High Temperature Low Stress Creep Research Articles

Microstructure and dislocation structure evolution during creep life of Ni-based single crystal superalloys

The high performance of Ni single crystal superalloys during high temperature low stress creep service, is intrinsically determined by the combined effects of microstructural evolution and the dislocation behaviour. In the field of the evolution of dislocation network, two main recovery mechanism based on dislocation migration dominate the process. One is superdislocations shearing into γ’ rafts through a two-superpartials-assisted approach. Another is the compact dislocations migrating along γ/γ′ interface. These two mechanisms are similarly climb-rate-controlled process. In this work, a model for the minimum creep rate based on thermodynamic and kinetic calculations and using an existing detailed dislocation dynamics model has been built by taking the dislocation migration behaviours as well as the rafted microstructure into consideration, which can well reproduce the ([100] tensile) creep properties of existing Ni superalloy grades, without the need to make the dislocation parameter values composition dependent.

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.

Crystal Plasticity Study of Heterogeneous Deformation Behavior in γ Matrix Channels during High Temperature Low Stress Creep of Single Crystal Superalloys

Abstract Single crystal nickel-based superalloys have high temperature creep resistance due to particle strengthening by high volume fraction of coherent γ′ precipitates distributed within nickel-based solid solution γ matrix. In the high temperature low stress regime, published experimental studies reveal that the creep deformation mechanism during the secondary stage is predominantly by dislocation glide in the γ matrix only, and that there is a preferential motion of dislocations in the matrix, oriented in a direction perpendicular to the stress. In this work, the crystal plasticity finite element method is employed to perform creep simulations on a representative volume element in the high temperature and low stress regime. A sine-hyperbolic–based material creep model was used for the matrix, while the precipitates are assumed to be elastic. A softening model incorporating the evolution of mobile dislocation density was used to capture the transition from secondary to tertiary creep. The predicted creep curves agree well with the published experimental measurements on single crystal superalloy CMSX-4. The simulations predict a higher creep strain distribution in the horizontal channel of the matrix (perpendicular to the applied stress) as compared with the vertical channel (horizontal to the applied stress). Local creep strain distributions in the channels were found to be greater than twice their average creep strain. The results provide key insights into the distribution of macroscopic creep strain in the local channels of the γ matrix to further aid in the microstructural design of creep-resistant superalloys.

Role of coherency loss on rafting behavior of Ni-based superalloys

The role of coherency loss on rafting of superalloys under high temperature low stress creep conditions is investigated by phase-field crystal plasticity simulations. It is demonstrated that coalescence, critically depending on the state of coherency between precipitate and matrix is crucial to understand the rafting behavior of superalloys. An explicit mechanisms is developed predicting coherency loss based on the plastic activity in the matrix. The simulations are verified using experimental creep test results.

Microstructural Degradation and the Effects on Creep Properties of a Hot Corrosion-Resistant Single-Crystal Ni-Based Superalloy During Long-Term Thermal Exposure

In the present study, the kinetics of microstructural degradation during long-term thermal exposure (LTTE) and the effects on creep deformation mechanisms of a hot corrosion-resistant single-crystal Ni-based superalloy with a low γ′ volume fraction and γ/γ′ lattice misfit were investigated in detail. The kinetic of γ′ coarsening in the experimental alloy conforms well to the Lifshitz–Slyozov–Wagner theory during LTTE at 900 °C up to 10,000 hours. The evolution of γ/γ′ lattice misfit during the LTTE was also investigated by a first attempt. The focused research emphasized on the influences of γ/γ′ lattice misfit evolution after the LTTE on the microstructural degradation, dislocation motion, and different creep mechanisms during high-temperature low-stress creep and high-temperature high-stress creep. The results show that the decreasing of the absolute values of γ/γ′ lattice misfit and change of γ′ size and morphology after the LTTE contribute to the weakening of barrier to the dislocation cutting process into γ′ precipitates during creep and the sharp reduction of stress-rupture lifetime at 950 °C/280 MPa after 1000 hours exposure. As the applied stress decreased to 230 MPa at 950 °C, the creep mechanisms change from the dislocation cutting through γ′ precipitates at high applied stress to the dislocation glide and climb around γ′ precipitates. The dislocation glide and climb by-pass deformation mechanism were not significantly influenced by the change of γ′ precipitates morphology and magnitude of γ/γ′ mismatch within 1000 hours thermal exposure, and the minimum creep rate and creep lifetime after 1000 hours thermal exposure were similar to that of the original heat-treated samples.

