Effects of Ni and Cu addition on cryogenic-temperature tensile and Charpy impact properties in austenitic 22Mn-0.45C–1Al steels

Austenitic high-Mn steels present a dominant deformation mechanism of TWinning Induced Plasticity (TWIP) or TRansformation Induced Plasticity (TRIP), which effectively enables be used for various cryogenic applications. This mechanism affects significantly tensile or impact properties, and can be carefully tuned by adjusting alloying compositions; however, the alloying effects of Ni and Cu have not been studied yet. In the present study, the steels were fabricated by adding Ni or Cu and their microstructural evolutions were examined for the quasi-statically-tensioned and dynamically-Charpy-impacted specimens. In the room-temperature tensile deformation, many twins were populated without any martensite. At cryogenic temperature, however, ε- and α′-martensite were formed together with twins in the 22Mn-0.45C–1Al (Base) and 1-wt.%-Ni-added (1Ni) steels, whereas they were not in the 1-wt.%-Cu-added (1Cu) steel. This TWIP or TRIP amount showed a good correlation with the stacking fault energy (SFE) calculated by considering Mn-segregation bands, and affected the tensile ductility. At cryogenic temperature, the TRIP occurred in the Base and 1Ni steels as SFEs of low-Mn bands decreased down to the TRIP range, whereas it did not in the 1Cu steel, and thus the Base and 1Ni steels showed the lower ductility than the 1Cu steel. For the cryogenic-temperature Charpy impact test, the martensite was not formed even at the heavily-deformed area near the notch tip as the time was not enough for inducing the martensitic transformation, while twins were populated. Since the twin fraction near the notch-tip area increased in the order of the 1Cu, 1Ni, and Base steels, it was expected that the Charpy impact energy increased in this order, but the Base steel showed the lowest energy because of the precipitation of fine M23C6-type carbides on grain boundaries.