Abstract
The deformation and damage behavior of the single crystal Ni-based alloy during elevated temperature creep were investigated through analyzing creep properties and microstructure. The results show that the creep life of Ni alloy under the condition of 1040 °C/137 MPa is 556 h, displaying an excellent creep resistance of Ni alloy. The creep feature of Ni alloy at steady state is the dislocation glide in γ phase and dislocation climb over the γ′ rafts. In the late stage of creep, the deformation feature of Ni alloy is that the γ′ rafts are sheared by the dislocations of cross-slip, which forms the Kear-Wilsdorf (K-W) locks to restrain the dislocation glide and cross-slip dislocation. The cross-slip dislocations cause the distortion of γ/γ′ rafts, thereby promoting the initiation of cracks along the γ′/γ interfaces and the interface fracture, which is the damage and fracture features of Ni alloy. The condition of σ>η/α is considered as the prerequisite for unstable crack propagation.
Science Press
Because of the excellent mechanical and creep properties under high-temperature service conditions, the single crystal Ni-based alloys are widely applied for production of blade parts of advanced aero-engine and industrial gas engin
Adding Re may enhance the temperature endurance of alloys. The feature of the second-generation and the third-generation Ni-based alloys is their composition with 3wt% and 6wt% Re, respectivel
Despite the excellent properties of single crystal alloys with high alloying extent under service conditions, the creep damage is still the main failure mod
During creep at high temperature, the γ′ phase in alloy changes into the raft configuration, which can impede the dislocation movemen
In this research, the Re-free single crystal Ni-based alloy was prepared. The creep behavior of Re-free single crystal Ni-based alloy at high temperature was investigated to explore the damage and fracture features of the alloy during elevated temperature creep through analyses of creep properties and microstructure.
The Re-free single crystal Ni-based alloys with [001] direction were prepared in a directional solidifying vacuum furnace under high temperature gradient by selecting crystal method. The experiment alloy was Ni-Al-7Ta-Cr-Co-4W-6Mo (mass fraction). Based on Laue back reflection method, all the orientations had the difference in orientation deviating from the [001] direction within 7°. Three sections of heat treatments were conducted in the following order: (1) 1280 °C/2 h+1300 °C/2 h+1315 °C/6 h+air cooling; (2) 1080 °C/4 h+air cooling; (3) 870 °C/24 h+air cooling. In addition, the γ and γ′ phases in the alloy had the negative lattice misfits.
After the heat treatment, the single crystal alloys were processed into creep specimens with the cross-section of 4.5 mm×2.5 mm and the gauge length of 20 mm on the (100) plane along [001] direction. After mechanical grinding and polishing, the creep properties of the specimens under various conditions were measured by the creep test machine with GTW504 model.
For the ground and polished specimens at different states, the solution of 20 g CuSO4+5 mL H2SO4+100 mL HCl+80 mL H2O was used for chemical corrosion. Then, the scanning electron microscope (SEM) of S3400 model was used to observe the morphology of alloys at different states. The films with the diameter of 3 mm and the thickness of 60 μm were prepared by grinding and polishing the specimens after creep for different durations. The specimens were thinned using the electrolyte with 7vol% perchloric acid and 93vol% ethanol by twin-jet polishing technique at 253 K. Then the morphology of alloys during creep was observed by the transmission electron microscope (TEM) of TECNA120 model. The creep mechanism at high temperature was also analyzed.
The creep curves of Re-free alloys under different conditions are in

Fig.1 Creep curves of Ni-based alloys under stress of 137 MPa at different temperatures (a) and under different stresses at 1070 °C (b)
In a flash of applying temperature and loading, the larger instantaneous strain and strain rate exist in the primary creep period of alloys with many activated dislocations in γ phase. Then, under the deformation strengthening, the strain rate is reduced and the creep enters into the steady state. The alloy maintains a constant strain rate, as described by Norton law, as follows:
(1) |
where A refers to a constant related to the structure, T means the thermodynamics temperature, R means the gas constant, Q stands for the creep activation energy, n means the stress exponent, and σA represents the applied stress. The strain rate of the alloys during steady state creep can be calculated according to the data in
After the heat treatment, the cuboidal γ′ phase in Ni-based alloys is embedded into the γ matrix and arranged regularly along the <100> direction, and the alloy morphology of (001) plane is shown in

Fig.2 Microstructure of (001) plane in Ni-based alloys after heat treatment
After the creep and rupture at 1040 °C/137 MPa, the sur-face morphologies of different areas of Ni-based alloys are displayed in

Fig.3 Schematic diagram (a) and surface morphologies (b~e) of different areas of Ni-based alloys after creep for 556 h and rupture at 1040 °C/137 MPa: (b) area B, (c) area C, (d) area D, and (e) area E in Fig.3a
In the area D in
The morphologies of Ni-based alloys after creep at 1040 °C/137 MPa for different durations are displayed in

Fig.4 Morphologies of Ni-based alloys after creep at 1040 °C/137 MPa for 10 h (a), 50 h (b), and 300 h (c)
The microstructure of Ni-based alloys after creep at 1040 °C/137 MPa for 350 h is displayed in

