Very High Cycle Fatigue Behavior of a Directionally Solidified Ni-Base Superalloy DZ4

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 MPam, 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.

Very High Cycle Fatigue of Ni-Based Single-Crystal Superalloys at High Temperature

“…This is in agreement with VHCF results obtained at room temperature on SX [38] and DS alloys. [39] It appears that the smaller the pore size is, the higher the VHCF life [see Figure 3(b) focused on AM1 and MCNG alloys], which is in agreement with previous studies done in the LCF [18,20,21] and HCF [18,19] regimes when environment has (almost) no impact. Since the casting pore size in the specimen volume (and more specifically, the tail of the pore size distribution toward larger sizes) is directly connected to the solidification process, [22] then the LMC solidification process shows a high improvement in the VHCF life in comparison to the Bridgman one.…”

“…When porosity is closed by the addition, before and/ or in the meantime of solutionning, of a HIP treatment, VHCF lifetime is highly improved, at least by a factor of ten [see Figure 3(b)]. These observations were already done in the VHCF regime at ambient temperature [39] and the LCF regime at 750°C, where it is thought that HIP treatment reduces plastic deformation. [40] Then, other metallurgical defects act as stress concentration sites and will be presented in the next section.…”

“…LCF results from Steuer et al study on AM1 (Bridgman and LMC) at 750°C [21] have been added in the graph, as well as VHCF results from Nie et al on the DS DZ4 alloy at room temperature. [39] This diagram shows that the FIP values fall onto one single power-law master curve whatever the alloy, temperature, and fatigue regime (i.e., LCF vs VHCF). This power-law relationship is described in Eq.…”

“…Crack initiation from the surface has not been observed for all SX and DS alloys failed between 10 [21,39] (Color figure online). and 10 9 cycles.…”

Effect of Basketweave Microstructure on Very High Cycle Fatigue Behavior of TC21 Titanium Alloy

“…Throughout their ultra-long time service, the components are subjected to cyclic load with high frequency and low amplitude, thus the very high cycle fatigue (VHCF) of titanium alloy has been drawing world wide attention [2,3]. Fatigue failure in high strength titanium alloys occurs beyond the conventional fatigue limit of 10 7 cycles, and the stress-number of cycles to failure (S-N) curves exhibit a step-wise shape [2] or a decreasing shape [4].…”