Revealing fracture mechanisms of medium manganese steels with and without delta-ferrite

Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, delta-ferrite and alpha'-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (alpha) plus austenite (gamma) and the other one of a layered structure with an austenite-ferrite constituent and delta-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced alpha'-martensite (alpha') interfaces. In contrast, the fracture surface of delta-ferrite containing steels shows a combination of cleavage in delta-ferrite and dimple/quasi-cleavage zones in the gamma-alpha (or gamma/alpha'-alpha) constituent. The embrittlement of delta-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to delta-ferrite and crack-arresting ability of the gamma-alpha layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing delta-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of delta-ferrite containing steels increases from similar to 13% to similar to 33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and delta-ferrite. Crown Copyright (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc. All rights reserved.