Abstract
The effect of casting pores on the very high cycle fatigue (VHCF) behavior of a directionally solidified (DS) Ni-base superalloy DZ4 is investigated. Casting and hot isostatic pressing (HIP) specimens were subjected to very high cycle fatigue loading in an ambient atmosphere. The results demonstrated that the continuously descending S-N curves were exhibited for both the casting and HIP specimens. Due to the elimination of the casting pores, the HIP samples had better fatigue properties than the casting samples. The subsurface crack initiated from the casting pore in the casting specimens at low stress amplitudes, whereas fatigue crack initiated from crystallographic facet decohesion for the HIP specimens. When considering the casting pores as initial cracks, there exists a critical stress intensity threshold ranged from 1.1 to 1.3 , below which fatigue cracks may not initiate from the casting pores. Furthermore, the effect of the casting pores on the fatigue limit is estimated based on a modified El Haddad model, which is in good agreement with the experimental results. Fatigue life for both the casting and HIP specimens is well predicted using the Fatigue Indicator Parameter (FIP) model.
Highlights
Solidified (DS) Ni-based superalloys are served as aero-engines turbine blades due to their exceptional high resistance to creep, oxidation, and corrosion, as well as good fracture toughness
When the stress intensity factor (SIF) of the casting pore is lower than its threshold, fatigue crack may initiate from the casting defects, but may not propagate to fracture, or the crack can initiate from other materials defects, such as persist slip, boundary [17]
When considering that most of the total very high cycle fatigue (VHCF) fatigue life is consumed by the crack initiation stage, fatigue life of DZ4 alloy can be estimated using a Fatigue Indicator Parameter (FIP) model that stage, fatigue life of DZ4 alloy can be estimated using a Fatigue Indicator Parameter (FIP) model that is based on the stress intensity factor close to crack initiation sites
Summary
Solidified (DS) Ni-based superalloys are served as aero-engines turbine blades due to their exceptional high resistance to creep, oxidation, and corrosion, as well as good fracture toughness. The size and the distribution of the defects vary substantially for different casting samples, resulting in the large scatter of the fatigue data, which is remarkably higher in very high cycle fatigue region. It obviously limits the application of this alloy as cyclically loaded components for which VHCF fatigue strength is required. Very high cycle fatigue behavior of the casting DZ4 alloys, which was developed to be used in gas turbine engines due to the low cost, were investigated in comparison with the HIP ones. It is expected to quantitatively evaluate the influence of the casting defects on fatigue properties, as well as to clarify the crack initiation mechanism
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