Re-containing Single Crystal Superalloy Research Articles

On dislocation networks and superdislocations in Re-containing nickel-based SX superalloy under different creep conditions

In this paper, the creep behaviors and deformation mechanisms of a 3 wt% Re-containing single crystal superalloy, are investigated under two tough conditions of 1120 °C/137 MPa and 850 °C/690 MPa. The detailed TEM observation on interfacial dislocation networks and superdislocations are analyzed to interpret the creep resistance. Results show that at 1120 °C/137 MPa, dislocation networks degrade into unstable structures and give rise to the universal shearing of superdislocations. At 850 °C/690 MPa, three mechanisms including superdislocation pairs, stacking faults, and bypassing loops jointly contribute to the plastic deformation. In addition, the comparison of superdislocations and creep performance of superalloys with different Re contents are concluded. In general, the deficiency of interfacial dislocation networks and γ′ phases in resisting dislocation climbing and shearing causes the poor creep performance, which could be improved by stabilizing interfacial dislocation networks with additional W contents. This paper guides the composition optimization of low-cost third-generation superalloys in the future.

Solidification and Evolution of β-NiAl Phase in a Re-Containing Single Crystal Superalloy

Solidification and Evolution of β-NiAl Phase in a Re-Containing Single Crystal Superalloy

Influence of Stress Level on the Stress-Rupture Property and Related Deformation Microstructure of One Ni-Base Re-Containing Single Crystal Superalloy in [001] and [111] Orientations at 1100 °C

Influence of Stress Level on the Stress-Rupture Property and Related Deformation Microstructure of One Ni-Base Re-Containing Single Crystal Superalloy in [001] and [111] Orientations at 1100 °C

Microstructural degradation after thermal exposure of a Re-containing single crystal superalloy

Microstructural degradation after thermal exposure of a 5% Re -containing single crystal (SX) superalloys has been studied. The alloy was subjected to thermal exposure at temperature of 1100 °C for periods of 100, 500, and 1000 h. The microstructure of the alloy was degraded after long-term thermal exposure, mainly in γ' coarsening, the precipitation topologically close packed (TCP) phases. Moreover, the stress-rupture properties of the samples before and after long-term thermal exposures were characterized. The results showed that the γ' coarsening and TCP formation after exposed for 500 h, which led to a damaged of the stress-rupture life time. Besides, the growth of pores during exposure also influences the stress-rupture life. • γ' coarsening and TCP phase precipitation are characterized after long-term exposure. • The TCP phase does not embrittle the alloy. • The severe γ' coarsening is the main factor in the degradation of the life time.

Influence of thermal exposure on microstructure and stress rupture property of a Re-containing Ni-based single crystal superalloy

In this study, the microstructural evolution and microstructure-property relationship of a Re-containing single crystal superalloy after long-term thermal exposure for up to 20,000 h at 900 °C have been investigated. The results showed that the microstructure of the experimental alloy significantly degraded after thermal exposure, including the coarsening of γʹ precipitates, the decrease of γʹ volume fraction and the morphology evolution of γʹ precipitates from cubic to spherical. The magnitude of γ/γʹ misfits decreased obviously and then remained stable. The rupture life of the alloy at 900 °C/330 MPa dropped dramatically after thermal exposure for 2000 h, which was mainly caused by the obvious reduction of γʹ volume fraction, the decreased magnitude of γ/γʹ misfits and the coarsening of γʹ precipitates. The rupture life of the alloy decreased slowly from 2000 h to 20,000 h, although γʹ size continued to increase from 680 nm to 1648 nm. The primary reason for this period was the unchanged γʹ volume fraction and the lower magnitude of γ/γʹ misfits.

