| The solidification process has great effect on the material's microstructure and its operational performance.Since the rapid solidification process is completely deviated from the equilibrium conditions,many hypotheses based on the equilibrium conditions for the classical solidification theories are not suitable and the properties of new magnetic materials,such as amorphous soft magnetic materials and nano-crystal permanent magnetic materials prepared by the rapid solidification process,are significantly improved,including the magnetic properties,strength,plasticity and corrosion resistance.Therefore,the research on the magnetic materials'microstructure evolution during the rapid solidification process plays an important role in exploiting new materials with supreme magnetic properties.During the rapid solification process,how to improve the glass forming ability?GFA?,proposing a quantitative,generalized and universal GFA criterion to predict a novel amorphous system and produce the larger sizes of bulk metallic glasses?BMGs?with a smaller critical cooling rate?Rc?for the industrial application are an headed problem in this field.Theoretically,the Rc should be the most direct,quantitative,and universal parameter for the GFA prediction.However,this analysis has one disadvantage,that is,Rc is usually too large to be measured precisely by the experiment,which becomes the bottleneck for the development of this method and limits its application in evaluating and forecasting GFA.How to get the accurate value of Rc and how to understand the intrinsic physical influence mechanism of GFA,such as the formation mechanism of amorphous phase,the amorphous transition and the competition process between the amorphous phase and crystalline phase,is a significant problem.Besides,annealing the BMGs prepared by the direct casting process provides a new manufacturing technique for the three-dimension large size nanocrystallines.However,the GFA and uniformity of the sample manufactured by the direct casting are far below the ideal requirement,which limits the material properties enhancement and its further application.How to improve the GFA and optimize the microstructure of alloy become the key to solve the problems.Moreover,it is difficult to study the microstructure evolution dynamically,directly and visually,due to the high temperature of the melt and the opacity of the metal and copper mould.With the development of computational materials science,computational simulation assisted with experiment has been a new direction in materials science and engineering.Compared with other simulation methods,the advantages of phase field method?PFM?are that its governing equations are written as unified equations in the whole simulated space and there is no need to track the interface,which is more convenient for studying the evolution of the microstructure,the internal mechanism and internal rules.However,it is rarly reported that the GFA proposal and the influence of technological parameters for microstructure evolution during the rapid solidification process are studied through PFM.Therefore,the focus of this dissertation are to get the Rc of Fe-B amorphous soft magnetic materials through two methods as the intrinsic criteria reflecting the GFA and competition process between the amorphous phase and crystalline phase on one hand and to research the influence of technological parameters on the microstructure evolution of Nd-Fe-B permanent magnetic materials during direct casting on the other hand.Firstly,the Rc calculated on the basis of the TTT curve by the isothermal PFM is taken as the intrinsic criteria to reflect the GFA during the rapid solidification process of Fe-B amorphous soft magnetic materials.Then,the influences of different activation energy E and Q for the interface mobility M?and nucleation rate P,respectively,on Rc and GFA are investigated for Fe75B25 binary amorphous alloy.The results show that E for M?had a little influence on the nose tip temperature and time for the TTT curves,under the current simulation accuracy.With the increase of Q for nucleation rate,the nose tip temperature increase,which mean Q controlls the nose tip temperature,while the higher Q can not get the perfect C shape.Under the thermal activation energy Q1,Q2 and Q3,the temperatures and times corresponding to the nose tip are 500 K,600 K,775 K and 6.2×10-4 s,4.5×10-4 s,9.5×10-4 s,respectively,and the calculated values of Rc are 1.57×106 K/s,1.93×106 K/s,7.35×105 K/s,respectively,which are in the range of 6.4×1056.8×106 K/s from experimental values.It can be found that the values of Rc calculated based on the TTT curves by the PFM are in good agreement with the reported experimental values,which verifies the feasibility of the glass prediction and the research of the intrinsic physical mechanism of the GFA by the PFM.Secondly,the glass transition and the competition between the amorphous phase and crystalline phase of Fe1-xBx?x=9,11,15,17,20,and 25?alloy during the rapid solidification process are simulated.The transition process of liquid phase to amorphous phase and crystalline phase is mainly investigated.When the temperature is higher than the glass transformation temperature?Tg?,the whole system is liquid,while the temperature is cooled to the Tg,the amorphous cluster forms quickly,with big formation rate and small growth rate,which is similar to the catastrophic nucleation.The influence of different cooling rates?d T/dt?on the competition between amorphous phase and crystalline phase for Fe-B alloy are analyzed.The simulated Rc are 4.3×106 K/s,4.0×106 K/s,3.25×106 K/s,2.0×106 K/s,3.4×106K/s,3.5×106 K/s for Fe91B9,Fe89B11,Fe85B15,Fe83B17,Fe80B20,Fe75B25,respectively.The Fe83B17 is the best glass forming alloy predicted by PFM,which is in good agreement with the experimental or calculated results in trend and verifies the feasibility of the glass prediction and experimental guidance by PFM.Thirdly,the temperature distribution of the direct casting sample is simulated and the temperature gradient is verified by PFM.The influences of the temperature gradient on the dendrite morphology,concentration distribution and the tip velocity for binary alloy during the direct casting are the focus.Under the applied temperature gradient,resulted by the rapid thermal conductivity of copper mould,the growth velocity and the microscopic growth morphology are changed greatly and a cone shape of dendrite,instead of the normal dendrite with four symmetric arms,can be found easily.The level of the temperature gradient has a significant effect on dendrite growth and solute distribution.With the increase of the temperature gradient,the growth velocity and segregation of dendrite increases.This model can give a specific prospection for the influence of temperature gradient on dendrite morphology.Finally,the microstructure evolution of Nd-Fe-B permanent magnetic materials during the direct casting,including the nucleation and coarsening process,is simulated by PFM.Due to the greater under-cooling,amorphous phase exists near the cool copper mould,partially mixed with T1 and T2 phases,and then followed by the uniform,small and nearly spherical T1and T2 grains.The coarse grain distributes more inhomogeneous at the solidification center.In whole,the microstructure and solute shows an apparent uneven distribution.The cooling rate has great effect on the thickness ratio between the glass zone,the refine grain zone and coarse grain zone.The larger cooling rate is,the smaller overall size,and the higher uniformity of the microstructure are.The applied magnetic field can refine the grain size to some extents,but the refinement effect is not obvious.This model can reveal the quantitative relationship between the manufacture parameters and dynamic microstructure evolution directly. |
PDF Full Text Request