Unusual relationship between impact toughness and grain size in a high-manganese steel

The high-manganese steels are important structural materials, owing to their excellent toughness at low temperatures. However, the microstructural causes for their unusual properties have not adequately been understood thus far. Here, we report a reversal relationship between impact toughness and grain size in a high-manganese steel and its unrevealed microscopic mechanisms, which result in an excellent low-temperature toughness of the steel. Our investigations show that with increasing grain size the impact toughness of the steel can be improved drastically, especially at low-temperatures. Advanced electron microscopy characterization reveals that the enhanced impact toughness of the coarse-grained steel is attributed to the twinning induced plasticity and transformation induced plasticity effects, which produce large quantities of deformation twins, epsilon(hcp)-martensite and alpha'(bcc)-martensite. Inversely, in the fine-grained steels, the formation of deformation twins and martensite is significantly inhibited, leading to the decrease of impact toughness. Microstructural characterizations also indicate that epsilon(hcp)-martensite becomes more stable than alpha'(bcc) -martensite with decreasing temperature, resulting in characteristic microstructures in the coarse-grained samples after impact deformation at liquid nitrogen temperature. In the coarse-grained samples under impact deformation at -80 degrees C, epsilon(hcp)-martensite transformation, alpha'(bcc)-martensite transformation and deformation twinning all occur simultaneously, which greatly improves the toughness of the steel. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

High-manganese steels are important structural materials known for their excellent low-temperature toughness. A reversal relationship has been observed between impact toughness and grain size in these steels, where increasing grain size can drastically improve impact toughness, especially at low temperatures. This improvement is attributed to twinning induced plasticity and transformation induced plasticity effects in the coarse-grained steel.