| With the advancement of the automobile lightweight,Magnesium alloy,as a kind of lightweight alloy with outstanding damping capacity,low inertia,good electromagnetic shielding effectiveness and green environmental protection,has been favored by many automobile manufacturers.The microstructure formed during solidification always plays a key role in determining the mechanical properties of alloys.Therefore,quantitative regulation of the evolution of the microstructure of Mg alloys is of far-reaching significance for controlling and optimizing the mechanical properties of Mg alloys.In this paper,a quantitative phase-field model for polycrystalline solidification and dilute multicomponent alloys is developed,which analyzed the effects of phase-field parameters such as spatial step size,time step size,anisotropic strength,noise intensity and coupling coefficient on the simulation results,and applied to Mg-Gd-Zn alloys.The dendrite growth laws were revealed from the perspectives of Lewis coefficient,initial undercooling,cooling rate,solidification rate,temperature gradient and nucleation undercooling in the optimal parameter range.The real-time dynamic process of the dendrite growth has been observed and recorded using in situ synchrotron radioactive technique,and the experimental results were compared and verified with the simulation results.The major work of this paper is listed as follows.(1)A multi-dendrites growth in non-isothermal solidification has been studied by establishing a polycrystalline and multicomponent phase-field model which coupled with temperature field.The effects of Lewis coefficient on the dendrite growth velocity,tip radius,concentration distribution and temperature distribution were revealed.It is found that the larger the Lewis coefficients,the more conducive to the release of solidification latent heat,the faster the growth velocity of dendrites,the smaller the tip radius,the higher the solute concentration of the liquid and solid phases,and the more uniform temperature field distribution.The effect of different initial undercooling on the growth of dendrites was studied.The results show that as the degree of subcooling increases,the driving force for dendrite growth is enhanced,which speeds up the growth of dendrites,increases the volume fraction of the solid phase,and increases the solute concentration of the liquid and solid phases,resulting in an increase in the segregation ratio.The growth law of dendrites under different orientation angles is analyzed.It is found that with the growth of dendrites,the interaction potential between dendrites increases,the growth rate of dendrites decreases,and the tip temperature and concentration increase.(2)The phase-field method is applied to calculate the dendrite growth kinetics during the non-isothermal solidification,and the results are compared with the classic JMAK theory and phenomenological theory.It shows that the single dendrite simulated results are in good agreement with prediction of JMAK theory.However,the prediction result of the transformation kinetics of polycrystalline solidification by JMAK theory is relatively larger than the phase-field simulation due to the neglection of the collision effect among dendrites with different orientations,while the phenomenological theory considers the interaction between dendrites,which are consistent with the simulation results.(3)The phase-field method is used to quantitatively predict the phenomenon of crystal transformation.Larger temperature gradient results in greater secondary dendrite arm space,smaller tip radius.Both the tip radius and the secondary dendrite arm space decrease by increasing the solidification rate.The growth law of dendrites in different crystal regions was revealed.The tip rate of equiaxed crystal and the volume fraction of solid phase increase with the increase of cooling rate in equiaxed zone.In the region of crystal transition(CT),when the solidification speed is fixed,the smaller the temperature gradient,the more conducive to the occurrence of CT.Under the condition of fixed temperature gradient,the probability of CT increases with the increase of solidification rate.In the columnar region,increasing the temperature gradient is not only beneficial to the reduction of dendrite spacing,but also can increase the growth rate of columnar dendrite.The CT transformation of dendrites was inhibited with the increase of critical nucleation undercooling,which also accelerated the columnar dendrite growth rate.The influence of cooling rate(0.025?0.25K/s)on the dendrite morphology has been investigated by using in situ synchrotron X-ray radiographic technique,and the experimental results were used to verify the simulation results.The experimental and simulation results demonstrate that the higher the cooling rate,the faster the growth rate of dendrite,the smaller the distance between the secondary dendrite arms and the tip radius,the larger the number of grains,the smaller the grain size,and the more significant the CT phenomenon.(4)The effects of cooling rate,temperature gradient,solute concentration and preferred orientation on the morphology of columnar dendrite were revealed.It is found that with the increase of cooling rate,temperature gradient and solute concentration,the growth rate of columnar dendrite increases,and the dendrite spacing decreases obviously.When the preferred orientation angle is increased,the growth rate of columnar dendrite decreases first and then increases.When ?>10°,the growth rate of dendrites decreases in turn.Quantitative research on competitive growth in the process of directional solidification was carried out.During the bi-crystal converging growth,if the angle is less than 10°,unfavorably oriented dendrite could block the favorably oriented dendrite,which results in an unusual overgrowth.However,when ? is less than10°,there is no unusual overgrowth phenomenon in divergent growth.In divergent growth,when ? is less than 10°,the inhibitory effect of the unfavorably oriented dendrites on the favorably oriented dendrites is weakened,and there is no unusual overgrowth phenomenon. |
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