Effect of Nb element on onset of deformation-induced martensitic transformation in iron: Insight from molecular dynamics simulations

Abstract

Niobium (Nb) is a crucial alloying element in advanced steels, influencing deformation-induced martensitic transformation (DIMT) in advanced high-strength multiphase steels. In this work, molecular dynamics (MD) simulations were used to investigate the effects of Nb on DIMT characteristics in iron, focusing on the onset strain (${\epsilon }_S$), final strain (${\epsilon }_F$) and complete transformation rate $({\eta }_C$) of DIMT at atomic level. MD simulations reveal that Nb stabilizes residual face-centered cubic (FCC) austenite at 300 K, maintaining approximately 56 vol% austenite with Nb concentrations between 0.05 and 0.4 at.%. Tensile simulations at 300 K for these Nb concentrations show the yield stresses ranging from 7.55GPa to 8.94GPa, attributed to a combination of phase transformation and dislocation mechanisms. Across these Nb concentrations, the ${\epsilon }_S$ remains consistent at approximately 4.34 %, and the ${\eta }_C$ is about 98 %. The formation of Lomer-Cottrell (LC) dislocations during yielding acts as a precursor for DIMT. Varying Nb content alters the competition between interface- and LC-triggered DIMT mechanisms. These findings provide valuable insights into the role of Nb in controlling DIMT and offer theoretical guidance for designing and developing high-performance Nb-alloyed steels.