To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single ...crystal superalloy containing 22 wt% refractory elements was investigated. Results show that for the experimental alloy, even the homogenization–solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ′ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ′ cubes. For specimens with primary aging heat treated at 1120 °C, γ′ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 °C, γ′ phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ′ phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ′ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ′ phase. As a result, the γ′-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ′ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then, high-temperature properties can be improved.
The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for ...the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.
Single crystal superalloys are widely used in high temperature sections of turbines, where they are able to withstand extended exposure to high temperatures. As these alloys are commonly cast and ...directionally solidified via the Bridgman process, the knowledge of their solidification path is of great importance. This paper gives an overview of aspects of the solidification path in superalloys, as studied experimentally and from a modeling and simulation point of view. The implications of microsegregation, and sequence of phase formation are discussed. Attention is given to the processes leading to the formation of interdendritic γ′, also often referred to as the γ/γ′ eutectic.
The effect of thermal exposure on microstructure and creep properties of a Re bearing single crystal superalloy is investigated. The alloy is exposed at 950°C for 100–2000h. The coarsening behavior ...of γ′ varies with local position. The γ′ particles in dendrite core and interior secondary arm form raft when aged longer than 100h. The γ′ in periphery of secondary arm can retain cuboidal morphology aged to 2000h in addition to normal growth. The γ′ in interdendrite region develops into irregular raft. The various behavior of γ′ is attributed to the segregation of refractory elements. The MC carbide demonstrates high stability even though it begins to decompose to M6C after 500h exposure. This is explained from its high contents of Ta and Nb. μ phase precipitates in both dendrite core and interdendrite region when aged more than 100h, but the μ phase in these two areas have different morphologies, which is attributed to their unique nucleation site and orientation relationship with matrix. The creep lives of exposed samples at 1000°C/280MPa decrease with increasing of aging time generally, especially, the life of 500h aged sample drops lower than that of 1000h aged sample. The degradation of creep life is accounted for based on coarsening of γ′ and deposition of dislocation during thermal exposure.
Aiming at the status of fewer researches on the anisotropy of single-crystal superalloy and micro-grinding temperature field, micro-grinding using the tool of < 1 mm in diameter to remove material is ...the final procedure of machining micro-parts. This paper firstly analyzes the anisotropy of surface-centered cubic structural material and establishes the theoretical model of elastic modulus and shearing modulus of single-crystal superalloy in the (001), (110) and (111) crystal plane. Afterward, it builds the three-dimensional simulation model of grinding temperature based on the Hill model, adopts any Lagrangian–Euler method to realize the simulation of micro-grinding temperature, gets the distribution of temperature field and changing situation during the micro-grinding through single-factor experiment and simulation analysis, and makes a study on the laws of different grinding depths, feeding rates, grinding velocities, different crystal planes {(001), (110) and (111)} and different crystal orientations in (001) crystal plane that affect the micro-grinding temperature, thus obtaining the crystal orientation which produces low grinding force and temperature. The result indicates that the grinding temperature increases with the increase in spindle speed, feeding rate and grinding depth; the grinding force and temperature are proportional to the elastic modulus and shearing modulus of the single-crystal material; the grinding temperature along (111) crystal plane is the highest, that of the (110) crystal plane is at the middle, and that along (001) is the lowest. When grinding in the (001) crystal plane, the normal grinding force and temperature along 110,
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crystal orientations are the highest, and those along 100, 010,
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crystal orientations are the lowest. And along with the aggravating of wear of abrasives on the grinding tool, the normal grinding force, tangential grinding force and grinding temperature all increase and the wear state of grinding tool can be judged online according to the signal of grinding force and temperature. These results provide an important evidence to do further research on the surface integrity of grinding single-crystal superalloy.
An in situ scanning electron microscope (SEM) tensile test for Ni-based single-crystal superalloy was carried out at 1000 °C. The stress displacement was obtained, and the yield strength and tensile ...strength of the superalloy were 699 MPa and 826 MPa, respectively. The crack propagation process, consisting of Model I crack and crystallographic shearing crack, was determined. More interestingly, the crack propagation path and rate affected by eutectics was directly observed and counted. Results show that the coalescence of the primary crack and second microcrack at the interface of a γ/γ′ matrix and eutectics would make the crack propagation rate increase from 0.3 μm/s to 0.4 μm/s. On the other hand, crack deflection decreased the rate to 0.05 μm/s. Moreover, movement of dislocations in front of the crack was also analyzed to explain the different crack propagation behavior in the superalloy.
