•A TMF test platform which can accurately control temperature and load was constructed.•The TMF behaviors were analyzed by macro-fine-micro multi-scale testing.•The TMF life and fracture morphologies ...of IP and OP conditions are obviously different.•Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase of OP condition.
Nickel-based single-crystal superalloys are extensively used in aeroengine hot-end components owing to their unique crystal structure and outstanding high-temperature mechanical properties. In this study, a thermomechanical fatigue (TMF) test platform capable of realizing precise temperature control and coordinated control of mechanical and temperature loads was constructed. The TMF of a second-generation nickel-based single-crystal superalloy (DD6) was analyzed in a temperature range of 500–950 °C at a stress ratio of 0.1 in two phases (in-phase and out-of-phase); moreover, the TMF tests were conducted at various stress levels, and the TMF life of the single-crystal specimens was determined. The OP TMF lifetime was considerably longer than the IP TMF lifetime at identical stress levels. The failure mechanism of the material in different phases was elucidated. The fracture and microstructures of the specimens were analyzed by SEM and TEM. For the IP specimens, the fracture mode was determined to be a quasi-cleavage fracture, and the γ' phase exhibited N-type rafting prior to fracture. For the OP specimens, the fracture was relatively smooth and flat. Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase.
With the development of nickel-based single crystal superalloys, the quantity of micro-pores generating in solution treatment has increased, and there is an urgent need to develop a solution ...treatment design method that takes account of both residual eutectics and micro-pores. Therefore, the effect of heating processing on the formation of micro-pores during solution treatment of a 4th generation nickel-based single crystal superalloy has been investigated using high-resolution transmission X-ray tomography and electron probe microscopic analyzer. In the temperature range below interdendritic γ' phases solvus, decreasing the heating rate can limit the formation of micro-pores significantly. However, heating method (continuous or stepwise), has a little effect on micro-pores formation. The influence of heating processing on micro-pores formation is associated with composition homogenization degree of the alloy before eutectics dissolve. Moreover, it is found that the dissolution of eutectics is most likely the reason for substantial formation of micro-pores. Finally, a designing thinking for heating processing of solution treatment for high generation nickel-based single crystal superalloys is proposed.
•Transmission X-ray tomography technique was used to characterize the micro-pores.•Severe Kirkendall effect during eutectic dissolution causes formation of H-pores.•Decreasing of heating rate can limit the formation of H-pores significantly.•Heating method of solution treatment has a little effect on H-pores formation.
•The PtAl coating effects on the high-cycle fatigue (HCF) behavior of SX superalloy with sheet specimens were investigated.•The coated alloy had lower HCF lives than the uncoated alloy at low maximum ...stresses (σmax < 590 MPa).•The fatigue crack initiation site of the coated alloy could transform at higher σmax than that of the uncoated alloy.•The brittle cracking of the coating and the grain boundaries in the IDZ/SRZ leaded to surface microcrack initiation and growth, facilitating the formation of crack initiation sites.
High-cycle fatigue (HCF) is one of the primary failure modes for turbine blades in aero-engines. Therefore, comprehending the effect of coating on the HCF property of single crystal (SX) superalloys is vital for the safe service of turbine blades. In order to mimic the degradation of the fatigue property for the turbine blades during service, this study investigated the effect of PtAl coating on the HCF property of a third-generation Ni-based SX superalloy using sheet specimens at 900 ℃ and different maximum stresses (σmax). The results indicated that both coating and σmax affected the fatigue properties and crack initiation process of SX superalloy. A transition in fatigue crack initiation site from the internal micropores to the surface cracks for both uncoated and coated alloys has been observed as the σmax decreased. However, the coating significantly increased the σmax required for the transition of the crack initiation site. Meanwhile, the HCF property of the SX superalloy was reduced with the deposition of PtAl coating at lower σmax, and the debit in HCF life was more pronounced with decreasing the σmax. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σmax. On the other hand, since the HCF life is controlled by the crack growth rate, grain boundaries in the interdiffusion zone (IDZ) and secondary reaction zone (SRZ) accelerated the growth of the surface crack under lower σmax, resulting in a shorter crack initiation stage and lower fatigue life compared to the uncoated alloy. This study will be helpful in accurately predicting the HCF life of the coated Ni-based SX superalloys.
