Microstructural Development and Deformation Mechanisms during Cold Rolling of a Medium Stacking Fault Energy TWIP Steel
Introduction
In recent years high-manganese austenitic steels have attracted great interest both in academia and the automotive industry because of the unusual combination of excellent ductility (with elongations up to 90% being reported) and high strength[1]. Their chemistry is usually composed of 14–30 wt% Mn with additions of C, Al, Ni, Si and/or N. The interesting mechanical properties have been attributed to the competitive deformation mechanisms those can occur in these alloys in addition to dislocation glide: (1) the formation of mechanical twins in the austenite grains, leading to twinning induced plasticity (TWIP), as a result of coherent twin boundaries acting as obstacles to dislocations and resulting in increased work hardening; (2) the transformation of austenite to ɛ- and/or α′-martensite, which have hcp. and bcc crystal structures respectively. This transformation leads to transformation induced plasticity (TRIP)[2]. The deformation mechanism(s) that will dominate depend(s) on the stacking fault energy (SFE)[3] of the material, which is a function of chemical composition[4], [5], [6], [7], [8], temperature[9], and microstructure[6].
Stacking faults[10], are an essential part of the deformation process in fcc materials, and their formation is governed by the SFE. In high-SFE materials deformation occurs by cross slip and the activation of slip systems, however in medium- to low-SFE materials these mechanisms are no longer energetically favourable and the dissociation of perfect dislocations into partials becomes favourable[11]. These dissociation reactions are nucleation points for the formation of mechanical twins and the formation of strain-induced martensite. The stacking faults are considered as local regions in the crystal where the regular stacking sequence has been interrupted[12]. Generally extrinsic stacking faults are considered as nucleation points for mechanical twin formation and intrinsic stacking faults result in the transformation of austenite to ɛ-martensite. The nucleation of α′-martensite is thought to be promoted at intersections of shear bands in the form of ɛ-martensite laths, mechanical twins or bundles of stacking faults[13], [14].
To enable optimal use of the excellent properties exhibited by TWIP steels, a good knowledge of the microstructural changes that occur during deformation and how these changes are related to the deformation mode(s) is crucial, but the current understanding in this regard is still insufficient. By studying the microstructure at different stages of cold rolling, a comprehensive understanding can be obtained for the microstructural changes that occur, and how these changes are related to the active deformation mode(s) as thickness reduction increases. Therefore, in this work the development of the microstructure and deformation mechanisms in a high-manganese steel with a medium stacking fault energy were studied at different stages of cold rolling. This was done by measuring the ferromagnetic behaviour of the material in cold-rolled samples with different thickness reductions, analysing the integral breadths of their XRD profiles, and studying the development of texture associated with cold rolling. The α′-martensite fraction, probability of stacking fault formation, domain size, internal strain, and texture development at different levels of cold rolling were discussed.
Section snippets
Experimental Methods
A high-manganese TWIP steel with a stacking fault energy of 42 mJ/m2 was used in this study. The SFE was calculated thermodynamically based on an approach by Bleck et al.[15] with the effect of Si and Al on the SFE being taken into account as (−7 mJ/m2 per wt% Si and + 10 42 mJ/m2 per wt% Al[16], [17]). The value is accurate within ±10 mJ/m2. The chemical composition is shown in Table 1.
As-received sheets had undergone, 2%, 5%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, 70% and 75% thickness reduction (
Magnetic measurements
Fig. 2(a) shows the magnetisation curves as a function of the magnetic field after different cold-rolling steps. The shape of the curves indicate a change from a predominantly paramagnetic state in the warm-rolled material (ɛvm = 0) to an increasingly ferromagnetic state as the strain increases, with the presence of a ferromagnetic phase already detectable for the sample with ɛvm = 0.19. For ɛvm ≥ 1.39 the curves show the largest fraction of a ferromagnetic phase; the magnetisation curves for
Discussion
The change in the form of the magnetisation curves from paramagnetic to partly ferromagnetic, is due to the formation of a ferromagnetic phase, which we believe to be α′-martensite. Based on previous work correlating the Stacking Fault Energy (SFE) to deformation mechanisms, transformation of austenite to α′-martensite in a high-Mn steel with a medium SFE of 42 mJ/m2 is not expected[5], [30]. Scott et al.[5] reported that microsegregation of Mn and C cause fluctuations in the SFE of the
Conclusion
A medium-SFE austenitic steel exhibits γ → α′-martensite transformation during cold rolling. The microstructural parameters Ds, Psf and ɛ at different levels of equivalent strain were determined by using a modified Williamson and Hall equation. The formation of a small fraction of α′-martensite has been measured at different equivalent strain levels and is related to the changes of the Psf as cold rolling progresses. Besides the formation of α′-martensite, the Schmid factors of the texture
Acknowledgements
This research was carried out under the project number MC41.5.08312 in the framework of the Research Program of the Materials Innovation Institute (M2i) (www.m2i.nl). It was also supported by Tata Steel who provided the sample materials. The authors also gratefully acknowledge Dr. L. Bracke and Dr. R. H. Petrov for helpful discussion, and N. M van der Pers and R. Hendrikx for helping with the X-ray measurements.
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2019, Materials Science and Engineering: ACitation Excerpt :For instance, during plastic deformation of metastable austenite, this phase, depending upon its composition thereby its stacking fault energy, may decompose partially into either ɛ- and/or α′-martensite in the so-called TRIP (TRansformation Induced Plasticity) and/or exhibit TWIP (TWinning Induced Plasticity) effects. These are competing deformation mechanisms and would markedly increase the strain hardening rate, strength and ductility [1–4]. In fact, the strain induced martensite formation and the mechanical twinning are kicked off to accommodate the undertaken additional strain.