The accommodation of martensitic phase transformation strains by the ferritic matrix in dual-phase steels

Abstract

Dual-phase (DP) steels are an important class of advanced high-strength steels (AHSS) and constitute a major share of steels for the automotive industry. A microstructure consisting of hard martensite embedded in a soft ferritic matrix gives them a good combination of strength and ductility. The martensite formation in the microstructure from austenite involves a shape and volume change, which is accommodated by the deformation of the surrounding ferritic matrix. This accommodation is known to impart typical characteristics in DP steels such as the absence of a yield point, continuous yielding and high initial work hardening rate. This thesis is an attempt to understand and model the aforementioned accommodation process in the ferritic matrix of DP steels. Traditionally, in predictive modelling of DP steel mechanical behaviour, the region of ferrite which undergoes deformation to accommodate martensitic transformation is taken into consideration as a constant thin layer of strain-hardened ferrite at the ferrite/martensite interface. This approach is shown to be inadequate for capturing local variations in ferrite deformation. Hence, electron backscatter diffraction (EBSD) experiments were carried out to study in detail the influence of various microstructural features on local variations in the transformation-induced deformation of ferrite. It was found that the crystallographic orientation of ferrite grains, martensite variant and its prior austenite grain (PAG) play an important role in determining the extent of transformation-induced deformation of ferrite. Taking a cue from this, a novel methodology comprising sequential experimental and numerical research on DP steels is developed which combines the results of PAG reconstruction, phenomenological theory of martensite crystallography (PTMC) and EBSD orientation data to estimate ferrite deformation due to every martensitic variant formed, via full-field micromechanical calculations on a virtual DP steel microstructure. Furthermore, the influence of self-accommodation during martensite variant formation on transformation-induced deformation of ferrite was also investigated. It is shown that the higher the number of variants which form from a PAG, the less the deformation caused by that PAG in the surrounding ferritic matrix. This is because of a decrease in the effective magnitude of the shear component of martensitic transformation during multi-variant transformation. The scientific findings presented in this work can be used for developing predictive models for the mechanical behaviour of not only DP steels but any multiphase steels which exhibit plastic accommodation and residual stresses in their microstructure due to martensitic phase transformation.