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Abstract
In this study, we investigated the creep deformation mechanism of a single-crystal high-entropy superalloy (HESA) with the spherical γ′ precipitates at 760 °C. Before the creep tests, long-term aging tests at 760 °C without load were conducted, which showed Ostwald ripening of the secondary γ′ precipitates up to 50 h. The creep tests revealed that in the range of 500 and 600 MPa at 760 °C, the creep deformation mechanism of HESA was independent of applied stress in both the primary and secondary creep regions. The deformation mechanism of HESA was further investigated under the condition of 760 °C and 520 MPa by performing creep interrupted tests and microstructural analysis. Scanning electron microscope observation showed elongated γ′ precipitates along the applied stress axis near the ruptured surface. This could have been caused by the multi-slip around <100> preceded by the lattice rotation into <100> along the tensile axis, which was confirmed by the electron backscatter diffraction analysis. Transmission electron microscope observation of the creep interrupted and ruptured specimens showed bypass and climb motion of dislocations in the 2-h interrupted, shearing of the γ′ precipitates by the paired straight dislocations in the 50-h interrupted, and shearing of the γ′ precipitates by both the straight and the curved paired dislocations in the ruptured specimens, respectively. The secondary γ′ precipitates do not affect creep behavior as long as the deformation mechanism is a bypass and climb motion of dislocations.
Highlights
Introduction nal affiliationsThe novel alloy design concept of “high-entropy” alloys (HEAs) offers the composition space of a single phase for alloy developments [1,2]
After the disappearance of the coarsened secondary γ0 precipitates, much finer secondary γ0 precipitates with uniform distribution were discerned in Figure 3g–h indicating that these secondary γ0 precipitates form during cooling from the aging treatment at 760 ◦ C
This Ostwald ripening in the secondary γ0 precipitates should be active in the creep tests
Summary
The novel alloy design concept of “high-entropy” alloys (HEAs) offers the composition space of a single phase for alloy developments [1,2]. HEAs with a single phase of face-centered cubic (FCC) structure have been reported to show excellent mechanical properties at cryogenic temperatures among the other alloys with a single phase of FCC structure [3,4,5]. To explain the deformation mechanism realizing the superior mechanical properties, many experiments and simulations have been reported. FCC single phase have not shown sufficient mechanical properties for actual use at higher temperatures [6]. To enhance mechanical properties at higher temperatures, the concept of high-entropy superalloys (HESAs) has been proposed from the composition space of HEAs [7]. The microstructure of HESAs consists of γ0 precipitates with an L12 structure
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