Further investigation on microstructure refinement of internal crack initiation region in VHCF regime of high-strength steels

Abstract

The profile samples prepared by focused ion beam (FIB) in crack initiation region (CIR) and fish-eye (FiE) region of failed specimens subjected to rotary bending (RB) and ultrasonic axial (UL) fatigue testing with various stress ratios (R) were observed by transmission electron microscopy (TEM) with selected area electron diffraction (SAD) detection for two high-strength steels. The grain size and the thickness of nanograin layer along the crack growth path in CIR underneath fine-granular-area (FGA) were measured for the cases of R < 0, and a normalized quantity d* based on the detected SAD patterns was introduced to quantitatively demonstrate the variation of the grain size. The results showed that the nanograin size near the origin (an inclusion) of crack initiation is smaller than that away from the inclusion. Nevertheless, there was no evidence of grain refinement in CIR for the cases of R > 0 and the FiE region outside CIR for either negative or positive stress ratio cases, which suggests that the formation of nanograin layer in the FGA region is due to the numerous cyclic pressing (NCP) process and the plastic deformation ahead of the crack tip may cause certain extent of microstructure deformation but is insufficient to form nanograin layer on crack surfaces.

Effect of basal texture on small crack behavior of magnesium alloy in very high cycle fatigue regime

“…The electron diffraction pattern of the layer is shown in the inset of Figure 9D and shows a typical appearance of concentric rings for polycrystalline materials. Similar fine-grained microstructure at the crack surface has also been reported in previous investigations in the VHCF range, 26,27 and its formation is supposed to be a result of numerous cyclic compressions of crack surface after crack initiation. The crack surface contacts and compresses with each other, and the materials near the crack surface undergo a large plastic deformation, which leads to the process of grain refinement.…”

Relationship between the microstructure and the heat treatment and creep behavior of Fe–33Ni–19Cr alloy

“…2 A large number of triple junctions in fine-grained specimens can limit crack propagation. [36][37][38][39][40][41][42] These results agreed with the critical length criterion for intergranular creep rupture 43 and are found to be applicable in Ni-based alloy 32,[44][45][46][47] and 316 stainless steel. 12,44 The percentage elongation results, which are the ductility, were slightly affected by the testing temperature.…”