(Top) We elucidated the mechanism of fatigue crack growth in a steel that exhibits ten-fold fatigue life compared to conventional steels. In this steel, highly reversible FCC/HCP transformation occurs as a cyclic plastic deformation mechanism, mitigating cyclic hardening at the crack tip. In addition, crystallographically faceted crack paths develop along specific crystallographic planes. These features make the cracks more difficult to open. As a result, the effective driving force for crack propagation is reduced, leading to a lower crack growth rate and consequently a longer fatigue life. We also found that an unusual HCP/BCC transformation occurs around the crack, which may contribute to fatigue life reduction. By suppressing the HCP/BCC transformation around fatigue cracks, we succeeded in further doubling the fatigue life.
(Bottom) We also developed a steel that combines excellent fatigue performance with weldability. By identifying a range of chemical composition that promotes a cyclic deformation mechanism capable of maintaining high fatigue durability while optimizing the solidification mode during welding, we developed a steel that avoids solidification cracking and exhibits approximately ten times longer fatigue life compared to conventional steels. In addition, we have successfully developed steels that combine high fatigue durability and weldability with improved corrosion resistance.
Currently, we are developing a fatigue damage characterization method that combines in situ observation of microcrack initiation and propagation with crystallographic orientation analysis, with the aim of experimentally establishing the relationship between dislocations, plastic deformation, and fatigue crack development.