Jae-Eun Jin

Transformation-induced plasticity in an ultrafine-grained steel: An in situ neutron diffraction study

An ultrafine-grained steel with an average grain size of about 350nm was developed. The tensile testing at ambient temperature shows a threefold increase in the yield strength compared to its coarse-grained counterpart. Moreover, the increase in the strength was achieved without the sacrifice of the ductility due to strain-induced martensitic transformation. The evolution of lattice strains and phase fractions of the austenite and martensite phases during the deformation was investigated using in situ neutron diffraction to provide a micromechanical understanding of the transformation-induced plasticity responsible for the combination of high strength and ductility.

Néel temperature of high Mn austenitic steels

The predictive equations for the Néel temperature (TN) of high Mn austenitic steels were reviewed and reevaluated using 116 different measured TN values. The previous equations gave good predictions for TN values of high Mn-low C austenitic steels, but not for those of medium and low Mn-high C austenitic steels, including TWIP steels with less than 20 at.% Mn. Therefore, an improved TN equation for medium and low Mn-high C as well as high Mn-low C was newly suggested. To improve the accuracy of the new equation, especially for high C TWIP steels, the TN values of three different high C TWIP steels were experimentally measured using a superconducting quantum interference device (SQUID) because there are few measured TN values of high C TWIP steels in the literature.

High ductility of ultrafine-grained steel via phase transformation

There is often a tradeoff between strength and ductility, and the low ductility of ultrafine-grained (UFG) materials has been a major obstacle to their practical structural applications despite their high strength. In this study, we have achieved a {approx}40% tensile ductility while increasing the yield strength of FeCrNiMn steel by an order of magnitude via grain refinement and deformation-induced martensitic phase transformation. The strain-rate effect on the inhomogeneous deformation behavior and phase transformation was studied in detail.

Microstructure and Mechanical Properties of Al-Added High Mn Austenitic Steel

Many studies about high Mn austenitic steels have been done due to their excellent mechanical properties such as tensile strength, uniform ductility, and wear resistance. In this paper, we tried to summarize the effects of Al addition on carbide precipitation, stacking fault energy (SFE), dynamic strain aging (DSA), tensile properties, and hydrogen delayed fracture in high Mn austenitic steels such as Hadfield and TWIP steels. The addition of Al suppressed the cementite precipitation due to the decreases in both activity and diffusivity of C in austenite. However, the κ′-carbide precipitation with a chemical formula (Fe, Mn)3AlC x may occur in the high Mn and Al austenitic steel. The addition of Al linearly increased the stacking fault energy of austenite. The Al addition increased the strain hardening rate in the Hadfield steels owing to the increase in the DSA effect caused by the increased solute C content, while it decreased the strain hardening rate in the high Mn TWIP steel because of the decreases in mechanical twinning caused by high SFE and in DSA effect caused by slow diffusivity of C atoms. The addition of Al improved the resistant property to the hydrogen delayed fracture in the high Mn TWIP steels.