Isothermal oxidation behavior of a new Re-free nickel-based single-crystal superalloy at 950℃

The isothermal oxidation behavior of a new Refree nickel-based single-crystal superalloy in air at 950 ℃ for 200 h was studied by scanning electron microscopy（SEM）with energy-dispersive spectroscopy（EDS）and X-ray diffraction（XRD）.The results indicate that oxidation kinetics obeys parabolic law approximately,and the mass gain increases rapidly during initial oxidation stage and then gradually slows down.The oxidation scales are composed of three layers：the outer layer mainly consists of NiO with a small amount of CoO;the intermediate layer is mainly composed of Cr_2O_3 with a small amount of spinel compounds such as CrTaO_4,NiCr_2O_4,CoCrAl_2O_4,CoAl_2O_4,and NiAl_2O_4;and the inner layer is composed of Al_2O_3.Inner Al_2O_3 layer suppresses the diffusion of elements between oxygen and alloy elements,slows down the alloy oxidation speed,and also suppresses the growth of the oxide scale and reduces the oxidation rate,which is agreeable with the oxidation kinetics.

Creep prediction model for nickel-based single-crystal superalloys considering precipitation of TCP phase

The microstructure of nickel-based single-crystal(SC) superalloys has a pivotal influence on their creep properties. The addition of the Re element not only enhances the long-term creep properties of nickel-based SC superalloys, but also results in the formation of a topologically close-packed(TCP) phase which is a harmful and brittle hard phase. Here, high-temperature creep interruption tests of a nickel-based SC superalloy that contains4.8 wt% Re were performed under various temperatures and stress conditions, and the evolution of microstructure during creep was observed by scanning electron microscopy(SEM). The volume fraction of the TCP phase was also extracted to explore the mechanism that controls the impacts of the TCP phase on the creep properties.According to the microstructure evolution mechanism, the influence of the TCP phase was attributed to the initial damage and critical shear stress of the material. A creep performance prediction model for nickel-based SC superalloys considering the precipitation of the TCP phase that is based on the crystal plasticity theory and a modified creep damage model was established. The simulation curves fit well with the experimental results and the errors between prediction creep life with test results are within 5%.

Microstructural Instability of an Experimental Nickel-Based Single-Crystal Superalloy

Microstructural instability with the precipitation of topologically close-packed(TCP)phases of an experimental nickel-based single-crystal superalloy has been investigated.A significant amount of σ phases are distinguished in the interdendritic region of the as-cast samples after thermal exposure at 900℃ for 1000 h.Theσphases are preferentially precipitated at the periphery of coarse γ/γ′eutectic,and their morphological evolution from needles to granules is observed.Microstructural analysis suggests that the local segregation of Cr and Ti at the periphery of coarse γ/γ′eutectic accounts for the formation ofσphases in the as-cast samples.After heat treatment with low solution temperature and short holding time,the dendritic segregation of alloying elements(i.e.,W,Re,Ti and Ta)and the volume fraction of γ′phase in the interdendritic region are similar to that of the as-cast samples.However,no TCP phases are present in the interdendritic region of the heat-treated samples after thermal exposure,which is primarily ascribed to the elimination of local segregation of Cr and Ti near the coarse γ/γ′eutectic.Moreover,small quantities ofμphases are precipitated in the secondary dendrite arm near the interdendritic region after thermal exposure,due to the increased volume fraction ofγ′phase and the concomitant enrichment of W and Re in theγmatrix.

An overview of rhenium effect in single-crystal superalloys

Nickel-based single-crystal superalloys are the key materials for the manufacturing and development of advanced aeroengines. Rhenium is a crucial alloying element in the advanced nickel-based single-crystal superalloys for its special strengthening effects. The addition of Re could effectively enhance the creep properties of the single-crystal superalloys; thus, the content of Re is considered as one of the characteristics in different-generation single-crystal superalloys. Owing to the fundamental importance of rhenium to nickel-based single-crystal superalloys, much progress has been made on understanding of the effect of rhenium in the single-crystal superalloys. While the effect of Re doping on the nickelbased superalloys is well documented, the origins of the socalled rhenium effect are still under debate. In this paper,the effect of Re doping on the single-crystal superalloys and progress in understanding the rhenium effect are reviewed. The characteristics of the d-states occupancy in the electronic structure of Re make it the slowest diffusion elements in the single-crystal superalloys, which is undoubtedly responsible for the rhenium effect, while the postulates of Re cluster and the enrichment of Re at the c/c0 interface are still under debate, and the synergistic action of Re with other alloying elements should be further studied.Additionally, the interaction of Re with interfacial dislocations seems to be a promising explanation for the rhenium effect. Finally, the addition of Ru could help suppress topologically close-packed（TCP） phase formation and strengthen the Re doping single-crystal superalloys.Understanding the mechanism of rhenium effect will be beneficial for the effective utilization of Re and the design of low-cost single-crystal superalloys.

A united tension/compression asymmetry micro-mechanical model for nickel-base single-crystal alloys

In recent years, the micro-deformation mechanisms of the tension/compression behavior for nickel-base single-crystal superalloys have been studied extensively and general agreements have been derived. Based on these researches, a new model called united tension/compression asymmetry micro-mechanical model (UTCAM) has been proposed, which can effectively estimate the initial yield strength of nickel-base single-crystal (SC) superalloys under different loading directions. Considering the combined effects of octahedral slip system and cubic slip system, slip control factor is introduced in the UTCAM to determine the type of the open slip system of nickel-base single-crystal superalloys during deformation, thus making this model cover a rather wide range of application. Furthermore, the UTCAM is applied to hot tension and compression tests of three typical nickel-base SC superalloys (PWA1480-593 ℃, RENE N4-760 ℃ and DD407-760 ℃). The predicted initial yield strengths of the nickel-base SC superalloys are in good agreement with the experimental results, and the UTCAM proves to be effective.

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.

High-Temperature Structural Stabilities of Ni-Based SingleCrystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like（Ni^-）, γ＇-forming Cr-like（Cr^-γ＇） and γ-forming Cr-like（Cr^-γ＇）, the optimal formula for single-crystal superalloys is established [Al–Ni^-12]（AlCr^-γ0：5Cr^-γ1：5）. The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al-Ni2-xCox]（Al1Ti0.25Ta0.25Cr1W0.25Mo0.25）, with x = 1.5, 1.75, 2 and 2.5（the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively）. The900 ℃ long-term aging follows the Lifshitz–Slyozov–Wagner theory（LSW theory）, and the Co content does not have noticeable influence on the coarsening rate of c0. The microstructure and creep behavior of the four（001） single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co（9.43 Co） shows the longest lifetime of about 350 h at 1050 ℃/120 MPa, and all the samples show N-type rafting after creep tests.