Impact of nanodiffusion on the stacking fault energy in high-strength steels

doi:10.1016/j.actamat.2014.04.062

Acta Materialia

Volume 75, 15 August 2014, Pages 147-155

https://doi.org/10.1016/j.actamat.2014.04.062 Get rights and content

Abstract

A key requirement of modern steels – the combination of high strength and high deformability – can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed.

Introduction

In order to combine high strength and high ductility in steels, adaptive microstructures are required, which allow the material to react only locally to potential failure mechanisms caused by high stress levels. This adaptation can occur by diffusionless (martensitic) transformations to another well-defined crystal structure – the so-called transformation induced plasticity (TRIP) mechanism; alternatively, materials showing twinning-induced plasticity (TWIP) make use of twin boundary formation, accommodating strain and creating obstacles for dislocation motion.

Both deformation processes can be realised by locally modifying the sequence of atomic layers in the affected region [1]. However, this is only possible if the energy to create the change in stacking sequence – the stacking fault energy (SFE) – is sufficiently low. Therefore, the SFE serves as an indicator for the occurrence of either of the aforementioned deformation mechanisms. The observation that adjusting the SFE influences the adaptation of the microstructure allows new routes in designing damage-tolerant, high-strength steels with tailored mechanical properties [2].

Changing the chemical composition of a material is the most suitable strategy for such an adjustment. Austenitic steels, for example, in which the face-centred cubic (fcc) crystal structure is stabilized by a high Ni or Mn content (above 20 wt.%) and a C content of the order of 1 wt.% form an important class of adaptive structural materials. It is generally accepted that the SFE in high-Mn steels is of the order of 20 mJ m⁻² and that a reduction by about 5 mJ m⁻² changes the dominant deformation mechanism from TWIP to TRIP [1]. However, compiling experimental results for the dependence of the SFE on the C content in steels reveals strikingly inconsistent trends: some sets of experiments show only a slight change [3], whereas others report a steep increase in the SFE with C content [4] (see Fig. 1(a)). The discrepancies between the various experiments deviate more than an order of magnitude from the desired accuracy of 5 mJ m⁻². Consequently, the reliability of determined values of the SFE (Fig. 1) is debated in the literature [3], [5].

Here, we demonstrate that knowing the defect-mediated local chemical composition is critical for understanding and interpreting the origin of the conflicting experimental data. This finding goes beyond the above cited studies, which assume a homogeneous atomic compositions of the steel samples. Instead, we bring experimental observations into context with the Suzuki [6] effect. The thermodynamics of segregation of solute atoms to stacking faults was first introduced several decades ago [7], [8]. The energetics involved in such a process can be interpreted as the chemical driving force for local diffusion, but also as a way to reduce the SFE as a function of temperature [9]. These concepts are predominantly reported for substitutional elements that are attracted by the stacking fault (SF). In contrast to this, the C atoms behind the discussion in this paper are interstitials that are repelled from the SF. While the absolute changes in the local concentrations might be smaller in such a case, the jumps happen on a different timescale and potential energy surface than vacancy-mediated bulk diffusion. In addition, we were able to prove by ab initio calculations that just a few atomic jumps of interstitial carbon out of the SF are sufficient to substantially lower its energy. In this respect, C probably behaves similarly to, for example, nitrogen in high-N steels, for which an increase in the SFE has also been revealed by ab initio [10]. Based on the insights obtained in this work, we provide experimental evidence for such a Suzuki effect and its relevance for macroscopic deformation in high-Mn steels.

Section snippets

Ab initio methodology

To resolve the controversy about the role that C plays for the SFE, we have first used density functional theory (DFT), which allows us to focus on the influence of C without being simultaneously confronted with the full complexity of multicomponent and magnetic steels. The DFT calculations were carried out with the Vienna ab initio simulation package (VASP) [14] using the projector-augmented wave basis set [15] and the PBE exchange–correlation functional. Explicit calculations of intrinsic

Ab initio results and interpretation

Our initial DFT calculations use the assumption that the C atoms are homogeneously distributed and that the presence of the SF does not trigger nanodiffusion of C atoms. The SFE of fcc Fe–C has been computed for different C concentrations [18] and a strong increase with the C content was observed (red line in Fig. 1(b)). The trend is consistent with the experimental assessment of Schramm and Reed [4], which is historically the first paper to report such a strong influence of C, though it was

Experimental methodology

To experimentally verify the proposed effect of the electron beam on nanodiffusion of C, we performed in situ cooling–heating experiments on an Fe–22Mn–0.6C wt.% (Fe–22Mn–3C at.%) sample in the transmission electron microscope. For this purpose, high-purity (electrolytically refined) Fe and Mn and pure graphite were melted in an induction furnace under an argon atmosphere and cast into a rectangular copper mould. To obtain a compositionally homogeneous microstructure, the solidified steel slab

Experimental results and discussion

A prerequisite for discussing the outcome of the in situ cooling–heating TEM measurements is the knowledge of the C distribution in the steel samples. A typical set of APT data is shown in Fig. 3. The left tomogram shows the distribution of C atoms, whereas the right shows an isodensity surface at 1.5 C atoms nm⁻³. Two one-dimensional concentration profiles have been provided, corresponding to sections within the cylindrical regions of interest as shown, each having a 10 nm diameter and a 100 nm

Summary and conclusions

The surprisingly large impact of nanodiffusion on the determination of the SFE revealed in this study has implications for the strategies employed to develop adaptive structural materials. In order to control the SFE – and therefore the deformation mechanisms – an approach that is often employed is to modify the global chemical composition of the alloy. The relationship between the global concentrations of alloying elements and the SFE is documented in mechanism maps [37]. The present study

Acknowledgements

The paper benefited from fruitful discussions with Bengt Hallstedt and Joachim Mayer. Funding by the collaborative research centre SFB 761 “Stahl-ab initio” of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. R.K.W.M. gratefully acknowledges the support of the Alexander von Humboldt Foundation through the award of a Humboldt Postdoctoral Fellowship.

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¹: Present address: Institute for Frontier Materials, Deakin University, 75 Pigdons Road, Geelong, VIC 3216, Australia.

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Impact of nanodiffusion on the stacking fault energy in high-strength steels

Abstract

Introduction

Section snippets

Ab initio methodology

Ab initio results and interpretation

Experimental methodology

Experimental results and discussion

Summary and conclusions

Acknowledgements

Acta Metall

Mater Sci Eng

Acta Mater

Acta Mater

Scr Metall

Acta Mater

Acta Metal

Ultramicroscopy

Mater Sci Eng A

Acta Mater

Supra-ductile and high-strength manganese-TRIP/TWIP steels for high energy absorption purposes

ISIJ Int

Deformation-mechanism map for nanocrystalline metals by molecular-dynamics simulation

Nat Mater

Effect of carbon on stacking-fault energy of austenitic stainless-steel

Metall Trans A

Stacking-fault energies of 7 commercial austenitic stainless-steels

Metall Trans A

The composition dependence of stacking fault energy in austenitic stainless steels

Metall Mater Trans A

Segregation of solute atoms to stacking faults

J Phys Soc Jpn

Thermodynamics of stacking faults

Metall Trans

Compositional effects on deformation modes annealing twin frequencies and stacking fault energies of austenitic stainless steels

Trans Metall Soc AIME

Effect of carbon fraction on stacking fault energy of austenitic stainless steels

Metall Mater Trans A