Strain partitioning and strain localization in medium manganese steels measured by in situ microscopic digital image correlation

 abstract

In situ microscopic-digital image correlation (μ-DIC) is used to investigate the strain partitioning and strainlocalization behavior in a medium manganese steel. Continuous yielding results from strain partitioning withhigher strain in the reverted austenite (𝛾R) islands and less strain in the tempered martensite (α′temp) matrix, bothin hot and cold rolled material. μ-DIC experiments are performed to further understand the effects of textureand grain morphology on strain partitioning which cannot be locally resolved through high resolution x-rayor neutron diffraction experiments. Apart from strain partitioning, strain localization is observed in hot rolledsamples within colonies of lamellar 𝛾R islands. This localization does not only depend on the crystallographicorientation, but also on the spatial alignment of an austenite island relative to the loading direction. The effectsof texture, spatial and colony alignment are interpreted within the concept of a relative grain size effect resultingin different yield stresses in the hot and cold rolled samples showing continuous yielding. Strain partitioning andstrain localization based on texture and spatial alignment can be extended to numerous dual phase morphologieswith similar texture, colony and spatial alignment effects.

Keywords:

Medium manganese steel In situ Microscopic-digital image correlation DIC EBSD Strain partitioning Strain localization.

Introduction

Due to increasing property requirements for metallic materials inautomotive applications, there is an increasing interest in medium Mn steels which fall under the 3rd generation of advanced high strength steels. With a manganese content between 3 and 12 wt.%, medium Mn steels show promising property combinations of high to ultra-high strength at sufficient total elongations [1–6]. The ultra-fine grain (UFG) microstructure of these steels is obtained through an intercritical annealing treatment, resulting in a complex multi-phase microstructure consisting of austenite, martensite/ferrite and sometimes delta ferrite. Especially the austenite islands contribute to the material’s ductility due to the higher strain hardenability enabled by enhanced disloca-tion accumulation, TRIP and/or TWIP effects. Careful compositional alloy design and appropriate selection of the intercritical annealing temperature and thermomechanical treatment results in the formation of different types of beneficial microstructures at room temperature [2,7,8]. Austenite growth during the intercritical annealing process takes place with the composition given by local equilibrium partitioning [9]. Accordingly, selecting different intercritical annealing tempera-tures results in austenite having different compositions and, hence,stacking fault energy values, thus affecting its deformation mecha-nisms and strain hardening response. The complex strain partitioning phenomena occurring among the various phases upon mechanical loading is the key to better understand the underlying mechanisms and tailor the mechanical behavior of medium Mn steels, thus, allowing further tuning of this emerging material class.

Conclusions

1. Designing medium Mn steels by selecting low intercritical annealing temperatures and long holding times results in avoiding recrystal-lization of α′temp and an increased fraction of 𝛾R. This prevents macro-scopical localization of strain in ferrite which would form during re-crystallization of α′temp and would promote the formation of Lüders bands and thus of a pronounced yield point. Increased holding times result in a sufficiently increased volume fraction of 𝛾R in order toprevent strain localization within the recovered α′temp grains.

2. In the present study continuous yielding and higher local von Misesstrain in 𝛾R than in α′temp were observed in both, the hot rolled and cold rolled intercritically annealed material. Strain is taken up by 𝛾R / α′f resh and α′temp at global strains beyond 10% in hot rolled and 5% in cold rolled samples. The strain within the 𝛾R is not entirely contributed from shape strain distortion.

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