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, 8 (1), 1452

Improvement in Creep Life of a Nickel-Based Single-Crystal Superalloy via Composition Homogeneity on the Multiscales by Magnetic-Field-Assisted Directional Solidification

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Improvement in Creep Life of a Nickel-Based Single-Crystal Superalloy via Composition Homogeneity on the Multiscales by Magnetic-Field-Assisted Directional Solidification

Weili Ren et al. Sci Rep.

Abstract

The improvement of the creep properties of single-crystal superalloys is always strongly motivated by the vast growing demand from the aviation, aerospace, and gas engine. In this study, a static magnetic-field-assisted solidification process significantly improves the creep life of single-crystal superalloys. The mechanism originates from an increase in the composition homogeneity on the multiscales, which further decreases the lattice misfit of γ/γ' phases and affects the phase precipitation. The phase-precipitation change is reflected as the decrease in the γ' size and the contents of carbides and γ/γ' eutectic, which can be further verified by the variation of the cracks number and raft thickness near the fracture surface. The variation of element partition decreases the dislocation quantity within the γ/γ' phases of the samples during the crept deformation. Though the magnetic field in the study destroys the single-crystal integrity, it does not offset the benefits from the compositional homogeneity. The proposed means shows a great potential application in industry owing to its easy implement. The uncovered mechanism provides a guideline for controlling microstructures and mechanical properties of alloys with multiple components and multiple phases using a magnetic field.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1
The structure illustration of the single-crystal nickel-based superalloy.
Figure 2
Figure 2
Creep behaviors of the single-crystal nickel-based superalloy at 180 MPa/980 °C. (a~c) The creep strain as a function of time with the different withdraw velocities (15, 20, and 25 μm/s) and magnetic-field intensities. (d) Creep life and (e) elongation with the magnetic-field intensity.
Figure 3
Figure 3
Solute distribution of alloying elements along the longitudinal section. The alloying elements are marked at the top left corner in each subgraph. The samples were solidified at 25 μm/s.
Figure 4
Figure 4
Microsegregation coefficient of alloying elements on the dendrite scale. The samples were solidified at 25 μm/s.
Figure 5
Figure 5
Atom-probe tomography (APT) analysis. The left-column graphs show each atom map of the atomic distribution within the γ/γ′ phase. The right-column graphs present the proximity histogram of the atom distribution across the γ/γ′ interface and within the γ/γ′ phases in the single-crystal nickel-based superalloy.
Figure 6
Figure 6
Two-dimensional iso-intensity mappings on the reciprocal space for the investigated superalloys. (a) 0 T. (b) 0.5 T. (c) 1 T. (d) 1.5 T. The line end within the iso-intensity mappings indicates the positions of γ and γ′ phases, according to which the lattice misfit was determined. The detailed procedure is stated in ref..
Figure 7
Figure 7
Precipitation phases in the superalloys prepared at 25 μm/s. (a1~a5) γ′ precipitation at the dendrite core. (b1~b5) carbides. (c1~c5) γ/γ′ eutectic phases (indicated with the green line by hand for the clear presentation).
Figure 8
Figure 8
Dendrite morphology and orientation. (a1~a4) The dendrite morphology. (a5) The primary dendrite-arm spacing. (b1~b5) The dendrite orientation characterized by EBSD photos and the inverse pole figures. The samples were solidified at 25 μm/s. The magnetic-field-assisted solidification destroys the single-crystal integrity, as indicated by an increase in column grains (marked by the yellow dash line).
Figure 9
Figure 9
Fractography and longitudinal microstructures near fracture surface in the samples crept at 980 °C/250 MPa (solidified at 25 μm/s). (a) 0 T. (b) 0.5 T. (c) 1 T. (d) 1.5 T. (e) 2 T. The enlarged areas in the (a) and (b) showed that the cracks initiate around the MC carbide and eutectic phases.
Figure 10
Figure 10
Rafting structures near fracture surface in the samples crept at 980 °C/250 MPa (solidified at 25 μm/s). (a) 0 T. (b) 0.5 T. (c) 1 T. (d) 1.5 T. (e) 2 T. The numbers 1 and 2 show the positions of 1 mm and 3 mm away from the fracture surface, respectively.
Figure 11
Figure 11
Dislocation characteristic in the samples crept at 980 °C/250 MPa (solidified at 25 μm/s). In the case of 0 T, there is dislocation gliding in the γ channel (a), dislocation cutting in the γ′ phase (b), and regular (c) and irregular (d) dislocation network at the γ/γ′ interface and in the γ channel. In the samples prepared under 0.5 T (e), 1 T (f), 1.5 T (g), and 2 T (h), the dislocation accumulated at the γ/γ′ interface and cut in the γ′ phase.
Figure 12
Figure 12
The effect of the magnetic field on the solute distribution in the Ni-40.38 wt.% Cu alloy. (a) 0 T. (b) 0.5 T. (c) 1 T. (d) 1.5 T. (e) 2 T.
Figure 13
Figure 13
The illustration for the affected zone of the multiple effects from the static magnetic field and their influence on the structure and solute distribution. It should be emphasized that the TEMF in the just-crystalized solid are not strong enough to modify the liquid-solid interface shape and the dendrite morphologies in the present study.

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