Measurements of γ/γ' Lattice Misfit and γ' Volume Fraction for a Ru-containing Nickel-based Single Crystal Superalloy

A conventional X-ray difFractometer has been used to determine the -y/y＇ lattice misfit and γ＇ volume fraction for a Ru-containing nickel-based single crystal superalloy at room temperature. The rocking curve was used to characterize the distribution of subgrains. The diffraction peaks obtained by w-20 scan were used to determine the γ/γ＇ lattice misfit and γ＇ volume fraction. A three peaks fitting model was proposed. The peak fitting results are in good agreement with the model. The X-ray diffraction results indicate that the nickel-based single crystal superalloy was not a perfect monocrystalline material, which is comprised of many subgrains; and each subgrain also consists of large numbers of mosaic structures. In addition, two anomalous reflection phenomena were found during the experiment and discussed with respect to their occurrence and impact on the measurement. The experimental results show that the γ/γ＇ lattice misfit and ~/r volume fraction will be various at the different regions of its dendritic microstructure. The average γ/γ＇ lattice misfit and γ＇ volume fraction of the experimental alloy are approximately-0.2% and 70%, respectively. Furthermore, the γ＇ volume fraction calculated by atom microprobe （AP） data is also basically consistent with the experimental results.

First-Principles Study of the Influence of Lattice Misfit on the Behavior and the Ductility Effect of Hafnium in Ni-Ni3AI System

Two Ni/Ni3Al-interface-contained cluster models with/without lattice misfit are studied by first-principles method to clarify the debates about the segregation behaviors of Hafnium （Hf） and explore the influence of lattice misfit on the ductility effect of Hr. It is found that though Hf prefers to substitute A1 rather than Ni in Ni3A1 phase within most of the investigated misfit range, its stronger preferring to Ni phase than NiaA1 phase makes it impossible to go into Ni3A1 phase to occupy A1 site in Ni-Ni3A1 alloys. Bond order analysis in Hf-free case shows that lattice misfit has different effects on the Griffith work of interfacial cleavage 27int/E and the maximum theoretical shear stress Zmax of Ni and Ni3A1, contributing to the existence of anomalous strength-temperature phenomena in NiaA1 alloys. However, the addition of Hf will make the 27int/E （or Zmax） of both Ni3A1 and Ni decrease （or increase） with lattice misfit, indicating that the addition of Hf may make the anomalous strength-temperature relationship in Ni3A1 region disappear locally.

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

Subject Code:E01 With the support from the National Natural Science Foundation of China,significant progress has been made in developing maraging steels with high performances by the research group led by Prof.LV Zhaoping(吕昭平)from the State Key Laboratory for Advanced Metals and Materials,University

The First-principles Study on the Occupation Behavior and the Ductility Mechanism of Zr in Ni-Ni_3Al System with Lattice Misfit

The influence of lattice misfit on the occupation behavior and the ductility effect of Zr in Ni-Ni3Al alloys were explored. It is found in energy analysis that the preferable site of Zr between Ni sublattice and Al sublattice will change under different lattice misfit, however, the Zr prefers to segregate Ni phase rather than Ni3Al phase in all lattice misfit range, which makes it impossible for Zr to go into Ni3Al phase to occupy Al sublattice in Ni-Ni3Al system. Bond order （BO） analysis shows that the localized ductility effect of Zr differs in different region, and the comparison between Zr-free and Zr-doped BO analysis successfully explain the mechanism of the embrittlement of Ni-Ni3Al alloys and the ductility effect of Zr.

Partitioning behavior and lattice misfit of γ/γ’phases in Ni-based superalloys with different Mo additions

A low-density single crystal(LDS) alloy with the composition of high Mo content was designed.The extra 1.5 wt% Mo was added in the Alloy A with the composition of Ni-6.5 Al-8.0 Mo-2.4 Cr-6.2 Ta-4.9 Co-1.5 Re-(0.01-0.05) Y(wt%) to study the influence of Mo on the lattice parameter and partitioning behavior.Scanning electron microscope(SEM) with energy-dispersive spectrometer(EDS),transmission electron microscopy(TEM)and high-temperature X-ray diffraction(HT-XRD) were used to observe the microstructure,analyze the elemental content and measure the lattice parameter of the alloys.The natural lattice misfit was calculated by lattice constants which were measured by HT-XRD at the temperature from 25 to 1150℃,and the results showed that the lattice misfit would be more and more negative with temperature increasing.It was found that 1.5 wt% addition of Mo will increase the absolute value of the lattice misfit of γ/γ’phases and the volume fraction of γ’,and at the same time,influence the elemental distribution in γ and γ’ phases,especially Re and Cr.Re has a higher partitioning ratio(k) after the addition of Mo.

The effect of rare earths on the solidification structure of Fe-36Ni invar alloy

The effect of Ce, La and mischmetal on the solidification structure of Fe-36Ni invar alloy was investigated. The results show that great amounts of high-melting point compounds （ Ce2O3, La202S and （ Ce, La）2O2S ） respectively formed in the alloy with the addition of Ce, La or mischmetal. Based on the theory of lattice misfit, the lattice misfit between the （0001） surfaces of Ce2O3,Ce2O2S and La2O2S and （100） surface of Fe-36Ni invar alloy were 6.21%, 5.77 % and 5.42 %, respectively, which are relatively low. Therefore, Ce2 O3, La2 O2 S and （ Ce, La） 2 O2 S could serve as the core of heterogeneous nucleation, improve the equiaxed grain ratio, reduce the equiaxed grain size and refine the solidification structure of alloy.