Effects of Hf and B on high temperature low stress creep behavior of a second generation Ni-based single crystal superalloy DD11

The as-cast and heat-treated microstructures and high temperature creep properties have been investigated in four experimental Ni-based single crystal superalloys containing various levels of Hf addition (0–0.4wt%) and B addition (0–0.02wt%). The experimental results indicated that the creep rupture life showed an improvement with individual addition of Hf, but it was decreased with individual addition of B. The elemental partitioning ratio and interfacial dislocation spacing of γ/γ′ were obviously changed with individual Hf or B additions. Meanwhile, the formation of secondary phases, such as the blocky MC carbide, script-like shape M3B2 phases, was observed in the creep samples, which was also closely related to the high temperature creep behaviors. The high volume fraction of residual (γ+γ′) eutectics was mainly attributed to the significant decrease of creep rupture life for the present experimental alloy containing both Hf and B additions. This study is helpful to better understand Hf and B's role of strengthening mechanism and to optimize Hf and B additions in single crystal superalloys.

Dislocation motion during high-temperature low-stress creep in Ru-free and Ru-containing single-crystal superalloys

Creep deformation of the two experimental single crystal superalloys at high-temperature low-stress (1140°C/137MPa) has been analyzed through transmission electron microscopy. Emphasis is placed on elucidating the dependence of dislocation motion on microstructural evolution. The detailed analysis demonstrated that the stacking fault energy of the γ matrix significantly decreased with the addition of Ruthenium (Ru). The stacking faults presenting in the γ matrix after heat treatment has been rarely reported previously. During the primary creep stage, the dislocations can easily cross-slip on the different {111} planes in the horizontal matrix and leave 60° dislocation loops on the (001) γ/γ′ interfacial plane. Furthermore, calculations demonstrated that it is difficult for the slipping dislocations to bow into the vertical γ matrix channel. In the early stages of steady state creep, the interfacial dislocations reoriented slowly from the 〈110〉 slipping direction to the 〈100〉 well misfit stress relief direction. On the other hand, few dislocations shearing into the rafted γ′ phase have been observed. In fact, during the middle stages of the steady state creep, although perfect dislocation networks have formed, some dislocations shearing into the γ′ phase have also been observed. In addition, the a 〈010〉 type superdislocations (some with non-compact core) have also been observed in the two experimental alloys. At last, the Ru-containing alloy possesses more negative lattice misfit, denser γ/γ′ interfacial dislocation networks and higher microstructural stability, thus can maintain a minimum creep rate in the steady state stage and have a longer creep life.

Interfacial dislocation motion and interactions in single-crystal superalloys

The early stage of high-temperature low-stress creep in single-crystal superalloys is characterized by the rapid development of interfacial dislocation networks. Although interfacial motion and dynamic recovery of these dislocation networks have long been expected to control the subsequent creep behavior, direct observation and hence in-depth understanding of such processes has not been achieved. Incorporating recent developments of discrete dislocation dynamics models, we simulate interfacial dislocation motion in the channel structures of single-crystal superalloys, and investigate how interfacial dislocation motion and dynamic recovery are affected by interfacial dislocation interactions and lattice misfit. Different types of dislocation interactions are considered: self, collinear, coplanar, Lomer junction, glissile junction, and Hirth junction. The simulation results show that strong dynamic recovery occurs due to the short-range reactions of collinear annihilation and Lomer junction formation. The misfit stress is found to induce and accelerate dynamic recovery of interfacial dislocation networks involving self-interaction and Hirth junction formation, but slow down the steady interfacial motion of coplanar and glissile junction forming dislocation networks. The insights gained from these simulations on high-temperature low-stress creep of single-crystal superalloys are also discussed.

Kinetics and Nanostructure Dependence of High Temperature-Low Stress Creep of Al and Al-0.3%Fe

The novel nanostructure of Al and Al{Fe were prepared by ball milling alumina with elemental Fe. The kinetics and nanostructure dependence of high temperature low stress Newtonian creep of Al and Al{0.3%Fe have been investigated and compared with the predications of the Nabarro{Herring (N{H) theory of directional difiusion. A simple theory based on the climb controlled generation of dislocations from a flxed density of sources is developed to explain the observed behavior. The dislocation density increases and subgrains form during the creep. Also, the presence of precipitates of FeAl3 reduces the creep rate of Al by absolute faster of 100 at the same stress and temperature, in spite of the fact that the grain size in the Al{0.3%Fe alloy is smaller by a factor of about 100 nm. The reduction of grain size to the nanometer scale improves their mechanical properties. Electron difiraction methods combined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies are a convenient and powerful technique for the characterization of the phases and grain structure of the resulting materials.

