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

Yukun Chang,Xiangnan Pan,Liang Zheng,Youshi Hong

doi:10.3221/igf-esis.49.01

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 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.

Highlights

Fatigue failure of engineering materials and structures bearing cyclic loading beyond 107 cycles, i.e. very-high-cycle fatigue (VHCF), may still happen in practical industrial applications [1,2,3,4]
The transmission electron microscopy (TEM) sample was prepared by focused ion beam (FIB) technique, and their results showed that the fine granular layer was observed in fine-granular area (FGA) region, whereas the fine polygonization was not observed in the location away from the FGA surface
In order to investigate the formation mechanism of FGA more deeply and comprehensively, Hong et al [16] first performed fatigue tests under different stress ratios via rotary bending (RB) and ultrasonic axial (UL) loading for two high-strength steels, the profile samples were prepared by FIB at the characteristic region of crack initiation of failed specimens, and subsequently the microstructure of the samples were examined by TEM

Summary

Introduction

Fatigue failure of engineering materials and structures bearing cyclic loading beyond 107 cycles, i.e. very-high-cycle fatigue (VHCF), may still happen in practical industrial applications [1,2,3,4]. In order to investigate the formation mechanism of FGA more deeply and comprehensively, Hong et al [16] first performed fatigue tests under different stress ratios via rotary bending (RB) and ultrasonic axial (UL) loading for two high-strength steels, the profile samples were prepared by FIB at the characteristic region of crack initiation of failed specimens, and subsequently the microstructure of the samples were examined by TEM Their observations revealed the existence of the thin nanograin layer of FGA under negative stress ratios, whereas the morphology of FGA was diminishing or even extinguishing under positive stress ratios without the evidence of nanograin feature. There was no evident grain refinement in CIR for the cases of R > 0 and the FiE region outside CIR for either negative or positive R cases, suggesting that the formation of nanograins in the FGA region is due to the NCP process and the plastic deformation ahead of crack tip may cause certain extent of microstructure deformation but is insufficient to form nanograin layer on crack surfaces

Methods

Results

Conclusion