| Abstrakt: |
Extremely low cycle fatigue tests, up to a total axial strain amplitude of 10%, were conducted on Fe-15Mn-10Cr-8Ni-4Si bidirectional transformation-induced plasticity (B-TRIP) steel. The fatigue life was approximately five times longer than that of SUS316 when the total strain amplitude was 4% or higher. The improved fatigue life of Fe-15Mn-10Cr-8Ni-4Si was attributed to reversible bidirectional γ ↔ ε transformation during fatigue deformation, which might mitigate fatigue damage. In contrast, the fatigue life tended to decrease with increasing strain rate when the strain rate was varied from 0.1 to 2.5%/s with a total strain amplitude of 10%. Fractography revealed that the fracture surface varied significantly with strain rate. At low strain rates, crystallographic fracture surfaces characterized by facets and secondary cracks were observed, whereas these features were not observed at high strain rates. Electron backscatter diffraction measurements of the postmortem microstructure showed that frequent ε-martensite formation occurred at low strain rates, whereas martensitic transformation was suppressed at high strain rates. The change in the specimen surface temperature was evaluated in terms of the Gibbs free energy difference between γ-austenite and ε-martensite (i.e.,ΔGγ↔ε), and the effect of strain rate on the extremely low cycle fatigue was discussed from the viewpoint of the deformation mechanism. At a low strain rate, the condition for B-TRIP to work effectively, that is, ΔGγ↔ε is negative but close to zero, was maintained over the entire life span. At a high strain rate, the deformation mechanism changed to one in which γ-austenite was dominant because of the increase in ΔGγ↔ε caused by self-heating; the fatigue damage mitigation mechanism provided by B-TRIP was less likely to occur at high strain rates, which reduced the fatigue life. [ABSTRACT FROM AUTHOR] |
