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Dislocation Network With Pair-Coupling Structure in {111} γ/γ' Interface of Ni-based Single Crystal Superalloy

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Dislocation Network With Pair-Coupling Structure in {111} γ/γ' Interface of Ni-based Single Crystal Superalloy

Yi Ru et al. Sci Rep.

Abstract

The γ/γ' interface dislocation network is reported to improve the high temperature creep resistance of single crystal superalloys and is usually found to deposit in {001} interface. In this work, a new type of dislocation network was found in {111} γ/γ' interface at a single crystal model superalloy crept at 1100 °C/100 MPa. The dislocations in the network are screw with Burgers vectors of 1/2 a<110> and most interestingly, they exhibit a pair-coupling structure. Further investigation indicates that the formation of {111} interface dislocation network occurs when the γ' raft structure begins to degrade by the dislocations cutting into the rafted γ' through the interface. In this condition, the pair-coupling structure is established by the dislocations gliding in a single {111} plane of γ', in order to remove the anti-phase boundary in γ'; these dislocations also act as diffusion channels for dissolving of the γ' particle that is unstable under the interfacial stress from lattice misfit, which leads to the formation of {111}-type zigzag interface. The formation of this network arises as a consequence of more negative misfit, low-alloying γ' particle and proper test conditions of temperature and stress.

Figures

Figure 1
Figure 1. Observations of the {111} interfaces and associated dislocation network in the specimen ruptured at 1100 °C/100 MPa.
(a) Observation of {111} interfaces with the electron beam incidence parallel to [110] (B//[110]); (b) TEM and High-Resolution TEM images of (formula image) interfaces; (c) TEM image of the (111) interface dislocation network composed of coupled dislocations; the bright-field TEM images of the network (d) using g = 002 and (e) using g = formula image; (f) Burgers vectors of the dislocations in the network, showing all the dislocations are screw.
Figure 2
Figure 2. TEM images of {111} interface dislocation network that is accompanied with the squared network.
(a) Dislocation density of both types of networks was similar in spite of different configuration; (b) the dislocations configuration transformation from {001} interface network to {111} ones.
Figure 3
Figure 3. Observation of the pair-coupling structure and its forming process.
(a) Coupled dislocations in {111} interface network and ones cutting into γ′, both with similar configuration; (b) forming process of the pair-coupling structure in the network.
Figure 4
Figure 4. TEM observations of the γ/γ′ interfaces and dislocation configurations deposited in the interface in different testing conditions.
Microstructure images of the specimens ruptured at (a) 1100 °C/100 MPa and (b) 1100 °C/130 MPa, showing the presence of {111} interface dislocation network and the pair-coupling dislocation structure; the network with coupled dislocations was not found in the specimen ruptured at (c) 980 °C/250 MPa; (d) after isothermally exposed of 1100 °C/100 h, the formation of {111} interface was associated with the dislocations moving in the interface.
Figure 5
Figure 5. Model for the formation of the {111} interface dislocation network.
Formation of {111} interface dislocation network occurs when the dislocations start to cut into the rafted γ′ through the {001} interface (see Stage I). The dislocations gliding in a {111} plane of γ′ phase become coupled to remove the APB (see Stage i→iii). The γ′ forming elements near to these dislocations continue to dissolve under the stress field induced by more negative misfit (see Stage II). Eventually in the Stage III, the network with coupled dislocations is completed in the zigzag {111} interface.

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