First-principles Calculations Of Interatomic Interactions And Molecular Dynamics Simulation Of N Diffusion In Rare-earth Nitrocarburized Layers

Carburizing, nitriding and nitrocarburizing are important methods tostrengthen the surface of iron based alloys. The introduction of C and N atomsenhances mechanical properties of the modified layers. Rare earth （RE） additionduring the processes of these thermochemical treatments reduces the processingduration, increases the thickness of the modified layer, and improves the surfacemechanical properties and microstructures. In the present work, first principlescalculations and molecular dynamics simulations are carried out to study thephysical fundamentals of rare earth thermochemical treatments. Our investigationsinclude four aspects as follows. The first is the behaviors of C （or N） atoms in Fewithout considerations of other alloying elements. The second is the interactions ofC （or N） atoms with other alloying elements and vacancy in nitrocarburized layersof steels. The third is the interactions of La with other solute atoms in rare earthnitrocarburized layers. The fourth is Fe N potential and the molecular dynamicssimulations of N diffusion.A modified layer with the thickness of58.6μm on174PH steel is obtained byplasma nitrocarburizing at500°C for4h. After the addition of rare earth elements,the thickness becomes66.3μm, and increases13.1%. High-resolution XPSspectrum of La3d at the depth of300nm indicates that lanthanum atoms exist inLa0+state.A C （or N） atom prefers to occupy an octahedral interstitial site in bcc Fe.Weak covalent bonds are formed between C （or N） atom and its neighbor Fe atoms.The strength of Fe C is higher than that of Fe N. There are some hybridizations inthe DoSs （Density of States） of the Fe atoms neighboring to C （or N） atoms. C （orN） atoms gain electrons, its neighbor Fe atoms lose electrons. Two foreigninterstitial atoms （FIAs, i.e. C or N） repel each other. As the distance between twoFIAs increases, the repulsion tends to decrease. C C interactions are the leastrepulsive, N N interactions are the most repulsive, and C N interactions just liebetween of them. During the processes of thermochemical treatments, the drivingforce of C （or N） atoms diffusion originates from the repulsions between two FIAs.A C （or N） atom and a vacancy attract each other. A C （or N） atom prefers tooccupy the first nearest neighbor octahedral interstitial sites of a vacancy. Avacancy can attract only three C atoms, or two N atoms, or a C atom and a N atom.A C （or N） atom and110self interstitial atoms （SIAs） repel each other.A FIA and a foreign substitutional atom （FSA, i.e. Al, Si, Ti, V, Cr, Mn, Co, Ni,Cu, Nb and Mo） repel each other. As the FIA FSA distance increases, the repulsion decreases and tends to zero. After the introduction of a vacancy, attractiveinteractions are formed between a FSA and a vacancy, and between a FIA, a FSAand a vacancy. C （or N） atoms and FSAs prefer to accumulate around vacancies,and form clusters. These clusters will grow up and form FSA carbides and nitrides.This is the vacancy induced formation mechanism of carbides and nitrides.When a single FSA stays in bcc Fe, the difference between La and other FSAsis great. Big positive values are obtained for the substitution energy of a La atom inbcc Fe and the sum of the Fe atoms’ relaxations around La atom. The repulsion ofLa atoms with C （or N） atoms is obviously greater than other FSAs. The interactionis attractive for La Cu, close to zero for La Co/Ni, and repulsive for La FSA （theFSA is Al, Si, Ti, V, Cr, Mn, Nb or Mo）, respectively. The DoSs of all FSAsneighboring to La present a hybridization because of the interaction of La atomwith another FSA. The bond populations of C （or N） atoms （first or second nearestneighboring to La） and its neighbor Fe atoms increase, which enhances the strengthof the covalent bonds.Based on embedded atom method, a many-body potential for N in Fe isdeveloped. The potential parameters are determined by fitting to first principlesdata, which includes energetics （solvation and interaction energies）, configurationsof a N atom with other point defects, and relaxations of Fe atoms close to N atom.This potential successfully reproduces the physical properties of N atoms in bcc Fe,γ′Fe4N and ε Fe₂N. The physical properties of N atoms in fcc Fe are predictedbased on our developed Fe N potential as follows. Octahedral sites are also thepreferred position of N atoms. When a N atom and a vacancy are the first nearestneighbors, they are attractive and form a stable configuration. A N atom with SIAs,and a vacancy with two N atoms also attract each other. Two N atoms lie as faraway from one another as possible.Based on the Fe N potential, the diffusion of N atoms in bcc Fe is simulatedby molecular dynamic methods. The vibrations and migrations of a N atom ondifferent octahedral interstitial sites are vividly observed. By fitting to theArrhenius equation, the N atoms diffusion constant and activation energy areobtained and in agreement with experimental data.