Kinetics Modelling Of ?' Precipitation Behaviour And Creep Behaviour In Ni-based Single Crystal Superalloys

This dissertation investigated the ?' precipitation behavior during heat treatment and the creep behavior under high temperature and low stress condition in Ni-based single crystal superalloys,then established quantitative models which describe corresponding kinetics behavior based on CALPHAD.The statistical analysis of size,volume fraction and number density of ?'precipitates in single crystal superalloys DD5 and DD91 was carried out using SEM and Image-Pro Plus.It could be concluded that the average size evolution of cuboidal?' precipitates fits LSW theory well,that the volume fraction of ?' precipitates keeps ascending,and that the number density of ?' precipitates keeps descending,during high temperature and low temperature ageings.The phenomenon is called as "fake coarsening" or "competitive growth".Based on classical nucleation theory,diffusion law and Gibbs-Thompson effect,a numerical model which accounts for the evolution of average size,volume fraction and number density of ?' precipitates during precipitation simultaneously was proposed subsequently.The thermodynamics parameters emerging in the model like chemical potential,diffusive coefficient and compositional distribution could be extracted form Thermo-Calc by CALPHAD effectively.The modelling results show that it can not only predict the evolution of average size,volume fraction and number density of ?' precipitates in multi-component systems during ageing accurately,but also account for the quasi-coarsening and the compositional segregation behavior.The microstructural evolution and the precipitate-dislocation modes in single crystal superalloy DD5 under high temperature and low stress creep test were studied by SEM and TEM.The underlying relationship between creep behavior and deformation characteristics was discussed later.The specific analysis of creep curves show that the creep deformation under this condition could be divided into decelerating stage and accelerating stage.During the primary creep stage,the periodical interfacial dislocation network around rafted ?' precipitates impedes further dislocation movement in ? matrix,thus lowering the creep rate.Subsequently,massive dislocations climb over rafted ?' precipitates,annihilating part of dislocations and receding the impediment of interfacial dislocation network(recovery/softening mechanism).This relative balance between dislocation multiplication and annihilation results in slower creep rate evolution.During the later creep stage,rafted ?' precipitates are cut through by superdislocations coupled with anti-phase boundary owing to microstructural degradation and dislocation pile-up around ?/?' interface.The stable dislocation network and L12 lattice structure would be destructed,accelerating creep deformation accordingly.A state-variable creep kinetics model which accounts for above characteristics was presented subsequently.The physical properties emerging in the model like diffusion activation energy and anti-phase boundary energy could be associated with CALPHAD,and could be calculated by Thermo-Calc.The results show that the model is able to simulate the deformation behavior under different high temperature and low stress creep conditions for single crystal superalloy DD5.