•The behavior of low cycle fatigue at typical temperatures was comprehensively investigated.•The life prediction model based on entropy increase theory is in good agreement with the experimental ...data.•This paper offers a methodology for finite cycle prediction of overall fatigue life.
Nickel-based single-crystal superalloys (Ni-SX) are prominently utilized in the fabrication of turbine blade materials for advanced aero-engines, owing to their commendable material characteristics. This paper delves into the low cycle fatigue (LCF) characteristics of a second-generation Nickel-based single-crystal superalloy across a temperature spectrum ranging from room temperature to 1000 °C. During this procedure, there is an escalation in the trend towards deformation homogenization, leading to a transition in fracture mode from shear fracture to normal fracture. A novel approach is introduced in the form of an entropy-based fatigue life prediction model, grounded in the correlation between cumulative plastic strain and entropy increase rate. Through a comparative examination with experimental data, the model demonstrates proficiency in precisely forecasting the fatigue life of Ni-SX under diverse strain amplitudes and temperature conditions.
In this work, the oxidation behaviors of DD6 Ni-based single-crystal superalloy and its compressive strength evolution after oxidation were investigated. The results showed that the thermal exposure ...time has a significant effect on the oxidation products, oxidation kinetics, Al elements diffusion and compressive strength evolution of DD6 superalloy. Firstly, the relationship between grain size change of (Ni0.9,Co0.1)O, the thickness of γ′-free zone and the exposure time was approximate power exponential law, while the relationship between weight change of DD6 superalloy and exposure time was approximate parabolic law since the densification of inner continuous Al2O3 layers hindered the diffusion of oxygen ions. Moreover, the black phases of AlN in the γ′-free zone formed during the oxidation process, and its quantity and size increased first and then decreased with the oxidation time extending, since the inner Al2O3 not only blocked the ion but also reacted with AlN. In addition, with the increase of oxidation time, the compressive yield strength of DD6 in 001 direction first decreased rapidly and then gradually stabilized at about 740 MPa.
•The oxidation mechanisms of DD6 superalloy at 1100 °C were discussed.•Relationship between size change of oxides, thickness of γ′-free zone, weight change and exposure time was established.•The Al-enriched phase in γ′-free zone and its evolution within the oxidation process were studied.•The evolution of compressive strength of DD6 superalloy after oxidation was first determined.
A second-generation nickel-based single crystal superalloys DD6 were creep tested in the 001 direction (within 15°) at 1100 °C/140 MPa. The specimen tested until rupture was investigated using ...scanning and transmission electron microscopy to determine the evolution of the dislocation behaviors during tertiary creep. It was found that the tertiary creep deformation was highly localized and inhomogeneous along the gauge length, and the types of superdislocations in γ′ rafts varied with the distances from the rupture surface, including individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. It can be concluded that γ′ rafts shearing events occur in the following sequence with the evolution of tertiary creep: individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. The origin of these transformations of superdislocation types and its influence on tertiary creep rate are discussed. It is proposed that at the microscopic level, a more reasonable explanation for the strain softening mechanism during the tertiary creep of nickel-based superalloys at high temperatures and low stresses is the emergence of new superdislocation types with higher mobility rather than the density rise of a single type of superdislocation produced during the later secondary creep stage.
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This paper proposed a new weight function modified critical distance method to predict the creep life of Ni‐based single‐crystal superalloy plate with film‐cooling holes through experimental and ...numerical research. The creep test and numerical simulation results both indicated that cracks originated around the hole, and the location of the maximum Mises stress point around the hole was consistent with the crack initiation point shown in the test results. Considering the stress gradient in the thickness direction, a critical fracture surface was defined, and the stress distribution at the critical fracture surface was defined using polynomials. An improved stress gradient function and weight function were introduced to calculate the effective stress at the critical fracture surface. The effective stress was used to predict the creep life of the plate with film‐cooling holes. Compared with the experimental results, the errors were all within the dispersion band of 1.5 times, indicating good prediction results.
Highlights
Creep tests and numerical simulations were conducted on specimens with FCHs.
A novel method for determining fracture damaged area was proposed.
The modified creep weight function was used to assess the effect of stress gradient.
A method for estimating the creep life of film‐cooling structure was proposed.
