The combined effects of uniaxial pressure and defects on the mechanism of martensitic transformation in pure iron: A molecular dynamics study

The as-produced martensitic steel fabricated by selective laser melting (SLM) is generally softer and weaker than the wrought specimen due to the residual austenite in the microstructure. To avoid additional post-heat treatment, this study proposes a strategy to reduce the residual austenite by applying additional pressure after each laser scanning. Therefore, the combined effects of uniaxial pressure and stacking faults (SFs) on martensitic transformation (MT) in the parent face-centered cubic (FCC) phase of pure Fe are investigated via molecular dynamics (MD) simulation. The simulation results show that the supercooling, uniaxial pressure, and pre-existing SF defects in the parent FCC phase provide chemical driving force, and mechanical driving force and reduce the energy barrier of MT, respectively. The Ms temperatures increase gradually with the pressure from 0 to 200MPa. The MT process consists of nucleation and growth. The heterogeneous nucleation occurs in the parent FCC phase with pre-existing defects and favoring intersecting-SF, SF, and defect-free FCC phase region in order. The homogeneous nucleation occurs in the parent FCC phase without pre-existing defect and the MT process is achieved by the growth and coalescence of the scattered crystal nucleus. The stress field generated by the dislocation lines at the intersecting region of SFs results in the lattice distortion of the atoms nearby, reducing the energy barrier of MT nucleation. The study verifies the feasibility of reducing residual austenite in SLM-ed martensitic steel by applying pressure during the cooling process of each layer.