| Eutectoid structure exists generally in the process of heat treatment,and its eutectoid growth mechanism as a kind of basic phase transition phenomenon of heat treatment process,widely presents in a variety of structural materials.Accurate to obtain the microstructure evolution and the law is the foundation in deep understanding of phase transformation mechanism and control the final formation of the microstructure.However,the previous researches of cooperative growth focused on the eutectic transformation process,and the study of eutectoid is not enough depth.At the same time,the experimental theory of nuclear growth is diverse,and the method of direct experimental observation was lacking.Therefore,in this article,from the aspects of simulation,the Fe-C binary alloy is used as the research object,a model of eutectoid nucleation growth consistent with various theories try to be established.The eutectoid transformation process of cooperative nucleation,cooperative growth and multi-island growth were studied by using the multi-phase field method.Firstly,based on the classical cooperative nucleation growth model of Fe-C alloy pearlite,according to two experimental theoretical phenomena,a)the carbon diffusion coefficient of ferrite,austenite,cementite at austenite boundary is larger than that in the internal grain and b)pearlite actual nucleation process is very short,the austenite interface diffusion and austenite boundary nucleation rate is analyzed,then an improved model of nucleation growth is proposed by introducing boundary diffusion and step growth rate control item.On this basis,the anisotropy and orientation relationship model was introduced,then the growth of different oriented multi-island lamellar was discussed.Then the numerical solution of the phase field and concentration for pearlite was given.Secondly,the reliability of the improved model is discussed by introducing different correction conditions step by step.Then,the optimal nucleation growth parameters of Fe-C alloy pearlite under this model were studied by using the improved model,and the regularity between nucleation growth parameters and nucleation rule of pearlite lamellae was studied.The anisotropy and orientation relationship were introduced for the improved model of nucleation growth.It was found that the anisotropy induced the slant growth of the lamellae,and the angle of tilted growth was the same as that of lamellar pearlite and austenite.In view of the uncertainty of the nucleation position,the nucleation and growth mechanism of the lamellae under different nucleation position models were discussed.The bifurcation growth phenomenon can be presented when the austenite grain nucleation was considered.However,the growth orientation relationship remains essentially constant.Thirdly,the multi-phase field model was used to simulate the single orientation pearlite.The growth mechanism of single orientation pearlite was analyzed,and different initial parameters were adjusted.The effect of lamellar spacing and isothermal temperature on the growth rate and morphology of lamellae was obtained.In view of the influence of anisotropy on the nucleation growth,the effect of anisotropy model on the growth of unidirectional pearlite is discussed.Compared with the isotropic results,not only the feasibility of tilting growth is obtained,but also the preferential orientation in cooperative growth.For the effect of carbide on the lamellar cooperative growth,the proeutectoid cementite is taken as the object of study,and the influence of different position and size of pro-eutectoid cementite on the cooperative growth of lamellae is considered.Fourthly,based on the improved model of pearlite nucleation growth,the evolution process of multi-island orientation lamellar growth behavior was studied by combining the nucleation process and the cooperative growth process.Then the lamellar growth law under different nucleation position model and different isothermal temperature was discussed.When the orientation of the two islands are different,the growth direction is dominated by the island of the larger angle.When the orientation difference of the two islands is small,the growth direction is dominated by the bisector of the two island borders.The presence of carbides can affect the growth of the lamellae.The larger the carbide area in the austenite grains,the more likely to limit the nucleation of the lamellae.Finally,the isothermal transformation experiments were carried out with different isothermal temperature and different isothermal time.The pearlite lamellar spacing and pearlite morphology of Fe-C alloy will be analyzed.Then the results were compared with the simulation results,and the effects of pearlite lamellar spacing,nucleation process,multi-island cooperative growth morphology and carbon-containing compounds were verified. |
