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Extremely low-cycle fatigue tests were conducted on the Fe-15Mn-10Cr-8Ni-4Si bidirectional-TRIP (B-TRIP) steel, up to an axial total strain amplitude of 10%. The fatigue life was about five times longer than that of SUS316 in the range of total strain amplitude of 4% or more. The improved fatigue life of the Fe-15Mn-10Cr-8Ni-4Si was attributed to the reversible bidirectional γ↔ε transformation during fatigue deformation that might mitigate the fatigue damage. On the other hand, the fatigue life tended to decrease with increasing strain rate when the strain rate was varied from 0.1 to 2.5%/s with the total strain amplitude of 10%. The fractography revealed that the fracture surface varied significantly with strain rate. At low strain rates, the crystallographic fracture surface characterized by facets and secondary cracks were observed, whereas these features were not observed at high strain rates. EBSD measurements on 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 ΔGγ→ε, and the effect of strain rate on the extremely low-cycle fatigue was discussed from the viewpoint of the deformation mechanism as follows. At low strain rate, ΔGγ→ε ≲ 0 (negative close-to-zero ΔGγ→ε), the condition for B-TRIP to work effectively, is maintained over the entire life span. At high strain rate, the deformation mechanism changes to one in which γ-austenite is dominant due to the increase in ΔGγ→ε caused by self-heating; the fatigue damage mitigation mechanism by B-TRIP is less likely to occur at high strain rates, resulting in a reduction in life.

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• In Copyright
  

Keyword: Fatigue, Fracture surface, Extremely low cycle fatigue, Microstructure, Martensitic transformation, Seismic damper

Date published: 2023-12-15

Publisher: 公益社団法人 日本材料学会

Journal:

• 材料 vol. 72 issue. 12 p. 858-865

Funding:

• JSPS 23K13227 (可逆双方向TRIPを活用した疲労き裂制御による耐疲労合金創製)
• JSPS 20K04170 (大地震を模擬した高速ひずみ速度の極低サイクル繰り返し変形による疲労損傷の解明)
• JSPS 19K14853 (疲労損傷のマルチスケール的観察が結びつける転位-塑性変形-き裂関係)

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.2472/jsms.72.858

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Updated at: 2024-09-30 16:30:40 +0900

Published on MDR: 2024-09-30 16:30:40 +0900