Effect of α‐Ferrite on Plasticity and Fracture Mechanisms of Dual‐Phase Ultrastrong Heterolamellar Steels

Lamellar heterostructure design is widely recognized as a potential method to improve strength and ductility simultaneously of high‐performance steel. However, the high mechanical contrast among soft/hard phase and inharmonic plastic deformation lead to complex fracture mechanisms and limit the improvement of plasticity. Herein, the plasticity and tensile fracture mechanisms of two types of δ‐ferrite and lath martensite heterolamellar steels with and without α‐ferrite are investigated. The sample without α‐ferrite shows a combination of cleavage in δ‐ferrite and dimple zones in the martensite. The microcracks initiate in the martensite early and the then the cleavage fracture occurs in the δ‐ferrite due to the stress concentration. In contrast, the sample with α‐ferrite has lower plasticity. Failure of the samples with α‐ferrite is govern by the microcracks initiating in the interface of martensite/α‐ferrite. Eliminating the impact of the effect of α‐ferrite, the total elongation increases from 8.9% to 11.6%, and the ultimate tensile strength (UTS) increases from 1567 to 1652 MPa. This study investigates the critical factor in plasticity control of dual‐phase heterostructure and promotes the development and application of lamellar structure.