Dislocation Configurations in Single-Crystal Superalloys during High-Temperature Low-Stress Creep

Dislocation configurations in two single-crystal superalloys during high-temperature low-stress creep (1100°C, 137 MP) were illustrated schematically with the use of transmission electron microscope (TEM). For an alloy with a small lattice misfit, the dislocations move in the combination of climbing and gliding processes. In the primary stage, the dislocations first move by slip in the g-matrix channels. When they reach the g¢ cuboids, they move by climb along the g¢ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, deviating from the deposition orientation of <110> to the misfit orientation of <100>. For an alloy with a large lattice misfit, the dislocations are able to move smoothly by cross slip in the horizontal g channels. The dislocation reorientation from the deposition orientation of <110> to the misfit orientation of <100> in the (001) interfacial planes can be completed in the primary creep stage.

Dislocation motion in the early stages of high-temperature low-stress creep in a single-crystal superalloy with a small lattice misfit

Dislocation configurations at different creep stages (1100 °C and 137 MPa) in a superalloy TMS-75(+Ru) were studied in transmission electron microscopy (TEM) and the movement path of these creep-produced dislocations could be fully illustrated. Due to the small value of γ/γ′ lattice misfit, these dislocations cannot glide in the horizontal γ matrix channels by cross slip, but they mainly move by climbing around the γ′ cuboids. In the primary stage, the dislocations first move by slip in the γ-matrix channels. When they reach the γ′ cuboids, they move by climbing along the γ′ cuboid surfaces. In the secondary creep stage, dislocation reorientation in the (001) interfacial planes happens slowly, away from the deposition orientation of 〈110〉 to the misfit orientation of 〈100〉. The velocity of the reorientation is lower and a perfect γ/γ′ interfacial dislocation network cannot be formed quickly. This factor results in a large creep rate of the alloy during the secondary creep stage. The path for dislocation motion during the early creep stages consists of the following sequences: (i) climbing along the γ′ cuboid surface, (ii) deposition onto the (001) γ/γ′ interfacial plane, and (iii) reorientation from the 〈110〉 direction to the 〈100〉 direction.

High‐temperature dislocation plasticity in the single‐crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single-crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a gamma/gamma'-microstructure consisting of gamma'-cubes (L12 phase, 80 vol.%) separated by thin gamma-channels (face-centred cubic). <100> tensile creep tests were performed at 980 and 1020 degrees C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high-temperature creep. It was found that in the early stages of creep, a(0)/2<011> dislocations form within the gamma-channels. Later on, dislocation networks form and gamma' cutting processes with a(0)/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high-temperature low-stress creep regime.

第4世代Ni基単結晶超合金のクリープ強度に及ぼす微量添加Yの影響

The effects of yttrium addition on high temperature properties of the 4th generation nickel-base superalloy were studied. Nickel-base superalloys, which possess both creep strength and oxidation resistance, are being required to satisfy high efficiency and reliability of jet engines. 4th generation superalloys contain platinum groups metals such as ruthenium. In order to satisfy creep strength and phase stability, however ruthenium worsens oxidation resistance. Yttrium doping was found to increase the oxidation resistance due to the improved adhesive strength of Al2O3 oxide, and the effects of yttrium doping on creep strength are established in this paper. Yttrium-added TMS-138 alloy (TMS-138Y) showed high creep strength of 4th generation superalloys, however the creep strength was slightly worse than the alloy without yttrium in the high-temperature low-stress creep condition. Microstructural observations confirmed the increase of Topologically Close Packed (TCP) phases in TMS-138Y. This was due to the formation of a yttrium-containing compound, Al0.06Ni0.73Co0.04Y0.18 (at.), which made incoherent interfaces that allowed TCP phases to precipitate by heterogeneous nucleation.

The effect of lattice misfit on the dislocation motion in superalloys during high-temperature low-stress creep

The development of dislocation configurations in two single-crystal superalloys during high-temperature low-stress creep (1100°C, 137 MPa) was investigated with the use of transmission electron microscopy. Detailed analysis showed that the lattice misfit has an important influence on the dislocation movement. For an alloy with a large negative lattice misfit, the dislocations are able to move smoothly by cross-slip in the horizontal γ channels. During subsequent formation of γ/γ′ rafted structure, the dislocations on the surface of γ′ cuboids rapidly re-orientate themselves from 〈110〉 to 〈100〉 direction and form a complete network. For an alloy with a small lattice misfit, the dislocations move by the combination of climbing and gliding processes, and the resultant γ/γ′ interfacial dislocation network is incomplete. A good explanation of the creep curves is obtained from these differences in the microstructures.

