Evaluation of Fracture Micromechanisms in a Fine‐Grained Dual Phase Steel during Uniaxial Tensile Deformation

Abstract

Ductile fracture mechanism in fine‐grained high‐strength dual phase steel in sheet form, which incorporated martensite particles in a soft ferrite matrix, was studied through extensive quantitative metallography, scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD) observations of polished sections as well as fracture surfaces analysis of failed specimens. The void characteristics in terms of area fraction, density, and average size were examined as a function of thickness strain in the sectioned specimens. Detailed microstructural analysis revealed that interface decohesion at triple junctions of ferrite–ferrite–martensite was the dominant void nucleation mechanism. EBSD analysis also revealed that void nucleation was predominantly promoted by the increase of ferrite–ferrite grain boundary misorientation with strain, especially at the boundaries incorporating adjacent martensite particles. Moreover, the study of voids nucleation and evolution behavior suggested that ductile fracture in this steel was nucleation controlled such that just before the final fracture incidence, a high density of voids would be nucleated or a sudden accelerated void nucleation could happen. Microscopic observations as well as statistical analysis confirmed this phenomenon.

EBSD study of micromechanisms involved in high deformation ability of DP steels

“…By this mechanism, the stored energy at the interface could be released and, therefore, the strain gradient within the microstructure might be decreased [32]. More detailed examination of the void evolution behavior is given elsewhere [33].…”

Micromechanics stress–strain behavior prediction of dual phase steel considering plasticity and grain boundaries debonding

“…Inhomogeneous deformation on a microscopic scale between the soft phase of ferrite and the hard phase of martensite should be accommodated at phase boundaries. So it is expected that there should be high stored energy at these boundaries due to the mentioned accommodation [29]. Initiation of voids creation can be predicted by the damage initiation and propagation capability of CZM interface elements.…”

Microstructural Modifications of Dual‐Phase Steels: An Overview of Recent Progress and Challenges

“…Given different martensite volume fraction, two major fracture mechanisms have been observed in association with the DP steel. [ 34–36 ] The first fracture mechanism was martensite cracking; the explanation was that quenching of steel and martensite transformation during the production processing could make initial microcracks at the martensite particle. These initial microcracks could grow by stress concentration together with dislocations pile‐up around martensite particles created by the misfit plastic strain between ferrite and martensite during tensile deformation.…”

“…Voids can form at ferrite–martensite interface almost with the presence of any amount of strain; they, however, more dramatically occur at the final stage of deformation, definitely before the fracture. [ 35 ] Figure 4 shows a schematic illustration of the above two cracking mechanisms. This figure denotes that the void coalescence would diminish by increasing of the V M value.…”

“…Consequently, the nucleation of microvoids is increased to the extent that the fracture surface would compose of high density of tiny dimples. [ 31,35,37 ]…”