Numerical Simulation Of The Directional Solidification Deformation And Experimental Study Of Recrystallization In Nickel-based Single Crystal Superalloys

Due to the need for higher inlet gas temperatures to achieve higher performance and efficiency,the external and internal geometries of single crystal blades are becoming increasingly complex,resulting in extremely complex stresses and strains in the blades during casting.The plastic deformation generated during the casting process of the blade will induce recrystallization(RX)during the subsequent heat treatment.The occurrence of recrystallization introduces high-angle grain boundaries in the single crystal,which significantly degrades the mechanical properties and requires the avoidance of such defects.It is still lack of systematic studies on the effect of different temperature deformation mechanisms on RX.Further,more detailed experimental work is needed to verify the accuracy of the deformation simulation of castings during directional solidification.Finally,there is no systematic study of factors influencing the deformation of single crystal castings during directional solidification.This thesis focuses on understanding the effects of different temperature deformations on RX formed after heat treatment,developing an accurate finite element model for predicting deformations during directional solidification,clarifying the factors influencing deformations in single crystal castings to achieve the goal of accurately predicting the location of recrystallization in single crystal castings,and providing scientific recommendations for preventing recrystallization formation in single crystal castings.The relationship between deformation and recrystallization in isothermal compression and directional solidification has been investigated in first-generation(SX)superalloys DD413.The deformation features at different temperatures were investigated using optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).Comparison of deformation and RX was conducted to determine the factors affecting the deformation of single crystal castings during directional solidification.The main contents of this study are as follows:(1)Pre-deformation(about 3.5%)was introduced to as cast SX specimens using isothermal compression at different temperatures.An investigation of the deformation mechanism on recrystallization was conducted using microstructure characterization at different scales.A unified criterion for recrystallization induced by deformation ranging from room temperature to solidus was proposed,which can also be used to evaluate the recrystallization tendency of alloys with different compositions.Based on the recrystallization features of the(110)planes of compressed samples with different orientations at multiple temperatures,the influence of the deformation mechanism on the RX grain boundary direction was obtained.(2)Inverse solution method was used to obtain a simulated temperature field that matched the measured temperature.The exact elastic constants of DD413 at different temperatures were obtained using the regression method.Based on Hill’s yield criterion,the yield anisotropy coefficients were calculated using the yield strengths of the four orientations.A small deviation exists between predicted and measured yield strength.Using the above model parameters and material parameters,a thermo-elastic-plastic model considering the anisotropic mechanical properties of nickel-based SX superalloys was developed to accurately predict the RX location in solid and cored castings.The accuracy of the finite element model was verified using TEM and EBSD.RX is mainly induced by high temperature plastic deformation during directional solidification based on the results of stress,strain,and RX in solid and cored castings.(3)Three castings of the same structure with different orientations were simulated for deformation,using the established finite element model.A comparison of the RX and deformation features of the three castings revealed the reasons for the increased deformation of fillet radii with primary orientation.Through variation of the RX height at the fillet radii with the secondary direction,it was determined that the strain magnitude depends on the strength of the material in spatial orientation at the fillet radii.The higher temperature gradient of the inner fillet radii leads to the accumulation of greater plastic strain at this location.Both EBSD and TEM results show that deformation below 600°C can exist in the casting.The distribution of dislocation density at different locations leads to difference of RX induced by deformation in different temperature range.(4)Aiming at the structural characteristics of actual SX blades,simulations and experimental studies were conducted for solid and cored castings with variations in fillet radii and gauge dimension,respectively.It is found that the plastic deformation of the fillet radii increases with the increase of the stress concentration factor of R.A larger gauge diameter on the upper side of the platform can effectively reduce plastic deformation at the fillet radii.The change in the spatial location of the casting affects the temperature gradient at the fillet radii.The region of high temperature gradient has a greater strain rate,which leads to the accumulation of greater plastic deformation at the fillet radii on the side of center rod.Simulation and experimental studies on a certain SX cored turbine blade show that the deformation of the blade in directional solidification is mainly concentrated at the fillet radii,gibbosity and hole.The deformation in different temperature ranges around the hole leads to different RX phenomena.The deformation at the fillet radii and gibbosity is not sufficient to induce RX.Finally,the effects of different process conditions on deformation were analyzed using finite element modeling,and suggestions were made to prevent RX in SX blades.