Fig.5 Morphology of Ni-based alloy after creep at 1040 °C/137 MPa for 350 h
Based on the analysis, it is considered that the dislocations in γ matrix glide to the interface to react with the dislocation networks, which may alter the direction of dislocation movemen
After creep at 1040 °C/137 MPa for 556 h and fracture, the microstructures in different areas of Ni-based alloys are shown in

Fig.6 Deformation morphologies of different areas of Ni-based alloys after creep at 1040 °C/137 MPa for 556 h and fracture: (a) far away from fracture; (b) beside fracture; (c) other area near fracture
The dislocation configurations of Ni-based alloys after creep and rupture at 1040 °C/137 MPa are shown in

Fig.7 Dislocation configurations of Ni-based alloys after creep at 1040 °C/137 MPa and rupture based on different operating vectors: (a) g=[], B=[110]; (b) g=[], B=[110]; (c) g=[020], B=[100]; (d) g=[], B=[110]
Because the γ and γ′ phases in the single crystal alloy possess the face centered cubic (fcc) structure, the activated dislocations during creep firstly glide into the {111} plane. When the dislocation glide on {111} plane is hindered, the cross-slip dislocations from {111} plane to {100} plane occur to form the K-W locks with non-plane core structure. Consequently, the dislocations in (100) plane of Ni-based alloys originate from the cross-slip dislocations, and the K-W locks may restrain the dislocation glide and cross-slip dislocation to increase the creep resistance of alloys.
Furthermore, the dislocation A displays the double line contrast due to its decomposition, and the anti-phase boundary (APB) can be observed. The decomposition reaction of dislocation A is [011]→(1/2)[011]+(APB)(100)+(1/2)[011].
The dislocation B can be observed under the operating vectors of g=[020] and g=[], as shown in Fig.
Microstructure observation indicates that the deformation mechanisms in the late creep period are the dislocation glide in γ matrix and the γ′ rafts sheared by dislocations. With proceeding the creep process, the γ and γ′ phases are alternately sheared by the initial/secondary dislocation glide due to the orientation difference. The γ and γ′ phases are firstly sheared by the initial gliding dislocation, and then sheared by the activated secondary gliding dislocation, which leads to the distortion of γ′/γ rafts. Meanwhile, the strain of alloys is increased with proceeding the creep process to aggravate the distortion of γ' rafts, which may promote the initiation and expansion of cracks along the γ′/γ interfaces until fracture. These phenomena are the deformation and damage features of Ni-based alloys in the late creep stage.
After creep at 1040 °C/137 MPa and rupture, the morpho-logies of the crack are shown in

Fig.8 Morphologies of Ni-based alloys after creep at 1040 °C/137 MPa for 556 h and rupture in the area beside fracture zone: (a) crack initiation, (b) crack propagation, and (c) crack expansion
In the late creep period, the micro-cracks are formed in the interfaces of γ/γ′ rafts. With proceeding the creep process, the micro-cracks in other areas near the fracture are expanded in γ/γ′ interfaces along the direction which is vertical to the stress axis, as shown in the area B of
Based on the microstructure observation, the γ′ phase being sheared by dislocations is considered as the deformation mechanism of Ni-based alloy in the late creep period. The initial and secondary dislocation glides cause the distortion of γ'/γ rafts, which promotes the crack initiation and propagation along γ/γ′ interfaces. The schematic diagrams of the initial/secondary dislocation glide on the (100) plane are shown in

Fig.9 Schematic diagrams of initial (a) and secondary (b) dislocation glides for crack initiation and propagation; schematic diagram of crack (c)
The dark zones in
With further proceeding the creep process, the micro-cracks are formed by accumulation of micro-holes, leading to the crack propagation along the interface. As shown in
Because the stability of the crack formation is related to the energy, the free energy change for forming the crack with length of 2c and displacement of a under the plane-strain condition can be expressed as follows:
(2) |
where μ stands for the shear modulus, ν means the Poisson's ratio, η is the given surface energy, L is the average spacing between the two γ' strengthening phases, and σ represents the applied stress. Based on ∂∆G/∂c=0, the minimum length c, namely critical size of crack cc, of the stable crack is determined as follows:
(3) |
where α is force constant. Because the distortion of γ′/γ rafts and the crack initiation are attributed to the cross-slip dislocations, based on
(4) |
According to
1) The creep life of the Re-free single crystal Ni-based alloys at 1040 °C/137 MPa is 556 h, displaying the excellent creep resistance.
2) The creep feature of Ni-based alloys during the steady state creep is the dislocation glide in γ phase and dislocation climb over the γ′ rafts. The deformation mechanism in the late creep period of Ni-based alloys at 1040 °C is that the γ′ rafts are sheared by dislocations which may cause cross-slip dislocation to form the Kear-Wilsdorf locks. The anti-phase boundary originating from the dislocations decomposes, leading to the hindrance of dislocation glide and cross-slip dislocation, thereby enhancing the creep resistance of Ni-based alloys.
3) In the late creep stage, the cross-slip dislocation causes the distortion of γ/γ′ rafts, promoting the crack initiation and propagation along interfaces, which is also the damage and fracture features of Ni-based alloys in the late period of creep. The condition of σ>η/α is the prerequisite of unstable crack propagation.
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