Strengthening Mechanisms in Some Single-Crystal Superalloys

The creep behavior and microstructure of several nickel-base single-crystal superalloys after high-temperature low-stress creep have been investigated. These alloys were designed with varying content of the alloying elements Mo and Ru. At 1100°C and 137 MPa, the large g/g¢ lattice misfit in negative with the addition of Mo leads to the formation of dense interfacial dislocation networks. These dislocation networks are effective to strengthen the alloys during creep by preventing the penetration of the g dislocations into the g¢ phase.

Bubble evolution and effects during tungsten processing

The sintered microstructure of potassium doped P/M tungsten is besides the usual residual porosity characterized by small nanosized spherical potassium filled bubbles in the diameter range between 10 and 50 nm. During the deformation of the sintered ingots the spherical bubbles change their morphology into elongated bubble tubes or ellipsoids. At intermediate annealings these potassium tubes decay above critical dimensions due to Rayleigh instabilities into rows of spherical bubbles which determine the recrystallization and the high temperature–low stress creep behaviour of doped tungsten wires. For a correct description of the occurring shape changes the relative density of the tungsten ingots at the beginning of the deformation steps as well as the deformation temperature and the temperature–time regime of intermediate annealings must be considered. In addition, there are significant differences between bubble sizes and bubble numbers located in the grain volume and at grain boundaries. For the occurrence of single bubbles a mechanism of “in situ spheroidization” during deformation will be proposed. Further reasons for discrepancies between experimental and modeled bubble dimensions at high true strains of deformation will be discussed.

On the formation of 〈010〉-dislocations in the γ′-phase of superalloy single crystals during high temperature low stress creep

The cutting of γ′-particles by dislocations during shear creep deformation of CMSX 6 superalloy single crystals loaded on the macroscopic crystallographic shear system 〈110〉{111} at T = 1025°C and τ = 85 MPa is analysed using transmission electron microscopy (TEM). The formation of a 〈010〉-dislocation in the γ′-phase was observed and investigated by means of stereo microscopy, line trace and g·b-analysis. Two γ-matrix channel dislocations with different Burgers vectors ( b) of type α 0/2〈110〉 jointly shear the γ′-particle where they form a superdislocation with a total Burgers vector of α 0〈010〉. The present paper provides microstructural evidence for this high temperature cutting process. The results are discussed in relation to the evolution of the dislocation structure in the γ-channels during primary creep. Peach-Koehler forces were calculated to explain the formation of the 〈010〉-dislocation.

On the formation and motion of 〈010〉 — dislocations during high temperature low stress creep of superalloy single crystals

The cutting of γ'-particles by dislocations during shear creep of superalloy single crystals loaded on the shear system 〈110〉111 at T = 1025 °C and τ = 85 MPa is analysed. It was found that two γ-matrix channel dislocations with different Burgers vectors of type a 0/2〈110〉 jointly shear the γ′-particle and compose a superdislocation with a Burgers vector of a 0〈010〉. Peach-Koehler forces were calculated which are associated with the birth of these superdislocations. The results of the numerical study show that the formation of 〈010〉 dislocations in the superalloy microstructure is energetically favourable.

Influence ofprecipitate morphology on the high temperature fatigue properties of SRR99 : Martin, J.W. and Henderson, M.B. Acta Metall. Mater. (1995) 43 (11), 4035–4043

The development of dislocation configurations in two single-crystal superalloys during high-temperature low-stress creep (1100 °C, 137 MPa) was investigated with the use of transmission electron microscopy. Detailed analysis showed that the lattice misfit has an important influence on the dislocation movement. For an alloy with a large negative lattice misfit, the dislocations are able to move smoothly by cross-slip in the horizontal γ channels. During subsequent formation of γ/γ′ rafted structure, the dislocations on the surface of γ′ cuboids rapidly re-orientate themselves from 〈1 1 0〉 to 〈1 0 0〉 direction and form a complete network. For an alloy with a small lattice misfit, the dislocations move by the combination of climbing and gliding processes, and the resultant γ/γ′ interfacial dislocation network is incomplete. A good explanation of the creep curves is obtained from these differences in the microstructures.

High temperature creep in Fe-3 pct Si

The kinetics and structure dependence of the high temperature-low stress creep of Fe-3 pct Si has been studied and compared with the predictions of current creep theories. It is shown that the steady state creep rate is diffusion controlled and exhibits a power law stress dependence in the temperature range 1393 to 1678 K and stress range 6.9×104 to 1.03×106 Pa. In this same range of experimental conditions the dislocation density present during steady state creep is essentially stress independent. It is shown that current creep theories are incompatible with the experimental results.