Analysis Of Interfacial Microstructure And Directional Coarsening Behavior Of Ni-Based Single Crystal Superalloys

Ni-based single crystal superalloys are widely used as advanced aircraft turbine blade materials for their excellent creep resistance behavior. They exhibit a remarkable character that theγcubic particles will transform into flat shapes (which are named rafts) under the combined influence of stresses and temperatures. This rafting behavior directly affects the creep fatigue life of Ni-based superalloys. The fracture surface is usually along the direction of rafting. Therefore, the directional coarsening mechanism is a key rule of precipitation hardening in Ni-based single crystal alloys.Ni-based single crystal superalloys are two-phase materials. The material is strengthened by a high volume fraction of hard cubicalγprecipitates (Ni3Al phase) embedded coherently in a softerγmatrix (Ni phase), where the volume fraction ofγphase may reach as high as 70%. The microstructure of theγ/γinterface as well as its evolution under external loading and temperature determines the mechanical properties of Ni-based superalloys. In this thesis, we study directional coarsening behavior of Ni-based superalloys as well as the interface microstructure and its evolution under the influence of temperatures and stresses, by using models and methods in different scales. The relationship between the microstructure evolution and the macroscopic mechanical behaviors is discussed. The main contents and results include:(1) The interface microstructure and its evolution under the influence of tensile loading and temperature are simulated by molecular dynamics. From the simulation we find that three dislocation network patterns, namely square, rectangle and triangle appear on the (100), (110) and (111) phase interfaces respectively. The three patterns of dislocation network change from regular to irregular or disappear under the influence of tensile loading and temperature. Different patterns of dislocation network show different degrees and patterns of damage. Theγrafts and mechanical properties are closely related to the damage process of the dislocation networks.(2) Taking into account the complex stress and failure mechanisms in-service of Ni-based single crystal superalloys, the structural evolution of dislocation networks are simulated under different ways of loading (tensile, shear, combined tensile and shear) by molecular dynamics. The results show that the damage processes of the dislocation networks are different for three different phase interfaces under different ways of loading. The square dislocation network at (100) phase interface is the most difficult to damage and disappear, so it is the most stable and strengthen interface effectively. The damage of this square dislocation network is mainly caused by [100] axial loading, which is the reason for the failure of Ni-based single crystal superalloys.(3) Based on atomistic simulation, the effects of strain rate and temperature on the structural evolution of interface dislocation networks and deformation mechanisms are investigated. The results indicate that the dislocation networks show different degrees of damage at different strain rates and temperatures, and the difficulty level ofγrafting is also different. Theγrafting occurs only in the smaller strain rate, and become easier at higher temperature. Moreover, dislocation motion and deformation mechanisms are different at different strain rates and temperatures, which lead to the change of maro-mechanical properties at interfaces. The yield strength and ultimate tensile strength increases with the increase of strain rate, while decreases with the increase of the temperature. The plasticity and ductility decreases with the increase of strain rate, while increases with the increase of the temperature.(4) The Eshelby's inclusion theory and Mori-Tanaka's mean field method are used to evaluate the elastic energy caused by a change in the shape ofγprecipitates. The shape stability and directional coarsening ofγprecipitate in Ni-based superalloys with two types of elastic constant are investigated. Moreover, the plastic strain caused by the formation of dislocation networks at theγ-γinterface is taken into account. The effect of matrix plastic strain on the shape stability of y precipitates is considered. The results suggest that the plastic strain plays an important role on the shape stability and rafting behavior. The predictions based on a simple elastic analysis do not always agree with the experimental results regarding shape stability and rafting behavior. However, if the plastic matrix strain is introduced, the results are perfectly consistent with experimental observations for alloys with two types of elastic constant.(5) A new three-phase micromechanical model based on the Eshelby's equivalent inclusion method has been developed to study the directional coarsening behavior in Ni-based single crystal superalloys. In this model, theγmatrix as two-phase inclusion, while theγprecipitate as the matrix phase. The von Mises stress, elastic strain energy density and hydrostatic pressure in different matrix channels are calculated to predict the directional coarsening behavior of different stress axis orientations. The calculated results indicate that the directional coarsening ofγprecipitate occurs when the external stress is applied along the [001] and [110] directions, respectively. The rafting direction depends on the sign of the misfit and the type of the external stress. However, no rafts occurs when the external stress is applied along the [111] direction.(6) The finite element method has been applied to calculate the Mises stress and strain energy density distributions of theγandγphases in Ni-based single crystal superalloys. The analysis of directional coarsening behavior ofγparticles and driving force are performed based on the element diffusion behavior. The effects of the temperature and external loading on the directional coarsening behavior are considered. The critical external loading for start-up creep dislocations is obtained. The results show that the application of an external stress leads to differential levels of Mises stress and strain energy density, and the largest value of the Mises stress and strain energy density appears at the corners of the matrix near the interface. Creep dislocations penetrate preferentially into the most highly stressed matrix channels where theγphase rafting is also enlarged. The Mises stress and strain energy density of theγandγphases increase with the temperature increasing, thus the creep and rafting becomes easier at a higher temperature.