The fracture toughness of martensite islands in dual-phase DP800 steel

Abstract

In situ microcantilever bending tests were performed on martensite islands in a dual-phase (DP) steel to extract the fracture toughness of martensite at the microscale and to understand damage initiation during forming of DP steels. All microcantilevers were produced through FIB milling. The martensite islands do not exhibit linear elastic brittle fracture; instead, significant ductile tearing is observed. The conditional fracture initiation toughness extracted by definition and by Pippan’s transfer criterion is Ki = 6.5 ± 0.4 MPa m1/2 and Ki,2% = 10.1 ± 0.3 MPa m1/2, respectively. The obtained value is well-represented by the strength-toughness trend of other ferritic steel grades. Considering the yield stress of the same martensite island, we found that crack initiation can occur only in very large martensite islands or in a banded or agglomerated martensite structure. Graphic abstract

Micromechanical Effect of Martensite Attributes on Forming Limits of Dual-Phase Steels Investigated by Crystal Plasticity-Based Numerical Simulations

“…On the other hand, the reduced capability of the material to undergo global strains with more martensite fraction is due to the brittle nature of martensite and the comparatively less force shifted to the ferrite grains. Furthermore, the martensite's decreased grain size provokes the surrounding ferrite's plastic deformation; therefore, the DP steel sample with a small martensite grain size showed comparatively better formability [33].…”

Martensite content effect on fatigue crack growth and fracture energy in dual‐phase steels

“…Martensite with a lower fracture toughness is responsible for crack propagation. 51 Due to this, the MVF increase promotes crack propagation through this phase, moving from a mechanism of transgranular fracture (typical of ferrite grains in a material with a low MVF) to a mechanism of mixed intergranular and transgranular fracture in martensite grains. When interacting with the crack front, these grains break close to the ferrite grain boundary or into several fragments.…”

Current trends in nanomechanical testing research

“…Complex mechanical behaviors and associated materials parameters have become accessible through advanced nanomechanical testing techniques relying on a preliminary structuring of the samples via focused ion beam (FIB) milling. The most popular application in the focus issue is measuring the local fracture behavior of materials [17][18][19][20]. In addition to providing a desired complex microsample geometry, the FIB micromachining technique allows one to visualize the deformation field that would otherwise be concealed under the surface [21].…”

“…As evidenced by the high number of related contributions, the use of the FIB has become essential in the field of nanomechanical testing [17,18,[20][21][22]. This has inevitably raised concerns about possible measurement artifacts due to ion damage incurred during the specimen preparation [20,[23][24][25].…”

“…Their development has been a dominant trend in nanomechanical research throughout the past decade, with their application seemingly only being limited by the high costs associated with the techniques. This trend is continued in the focus issue, which features applications both inside scanning electron microscopes (SEM) [17,18,21,22] and transmission electron microscopes (TEM) [23,24].…”