On the strength of grain and phase boundaries in ferritic-martensitic dual-phase steels

Failure mechanisms in DP600 steel: Initiation, evolution and fracture

“…The higher hardness of the interfacial regions of the ferrite grains [24] together with a higher concentration of alloying elements, as previously explained, may produce an effective barrier for the dislocation movement. The higher dislocation density in the F-M and F-F boundary [21,22,24] makes it more difficult for dislocations transmission through the boundaries [24] during plastic deformation and therefore, increases further the dislocation density at these narrow bands around the martensite islands.…”

“…The in-grain orientation gradient is higher for the smaller ferrite grains surrounded by martensite islands [21]. Additionally, the induced plastic deformation due to the martensitic volume expansion may induce workhardening within the ferrite grains [24] and thus, increase the dislocation densities adjacent to the F-M interface. Consequently, higher local stresses are required to plastically deform smaller ferrite grains.…”

“…The higher dislocation density in the F-M and F-F boundary [21,22,24] makes it more difficult for dislocations transmission through the boundaries [24] during plastic deformation and therefore, increases further the dislocation density at these narrow bands around the martensite islands. As a result, strain heterogeneity increases within the ferrite grains and higher levels of deformation may occur close to the interfacial regions in smaller ferrite grains.…”

“…Therefore at an applied large plastic strain, the martensite phase is subjected to a relatively large local stress, close its yield strength, that will result in a local plastic deformation. Additionally the carbon is heterogeneously distributed in the martensite islands [24], the boundary regions of the martensite islands are prone to failure in a form of crack initiation due to higher carbon concentration and the reduced ductility compared to the interior of the islands. This may explain the fact that cracks formation are mostly observed at the boundary region of the martensite islands as shown in Fig.…”

“…The larger martensite islands experience more deformation heterogeneity due to the higher variations in carbon content [24], a non-uniform distribution of alloying elements (e.g. Mn) [28,29] The gradient of carbon content between the interface and the centre of the martensite island can affect the hardness and dislocation configurations and consequently, the formability of the grains in different locations [24].…”