, generally as a precursor of the formation of other non-equilibrium or metastable structures, has a severe effect on the subsequent phase transformations. Herein, a more flexible kinetic and microstructural predictive modeling for the key austenite-to-ferrite transformation of Fe-C-Mn-Si steels was developed,in combination with the classical nucleation theory, the general mixed-mode growth model based on Gibbs energy balance, the microstructural path method and the kinetic framework for grain boundary nucleation. Adopting a bounded, extended matrix space corresponding to a single ferrite grain, both softimpingement and hard-impingement can be naturally included in the current modeling. Accordingly, this model outputs the ferrite volume fraction, the austenite/ferrite interface area per unit volume, and the average grain size of ferrite, which will serve as the input parameters for modeling the subsequent bainite or martensite transformations. Applying the model, this work successfully predicts the experiment measurement of the isothermal austenite-to-ferrite transformation in Fe-0.17 C-0.91 Mn-1.03 Si(wt%) steel at different temperatures and explains why the final-state average grain size of ferrite has a maximum at the moderate annealing temperature. Effectiveness and advantages of the present model are discussed arising from kinetics and thermodynamics accompanied with nucleation, growth and impingement. | S.J.Song W.K.Che J.B.Zhang L.K.Huang S.Y.Duan F.Liu | 2019 | Journal of Materials Science & Technology2019，35，8： | 4 |
| 2 | Uncovering the softening mechanism and exploring the strengthening strategies in extremely fine nanograined metals:A molecular dynamics study显示文摘The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less than 10 to 20 nm),which limits the development of ultrahigh-strength materials.In this work,using columnar-grained nanocrystalline Cu-Ag‘samples’,molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies(e.g.,formation of solid solution or grain boundary(GB)segregation)in extremely fine nanograined metals.Accordingly,the softening of pure metals is induced by atomic sliding in the GB layer,rather than dislocation activities in the grain interior,although both occur during deformation.The solid solution lowers the stacking fault energy and increases the GB energy,which leads to the softening of NC metals.GB segregation stabilizes GB structures,which causes a notable improvement in strength,and this improvement can be further enhanced by optimizing the solute concentration and GB excess.This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahighstrength materials. | H.R.Peng Z.Y.Jian C.X.Liu L.K.Huang Y.M.Ren F.Liu | 2022 | Journal of Materials Science & Technology2022，，14： | 0 |
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