Micro-plasticity of medium Mn austenitic steel

Significance 

In recent years, the world’s curiosity and interest in medium Mn (4-7%) steels have grown exponentially, especially due to their specific strain hardening behavior. This property enables the steels to possess an extremely desirable strength-ductility balance. What makes medium Mn steels unique relative to other advanced high strength steels, is that these characteristics are achieved with the minimal addition of alloys. For instance, high Mn twinning-induced plasticity (TWIP) steels in spite of having similar properties with the steel above require a significant addition of alloys to achieve their properties. Pohang University of Science and Technology researchers in South Korea in collaboration with Javad Mola at Technische Universitat Bergakademie Freiberg in Germany investigated the deformation mechanisms of a medium Mn austenitic steel and explored its potential benefits regarding micro-plasticity. Their research work is published in Acta Materialia.

To realize their objectives, the researchers conducted an experimental procedure involving hot‑rolled steel with a composition of Fe-1.2%C-7.0%Mn (in wt. %). Electron backscatter diffraction technique was used to select grains with specific orientations. Nano-indention tests and in situ micro-pillar compression tests were conducted using Hysitron PI 85 SEM Pico-Indenter. The researchers then used transmission electron microscopy to investigate dislocation interactions and the evolution of the deformation microstructure after nano-indentation and micro-pillar compression tests.

Coarse austenite grains (> 50 µm) with the surface normal oriented close to the [001] and [111] directions were selected for the nano-indention and micro-pillar compression tests. The results showed that deformation twinning was favored in the crystals that had a [001] orientation, whereas perfect dislocation glide was more pronounced in [111]-oriented crystals.

The researchers reported that the stacking fault energy (SFE) of the medium Mn austenitic steel was estimated to be in the range of 28-34 mJ·m-2. Within this specified SFE range, only twinning and dislocation gliding could be observed during deformation.

They also revealed the steel grain orientation-dependence of the deformation mechanism during nano-indention and micro-pillar compression. The analysis based on Schmid’s law was further used to explain the deformation behaviors that had been observed during the experimental procedure. On the basis of the Schmid factor analysis, when the loading was along the [111] direction, the deformation twinning was suppressed. On the other hand, when the orientation for compressive loading is in the [001] direction, the Schmid factor for leading partial dislocation is larger than that for the trailing one, hence resulting in creation of wide stacking and twin formation. Therefore, during the compression of the [001]-oriented micro-pillar, the rapid twin growth led to large strain bursts. The observations offer strong support for the hypothesis that deformation twinning is a plasticity enhancing mechanism activated during the deformation of medium Mn steel.

Micro-plasticity of medium Mn austenitic steel: Perfect dislocation plasticity and deformation twinning. Advances in Engineering

About the author

Dr. Ir. Bruno C. De Cooman obtained his doctorate at Cornell University. Prior to his position of VP for R&D with NLMK, Dr De Cooman had an international career in materials R&D management at various industrial laboratories and academic institutions. His expertise includes scientific research and technology development related to advanced steel processing and products for automotive, electrical, engineering and constructional applications.

About the author

Eun Jung Seo received B.S. degree in Materials Science and Engineering from Gyeongsang National University, South Korea. She received her M.S. and Ph.D degrees in Graduate Institute of Ferrous Technology from Pohang University of Science and Technology (POSTECH). Since 2017, she has been working as a postdoctoral researcher at Advanced Steel Processing and Products Research Center (ASPPRC), Colorado School of Mines (CSM). Her research focused on the physical metallurgy of ultra-high strength automotive sheet steels. She is currently involved in the research on the effect of micro-alloying elements on the microstructure and mechanical properties of thermo-mechanically processed medium carbon bar steels.

About the author

Dr Mola is a scientific assistant and lecturer at the Institute of Iron and Steel Technology of Technische Universität Bergakademie Freiberg, Germany. He leads the institute’s research activities in the solid state and supervises the Metallography and Phase Transformation Laboratory.

His research is focused on the microstructure-property relationships in materials, especially ferrous alloys. He has used his knowledge of the physical metallurgy in the design of different classes of advanced high-strength steels.

About the author

Jin-Kyung Kim is currently a research professor in the Department of Energy Science at Sungkyunkwan University (SKKU), Korea. He worked as an Alexander von Humboldt fellow at Max Planck Institute for Iron Research, Germany. He holds a BS degree from Seoul National University, Korea, MS and PhD degrees from POSTECH, Korea.

His main research interests include the physical metallurgy of structural materials and the applications of transmission electron microscopy (TEM). The core theme of his research is to reveal underlying mechanisms of advanced alloys such as advanced high strength steel, Mg alloy and high entropy alloy with a main emphasis on their deformation behavior.

About the author

Lawrence Cho received B.S. degree in Materials Science and Engineering from Pohang University of Science and Technology (POSTECH), South Korea. He received his M.S. and Ph.D degrees in Graduate Institute of Ferrous Technology from POSTECH. Since 2017, he has been working as a postdoctoral researcher at Advanced Steel Processing and Products Research Center (ASPPRC), Colorado School of Mines (CSM).

His research focused on the physical metallurgy of coated, advanced high- to ultra-high strength steels. During his Ph.D studies and postdoctoral appointments, he was involved in the University-Industry collaborative research programs with a strong focus on the application of fundamental steel science to the practical challenges related to the development of these new and advanced materials for the automotive industry.

 Reference

Eun Jung Seo, Jin Kyung Kim, Lawrence Cho, Javad Mola, Chang Yeol Oh, Bruno C. De Cooman. Micro-plasticity of medium Mn austenitic steel: Perfect dislocation plasticity and deformation twinning. Acta Materialia, Volume 135, 15 August 2017, Pages 112-123.

 

Go To Acta Materialia 

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