2018 年 104 巻 7 号 p. 393-399
Fe-30Mn-4Si-2Al alloy (mass%) was reported to show excellent low cycle fatigue properties. We investigated fatigue characteristics of the Fe-30Mn-4Si-2Al alloy as a function of accumulative plastic strains, comparing with the low cycle fatigue test results of Fe-28Mn-6Si-Cr-0.5Nb C alloy and SUS304 steel. The obtained results are shown below. The fatigue life of Fe-30Mn-4Si-2Al alloy is the longest in all the strain ranges as compared with Fe-28Mn-6Si-5Cr-0.5NbC alloy and the SUS 304 steel. In particular, it has a long life in test of high strain amplitude. The εpa - Nf characteristics of Fe-30Mn-4Si-2Al alloy show a straight relationship (εpa = Cp/NfKp). The result that the Manson-Coffin rule holds was obtained. In addition, Cp = 5.62, Kp = 0.72, which is an extremely high value. The fatigue damage value D obtained from the Manson-Coffin equation of Fe-30Mn-4Si-2A alloy was almost 1, similar to Fe-28Mn-6Si-5Cr-0.5NbC alloy or SUS 304 steel. However, the relationship between the accumulative plastic strain λp and fatigue life N is much higher than the limit λp of Fe-28Mn-6Si-5Cr-0.5NbC alloy obtained in the previous report. In particular, the results of εta = 2.0% and 1.4% were 20 times the limit λp. It was found that the excellent low cycle fatigue life of Fe-30Mn-4Si-2Al alloy is caused by the much slower accumulation of plastic strain and the extremely high values of Cp and Kp. The above results show that the repetitive motion of partial dislocation progresses slowly as ε martensite repeats normal and reverse transformation and the developmental process of repeated deformed tissue and fatigue crack propagates in a zigzag along γ/ε interface. As a result, it agrees with previously reported fact that crack growth is suppressed.
