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Origins of back stress strengthening in Fe-22Mn-0.6C (-3Al) TWIP steels

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Highlights

  • Back stress in 22Mn0.6C and 22Mn0.6C3Al TWIP steels was compared using LUR tests.

  • GND distributions across microstructure of steels were determined using EBSD data.

  • Deformation twins were less effective for GND accumulation than grain boundaries.

  • Deformation twins were not leading features responsible for high back stress.

  • High back stress originated from pile-ups of high GNDs near grain boundary regions.

Abstract

The origins of back stress strengthening were investigated in Fe-22Mn-0.6C and Fe-22Mn-0.6C-3Al TWIP steels. In contrast to reports found in literature, this study revealed that the high back stress developed in these steels is primarily caused by pile-ups of high density of geometrically necessary dislocations near the grain boundary regions rather than at deformation twins.

Introduction

Twinning-induced plasticity (TWIP) steels possess excellent tensile properties and high work hardening rates, making them important candidate materials for the automotive industry [1]. In contrast to dislocation-based metals or alloys, the unique character of TWIP steels is that deformation twins are formed during straining. However, due to the complex deformation mechanisms active in such TWIP steels, there is still a debate about the role of deformation twinning as it relates to the strengthening. It is generally accepted that deformation twins, as planar obstacles, increase the total dislocation density, and thereby increase dislocation hardening, which refers to the so-called dynamic Hall-Petch effect [2]. According to the stress partitioning proposed by Cottrell for cyclic loadings, the total flow stress can also be decomposed in terms of short-range effective stress and long-range back stress [3]. Particularly, it is believed that the dislocation pile-ups near/at twin boundaries acting as “hard inclusions”, lead to back stress strengthening and hardening in TWIP steels [[3], [4], [5], [6]].

Through Bauschinger-type mechanical tests, different studies have reported that the back stress accounts for about 68%, 50%, 75% or 20% of the flow stress of a Fe-22Mn-0.6C TWIP steel [3,[6], [7], [8]]. The scatter in these measured values is due to the extreme sensitivity of the back stress to the selected strain offset on the reverse loading curve [6]. Additionally, previous works have implemented equations proposed by Cottrell-Kulmann-Wilsdorf-Laird [9] or Dickson et al. [10] to calculate the back stress. However, Yang et al. [11] recently revealed that the above equations are physically problematic, and therefore proposed an alternative equation, which reduces the scatter in the measured back stress values.

The traditional view assumes that the deformation twins generated during deformation are the primary mechanism responsible for the high back stress in TWIP steels [3,5,7,12]. Based on this assumption, Bouaziz et al. [3] explained the measured high back stress in TWIP steels by an analytical model, considering dislocation pile-ups at the deformation twins. This model has been adopted by other researchers [4,5,12]. However, some works have also questioned the role of twins in alloys [13,14]. For example, Konopka et al. [13] showed that the increase of the frequency of twin boundaries results in a reduction of the flow stress in copper and austenitic steel with similar grain sizes since twin boundaries act as weak barriers to be the dislocation movement and as sources of dislocations. Recently, Malyar et al. [14] reported no significant storage of geometrically necessary dislocations (GNDs) in the crystals and at a single coherent ∑3 twin boundary of Cu bi-crystals, independent of the compression direction, as revealed by in-situ Laue microdiffraction compression experiments.

In fact, the back stress is believed to originate from pile-ups of GNDs against barriers, rather than from the randomly distributed statistically stored dislocations (SSDs) in materials [15]. Consequently, a detailed analysis of GNDs in a large region of interest is crucial for revealing the origins of the back stress.

In this study, we aim to clarify the role of deformation twins on the back stress strengthening in TWIP steels. The back stress, deformation twins and GND distribution across microstructure were quantified by detailed experiments. To achieve the above aim, two typical TWIP steels, i.e. Fe-22Mn-0.6C (hereafter 22Mn0.6C) and Fe-22Mn-0.6C-3Al (hereafter 22Mn0.6C3Al), which have different stacking fault energies (SFEs): 22 mJ m-2 and 37 mJ m-2 [16], respectively, were employed; bulk compositions measured by using optical emission spectrometry are provided in Table 1. The 22Mn0.6C steel deforms through dislocation planar slip and deformation twinning, while the deformation of the 22Mn0.6C3Al steel is mainly accommodated by planar slip of dislocations, with only individual deformation twins present at very high strain levels [16]. Consequently, a comparison conducted between the two TWIP steels can shed light on the role of the deformation twins on the back stress strengthening and elucidate the origins of the back stress in TWIP steels.

Section snippets

Materials and experimental procedures

Ingots of 22Mn0.6C and 22Mn0.6C3Al TWIP steels were homogenized at 1423 K for 2 h in a protective argon gas atmosphere and hot-rolled at temperatures between 1375 K and 1223 K into 3 mm thick plates. Each plate was then cold-rolled at room temperature to obtain 1.5 mm thick sheets. Finally, the cold-rolled 22Mn0.6C and 22Mn0.6C3Al steel sheets were annealed at 1123 K and 1173 K, respectively, for 1 h in a vacuum tube furnace, followed by water quenching to prevent carbide precipitation.

Evolution of back stress

The initial microstructure of the two TWIP steels consists of fully-recrystallized grains, as shown in Fig. 1(a)-(b). Fig. 1(c) shows the grain size (including annealing twins) distribution analysis, which was fitted with a lognormal distribution function, giving average grain sizes of 4.9 and 4.3 μm for 22Mn0.6C and 22Mn0.6C3Al, respectively. The similar grain sizes in the two studied steels allow for evaluation of the role of deformation twins in back stress strengthening.

Fig. 2 compares the

The origins of back stress

By comparing Fig. 2, Fig. 3, it is noted that the slightly higher GND density does produce a slightly higher back stress increment (14 MPa) within the 22Mn0.6C3Al steel at 0.06 strain. In comparison, at 0.26 true strain, the formation of large amount of deformation twins induces 16% higher GNDs, producing 80 MPa (about 15%) higher back stress increment within the 22Mn0.6C steel (Fig. 2, Fig. 4). In view of Fig. 2(c), it is noted that a high back stress can still be obtained in the 22Mn0.6C3Al

Conclusions

In summary, the origins of back stress strengthening were investigated in 22Mn0.6C and 22Mn0.6C3Al TWIP steels. By comparison between two steels, the LUR experiments demonstrated that the deformation twins are not the leading contributors to high back stress in TWIP steels, which is contrary to the conventional view. This result was also revealed directly based on microstructural observations showing that deformation twin boundaries are less effective for storage of the GNDs than grain

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Originality statement

I write on behalf of myself and all co-authors to confirm that the results reported in the manuscript are original and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor has been accepted for publishing in another journal. The author(s) confirms that the research in their work is original, and that all the data given in the article are real and authentic. If necessary, the article can be

CRediT authorship contribution statement

Huihui Zhi: Methodology, Formal analysis, Investigation, Writing - original draft. Stoichko Antonov: Writing - review & editing, Resources. Cheng Zhang: Investigation. Zihui Guo: Investigation. Yanjing Su: Conceptualization, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB0700505), the National Natural Science Foundation of China (Grant No. 51571028) and the 111 Project (Grant No. B170003).

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