Deformation and fracture mechanisms of high strength medium Mn steel

Abstract

Metallic materials are a major workhorse of modern society due to their excellent performance-cost synergy. Modern technologies and industries not only significantly depend on current metallic materials, but raise higher demands for even stronger and tougher metals to withstand more arduous operating conditions and aggressive environments. Medium manganese (Mn) steels represent a new class of advanced high strength steels (AHSS) characterized by lean alloying contents and high strength-ductility combination, making them a promising candidate to meet the challenging requirements from various applications. The mechanical properties of medium Mn steels are governed by the deformation and fracture mechanisms. Therefore, the overall objective of the present thesis is to elucidate the deformation and the fracture mechanisms of medium Mn steels and to further promote their development and applications. The plastic deformation of medium Mn steels to a large extent depends on the transformation induced plasticity (TRIP) effect, which is realized by deformation-induced martensitic transformation and is controlled by the stability of retained austenite. Thus, this thesis, firstly, identified the role of fraction and stability of retained austenite on the mechanical properties of medium Mn steels, followed by investigating the phase transformation mechanisms, the process-microstructure relation, and the efficient strategies to optimize their properties of medium Mn steels. Different from the traditional view that the optimum properties of steels containing austenite phase are obtained when the volume fraction of austenite is maximum, the present study indicated that other aspects, especially the stability of austenite, significantly influence the performance of medium Mn steels. The activation of ferrite transformation is found to be an efficient way to optimize the stability of austenite. In contrast to the existing research that ferrite transformation is almost impossible in medium Mn steels, the present work showed that it can be activated by carefully tailoring the microstructure, despite the sluggish transformation kinetics. The successive activation of austenite reverted transformation and ferrite transformation leads to a heterogeneous austenite microstructure, contributing to a control-realized TRIP effect and a significant improvement on both strength and ductility. The deformation mechanisms of medium Mn steels in different loading directions were further studied. The mechanical properties and the corresponding deformation behaviors were found to be a function of the loading direction in medium Mn steels with anisotropic microstructure. In particular, the orientation-dependent Lüders deformation was, for the first time, observed in the present thesis. The reliability and application of medium Mn steels directly rely on their fracture toughness. The present thesis subsequently studied the fracture mechanism of medium Mn steels. It was shown that improved fracture resistance in steel with an ultrahigh yield strength of nearly 2GPa can be achieved by activating delamination toughening coupled with TRIP toughening. Delamination toughening, associated with intensive but controlled cracking at Mn-enriched prior-austenite grain boundaries normal to the primary fracture surface dramatically improves the overall fracture resistance. The present “high-strength induced multi-delamination” strategy offers a different pathway to develop engineering materials with ultrahigh strength and superior toughness at economical materials cost.