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

Abstract

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.