Ordering Of Interstitial Atoms Research Articles

Carbon Ordering in Martensite Lattice Under External Stress: Thermodynamic Theory and Molecular Dynamics Simulation

Rapid cooling of the Fe–C fcc solution leads to the formation of a phase called martensite, which has high mechanical properties. It is well known that under stress a martensite has the ability to plastically deform within a small range, despite the large value of the macroscopic yield strength. This effect is explained in different ways, mainly on the basis of dislocation theory. We study the impact of external stress on ordering of interstitial atoms in Fe–C solid solutions and formation of tetrahedral distortion of martensite lattice. Molecular dynamics with embedded atom (EAM) interatomic potential is used. According to the simulations, interstitial carbon atoms migrate under external stress applied along the tetragonality direction from z‐sublattice, and tetrahedral distortion of lattice changes its orientation to other ones when the stress exceeds some critical value. This result enables us to justify a new look at the nature of the martensite plastic deformation. The influence of temperature and carbon content on the critical stress value is investigated.

Neutron diffraction study of the formation kinetics of ordered antiphase domains in titanium carbohydride TiC x H y

The kinetics of formation and growth of ordered antiphase domains (APDs) in titanium carbohydride TiC0.50H0.21 has been investigated by neutron diffraction. A model of ordered APDs is proposed. It is established that the pronounced ordering of interstitial atoms and APDs begin at 450°C. It is shown that the period of ordered APDs (Р ≈ 10–12) is independent of the exposure time at a constant temperature. It is found that the temperature of ordered APDs, TOAPD, increases nonlinearly with an increase in the carbon concentration in the range 0.50 ≤ C/Ti ≤ 0.70. The formation temperature of ordered APDs is found to correlate with the concentration dependence of the order–disorder transition temperature and be 0.60 of the order–disorder transition temperature: TAPD = 0.60ТС.

Application of the cluster variation method to ε-Fe 2N 1-z and ζ-Fe 2N: ordering of interstitial atoms

The regular and the irregular trigonal prism approximations of the cluster variation method (CVM) are applied to investigate the ε-Fe 2N 1− z and the ζ-Fe 2N phases. The ε-Fe 2N 1- z /ζ-Fe 2N phase boundary, the types of interstitial ordering and the lattice parameters of the ε and ζ phases are calculated and compared with the available experimental data. In the composition range from 25 to 33 at.% N, long-range order occurs. The changes in the N distributions are associated with the strong repulsive interactions between neighbouring N atoms. The CVM results show that with increasing the N content towards 33 at.% N, the repulsive N–N interactions favour the distortion of the iron host lattice, thereby increasing the nearest-neighbouring distance between occupied sites, i.e. transformation to the ζ-Fe 2N phase. The calculated interstitial site occupations are in good agreement with those proposed on the basis of experimental data obtained by Mössbauer spectroscopy and neutron diffraction.

Stress-induced ordering of interstitial atoms due to dislocation motion

Abstract The variation of the activation energy of plastic deformation with strain rate was investigated in iron-carbon alloys as a function of interstitial concentration, pre-strain, temperature and grain size. The activation energy was measured directly by a differential temperature technique and was found to be 12 to 14 kcal/mole a t fast strain rates, rising to 20 kcal/mole a t slower strain rates. The stress dependence of strain rate was investigated in two strain rate ranges and was found to be a linear function in the slow strain rate range and a logarithmic function in the fast strain rate range. It was concluded from the data that this investigation was a direct verification of theories of Schoeck and Seeger and Eshelby of stress-induced ordering of interstitial atoms due to the stress fields of moving dislocations. The observed minimum in the activation energy versus strain rate relationship proves that the activation energy of the usual low-temperature rate-controlling mechanism does not increa...