Mixed type brittle fracture in 1.5 GPa dual-phase steel via {100} ferrite cleavage cracking

In the present work, we clarify the fracture mechanisms in a dual-phase (DP) steel consisting of-75% martensite and exhibiting tensile strength of 1.5 GPa. Generally, the DP steels are considered resistant to brittle fracture due to the crack-arresting behavior of the ferrite. However, contrary to conventional wisdom, we report a pre-dominant brittle fracture in the DP steel and clarify the associated damage mechanisms. Crack initiation occurs via martensite cracking along the prior austenite grain boundaries. The crack propagation in ferrite primarily occurs via {100} brittle cleavage cracking. Occasional void coalescence is also observed in ferrite. The brittle fracture observed in martensite is a mixed mode fracture consisting of both intergranular fracture and trans -granular cleavage cracking along {110} planes. Furthermore, it is observed that the morphology of the martensite crack (sharp and blunt crack) influences ferrite cleavage cracking. The ferrite damage mechanisms associated with the sharp and blunt crack have been elucidated. Finally, the micro mechanism for the occasional ductile ferrite fracture has been explained.