▶ Crystal plasticity calculations of the induced stress/strain fields in the vicinity of TCP-phases. ▶ Influence of TCP-phase on the local γ′ microstructure evolution. ▶ Influence of constitutive ...equations for TCP-phase.
The influence of topologically close packed-phase (TCP-phase) precipitation on the γ/γ′ microstructure evolution of a single crystal superalloy has been studied. Four representative configurations of needle-shaped μ-phase particles observed in the MC2 alloy after creep deformation at 1050°C/160MPa have been selected.
Finite element modeling was performed on the selected configurations using a crystal plasticity framework and a viscoplastic approach.
The local calculated stress/strain fields in the vicinity of TCP-phase precipitates, especially stress concentration, allow an interpretation of the observed microstructural evolutions (γ′-precipitate orientation and thickness).
Moreover, our numerical simulation brings a fruitful contribution to the understanding of the phenomenon often observed beyond needle-like μ-phase: the formation of a distorted γ/γ′ microstructure in lieu of the expected γ/γ′-rafted structure.
Two experimental single crystal superalloys, Ru-free alloy and Ru-containing alloy with 001 orientation, other alloying element contents being basically kept same, were cast in the directionally ...solidified furnace. The effect of Ru on the stress rupture properties of the single crystal superalloy was investigated at (980°C, 250 MPa), (1100°C, 140 MPa) and (1120°C, 140 MPa). The results show that Ru can enhance high temperature stress rupture properties of single crystal superalloy. The improvement effect of Ru addition on stress rupture properties decreases with increasing test temperature. The γ′ coarsening and rafting directionally are observed in Ru-free alloy and Ru-containing alloy after stress rupture test. Needle shaped TCP phases precipitated in both of alloys after stress rupture test at (1100°C, 140 MPa) and (1120°C, 140 MPa) and no TCP phase was observed in both of alloys after stress rupture test (980°C, 250 MPa). The precipitate volume fraction of TCP phases is significantly decreased by the addition of Ru. At last, the relationship between the microstructure change with Ru addition and improvement of stress rupture properties was discussed.
ABSTRACT
This research looked at the effect of crystallographic orientation and temperature on the fatigue crack growth rate and the resulting fracture surface morphology in PWA1484 single crystal ...superalloy. Two groups of single edge notch tension specimens, one group with controlled secondary orientations and one group with uncontrolled secondary orientation, were tested at temperatures from 649°C to 982°C at R‐ratios of 0.1 and 0.7. It was found that the effect of temperature on the crack growth rate becomes more pronounced as the crack driving force increases while the secondary orientation and R‐ratio effects on the crack growth rate increase with increasing temperature. Two types of crack surface morphology were seen during fractography. The first was a precipitate avoidance (γ′ avoidance) morphology that was rolling but still predominantly flat when observed on a larger scale. In primary oriented specimens, this fracture mode tended to follow the precipitate/matrix faces (microscopically cubic) while macroscopically staying essentially normal to the applied loads. The second mode was a form of cleavage (γ′ shearing) and occurred predominantly on octahedral crystallographic planes.
The rapid development of numerical modeling techniques has led to more accurate results in modeling metal solidification processes. In this study, the cellular automaton-finite difference (CA-FD) ...method was used to simulate the directional solidification (DS) process of single crystal (SX) superalloy blade samples. Experiments were carried out to validate the simulation results. Meanwhile, an intelligent model based on fuzzy control theory was built to optimize the complicate DS process. Several key parameters, such as mushy zone width and temperature difference at the cast-mold interface, were recognized as the input variables. The input variables were functioned with the multivariable fuzzy rule to get the output adjustment of withdrawal rate (
) (a key technological parameter). The multivariable fuzzy rule was built, based on the structure feature of casting, such as the relationship between section area, and the delay time of the temperature change response by changing
, and the professional experience of the operator as well. Then, the fuzzy controlling model coupled with CA-FD method could be used to optimize
in real-time during the manufacturing process. The optimized process was proven to be more flexible and adaptive for a steady and stray-grain free DS process.
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