Effect of Orientation on Stress-Rupture Property and Related Deformation Microstructure of a Ni-Base Re-containing Single-Crystal Superalloy at 900 °C

Stress-rupture properties of a Ni-base Re-containing single-crystal superalloy with three orientations have been tested under 900 °C/445 MPa. An obvious anisotropy of stress-rupture property is attributed to orientation reliant deformation microstructure. The good strength in [001] orientation is attributed to the rapid multiplication of dislocations active in horizontal channels and later γ’ cutting via dislocations pair coupled with anti-phase boundary. The microtwin formation largely limits the strength and plasticity as a result of the continuous shearing across γ/γ’ microstructure by {111}〈112〉 slip activated in [011] orientation. The property in [111] orientation results mainly from the lateral cross-slip movements of the screw dislocations within connected matrix channels as well as the precipitate shearing by coplanar dislocations. Microcracks all initially originate from the interdendritic micropores in three orientations. The critical temperature of stress-rupture anisotropy could be increased by a high level of refractory solutes especially Re.

Formation and evolution of porosity during high temperature creep of a nickel-based single crystal superalloy

A Re-containing single-crystal superalloy was used to research the high temperature low stress creep behavior. Transmission electron microscope, scanning electron microscope and some other research methods are employed. The results and analysis are summarized below: Two mechanisms for the steady creep are found in this experiment. The volume fraction of pores after creep test at 1100°C increased more than 2 times compared with that before test, but the increasing at 1000°C is relatively small, which reveals that temperature has an great influence on the formation of pore during creep; There are two types of pores associated with fracture during the creep process. One is the casting shrinkage located between the interdentritic, which is formed in the solidification of the alloy. Another type of pore is nucleated and growing during the creep deformation.

Evolution of TCP Phase During Long Term Thermal Exposure in Several Re-Containing Single Crystal Superalloys

The application and component designs of single crystal superalloys are restricted by the precipitation of topologically closed packed (TCP) phases, which can deteriorate the microstructural stability of the alloys severely. Limited researches concerning the type and morphology evolution of TCP phases under elevated temperature conditions have been reported previously. In the present work, three Re-containing single crystal alloys were designed to investigate TCP phase evolution via long term isothermal exposure tests at 1120 °C while the effects of Re on the microstructural characteristic and elements segregation were also clarified. The results showed that the addition of Re increased the instability of the alloys and the volume fraction of the TCP phases exceeded 5 vol% when the Re content reached 3 wt%. The increasing Re content had also raised the precipitation temperature of TCP phases but it did not change the type of them after long term aging; all the TCP particles were identified as μ phase in this study. Moreover, the elements segregation became considerably serious as Re addition increased constantly, which brought about various morphologies of the μ phase in the experimental alloys. In particular, the rod-like and needle-like μ phases demonstrated the typical orientation within γ matrix while the blocky μ phase was dispersedly distributed in the space. No specific orientation relationship could be observed in the μ phase when the addition of Re exceeded certain threshold value.

Effects of heat treatment on microstrucutures of a Re-containing Ni-based single crystal superalloy

Effects of heat treatment on microstrucutures of a Re-containing single crystal superalloy have been investigated. The results showed that (γ+γ′) eutectic in as-cast alloy is practically dissolved with solution heat treatment, and the degree of segregation is distinctly decreased. The size, morphology and distribution of γ′ phase are mightily affected by the first-stage ageing treatment temperature. When the first-stage ageing treatment temperature lower than 1160°C, the shape of the γ′ phase is cubic. But the γ′ phase shape is transformed to sphericity when the temperature higher than 1180°C. The γ phase channel width is significantly increased, and the fine γ′ phase precipitated in the γ phase matrix channel.

Creep Properties and Microstructural Evolution at 760°C/785MPa of a Re-Containing Single Crystal Superalloy

Intermediate temperature creep properties are considered a key indicator of single crystal superalloys used for turbine blades of aircraft engines. The interrupted and ruptured creep tests were carried out in a second generation single crystal superalloy under the conditions of 760°C/785MPa. The creep rupture life as well as minimum creep rate were also in the same level of those in CMSX-4 and PWA1484. The microstructural evolution at different creep stages were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed the γ’ phases kept the cuboid morphology mostly until the creep rupture, and super lattice stacking faults (SSFs) extended along [-1 1 0] and [-1-1 0] orientations within the γ’ precipitate were the typical dislocation configuration.

