Original ArticlesTension/compression asymmetry in creep behavior of a Ni-based superalloy
Introduction
Orientation and temperature dependence of yield stress or CRSS (Critical Resolved Shear Stress) and tension/compression anisotropy of the yield stress or CRSS have been shown by Shah and Duhl (1), Heredia and Pope (2), and Miner et al. (3). Tension/compression asymmetry in the yield strength of Ni-based superalloys has been explained in terms of the “core width effect” (4). Shah and Duhl (5) observed the tension/compression asymmetry in creep deformation, which is similar to that observed in the yield strength, and indicated that it can be attributed to cross slip and dislocation core-constriction mechanisms associated with octahedral slip. However, little is known about the mechanism of tension/compression asymmetry in creep.
In the present study, single crystals of a Ni-base superalloy were subjected to tensile and compressive creep tests. Tension/compression asymmetry in creep behavior was examined in detail for each orientation.
Section snippets
Experimental procedure
The investigation was carried out on an experimental Ni-based superalloy. Its chemical composition is listed in Table 1. Single crystals of this alloy were grown from a melt by the modified Bridgman method. After the analysis of the crystallographic orientation by the back-reflection Laue method, the tensile and compressive specimens for creep tests were cut from the as-grown crystals by a spark cutter. The specimens were solutionized at 1255°C for 10 h aged at 1100°C for 10 h. They were then
Results and discussion
The mean edge length of primary cuboidal γ′ precipitates was 0.40 μm and the volume fraction was 60% following the aging treatment. As shown in Figure 1, superfine secondary precipitates were observed in the matrix channel. Figure 2 shows the tensile and compressive creep curves for each orientation. In the Ni-based superalloy tested in this study, {111}<112>-type slip systems were found to be operative during creep at 700°C (6); therefore, the shear strain for {111}<112>-type slip systems
Conclusion
Tension/compression asymmetry of creep strength in [001] and [011] orientations is attributed to the directional-formation characteristic of a mechanical twin. In [11] specimens, creep strain is created by dislocation motions in the matrix channel; therefore, tension/compression asymmetry of creep curves was not observed.
References (17)
- et al.
Acta Metall.
(1986) - et al.
Scripta Metall.
(1987) - et al.
Acta Metall. Mater.
(1996) - et al.
Scripta Metall. Mater.
(1994) - et al.
Scripta Mater
(1999) - et al.
- et al.
Metall. Trans. A.
(1986) - et al.
Metall. Trans. A.
(1979)