The role of austenite stability on the change of fracture mode in a dual-phase medium Mn steel having a lamellar microstructure

Abstract

Reduction of austenite stability in lamellar-type medium Mn steels consisting of α′-martensite and γ-austenite causes a change in fracture mode from ductile to brittle. In the present study, we investigate the contributing mechanisms to the appearance of such behavior using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The transformation-induced plasticity (TRIP) effect occurs actively during deformation, and the deformation-induced α′ is found to form epitaxially on the pre-existing α′. As a result, the extension of α′ lamella area is observed after tensile deformation. Furthermore, the previous α′/γ interphase boundary remains as a low-angle boundary composed of an array of [110] edge dislocations between the pre-existing α′ and the deformation-induced α′. Increased γ stability by carbon partitioning leaves γ lamella in between neighboring α′ lamellae. Our results show that the variation in fracture mode depending on γ stability can be correlated to different TRIP responses which leads to the changes of dislocation pile-up stresses at the high angle boundaries during tensile deformation. • Improved γ stability changes fracture mode from brittle to ductile mode in a dual-phase medium Mn steel. • TRIP effect is occurred by the migration of α′/γ interface to γ side, leading to the increases in α′ fraction and pile-up length. • The traces of former α′/γ interfaces remain as low angle boundaries characterized by arrays of edge-typed dislocations. • Brittle fracture is a consequence ofhigher flow stress and larger dislocation pile-up length induced by rapid TRIP rate.