Micro-damage initiation in ferrite-martensite DP microstructures: A statistical characterization of crystallographic and chemical parameters
Graphical abstract
Introduction
DP steels comprise one of the advanced high strength steel (AHSS) grades, consisting of hard martensite islands embedded in a soft ferritic matrix. Deformation of a DP-steel microstructure leads to characteristic heterogeneity of the strain distribution, triggered by the mechanical phase contrast at the early stages of deformation [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. This heterogeneity increases with increasing global deformation level, promoting hot spots of strain localization, where damage is most likely to initiate [3], [5], [9], [11], [12]. Eventually, DP-steels fail in a ductile manner, following nucleation, growth, and coalescence of the damage features that develop progressively with increasing strain [13].
Different parameters may influence the strain distribution within the DP-steel microstructure, particularly, the average size of the martensite islands and their global distribution within the ferritic matrix [14], [15]. More detailed studies investigated the influence of the local distribution and morphology of both phases on the micro-strain localization, identifying strain peaks at critical microstructural configurations [1], [7], [9], [11]. Different mechanisms have been reported for damage initiation, including martensite cracking, damage at interphase boundaries, and void formation inside of ferrite. Though many damage-related studies were concerned with the influence of varying DP-steel parameters on the relative contribution of each mechanism, only few have attempted to define descriptive models for the damage initiation mechanism [16].
Most of the available systematic investigations on DP-steel damage do not provide the relevant crystallographic orientation dependency. On the other hand, in-situ micro-mechanical studies utilizing electron backscatter diffraction (EBSD) often lack the statistical relevance and are limited in terms of the analyzed strain levels due to surface topography effects. Therefore, even though an enormous library of relevant micro-mechanical studies exists, more efforts are still required to attain a microstructural model for damage evolution in DP steels [17] taking into account, at best, the 3-dimensional character of morphological, crystallographic, and chemical factors.
The present study aims to identify more comprehensively independent microstructural features underlying both strain localization and damage. To this end, details of damage initiation mechanisms under monotonic loading in the DP steel microstructure were observed using advanced SEM and FIB observation techniques. In order to identify grain and interphase boundaries of low fracture strength, micro-crack propagation was additionally characterized after cyclic deformation. Furthermore, elemental segregation at selected interfaces was assessed using atom probe tomography (APT) to evaluate the chemical aspect of grain or phase boundary embrittlement. In order to observe damage initiation at martensite interfaces already at low levels of deformation, an optimized model DP-steel was utilized with a relatively coarse microstructure and sufficient mechanical phase contrast.
Section snippets
Material
Hot-rolled plain C-Mn steel was utilized, with a chemical composition of Fe-0.14C-1.93Mn-0.42Cr-0.25Si (in wt%), and an initial bainitic-martensitic phase composition. A heat treatment process was applied to the specimens under vacuum, using a dilatometer DIL 805 A/D (TA Instruments). The applied process consisted of two isothermal holding steps, first at 840 °C for 90 s for complete austenitization, followed by partial ferritic transformation for 4 min at 650 °C. The material was finally quenched
Damage induced by monotonic deformation (post-deformation observations)
Two principle damage mechanisms were characterized under the applied monotonic tensile deformation, namely, martensite cracking (DM), and martensite-ferrite interphase damage (DINT). They constitute 50%, and 30% of the characterized features, respectively. The DM mechanism corresponds to any damage feature that is mostly enclosed inside of martensite. Features observed at the interphase boundaries – extending into the ferrite phase – belong to the DINT mechanism.
Discussion
The present investigation is particularly relevant to the initiation phase of damage in DP-steels, as suggested by the small length scale of the inspected damage features, and the respectively low deformation levels applied. In the following, the role of interfaces is particularly discussed with respect to damage initiation. First, critical microstructural configurations of high damage susceptibility (Fig. 5) are discussed regarding their evolution during the applied heat treatment. In 4.2
Conclusions
The investigations carried out enabled to define those ferrite-martensite microstructural configurations where damage is most likely to initiate. During the intercritical annealing, allotriomorphic ferrite grows along grain boundaries developing zones of concave ferrite-martensite interphase boundaries. These zones act as notches within the martensite islands, enhancing strain localization and subsequent martensite cracking along the prior austenite grain boundaries (PAGbs). PAGbs are
Acknowledgment
This research was carried out under project number T22.2.13497 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl).
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