In the present study, high-cycle fatigue (HCF), thermal fatigue and tensile anisotropy behavior of SRR99 Ni-based single crystal superalloy were investigated. Based on the fractograph analysis and microstructure observation, the deformation and rupture mechanisms are discussed.
The single crystals with different orientation were cast by directional solidification process. Optical microscopy observation shows that single crystals were solidified in the form of dendrite. The growth direction was primarily along <001>. After heat treatment, the eutectic region was eliminated and the microstructure was homogenized.
Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700℃, 760℃, 850℃ and 900℃ in air atmosphere. The results demonstrate that conditional fatigue strength of smooth specimens reaches the maximum at 760℃ and decreases with increasing temperature. The SRR99 alloy became more notch sensitive with increase of temperature while the notch sensitivity slightly declines at 900℃. Analysis on fracture surface showed a trend of cleavage rupture at higher temperatures regardless the stress amplitude. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through scanning electron microscope (SEM) and transmission electron microscope (TEM) observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal ′ precipitates tended to dissolve into the matrix channels, meanwhile a large number of secondary ′ particles were formed in the matrix in specimens fatigue tested at 700℃. Interfacial dislocations were frequently presented in the specimens after high-cycle fatigue tests and the ′ phase sheared by single dislocation and stacking faults was observed occasionally.
Thermal fatigue tests were performed on the SRR99 superalloys. The experimental results showed that the crack growth rate increases with the rise of upper temperature. Optical microscopy (OM) and SEM observation reveals that multiple small cracks primarily initiated at the notch tip regardless of the temperature range. Subsequently, only one or two of the small cracks can continue to grow and finally form the main cracks. The primary crack generally propagated along a preferential direction, as a result of which the crack propagation behavior became insensitive to dendrite orientation. Crack growth behavior varies distinctly in different alloys. In general, thermal fatigue cracks propagate along a preferential direction with the dendrite growth direction in single crystal superalloy, however crack in Mar-M002 alloy propagated generally along the interdendritic region.
Single crystal specimens of a nickel-base superalloy SRR99 with orientation near <001>, <011> and <111> were studied in tension test at the temperature range from 20 to 1100℃. The experimental results show that the alloy has a peak temperature at which the yield strength reaches the maximum. Moreover, the yield strength and elastic modulus have an orientation dependence at all tested temperatures. Based on the TEM observation, the primary deformation mechanism was characterized by the matrix dislocation propagation on the octahedral slip systems and shearing of particles by dislocation pairs, loops and superlattice stacking faults. Meanwhile, the cross-slip of screw dislocations within was observed, which was thought to account for the tensile anisotropy. Fracture behavior was investigated by OM and SEM. It is of interest to note that the fracture surface in the three orientation specimens tested at the intermediate temperature belongs to {111} crystal plane.
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