Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50 uniform elongation

摘要

A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.