Effect of Ruthenium on Precipitation Behavior of the Topologically Close-Packed Phase in a Single-Crystal Ni-Based Superalloy During High-Temperature Exposure

The precipitation behaviors of the topologically close-packed (TCP) phase in the Ru-free and Ru-containing single-crystal Ni-based superalloys have been investigated. All the TCP phases have been identified as the ?? phase with large numbers of planar defects by using a scanning transmission electron microscope (STEM). The addition of 3?wt pct Ru effectively suppresses the nucleation of the ?? phase because the addition of 3?wt pct Ru not only decreases the supersaturation of ?? phase but also increases the elastic strain energy for the nucleation of the ?? phase. Additionally, the addition of 3?wt pct Ru makes more Re enter the ?? phase during its growth and the critical nucleus radius of the ?? phase becomes larger. Therefore, it is found that the addition of 3?wt pct Ru decreases the growth rate of the ?? phase.     

The Effect of Long-Term Thermal Exposure on the Microstructure and Stress Rupture Property of a Directionally Solidified Ni-Based Superalloy

Microstructural degradation and microstructure-property relationship during long-term thermal exposure in a directionally solidified Ni-based superalloy are systematically studied. The coarsening kinetics of γ′ precipitation conforms well to the LSW model during the long-term thermal exposure. The detailed time dependence of MC decomposition during the long-term thermal exposure is revealed. Grain boundary coarsening was mainly facilitated by γ′ and M₂₃C₆ precipitates coarsening in GBs region, and the GB coarsening kinetics conforms well to the JMAK theory. During different stages of the thermal exposure, dominant factors for the decrease of stress-rupture lifetime vary due to the evolution of multiple microstructures (γ′ coarsening, MC decomposition, and grain boundary coarsening).     

Microstructural Parameters Controlling High-Temperature Creep Life of the Nickel-Base Single-Crystal Superalloy MC2

The influence of both topologically close-packed (TCP) phase precipitation and pores on the creep life of a single-crystal superalloy has been studied at 1323?K (1050?°C)/160?MPa. Despite very reproducible primary and secondary creep stages, the creep life is scattered for this specific condition where a very steep tertiary creep stage is observed, corresponding to a highly localized failure process. Image processing was performed after failure to determine the stereological parameters characterizing pores and TCP-phase particles. It was determined that pores are major determinants of creep life under these temperature and stress conditions. It was also observed that the average surface area or the density of pores is not sufficient to explain creep life variability. A homogenization method including modified ??/???? microstructure area surrounding pores and TCP-phase particles was developed and correlated to creep life. It is shown that the greater the extent of the modified microstructure, the lower the creep life. Moreover, a better understanding of the TCP-phase role in controlling the creep life was obtained: TCP-phase particles modified the local stress field and disturbed the local ??/???? microstructure. They enhance the generation of vacancies and subsequent nucleation and growth of pores.     

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.     

Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.     

Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant (p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.     

Microstructural Characterization of Spray Formed FGH4095 Superalloy During Hot Deformation

 In order to study the hot workability and to optimize the processing parameters for spray formed FGH4095 superalloy, thermal compression tests for spray formed FGH4095 superalloy have been finished by using a Gleeble 1500 thermal simulated test machine at the strain rates of 0. 01-10. 0 s-1 and temperatures of 1050-1140 ℃. The effects of strain rate and deformation temperature on the true stress-true strain curves and microstructure evolution were investigated. The results show that the generation of dynamic recrystallization (DRX) depends sensitively on deformation temperature. When the temperature was lower than 1080 ℃, long and narrow necklace grains were shown in the microstructure. When the temperature increased to 1140 ℃, new recrystallization grains were generated. The size and shape of γ′ precipitates in the grains have a very important effect as factors of hindering sufficient migration of dislocations on plastic deformation. The result of thermal processing map is in accord with the microstructure observation, and the best material thermal processing temperature is above 1128 ℃.     