Influence of substituting Mo for W on solidification characteristics of Re-containing Ni based single crystal superalloy

The influence of substituting Mo for W on solidification characteristics of Re-containing single crystal superalloy was investigated via experimental analysis and thermodynamic calculation. W-rich single crystal superalloy (Alloy W) and Mo-rich single crystal superalloy (Alloy Mo) both had typical dendritic microstructure, while interdendritic precipitations of Alloy W and Alloy Mo were γ′ phase+γ/γ′ eutectic phases and γ′ phase+NiMo phase, respectively. Meanwhile, W significantly segregated to dendritic region in Alloy W, while Mo segregated to interdendritic region in Alloy Mo. Moreover, substituting Mo for W obviously decreased the microsegregation degree of Re and Al elements. The influence of substituting Mo for W on microsegregation behavior of alloying elements (e.g. Re) effectively decreased the tendency of forming freckle during solidification. Furthermore, substituting Mo for W decreased liquidus temperature and solidus temperature of single crystal superalloy.

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83μm and 3.98μm for the specimen annealed at 1050°C for 10min, which, after 16h, had grown to the average values of 3.22μm and 31.17μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

Solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys with thermal and metallographic analysis

To clarify the solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys, four experimental alloys with varied contents of Re and Ru were investigated by differential scanning calorimetry (DSC) and metallographic techniques. To obtain the γ′-solvus temperatures, the stepwise solution and aging heat treatments were used. DSC analysis shows that Re leads to the increase in freezing range and γ′-solvus temperature. On the contrast, Ru only has negligible influence on the freezing range, but leads to the lower γ′-solvus temperature. In comparison with Ru, Re leads to more severe segregation and higher eutectic fractions in as-cast microstructures. Furthermore, the castability and phase stability of Ni-based superalloys were analyzed by the results of DSC and metallographic analysis, such as freezing range, critical nucleation temperature, γ′-solvus temperature and eutectic fractions. It shows that Re leads to the wider freezing range and lower critical nucleation temperature, indicating the worse castability of Re-containing Ni-based single-crystal superalloys. The characteristic phase transformation temperatures of Re-containing single-crystal superalloy are determined by DSC analysis. For as-cast samples, the liquidus, solidus, critical nucleation and eutectic reaction temperature are clearly shown in DSC curves. After a stepwise solution and aging heat treatments, the higher amount of γ′ phase with cubic morphology can be obtained. Finally, the precipitation temperature of γ′ phase is determined by DSC analysis for the heat-treated samples.

Microstructure and Element Segregation of Ni-Base Superalloy Casting with Radiation and Liquid-Metal Cooling

A Re-containing single-crystal superalloy was processed with conventional radiation high rate solidification (HRS) and liquid-metal cooling (LMC) techniques, respectively. Compared with the HRS castings, LMC castings exhibited the greatly refined dendrite structures and lower segregation. At the withdrawal rate of 50 μm/s, the primary and secondary dendrite arm spacing of LMC casting was 103 and 19 μm, which were reduced by 60 % and 53 % compared to the counterpart of HRS castings. Moreover, higher withdrawal rates significantly shortened the characteristic diffusion distance and reduced the residual segregation of refractory elements after homogenization-solution treatments, which is beneficial for improving the local phase stability of Re-containing superalloys.

