The effect of nanopores on the mechanism of martensitic transformation in pure iron during the heating-cooling cycle: A molecular dynamics study

Numerous molecular dynamics (MD) simulations have been performed to investigate the effect of the nanopores on the martensitic transformation (MT) of pure iron (Fe) during the heating-cooling cycle. As per the simulation results, nanopores can obviously affect the difficulty on MT during the cooling process in two ways: On the one hand, the volume ratio of nanopores to simulation box model (Rp/b) can influence the distribution configuration of stacking fault (SF) to change the type of austenite microstructure during the heating process owing to the obstruction of the nanopores on the dislocation slip. The effect of nanopores is not obvious when Rp/b is less than 2.22 %. The main type of austenitic changes from defect-free/simple-SF types to the intersecting-SF type and, finally, to the dislocation-cell type with an increase in Rp/b from 2.22 % to 17.9 %. The dislocations in intersecting-SF type and dislocation-cell type austenitic microstructures can provide the nucleation site of MT, thus increasing the MT temperature. On the other hand, the high surface energy of the nanopores caused by the incomplete atomic lattice can provide the necessary energy for the nucleation of martensite, which then promotes the occurrence of MT during the cooling process. When the nanopores intersect with the dislocation wall in dislocation-cell type austenitic microstructure, martensite nucleates at the nanopores surface and then grows along the dislocation wall. In the previous investigation of selective laser melting martensitic steel, the nanopore in the microstructure was generally believed harmful to the mechanical property of specimen; however, in this study, it has been observed to promote the MT during solution-quenching heat treatment, which improves the mechanical property by improving the content of martensitic phase. This provides a new perspective in determining the significance of nanopores in microstructures.