An In Situ X-ray Tomography Study on the Stress Corrosion Behavior of a Ni-Based Single-Crystal Superalloy

Metallurgical and Materials Transactions A - The present study was devoted to investigating the stress corrosion behavior of Ni-based single-crystal superalloys using in situ three-dimensional...     

Influence of Melt Superheating Treatment on Solidification Characteristics and Rupture Life of a Third-Generation Ni-Based Single-Crystal Superalloy

The influence of melt superheating treatment on the melt properties, solidification characteristics, and rupture life of a third-generation Ni-based single-crystal superalloy was investigated to reveal the critical temperature range of melt structure evolution and its effect on rupture life. The results showed that the viscosity of superalloy decreased but the surface tension increased with increasing superheating temperature. Two characteristic temperature points where the melt viscosity and undercooling degree suddenly change were determined to be 1600 °C and 1700 °C, respectively. Similarly, the stability of the solidification interface firstly improved and then weakened with increasing superheating temperature. The dendrite arms were well refined and the segregation was reduced at 1700 °C. In addition, the rupture life obtained at 1100 °C and 137 MPa increased by approximately 30 pct, approaching the rupture life of the corresponding superalloy containing 2 pct Ru, with increasing superheating temperature from 1500 °C to 1700 °C. When the melt was further heated to 1800 °C, the rupture life decreased. The evolutions of solidification characteristics and rupture life with increasing melt superheating temperature were attributed to changes in the melt structure.     

Microstructural and Tribological Behavior of Pack-Borided Ni-Based Hastelloy C-276 Superalloy

Metallurgical and Materials Transactions A - Although the toughness and corrosion resistance of Ni-based superalloys are high due to their face-centered-cubic structure, their surface hardness and,...     

Effect of Processing Parameters and Shape of Blade on the Solidification of Single-Crystal CMSX-4 Ni-Based Superalloy

Metallurgical and Materials Transactions B - The effect of mold withdrawal velocity, heater and pouring temperatures and the thickness of blade root on the solidification parameters and shape of...     

Microstructure Evolution and Analysis of A [011] Orientation, Single-Crystal, Nickel-Based Superalloy During Tensile Creep

By means of the elastic?Cplastic finite-element method (FEM) for calculating the distribution features of the von Mises stress and strain energy density, the influences of the applied stress on the von Mises stress of the ????/?? phases and the rafting of the ???? phase for the [011] orientation, single-crystal, nickel-based superalloy are investigated. The results show that, after being fully heat treated, the microstructure of the [011] orientation, single-crystal, nickel-based superalloy consists of the cuboidal ???? phase embedded coherently in the ?? matrix, and the cuboidal ???? phase on (100) plane is regularly arranged along a 45?deg angle relative to the [011] orientation. Compared with the matrix channel of [010] orientation, the bigger von Mises stress is produced within the [001] matrix channel when the tensile stress is applied along the [011] orientation. Under the action of the larger principal stress component, the bigger expanding lattice strain occurs on the (001) plane of the cuboidal ???? phase along the [010] direction, which may trap the Al, Ti atoms with a bigger atomic radius for promoting the directional growth of the ???? phase into the stripe-like rafted structure along the [001] orientation. The changes of the interatomic potential energy, misfit stress, and interfacial energy during the tensile creep are thought to be the driving forces of promoting the elements?? diffusion and directional growth of the ???? phase.     

Evolution of Micro-Pores in a Single-Crystal Nickel-Based Superalloy During Solution Heat Treatment

Evolution of micro-pores in a third-generation single-crystal nickel-based superalloy during solution heat treatment at 1603 K (1330 °C) was investigated by X-ray computed tomography. 3D information including morphology, size, number, and volume fraction of micro-pores formed during solidification (S-pores) and solution (H-pores) was analyzed. The growth behaviors of both S-pores and H-pores can be related to the vacancy formation and diffusion during heat treatment.