Deformation Mechanism Of Fe And Fe-C Alloys By Molecular Dynamic Simulations

Steels are the most widely used structural material at present.The mechanical properties of steels can be significantly improved by adding different kinds of elements or adopting different heat treatment processes.Thus,steels are widely used in automobile,ship,aviation,and other industries.Steels with both high strength and good ductility have been one of the research targets pursued by researchers all over the world.In this work,the deformation mechanisms of FCC Fe and Fe-C alloys under uniaxial tensile loading condition are studied by molecular dynamic simulations.The effect of carbon atoms on the deformation mechanisms and mechanical properties are also investigated.The main contents and conclusions are summarized as follows:1.The deformation mechanisms of pure Fe under different uniaxial loading strain rate conditions are studied and compared with experiment results.The results show that at a strain rate of 10¹⁰ s^-1,FCC matrix transforms into BCC phase following the Nishiyama-Wassermann（NW）orientation relationship（OR）.Further strain can induce the BCC→HCP→BCC and BCC→FCC→BCC transformation serials obeyed the Burgers OR and the Pitsch OR,respectively.At a strain rate of 5×10⁹s^-1,after the martensite transformation,a part of BCC phase undergoes BCC→HCP→BCC transformation.At lower strain rates,such as 10⁹ s^-1 and10⁸ s^-1,only the martensite transformation is observed.2.The deformation mechanisms of Fe-C alloys with a carbon content of 0.5 at.%,1.0 at.%,and 1.5 at.%at a strain rate of 10¹⁰ s^-1 are studied and compared with that of pure Fe.The results show that the addition of carbon atoms can influence the phase transformation but can not change the orientation relationships and the variants between different phases during the pressure-induced phase transformation.With the addition of carbon atoms,the FCC→BCC and BCC→HCP phase transformations occur at lower strains and more stacking faults are observed in the HCP phase.Besides,carbon atoms can pin the grain boundaries when the phase grows with the movement of phase interfaces,leading to a finer grain size of the bulk material.3.The position of carbon atoms in different phases during phase transformations in Fe-0.5 at.%C is studied.The results show that carbon atoms in octahedral interstitial sites in FCC matrix locate in one out of three octahedral sublattice in BCC phase.When loading along the?110?direction that is perpendicular to the c axis,carbon atoms locate in basal crowdion sites in HCP phase and hexahedral sites in BCC products after phase transformations.When loading along the?110?direction that is not perpendicular to the c axis,carbon atoms locate in octahedral sites in HCP phase and BCC phase.The octahedral site is the most stable site in FCC,BCC and HCP structure.But pressure-induced transformations can still trap the carbon atoms in other interstitial sites.Carbon atoms in basal crowdion sites in HCP phase and hexahedral sites in BCC phase can hinder the movement of dislocations more effectively than octahedral sites in HCP phase and octahedral sites in BCC phase.