Application Of Analytical Model For The Study Of Interface-controlled Solid-state Phase Transformation Kinetics

In the production of very many high-performance metal materials, the solid-statephase transformation plays an important role. With the development of preparationtechniques of metal materials, the kinetic behaviors of solid-state phasetransformations deviate from the classical model predictions. Further, the solid-statephase transformation is an important mean to directly adjust the microstructure ofmetal materials, and thus tune the final properties and the performance of materials.Therefore, it is both physically meaningful and industrially useful to study the kineticsof solid-state phase transformations. In this thesis, investigations on the overallkinetics of crystallization of metallic glasses and polymorphic transformation werecarried out. During theoretical analysis, by analyzing the influence factors underdifferent conditions and removing the theoretical assumption in classical model,general models for solid-state phase transformation kinetics is proposed. Inexperimental study, adopting the thermal analysis, e.g., DSC and DIL, the overallkinetic informations can be measured for different solid-state transformations; withthe help of experimental methods for phase and microstructure analyses, the specificmechanism of phase transformation can be determined. Accordingly, the applicabilityof the models can be tested by the experimental results. The main conclusions are asfollows:1On the basis of analytical model, several new kinetic analysis recipes wereproposed. The activation energies for nucleation and growth can be obtainedfrom the linear regression by using the analytical expression for the overalleffective activation energy. An iterative method was proposed for thedetermination of impingement mode by using the normalized treatment fortransformed fraction. These methods were applied successfully for thedetermination of kinetic parameters in several metallic glasses.2The general expression for the “temperature integral” and the initialtransformation temperature were incorporated into the analytical model, so themodel can be applied to the more general cases. For transformation withdifferent activation energy, the accuracy of the model can be guaranteed byselecting the optimal approximations. If the initial transformation temperature ishigh enough to influence the kinetics, the new model still predicts the transformed fraction and the kinetic parameters very well. The crystallizationkinetics of Ti50Cu42Ni8was investigated by the new model.3According to the philosophy of analytical model, the summation/producttransition was extended. On this basis, the model was proposed for describingthe transformation involving multi synchronous or asynchronous processes. Themodel can be used to explain many kinetic phenomena: the mechanism ofAvrami nucleation can be assumed to be a combination of a positivesub-process with continuous nucleation and various gradually dominatingsub-processes; the transformation subject to continuous nucleation can beregarded as the total effect of infinitely many sub-processes due to sitesaturation starting at different times; both the abnormally fast transformationand the abnormal Avrami exponent are due to the incubation period. Finally, thetime asynchronous model was applied successfully to describe thecrystallizations of Mg-Cu-Y and Zr-Cu-Al amorphous alloys.4An extended analytical model is derived for isochronal solid-state phasetransformation assuming interface-controlled growth mode. In the modeling,incorporation of thermodynamic factor into kinetics of nucleation and growth isperformed, so that the model can be used to describe the transformationoccurring either near or far from the equilibrium state. Using a special treatmentfor the “temperature integral”, the model can be used to depict thetransformation during either continuous heating or continuous cooling.Applying the extended model, the overall kinetic behavior of austenite/ferritetransformation in binary substitutional Fe-based alloys upon cooling, measuredby dilatometry, was described successfully.5Staring from the rate equation and incorporating the different kinds ofthermodynamic energy terms, a thermo-kinetic model is derived to describe thetransformation kinetics for both isothermal and isochronal transformation. Themodel predicts a temperature range for complete isothermal transformation, asluggish stage in isochronally conducted transformation and atemperature-dependent stage in non-isothermally conducted transformation. Thekinetics of austenite/ferrite transformations in two binary substitutionalFe-based alloys (i.e. Fe-3Mn and Fe-2Co) was described successfully by thenewly proposed model.6Based on the Vogel-Fulcher-Tammann relationship for supercooled fragile liquids, a temperature-dependent apparent activation energy is introduced intothe solid-state phase transformation kinetics, and a general model for describingthe crystallization of metallic glasses was derived. According to new model, amore reliable and accurate kinetic recipe was also proposed. The informationsabout the liquids fragility can be obtained by applying it to the experimentaldata of crystallization. Finally, the crystallization kinetics of Zr-Cu-Al-Ni bulkmetallic glass is studied and interpreted well by using the new model.