Research On Microstructure Evolution Of A Second Generation Nickel Based Single Crystal Superalloy

Nickel based single crystal superalloys have been widely preferred as turbine blades in aero-engine which service under remarkable stress at high temperature.Their superior mechanical properties at high temperature derive from the specific microstructure of this kind of alloys;large amount of γ’ phase dispersively precipitated in γ phase.To improve stability of superalloys in service,ten or more alloying elements are added.The effect of alloying elements on phase structure and defects directly determines the high temperature performance of superalloys.Development of nickel based single crystal superalloys requires understanding of the strengthening effect of alloying elements,studies on microstructure which exhibit optimal performance,determinations of optimum melting,machining and heat treatment processes.In this work,high-temperature oxidation,aging,migration of phase boundary,interaction between critical alloying elements and defects,and the variation of superalloys’ properties with microstructural evolution were studied.The main work and achievements are listed as follows:（1）Oxidation process of nickel based single crystal superalloy under low pressure at high temperature were in-situ studied.Oxidation process and phase transformation from γ’ to γphase induced by oxidation under 5*10-2 Pa at 850℃ were directly observed on a heating holder in an environmental transmission electron microscopy（TEM）and the details were studied by high resolution TEM,selected area electron diffraction（SAED）,energy dispersive X-ray spectroscopy（EDS）,and electron energy loss spectroscopy（EELS）.Oxidation product of γ ’phase is composed of γ-Al₂O₃ crystallites about 1～3 nm in diameter with a strong crystallographic texture（（111）γ-Al₂O₃//（001）γ’,[1（?）0]γ-Al₂O₃//[1（?）0]γ’ or[1（?）0]γ-Al₂O₃//[1（?）0]γ’）,while oxidation product of γ phase is mainly composed of NiO nanoparticles.It was demonstrated at nanoscale that the destruction of ordered structure of γ’ phase and the generation of γ-Al₂O₃ crystallites was simultaneously happened which indicated that oxidation of Al atoms in γ’ phase led to γ’-γ phase transformation.In addition,EELS results revealed that oxygen diffused preferentially through the γ/γ’ phase interface in superalloys.（2）Crystalline structural evolution of nickel based single crystal superalloy during aging was carried out by in-situ X ray diffracmeter.During heating,as temperature increase,the lattice of γ and γ’ phase increase,absolute value of lattice misfit decrease.While during aging,redistribution of alloying elements would also result in the slight expansion of the two phases’lattice with misfit almostly unchanged.（3）A new method for preparing TEM specimen of bulk materials on heating chips was successfully developed.Appropriate area of TEM sample was transferred on chips in FIB to ensure larger thin area,less contamination of Ga ions,and stability of the specimen.This method can significantly shorten the time for sample preparation,especially those experiments requiring specific observation area,and suitable for other conductive bulk materials.（4）In situ study on the interaction between γ/γ’ phase boundary and interfacial dislocations was carried out.Migration of phase boundary resulted from temperature variation was directly observed and analyzed on a heating holder in a spherical aberration corrected TEM.The results indicated that migration of phase boundary can change the location and shape of the interfacial dislocations which always lied on the interface without escape.The angular interfacial grooves resulted by single dislocations indicated that the hinderance of dislocations to interface is restricted in a limited area.Therefore,combined with the EDS analysis of the dislocation core,we proposed a new formation mechanism of interfacial groove;the Re segregation at interfacial dislocation core significantly impede diffusion of y’-forming elements Ni and Al,retarding growth of γ’ near the dislocation,thus an interfacial groove formed.（5）Distribution of critical element rhenium（Re）along the interfacial dislocation network in nickel based single crystal superalloy DD5 was unveiled.By using high resolution scanning transmission electron microsopy（STEM）and Super-X,quantitative compositional analysis of dislocation cores was achieved and Re was found to be segregated at the cores.We found that heat treatment can control the Re concentration of interfacial dislocation core:a)the slower the cooling rate,the higher the Re concentration;b)the longer the aging time,the more even the Re content at the dislocation cores,while the average Re concentration at the cores would not be influenced;c)the aging time required for a specific Re concentration at the dislocation cores can be significantly shortened by firstly slow cooling then rapidly reheating.（6）The effect of Re segregation at interfacial dislocation core on nickel based single crystal superalloy have been investigated.Dislocation motions in samples with different Re concentration at interfacial dislocation core were revealed by using in-situ straining holder in TEM.Re segregation at dislocation core can remarkably strengthen the y/y’phase boundary.The blocking effect of interface with higher Re content was much higher than those with less Re content;dislocations can straightly shear the γ/γ’ phase boundary with less Re,while in strong contrast,dislocations’ motion were blocked by the interface with higher Re and limited in the y phase.Combination of molecular dynamics and Monte Carlo method indicated that Re segregation has limited effect on the dislocation structure,but can significantly decrease its energy（dislocation energy drops 152.7 eV per nm on average after adding 30wt.%Re）and stabilize the interfacial dislocation network,and complex the elastic stress field near dislocations impacting their motion.Our work on oxidation,aging,migration of phase boundary,and interaction between phase boundary and interfacial dislocations may pave the way for understanding of high-temperature microstructural evolution and the strengthening mechanism of nickel based single crystal superalloys,and may contributed to optimize the microstructure,heat treatment,composition and the performance of superalloys.