Superior strength and ductility in a low density duplex steel studied by in situ neutron diffraction

doi:10.1016/j.msea.2020.140252

Materials Science and Engineering: A

Volume 799, 2 January 2021, 140252

https://doi.org/10.1016/j.msea.2020.140252 Get rights and content

Abstract

A low density, high strength medium-Mn steel was processed to produce a bimodal duplex microstructure consisting of coarse grained γ-austenite with fine δ-ferrite grains decorating the γ-austenite boundaries. Using a combination of ex situ analysis and in situ neutron diffraction, the deformation mechanisms and lattice strains within each phase were identified for specimens undergoing uniaxial tension during room and elevated temperature loading up to 473 K. The coarse-grained high stacking fault energy γ-austenite deformed by dislocation glide, providing work hardening and ductility. Simultaneously, the fine grained δ-ferrite produced an elevated yield strength by strengthening the steel via a composite reinforcing mechanism. Neutron diffraction reveals that the yield strength reduction at elevated temperatures is due to a reduction in the δ-ferrite strength. The resulting combination 1200 MPa ultimate strength and 0.3 ductility achieved in this microstructurally engineered bimodal duplex steel exceeds that of typical hot worked medium-Mn steels.

Introduction

Low-density steels are of significant interest in both automotive and military applications for vehicle implementation [[1], [2], [3]]. These steels are derived from Robert Hadfield's original investigation of a Fe–13Mn-1.2C (in wt.%) steel with elevated toughness, high work hardening, and optimal wear resistance [4]. Medium-Mn (5–15 wt %) steels are currently under intensive investigation due to the potential to obtain high strengths (ultimate tensile strength > 1.2 GPa) and considerable plasticity (percent elongation > 20%) [5,6].

Recent efforts have focused on enhancing ductility via twinning induced plasticity (TWIP) or transformation induced plasticity (TRIP) [[7], [8], [9], [10], [11], [12]]. TWIP and TRIP steels however are well known to exhibit inverse strain rate sensitivity, high temperature sensitivity, and dynamic strain aging (DSA) [8,9,13,14]. A medium-Mn alloy can be designed to avoid these potentially detrimental characteristics while maintaining good strength and ductility through tailoring the chemistry to stabilize dislocation glide (i.e. slip). Distinctions between the deformation modes are determined by calculation of the stacking fault energy (SFE) of the γ-austenite [[7], [8], [9], [10], [11]]. The different deformation characters are approximately delineated as TRIP for SFE <20 mJ/m², TWIP for 20–35 mJ/m², and slip for SFE ≥35 mJ/m².

Within the slip family of Mn-steels, the microstructure and deformation response is tailored by alloying with aluminum, silicon and chromium [1,15,16]. A subset of these Mn-steels contain elevated aluminum levels for density reduction to obtain steels with high specific strengths [[15], [16], [17], [18]]. The microstructure is engineered by appropriately balancing ferrite stabilizers including Al, Cr, and Si with γ-austenite stabilizing elements Mn and C to obtain structures such as single-phase austenitic, duplex δ-ferrite + γ-austenite, and triplex δ-ferrite + γ-austenite + κ-carbide. Of these, the duplex microstructures are of particular interest due to their potentially favorable combination of strength and ductility [[19], [20], [21], [22], [23]]. These duplex Mn-steels are typically likened to duplex stainless steels and the increased aluminum, included to stabilize the δ-ferrite, is anticipated to provide some corrosion resistance [24].

While it is known that duplex medium Mn-steels exhibit a suite of excellent mechanical properties, the role of different microstructural features and specific deformation mechanisms is an outstanding question. Advances in in situ neutron diffraction provide a novel tool set for probing these issues [25,26]. Recent efforts in steel utilizing these capabilities have largely focused on room temperature deformation, twinning mechanisms in TWIP, or phase evolution in TRIP alloys [[27], [28], [29], [30], [31], [32], [33], [34], [35]]. Here, advanced in situ neutron diffraction methods are combined with ex situ microstructural analysis to study the deformation pathways in a newly developed medium-Mn duplex steel with low density. This analysis reveals the temperature dependent origins of enhanced strength and ductility as compared to conventional duplex steels as well as other hot worked Mn-steels.

Section snippets

Alloy preparation

The alloy was vacuum induction melted and cast into 25 kg ingots with sizes of 14.5 × 36.0 × 5.0 cm³. Chemical analyses were obtained on the cast material by inductively coupled plasma spectrometry after sample dissolution in hydrochloric and nitric acid; carbon content was determined using gas combustion analysis. The steel casting was normalized at 1523 K, held at temperature for 2 h, and air cooled to 298 K. The normalized casting was subsequently hot rolled by heating to 1373 K, rolling,

Tensile properties

Representative ex situ true stress-true strain curves from experiments at 298 K and 473 K are shown in Fig. 3. Overall, the alloy shows good strength and ductility relative to the goals set by the U.S. Department of Energy for 3rd generation advanced high-strength steel. This steel exhibits a theoretical density of 7.12 g/cm³, resulting in a 9.6% reduction in density relative to pure Fe and providing a further improvement of the specific strength. For comparison, a true stress-true strain curve

Summary of deformation sequence

Taken together, the suite of ex situ experiments and in situ neutron diffraction studies demonstrate the development of a lightweight medium-Mn steel with high strength and ductility. The combined analyses provide a micromechanical picture of the deformation processes and strengthening mechanisms. A mechanistic description of deformation process is as follows.

Initially as load is applied the large γ-austenite grains undergo microyield. The comparatively low strength of the γ-austenite phase as

Conclusion

Demand for low density and high strength steels for automotive and military applications has driven significant interest in medium-Mn steels. In order to achieve gains in alloy design a detailed understanding of the deformation characteristics is required. Here a combination of ex situ characterization and in situ neutron diffraction experiments are employed to study the deformation mechanisms in a hot worked medium-Mn duplex steel.

This coupled approach to evaluating the deformation revealed

CRediT authorship contribution statement

Daniel J. Magagnosc: Investigation, Formal analysis, Visualization, Writing - original draft, Writing - review & editing. Daniel M. Field: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Christopher S. Meredith: Investigation. Timothy R. Walter: Investigation. Krista R. Limmer: Conceptualization, Project administration, Writing - review & editing. Jeffrey T. Lloyd: Conceptualization, Project administration, Writing - review & editing.

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.

Acknowledgments

A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by Oak Ridge National Laboratory. The authors thank Matthew Frost, Yan Chen, and Ke An for their assistance executing the neutron diffraction experiments and analyzing the data. The authors would like to thank Micah Gallagher, Steven Marsh, and Mike Aniska for their assistance in preparing samples for tensile testing.

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Published by Elsevier B.V.

Superior strength and ductility in a low density duplex steel studied by in situ neutron diffraction

Abstract

Introduction

Section snippets

Alloy preparation

Tensile properties

Summary of deformation sequence

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

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