Effect of Mo Addition on Microstructural Characteristics in a Re-containing Single Crystal Superalloy

The effect of Mo addition on microstructural characteristics of a nickel-base single crystal superalloy containing 4 wt% Re was investigated. The γ/γ’ partitioning ratios determined by energy dispersive spectrometer attached to a transmission electron microscope showed that the addition of Mo enhanced the partitioning of Re,W and Cr in the g matrix while decreased the concentration of Ta in the matrix. Synchrotron radiation diffraction was adopted to measure the γ/γ’ lattice misfit at room temperature. The results indicated that Mo addition changed the γ/γ’ lattice misfit towards larger negative as well as increased the tetragonal distortion of the γ lattice. Additionally,Mo addition led to microstructural instability and altered the precipitation behavior of topologically close-packed phases during 1100 C exposure. Instead of precipitating directly from the matrix,the μ phase was observed to be converted from the α phase which precipitated preferentially as a metastable intermediate in the alloy with high Mo content.

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.

Phase Evolution and Thermal Expansion Behavior of aγ′Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

The phase evolution and thermal expansion behavior in superalloy during heating play an essential role in controlling the size and distribution of precipitates,as well as optimizing thermomechanical properties.Synchrotron X-ray diffraction is able to go through the interior of sample and can be carried out with in situ environment,and thus,it can obtain more statistics information in real time comparing with traditional methods,such as electron and optical microscopies.In this study,in situ heating synchrotron X-ray diffraction was carried out to study the phase evolution in a typicalγ′phase precipitation strengthened Ni-based superalloy,Waspaloy,from 29 to 1050°C.Theγ′,γ,M_(23)C_(6)and M C phases,including their lattice parameters,misfits,dissolution behavior and thermal expansion coefficients,were mainly investigated.Theγ′phase and M_(23)C_(6)carbides appeared obvious dissolution during heating and re-precipitated when the temperature dropped to room temperature.Combining with the microscopy results,we can indicate that the dissolution of M_(23)C_(6)leads to the growth of grain andγ′phase cannot be completely dissolved for the short holding time above the solution temperature.Besides,the coefficients of thermal expansions of all the phases are calculated and fitted as polynomials.

Microstructure and stress-rupture property of an experimental single crystal Ni-base superalloy with different heat treatments

The influence of heat treatment on the microstructure and stress-rupture property at 1,100 °C/140 MPa was investigated in a 5.0 wt% Re containing experimental single crystal Ni-base superalloy. The results indicate that the γmorphology is nearly cuboidal in the dendrite core after conventional heat treatment. The lattice misfit of alloy becomes more negative after modified heat treatment and results in more cuboidal γ precipitates than that after conventional heat treatment. The increased stress-rupture life after modified heat treatment is attributed to higher γ volume fraction, more negative lattice misfit, well-rafted structure, and narrower c channel width.

Formation of coherent BCC/B2 microstructure and mechanical properties of Al-Ti-Zr-Nb-Ta-Cr/Mo light-weight refractory high-entropy alloys

The present work investigated the formation of coherent body-centered-cubic(BCC)/B2 microstructure in Al-Ti-Zr-Nb-Ta-Cr/Mo refractory high-entropy alloy system,in which three alloys with the composition of Al_(2)Ti_(6) Zr_(2)Nb_(3)Ta_(2)(Cr/Mo)_(1)were designed.These as-cast alloy ingots were homogenized at 1573 K for 2 h and then aged at different temperatures for 24 h.It is found that the BCC/B2 coherent microstructure with cuboidal BCC nanoprecipitates into the B2 matrix would be formed in873 K-aged Al_(2)Ti_(6) Zr_(2)Nb_(3)Ta_(2)Cr_(0.5)Mo_(0.5)alloy with half Cr and half Mo,which is ascribed to a moderate lattice misfit(ε=0.94%)between BCC and B2 phases.It is due to the special coherent microstructure that results in a prominent mechanical property with the highest compressive yield strength(σ_(YS)=1314 MPa)and good plasticity among these alloys,while coarse Cr2 Ti and Zr_(5)Al_(3)particles are existed in 873 K-aged alloy with Cr1.After 1073 K aging,all these alloys with Cr_(1),Cr_(0.5)Mo_(0.5)and Mo_(1)consist of BCC,B2 and Zr_(5)Al_(3),in which BCC/B2 coherent microstructure is destabilized.The strengthening mechanism of BCC/B2 coherent microstructure in 873 K-aged alloy with Cr_(0.5)Mo_(0.5)was also discussed.

Microstructure Evolution of Primary γ′ Phase in Ni3Al-Based Superalloy

In this work,water cooling,air cooling(AC)and furnace cooling(FC)were applied to investigate the effect of cooling rate on microstructure evolution of primaryγ′in a newly designed Ni3Al-based alloy.The results showed that nucleation rate of primaryγ′increased with increasing cooling rate.In addition,higher cooling rate shortened growth period of primaryγ′,which made its morphology close to the initial precipitatedγ′.For AC and FC specimens,due to the lower cooling rate,primaryγ′possessed longer growth period and its morphology was mainly due to the evolution of lattice misfit betweenγand primaryγ′.Meanwhile,growth of primaryγ′depended on lattice misfit distribution between its corner and edge area.Moreover,primaryγ′morphologies of sphere,cube and concave cube with tip corners were illustrated by considering interaction between elemental diffusion and elastic strain energy.