Phase-field Simulation Of The Growth Of Grains In A Binary Alloy During Phase Transformation

The solidification of materials is a complicated process,by which the formed microstructure determines the final using performance of the material.Research and regulation of the microstructure can provide a theoretical basis for the preparation of high-performance materials and obtain the required material.The phase field method can directly simulate the formation of the microstructure of the material,which is an effective tool for microstructure simulation.In this paper,the liquid-solid phase transition and solid-solid phase transition of binary alloys are simulated based on the multi-phase field model.The main research contents and conclusions are as follows:1.Taking the Al-Cu binary alloy as an example,based on the three-dimensionalmulti-phasefieldmodel,theevolutionofthe microstructure in the liquid-solid phase transition process is simulated:?1?The competitive growth of multiple grains during isothermal solidification was simulated by applying parallel computing techniques.In addition,thephasefieldsimulationresultswereverifiedbythe experimental method.The simulation results show that the competitive growth of equiaxed dendrite is divided into two types:the first occurs during the process of competitive growth,the tips of primary dendrite on different grains taking part in the competition stop growing in their optimal growth direction;the second also occurs during competitive growth,the tips of primary dendrite which participate in the competition on different grains never stop growing in their optimal growth direction.The dendritic morphologies of the first competition growth type are divided into two types.Primary dendrites of grains taking part in the competition stop growing in their optimal growth direction and the competition plane enlarges when neither one wins the competition.However,when one wins the competition,the primary dendrites of grains with superiority go through the blocking grains and continue to grow in their optimal growth direction.The primary dendrites of inferior grains stop growing in their optimal growth direction and then instead grow in those areas without obstacles.The dendritic morphology of the second competition-growth type is shown to be the deformation of primary dendrites,which are mainly represented as the deflection and bending observed from different views.The simulation result agreed well with the experimental results.?2?Take the two-layer columnar crystal as an example.The influence of grain orientation on competitive growth of dendrites under different competitive modes was investigated aiming at competitive growth of columnar crystals on different planes.Additionally,the result of phase-field simulation was verified by applying cold spray and directional remelting.The result of a simulation experiment showed that the growth of dendrites on different planes is shown as follows:the dendrites whose orientation had a certain included angle with the direction of temperature gradient were restricted by those growing along the temperature gradient,and ceased to grow.Moreover,the larger the included angle between grain orientation and temperature gradient,the earlier the cessation of dendrite growth.The secondary dendrites of dendrites whose grain orientation was parallel to the temperature gradient flourished with increasing included angles between the grain orientation and temperature gradient.The competitive growth of columnar crystals on different planes was illustrated as follows:the larger the included angle between grain orientation and temperature gradient,the easier the dendrites whose orientation showed a certain included angle with temperature gradient inserted between those grew parallel to temperature gradient and the better the growth condition thereafter.Some growing dendrites after intercalation were deflected to the temperature gradient and the larger the included angle,the lower the deflection.The morphologies of competitive growth of dendrites obtained through simulation experiments canalsobefoundfrommetallographsinpracticalsolidification experiments.It implies that the two modes of competitive growth of dendrites characterised in the simulation experiment do exist and frequently appear in practical solidification processes.?3?The study simulated the 3-d competitive growth of monolayer and two-layer columnar crystals in the directional solidification process.In this way,we studied the influences of the growth orientation angle?₁₁and azimuth?_A of grains of dendrites not preferentially growing in the directional solidification upon the secondary dendrites on preferentially growing dendrites.In the simulation process,two single-factor influence experiments were designed for the monolayer and two-layer columnar crystal structures.The simulation results showed that,when 10o??₁₁<45oand?_A<45o,the secondary dendrites on the preferentially growing dendrites in monolayer and two-layer structures underwent the same changes as?₁₁1 and?_A for non-preferentially growing dendrites.The changes are summarised as follows:as?₁₁1 was enlarged,the growth direction of the secondarydendritesonthepreferentiallygrowingdendritesatthe converging grain boundary?GB?presented an increasing inclination to that of preferentially growing dendrites;with increasing?_A,the growth direction of the secondary dendrites on the preferentially growing dendrites at the converging GB exhibited larger deflection,and the secondary dendrites grew with branches;the secondary dendrites on the preferentially growing dendrites at diverging GBs grew along a direction vertical to the growth direction of the preferentially growing dendrites.When?₁₁<10oand?_A<45o,and at?₁₁=45oand?_A=45o,the two-layer columnar crystals exhibited a more complex morphology than that of their monolayer counterparts.This was caused by the grain locations and the interaction between grains in two-layer structures.2.Taking Fe-C as an example,based on the multiphase field model,the evolution of microstructure in solid-solid phase transition is simulated.In this paper,the phase-field method is used to simulate the morphology of lamellar pearlite microstructure,and investigate the growth rule of lamellar pearlite microstructure under the condition of different initial eutectoid lamella spacing and different initial undercooling.The simulation results show that with the increase of the initial lamella spacing when the undercooling is kept constant,the amplitude of eutectoid lamella increase,eutectoid lamellar morphology would exhibit as follows:regular symmetric lamellar?irregular asymmetric shape?rod;The lamella spacing is kept constant,when the initial undercooling is in the range of 19.6²0K,with the decrease of the initial undercooling,the diffusion ability of carbon atoms in eutectoid steel increase,lose stability with the lamella spacing,it cause that the eutectoid lamellar changed from regular shape to irregular shape;when the initial undercooling is in the range of 20²2K,with the increase of initial undercooling,the diffusion ability of carbon atoms in eutectoid steel decrease,the concentration of carbon atoms in the cementite phase front can not be make it grow symmetrically,cementite phase grow to the place where have high carbon concentration,which cause a phenomenon of tilt growth and lamellar merging.