Stress dependence of the creep behaviors and mechanisms of a third-generation Ni-based single crystal superalloy

Elevated temperature creep behaviors at 1100℃ over a wide stress regime of 120-174 MPa of a thirdgeneration Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close-(about 100 nm) and sparse-(above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, ao<010> superdislocations with low mobil让ies were frequently observed under a lower stress to pass through γ precipitates by a combined slip and climb of two ao<110> superpartials or pure climb. However, ao<110> superdislocations with higher mobility were widely found under a higher stress, which directly sheared into y precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137-174MPa, while the dislocation dim bing mechanism was operative throughout the whole creep stage un der a stress of 120 MPa, resulting a superior creep performanee.

Preparation of Inoculants Used in Superalloy and Analysis of the Atomic Matching Models

The mechanisms of grain refinement were investigated on two kinds of grain refiners used in Ni-Fe based superalloys and complete atomic matching modes were constructed in this study. It is found that there are at least three matching crystal planes having the small lattice disregistry between the refiner and the nucleated phase, which can lead to grain refinement of γ matrix. The results indicate that the （0001）, （01-10） planes of CrFeNb have a fine crystallographic matching relationship with the （111）, （110） planes of γ matrix. The disregistry of （0001）CrFeNb//（111 ）γ, （011-0）CrFeNb//（111）γ and （0110）CrFeNb//（110）γ is 3.34%, 6.60% and 5.90%, respectively. The （0001）, （0110） planes of Co3FeNb2 and （111）, （110） planes of γ matrix also have this relationship. The disregistry of （0001）Co3FeNb2//（111）γ, （0001）Co3FeNb2//（110）γ, （0110）Co3FeNb2//（111）γ and （0110）Co3FeNb2//（110）γ, is 4.45%, 9.35%, 8.38% and 6.12%, respectively.

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r^(3) vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r^(2) vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.

A correlative multidimensional study ofγ’precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum(Ta)content(2 Ta,5 Ta and 8 Ta in wt%)was investigated.Ta addition significantly affected theγ’precipitate morphology,γ/γ’lattice misfit and microstructural stability during long-term aging.Results showed that the partitioning behaviors of solutes were enhanced by Ta addition,meanwhile,the reversal partitioning behavior of W was triggered which partitioned fromγ’precipitate to matrix.The elemental concentration redistribution caused variations in lattice misfit from positive to negative,the values of lattice misfit were measured to be 0.16%for 2 Ta alloy,then decreased to-0.07%for 5 Ta alloy and negatively increased to-0.23%for 8 Ta alloy.These variations in the lattice misfit were reflected on the transition ofγ’morphology from round-cornered cuboidal shape to cuboidal with sharp corners,accomplished with increasing shape parameter ratioη.Consequently,the optimalγ’shape could be obtained at lattice misfit of approximately 0.3%.Theγ’coarsening investigation at 900℃(up to 2000 h)indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy,accompanied by the enhanced capability of resistingγ’coalescence.By incorporating the calculated interfacial energy,computational modeling,Thermo-Calc and PrecipiCalc,were employed to elucidate theγ’kinetic pathways,the simulation results agreed with experiments,indicating that the model and parameters were reasonable.Additionally,it was found that there was no overlap betweenγ’nucleation and coarsening when theγ/γ’interfacial energy increased to a critical value.

Tailoring Microstructure and Microsegregation in a Directionally Solidified Ni-Based SX Superalloy by a Weak Transverse Static Magnetic Field

A weak transverse static magnetic field(WTSMF,0–0.5 T)is applied to the directional solidification process of a DD3 Ni-based SX superalloy,aiming to tailor the microstructure and microsegregation of alloys.The mechanisms of microstructural refinement and microsegregation distribution caused by a WTSMF during directional solidification are discussed.It is shown that the primary dendrite arm spacing is rapidly reduced from 181 to 143μm,and the average size ofγ′phase is significantly refined from 0.85 to 0.25μm as the magnetic field increases from 0 to 0.5 T.At the same time,the volumefractions ofγ/γ′eutectic and the segregation coefficient are also gradually decreased.The 3D numerical simulations of the multiscale convection in liquid phase show that the modifications of the microstructure and microsegregation in DD3 are mainly attributed to the enhanced liquid flow caused by thermoelectric magnetic convection(TEMC)at dendrite/sample scale under the WTSMF.The maximum of the TEMC increases with increasing the magnetic field intensity.This work paves a simple way to optimize the microstructure and microsegregation in directionally solidified Ni-based SX superalloys without changing the processing parameters and composition.

Enhanced comprehensive properties of stereolithography 3D printed alumina ceramic cores with high porosities by a powder gradation design

Ceramic cores with complex structures and optimized properties are critical for hollow turbine blades applied in aeroengines.Compared to traditional methods,additive manufacturing(AM)presents great advantages in forming complex ceramic cores,but how to balance the porosity and strength is an enormous challenge.In this work,alumina ceramic cores with high porosity,moderate strength,and low high-temperature deflection were prepared using stereolithography(SLA)3D printing by a novel powder gradation design strategy.The contradiction between porosity and flexural strength is well adjusted when the mass ratio of the coarse,medium,and fine particles is 2:1:1 and the sintering temperature is 1600℃.The fracture mode of coarse particles in sintered SLA 3D printing ceramic transforms from intergranular fracture to transgranular fracture with the increase of sintering temperature and the proportion of fine powders in powder system.The sintered porosity has a greater influence on the high-temperature deflection of SLA 3D printed ceramic cores than grain size.On this basis,a"non-skeleton"microstructure model of SLA 3D printed alumina ceramic cores is created to explain the relationship between the sintering process and properties.As a result,high porosity(36.4%),appropriate strength(50.1 MPa),and low high-temperature deflection(2.27 mm)were achieved by optimizing particle size gradation and sintering process,which provides an insight into the important enhancement of the comprehensive properties of SLA 3D printed ceramic cores.