Abstract
Martensite/ferrite (M/F) interface damage plays a critical role in controlling failure of dual-phase (DP) steels and is commonly understood to originate from the large phase contrast between martensite and ferrite. This however conflicts with a few, recent observations, showing that considerable M/F interface damage initiation is often accompanied by apparent martensite island plasticity and weak M/F strain partitioning. In fact, martensite has a complex hierarchical structure which induces a strongly heterogeneous and orientation-dependent plastic response. Depending on the local stress state, (lath) martensite is presumed to be hard to deform based on common understanding. However, when favourably oriented, substructure boundary sliding can be triggered at a resolved shear stress which is comparable to that of ferrite. Moreover, careful measurements of the M/F interface structure indicate the occurrence of sharp martensite wedges protruding into the ferrite and clear steps in correspondence with lath boundaries, constituting a jagged M/F interfacial morphology that may have a large effect on the M/F interface behaviour. By taking into account the substructure and morphology features, which are usually overlooked in the literature, this contribution re-examines the M/F interface damage initiation mechanism. A systematic study is performed, which accounts for different loading conditions, phase contrasts, residual stresses/strains resulting from the preceding martensitic phase transformation, as well as the possible M/F interfacial morphologies. Crystal plasticity simulations are conducted to include inter-lath retained austenite (RA) films enabling the substructure boundary sliding. The results show that the substructure boundary sliding, which is the most favourable plastic deformation mode of lath martensite, can trigger M/F interface damage and hence control the failure behaviour of DP steels. The present finding may change the way in which M/F interface damage initiation is understood as a critical failure mechanism in DP steels.
Highlights
Facilitated by the economically viable thermo-mechanical processing procedures, low alloying requirements and excellent mechanical properties, dual-phase (DP) steels consisting of a ferrite (F)/(lath) martensite (M) microstructure are nowadays among the most attractive advanced high strength steels (AHSS) for automotive applications [1]
The overall sliding of the martensite island γhp is defined by the effective shear deformation component of the M/A laminate along the habit plane, used to quantify the substructure boundary sliding activity
The results shown so far demonstrate that, when shear is applied parallel to the subgrain lath martensite boundaries, the M/F interface damage initiation is controlled by the substructure boundary sliding of the martensite island
Summary
Facilitated by the economically viable thermo-mechanical processing procedures, low alloying requirements and excellent mechanical properties, dual-phase (DP) steels consisting of a ferrite (F)/(lath) martensite (M) microstructure are nowadays among the most attractive advanced high strength steels (AHSS) for automotive applications [1]. Site island connectivity [4] and by grain refinement [5], which often result in a ductility decrease. This trade-off acts as a critical constraint for further improvements of DP steels towards manufacturing of light-weight complex structural components. M/F interface damage is usually considered to originate from the large phase contrast between the ferrite matrix and martensite islands, which act as hard barriers for the dislocation glide in the ferrite [10]. Dislocations gradually accumulate along the M/F interface, leading to a locally high stored energy. After reaching a certain threshold, the stored energy is released [11,12], eventually inducing void nucleation along the M/F interface [13]
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