Researchers at Yonsei University’s Department of Materials Science and Engineering have recently developed a new Iron (Fe) – high Manganese (Mn) alloy that is both strong and ductile. Young-kook Lee’s team investigated the tensile properties of three Fe-Mn austenitic steels by varying the carbon concentrations. What the team found was that the strength and ductility (stretchable) of the alloy improved in accordance with increasing amounts of the carbon concentrations1.
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Metals alloys that exhibit both superior strength and ductility are highly desired in a variety of applications due to their improved mechanical properties. The strengthening of alloys by use of traditional methods such as grain refinement, strain hardening and dispersion hardening were found to reduce the ductility of the alloy, which is referred as the strength-ductility trade-off.
Austenitic steels made up of 16-26% chromium (Cr) and up to 35% nickel, are corrosion resistant, unhardened by increased temperature and non-magnetic, and are therefore used in several different applications ranging from aircrafts to food processing industries2.
Twinning-Induced Plasticity (TWIP) steels are a class of austenitic steels with high manganese (Mn) content (17 - 24%) are widely used especially in automotive applications due to the high-energy absorption and high stiffness, which can greatly improve the crash safety of vehicles3.
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TWIP steels combine high strength and ductility as a result of their high strain-hardening rate (SHR), which is caused by active mechanical twinning; a form of plastic deformation, in γ austenite with low stacking-fault energy (SFE), a determining factor of wear resistance1. Mechanical twins act as an obstacle during dislocation movement by sub-dividing grains during plastic deformation1.
Previous research in the material science field has indicated that factors such as grain size, chemical composition and temperature-dependent SFE greatly influence mechanical twinning. For example, mechanical twinning has been found to be reduced with reducing grain refinement because of the increase in critical twinning stress1.
Despite the excellent mechanical properties offered by the TWIP steel, the design of TWIP steel until now was only based on the optimal chemical composition and grain size required to achieve a low SFE value at room temperature, favorable for active mechanical twinning and high γ stability required to eliminate strain-induced martensitic transformation1.
TWIP steels, like other alloys, were also shown to exhibit the strength-ductility trade-off, therefore, designing the TWIP steel just based on the SFE and grain size is not enough. Young-kook Lee’s team investigated the impact of a new factor, carbon concentrations on the tensile properties of TWIP steels. The team studied the tensile properties of three Fe-MN austenite steels with similar stacking faulting energy and grain size at different carbon concentrations1.
The tensile properties of the Fe-Mn steels [Fe-31Mn (0C), Fe-29Mn-0.3C (3C) and Fe-25Mn-0.6C (6C)] were assessed by the engineering stress strain curves, total elongation (TE) Vs ultimate tensile strength (UTC) plots, true stress-strain (σ − ε) and strain-hardening rate (SHR, dσ/dε) curves at room temperature1.
The yield strength (YS) values of 160, 258 and 312 MPa for 0C, 3C and 6C respectively showed that the YS increases with increase in carbon concentrations in the TWIP steels. Increase in carbon concentrations from 0.002% to 0.62% resulted in the UTS and total elongation to improve from 532 to 1000 MPa and from 60% to 79% respectively1.
Furthermore, the microstructure evolution of the TWIP steels tested at different C-J stages were studied using an electron backscatter diffractometer (EBSD) 1. The studies revealed that the mechanical twins were not observed at any stages of the 0C and therefore, the deformation mode in the 0C steel is determined to be transformation-induced plasticity (TRIP) 1.
Whereas, the 3C and 6C showed mechanical twins and hence, the deformation in the 3C and 6C is TWIP. These 0C, 3C and 6C results show that the increasing C concentrations triggered mechanical twinning earlier and generated a higher twin fraction at the same true strain which resulted in higher SHR1.
The increase in C concentration was found to increase both the strength and ductility, rather than cause a trade-off between either one of those characteristics1. The results from this study showed that increasing C concentrations, along with SFE and grain size, can lead to the transformation of the deformation mode from TRIP to TWIP, therefore, suggesting a new parameter that needs to be considered during the alloy design of Fe-Mn TWIP steel1.
References:
- “Design for Fe-high Mn alloy with an improved combination of strength and ductility” S. Lee, J. Han, et al. Scientific Reports. (2017). DOI:10.1038/s41598-017-03862-y.
- “Stainless Steel” - Encyclopedia Britannica
- “High strength Fe-MN-(Al, Si) TRIP/TWIP steels development – properties – application” O. Grassel, L. Kurger, et al. International Journal of Plasticity. (2000). DOI: 10.1016/S0749-6419(00)00015-2.
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