Study Of Secondary γ’ Precipitats And Cabides Of A Second Generation Single Crystal Superalloy

Ni-based single crystal(SC)superalloys are widely used as turbine blades and vanes of advanced aero-engine due to their superior high temperature mechanical properties,resistance to corrosion and oxidation.Due to long-term exposure at high temperature,creep damage caused by centrifugal stress is one of the main failures of single crystal blades.In order to provide the experimental basis for alloy design and development,a[001]oriented nickel-base single crystal alloy was selected as the research object in this paper.The creep behavior of this alloy under the coupling effects of temperature and applied stress was investigated,which revealed the evolution of microstructure characteristics after different creep stages.The microstructural evolution and diffusion of alloy elements and the relationship between these and the degradation of creep properties were explained,which provided the crucial experimental evidence to understand the creep damage mechanism.In this paper,base on electron transmission microscope and scanning transmission microscope,combined with quantitative Scanning Transmission Electron Microscopy(QSETM)code simulation,the creep behavior of a 2nd generation Ni-based SC superalloy containg 3%wt Re were investigated after creep testing at 1100℃ and 137MPa.1.Base on spherical-aberration scanning transmission electron microscope(Titan G2 80-200 Chemi STEM,FEI),the evolution of secondary γ’ precipitates in the nickel-base single crystal alloy were studied after different creep time.The main results are summarized as follows:(1)After creep testing at 1100℃ and 137MPa,two types of the secondary γ’precipitates are formed:one precipitates and randomly distributes in the γmatrix(Type 1,2nd γ’γ),which has been observed by other research groups.The other precipitates and uniformly distributes at nodes of dislocation networks adjacent to γ/γ’ interfaces(Type Ⅱ,2nd γ’d),which has not been repoted in the literature.(2)The two types of the precipitates displayed different evolution of morphologies and zises with prolonged creeping time.① For the 2nd γ’γprecipitates,the morphology and size show a gradient distribution from the center of the y matrix to regions close to the γ/γ’ interface(cuboidal→spherical,large → small).The morphology evolves from being spherical →cuboidal → butterfly-like with prolonged testing time.② The 2nd γ’d precipitates have a relatively uniform morphology(cuboid)and size,which do not change with testing time.(3)The different morphologies of the two types of 2nd γ’ precipitates are due to the different atomic transport ways.The elemental diffusion of the 2nd γ’γ(Type Ⅰ)precipitate was an uphill process.The elemental diffusion of the 2ndγ’d(Type Ⅱ)precipitate was pipe diffusion along dislocation.(4)The different paths for diffusing atoms in the two types of 2nd γ’ precipitates lead to differences of elemental distribution.The segregations of γ-rich andγ’-rich elements are more distinctly separated in the 2nd γ’γ precipitates than the 2nd γ’d precipitates.The uphill diffusion of the secondary γ’ precipitates lead to convergent diffusion of Ni,Al and Ta towards the precipitates and divergent diffusion of Co,Cr,W,Re and Mo away from the precipitates.(5)The shape orientations of the two type 2nd γ’ precipitates with γ matrix are different due to the formation sites of 2nd γ’ precipitates.When they have similar size,both viewed from the[001]direction,The 2nd γ’γ precipitate has a shape orientation of orientation of[100]/[010]/[001].The 2nd γ’d precipitate particles are formed in the orientation of[110]/[110]/[001].The two types of 2nd γ’ precipitates retain complete coherency with the matrix.The 2nd γ’γprecipitate has a weak ordering,and the 2nd γ’d precipitate has higher level of ordering.(6)The experimental results have shown the formation of the 2nd γ’ precipitates decrease the creep lifetime of the superalloy.We subjected the alloys to a creep test under 137MPa tensile stress at 1100℃ for 20h,followed by furnace cooling(at～250C).In this condition the 2nd γ’ precipitates were formed.The samples were then subjected to a second creep test under 137MPa tensile stress at 1100℃ till rupture,followed by furnace cooling.The results show that its life dramtically decrease(～56h).2.Based on electron transmission microscope,the evolution of carbides in a Ni-based single crystal superalloy and elements distributions in M23C6 carbide have been investigated.Combined experimental HAADF-STEM images and QSTEM code simulation,the site occupation of metallic elements in M23C6 carbide was analysised.The main results are summarized as follows:(1)After interrupted creep testing at 1100℃ and 137MPa,MC carbide that is rich in Ta has been decomposed.P phase precipitates in the interface of MC carbide and it is rich in Cr,Re,Co and Mo,with a small amount of W.(2)After creep fracture testing,large populations of fine M23C6 carbides have been formed.Super X-EDS mapping technique was conducted.It can be seen that high concentrations of Cr,Re and a small amount of Mo is prone to segregate in the M23C6 carbide.The morphology of the M23C6 carbide evolves from irregular shape to regular shape with prolonged testing time.Some of M23C6 carbide with regular shape is mainly octahedron carbide particle with {111} crystallographic planes.(3)Based on experimental HAADF-STEM images and QSTEM code simulation of M23C6 carbide,the Cr,Re and Mo are found to prefer to substitute Cr at the 8c sites in the M23C6 structure,followed by the 48h,4a and 32f sites in order.Based on the lower bond order criterion,using first-principles calculations,we calculated the cohesive energies of Re,Mo and W at the 8c site.The cohesive energies are-8.7210 eV/atom for Re,-8.6690 eV/atom for Mo,and-8.7204 eV/atom for W.Therefore,the alloying elements have the preferential sequence of Re>W>Mo in substituting the 8c sites.