Impact of ε-martensite on the fracture resistance of high-Mn steels1

The formation of ε-martensite is generally considered as detrimental to cracking resistance. The essential work of fracture (EWF) of high-Mn steels with ε-martensite has been determined in the near plane-stress regime. The analysis of the thickness dependence of EWF shows that an alloy with more ε-martensite involves a higher work of material separation and a lower contribution from crack-tip necking. Therefore, the presence of ε-martensite leads to a lower cracking resistance at large thickness, but to an enhanced fracture toughness at small thickness. The beneficial effect of ε-martensite to the work of fracture is explained by an increased strength without any reduction of void spacing, and related to the ε-martensite/austenite plastic co-deformation and to microcrack blunting and crack branching. The key finding of the study is thus that the best compromise depends on the structural application which imposes the strength and stiffness requirements, hence the thickness.