Effects of alloys elements, temperature, and strain rate on the microstructure and low temperature deformation behavior of cryogenic high-Mn alloys

Abstract

Low temperature deformation behaviors of five cryogenic high-Mn alloys CAM-1 (Fe-30Mn-5Al-0.3C), CAM-2(Fe-25Mn-5Al-5Ni-0.3C), Cl(Fe-30Mn-1.2Al-0.1C), C3(Fe-30Mn-1.2Al-0.3C), and Fe-27Mn binary alloy [wt.\%] have been investigated. Austenitic high-Mn alloys(CAM-1, CAM-2, Cl, and C3) show high strength and excellent ductility at low temperatures. Especially, CAM-1, CAM-2, and C1 alloys exhibit inverse elongation behaviors with decreasing temperature from room temperature to 77 K, which was attributed to gradual formation of strain-induced deformation twinning during tensile testing. The amount of deformation twins formed during plastic deformation was not the major factor for maximum elongation, but the optimum work hardening rate by the gradual formation of deformation twins played an important role. Alloy C3 shows a peak in elongation at the temperature between 233 K and 77K due to the optimum formation rate of deformation twinning (TRIP In the wide sense). Fe-27Mn binary alloy possessing the lower yield strength, compared to other austenitic high-Mn alloys, shows a high rate of work hardening in the plastic deformation region resulting in the high ultimate tensile strength. This Fe-27Mn alloy also exhibits a peak in elongation between RT and 77K. The peak elongation for the Fe-27Mn alloy was due to the rapid work hardening by the formation of $hcp \epsilon$ martensite. The volume fraction of $hcp \epsilon$ martensite formed in the Fe-27Mn alloy after tensile testing at 77 K was 86\%. Total strain-controlled low cycle fatigue testing of CAM-2 alloy showed that the fatigue resistance at 77 K was superior to that at room temperature in the entire fatigue life range tested. The reason exhibiting the longer fatigue life at 77 K than that at 298 K for the CAM-2 alloy was the significant increase in the fatigue ductility coefficient with decreasing temperature; $\epsilon_f^\prime$ increased from 23\% at RT to 54\% at 77 K. The increase in the fatigue ductility coeffi...