Fatigue Behavior in Monocrystalline Ni-Based Superalloys for Blade Applications

Abstract

Ni-based superalloys are used for turbine airfoil applications due to their excellent high temperature properties. Alloy design has historically focused on creep resistance as the critical design limiting parameter. Recently, the focus has shifted to include fatigue resistance, resulting in the need to better understand the effects of alloy microstructure on fatigue crack initiation and propagation. In this study, sustained peak low cycle fatigue tests of coated monocrystalline René N5 specimens oriented with the loading axis along [001] were conducted in strain control in the temperature range of 980-1100°C. Tests were performed with either a tensile dwell (hold) time or a compressive dwell time. Fracture surfaces were characterized via scanning electron microscopy (SEM). Microstructural analysis was performed using a high resolution SEM. Far field deformation mechanisms were examined perpendicular to the loading axis and parallel to the loading axis using transmission electron microscopy. The occurrence of creep-fatigue interaction was characterized by the different types of rafting seen, and the deformation mechanisms varied between specimens with compressive hold time and tensile hold time. This project is a collaborative effort with GE Aviation and is funded by the AFOSR under the MEANS2 program.

Crack Progression during Sustained-Peak Low-Cycle Fatigue in Single-Crystal Ni-Base Superalloy René N5

“…The failure crack progression on (001) is much deeper than the total thickness of the coating and interdiffusion layers, which is about 100 lm. Similar fracture surfaces were observed after compressive dwell fatigue tests at 1253 to 1373 K (980 to 1100°C), [7] and the features are consistent with those observed at high temperature and low frequency. [1] At lower temperatures, cracks initiate and propagate on {111} crystallographic plane along deformation twins.…”

“…The deformation behavior depends on temperature, frequency, and environment. [1][2][3][4][5][6][7][8] It has been reported that creep deformation plays a significant role at higher temperatures, above about 1223 K (950°C), due to the large amount of the stress relaxation during the hold. At lower temperatures, formation of shear bands and twinning after crack initiation causes the failure.…”

“…[1][2][3][4][5][6][7][8] In this article, we will refer to isothermal fatigue with hold time at peak stress(es) as SPLCF and nonisothermal fatigue with or without hold time at peak stress(es) as TMF. The deformation behavior depends on temperature, frequency, and environment.…”

Cyclic Dwell Fatigue Behavior of Single Crystal Ni-base Superalloys With/Without Rhenium

“…Yablinsky and colleagues [10] have reportedly observed similar discontinuous p-type rafts in René N5 single crystals tested under zero-compression dwell fatigue at 1100°C. It has been established from previous studies that the formation of p-type rafts occurs in alloys with a negative -' misfit, when subjected to a compressive stress applied along [001].…”

“…Rafting of the -' microstructure has been primarily studied under constant load creep conditions [13][14][15]. Rafting can also occur under TMF loading conditions [16] or during strain-controlled isothermal fatigue tests incorporating tensile or compressive dwell periods [10]. Progressive ' coarsening, whether directional or isotropic in nature, can result in significant changes in the cyclic stress-strain response of the material and hence the fatigue failure mechanism [17].…”