INFLUENCE OF Pt ON THE CREEP RUPTURE PROPERTIES OF Ni-BASED SINGLE CRYSTAL SUPERALLOY

The Ni- based single crystal superalloys are considered to be the major materials for advanced areo- engine blades. In order to improve the high temperature properties of Ni- based single crystal superalloys,many refractory elements are introduced into this kind of alloys. Recently Pt has been suggested to be the alloying elements of advanced Ni-based single crystal superalloys. However, there are no researches for the effects of Pt on creep rupture properties of advanced single crystal superalloys. In this work, the influence of Pt element on the creep rupture properties of a Re-containing single crystal superalloy was investigated. The high-temperature creep rupture properties of the Pt-containing Ni-based single crystal superalloy at 1100 ℃, 180 MPa and 1000 ℃, 310 MPa were investigated. The deformation microstructure and the morphology of dislocations were studied by SEM and TEM.The results show that the creep rupture life of Pt-containing superalloy decrease slightly at 1100 ℃, 180 MPa and decreased obviously at 1000 ℃, 310 MPa. The fracture models of different alloys are all ductile fracture, and many irregular microviods and microcracks can be observed in the fracture surfaces. After high temperature creep deformation, regular dislocation networks formed at the g/g' interfaces. The differences of creep rupture properties among those alloys are that Pt element may promote the formation of TCP phase, and the interface between the TCP phase and g matrix may be favorite sites of the initiation of microvoids and microcracks.

Kinetics of cellular discontinuous precipitation in a Re-containing single crystal superalloy

Cellular recrystallization behavior of a second generation single crystal superalloy DD6 was studied experimentally by annealing at sub-solvus temperature of 1100°C in this work. During cellular growth, the microstructure in the direct vicinity of recrystallization reaction front was investigated by combined electron microscopy techniques. The results exhibited strong recovery and subgrain formation in the grit-blasted original matrix. Compositional mapping measured across the advancing recrystallization reaction front shows that a thin layer of γ-forming elements always exists in the recrystallization reaction front. By diffusion ahead and inside the reaction front, the chemical deviation from local equilibrium at the interface results in a force pulling the grain boundary forward.

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 γ 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 σ phase which precipitated preferentially as a metastable intermediate in the alloy with high Mo content.

Effects of elemental vaporization and condensation during heat treatment of single crystal superalloys

Vaporization and subsequent condensation of Ni and Al is shown to play a dominant role in the evolution of microstructure during heat treatment of an Re-containing single crystal superalloy. Thus the traditional view that matter is not exchanged with the environment – so that the thermodynamic system is closed – is shown to be incorrect. The extent of depletion of Ni and Al alters the surface chemistry, causing formation of refractory-rich precipitates and destabilization of the matrix phase, γ.

Modeling of Precipitation Kinetics of TCP-Phases in Single Crystal Nickel-Base Superalloys

The precipitation of brittle so-called TCP-phases is critical for the application of Re-containing single crystal superalloys. In this work a fully multicomponent precipitation model is presented, which is capable of simulating the precipitation process of the TCP-phases in superalloys considering complex precipitation sequences with several metastable phases. The model is coupled to multicomponent thermodynamic CALPHAD calculations and relies on multicomponent diffusion models based on the TC-API interface of the software DICTRA. The required mobility database has been newly developed and covers all relevant alloying elements of the Ni-base superalloys including rhenium (Re) and ruthenium (Ru). It is well known that adding Ru strongly reduces TCP-phase precipitation. Based on the developed precipitation model, possible mechanisms are investigated to explain this effect and it is concluded that Ru mostly influences the nucleation rate by a combined influence on interface energy, “reverse partitioning” and γ’-phase fraction.

Influence of temperature on tensile behavior and deformation mechanism of Re-containing single crystal superalloy

Tensile properties of a Re-containing single crystal superalloy were determined within the temperature range from 20 to 1 100 °C with a constant strain rate of 1.67×10 −4 s −1. From room temperature to 600 °C, the yield strength increases slightly with increasing temperature. The yield strength decreases to a minimum at 760 °C, while a maximum is reached dramatically at 800 °C. The elongation and area reduction decrease gradually from room temperature to 800 °C. Above 800 °C, the yield strength decreases significantly with increasing temperature. The γ′ phase is sheared by antiphase boundary (APB) below 600 °C, while elongated SSF (superlattice stacking fault) is left in γ′ as debris. At 760 °C the γ′ phase is sheared by a/3〈112〉 superpartial dislocation, which causes decrease of yield strength due to low energy of SSF. Above 800 °C, dislocations overcome γ′ through by-passing mechanism.