Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel

We study grain boundary embrittlement in a quenched and tempered Fe Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper embrittlement at 450 degrees C. Tempering at 600 degrees C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.