Formation And Evolution Of Secondary Phase In Fe-Ni-based Alloys

Upon processing of corrosion resistant Fe-Ni-based alloy, the formation and evolution of second phase have a significant influence on its corrosion-resistance and mechanical properties. A comprehensive study on the formation and evolution of second phase during processing and correspondingly theoretical knowledge is very important. In this thesis, based on theoretical analysis and experiments, the formation and evolution of second phase during processing of Fe-Ni-based N08028(028) alloy(i.e. solidification, heat treatment and hot deformation) are investigated. By analyzing the thermodynamic and kinetic factors for the formation and evolution of second phase under different conditions, correspondingly thermo-kinetic models are proposed. The formation and evolution rules of second phase are investigated by experimental study of solidification, heat treatment and hot deformation of the studied alloy under different conditions. The evolutionary mechanism of second phase is revealed by the combination of theoretical analysis and experimental study. The main conclusions are as follows:1. Based on thermodynamic calculation and analysis of elemental segregation during solidification, the formation of the interdendritic σ phase is found to be attributed to the eutectic reaction during the last stage solidification. The solidification path and microstructure evolution of 028 alloy subjected to different cooling rates are investigated by theoretical and experimental methods. An analytical model is developed to quantitatively predict the evolution of the second phase. Evolution of the size and fraction of the second phase with the cooling rate has been quantitatively presented in virtue of above model, and good agreement with the experiment result is obtained.2. Based on Johnson-Mehl-Avrami(JMA) theory and the classical dissolution model for single-particle system, an analytical model for the secondary phase dissolution in multi-particle system has been developed. The interactions of solute diffusion fields in front of the secondary phase/ matrix interface are considered in the current model, which can describe the evolution of volume fraction of the second phase with time for isothermal dissolution. The current model is derived from the diffusion-controlled transformation theory while the model parameters are time-dependent and physically realistic. The predictions of the current model agree well with the real process, which confirms the validity of the model.3. Dissolution behaviors of interdendritic σ-phase in the as-cast and forged 028 alloy during heat treatment have been investigated. By introducing an equivalent initial radius into the kinetic model, the dissolution processes during heat treatment are performed and the result is in good agreement with the experimental data. A recipe derived from transformed fraction is applied to evaluate the effective activation energy for dissolution(i.e. activation energy for diffusion) and the evaluated values agree well with the reported ones. Based on the prediction of the kinetic model, it is found that the prior hot deformation can accelerate the dissolution process. Moreover, the activation energy for dissolution decreases slightly and the dominant factor of acceleration is attributed to the decreased particle size.4. Hot deformation behavior of the 028 alloy with various initial microstructures has been investigated. It is observed that the deformation temperature, strain rate and initial microstructure have a significant influence on microstructural evolution and flow stress. During hot deformation, the initial microstructure with more amount of σ-phase is found to be able to increase the flow stress, which can be explained by that the dislocation movement is hindered by the σ-phase. By analyzing the microstructural evolution, dynamic recrystallization is found to be responsible for the flow stress softening of the alloy. Considering the influence of initial microstructure, the artificial neural network(ANN) model with a back-propagation learning algorithm is established to predict hot deformation behavior of the alloy. Furthermore, the Arrhenius-type constitutive equations are modified by compensation of strain and initial microstructure to describe the flow stress. Comparison between predicted results from the two approaches, the ANN model has a better prediction precision of flow stress in hot deformation behavior of the 028 alloy with various initial microstructures.5. Based on classical thermodynamics and kinetics, a thermo-kinetic model has been developed to describe the two-stage precipitation behavior including intergranular and intragranular precipitations, in combination with the JMA theory. The current model has been successfully used to describe the precipitation process of σ phase in the 028 alloy, where a typical two-stage precipitation behavior is observed and the predominant transformation mechanism is found to change from intergranular to intragranular precipitation. Evolutions of the thermodynamic driving force and the kinetic activation energy for the two stages are calculated. Once the driving force of precipitation is enhanced, the corresponding activation energy is found to become smaller. It is concluded that the difference of driving force and activation energy between intergranular and intragranular precipitations results in the occurrence of two-stage precipitation.