Papers by Keyword: Interstitial

Abstract: Anelastic relaxation measurements were performed in a Nb-46wt%Ti alloy, in the temperature range of 300 to 700 K, using a torsion pendulum operating at an oscillating frequency near 2.0 Hz. The samples were measured in different conditions: cold worked, annealed in ultra-high vacuum and doped with several quantities of nitrogen. The relaxation spectra obtained were resolved into their component peaks, corresponding to the different kinds of interaction of the interstitial solutes with the metallic matrix. The relaxation parameters of each process were calculated using Debye’s elementary peaks.

Abstract: Effect of grain boundary on strain ageing behaviour of Nb-bearing ULC steel sheets has been studied at the aging temperature from 70 to 220°C, using 2% pre-strained specimens with different ferrite grain sizes of 9.5μm and 183μm. Two different hardening stages were exhibited in the fine-grain specimen, whereas only a single hardening stage was shown in the large-grain specimen. The increase in YP of the first hardening stage was around 30MPa; the activation energy of this stage was estimated to be from 83 to 86kJ/mol, which is close to that of body diffusion of carbon atoms in α-Fe. The increase in YP of the second hardening stage reached 90MPa; the activation energy was 135kJ/mol, which is close to that of body diffusion of Fe atoms in grain boundary and precipitation of η-carbide. From TEM observations and nanoindentation analyses, it was inferred that the dominant mechanism could be dislocation pinning by carbon atoms for the first hardening stage, and grain boundary hardening or hardening around it for the second.

Abstract: We present numerical simulations of nucleation kinetics of vacancies and interstitials during RTA and we study the impact of annealing temperature on bulk micro defect concentration. Since the concentration of vacancies and oxygen and also its diffusion kinetics are significantly different inside Czochralski silicon, we assume the nucleation of vacancies and oxygen independent on each other. We show that different populations of voids formed during RTA can influence formation of oxygen precipitate nuclei. According to classical nucleation theory the homogeneous nucleation dominates around temperatures 500 °C while the calculation of oxygen diffusion into the voids shows that the oxygen clusters over the critical size can be formed above temperatures 700 °C. The nuclei concentration of BMD is thus the superposition of homogeneous nucleation below 700 °C and heterogeneous one prevailing above 700 °C.

Abstract: Molecular dynamics and molecular statics have been used to explore the transition between the partially Zener ordered state of carbon in octahedral sites. In this communication we have specifically used isothermal molecular dynamics with a recent Fe-C EAM potential to examine the observed high temperature transition from the Zener ordered state where carbon resides on only 1/3 of all octahedral sites to a state where all octahedral sites are available for occupation. It is shown that the Zener ordered state begins to disorder at temperatures well below the transition temperature and that this disordering occurs without any spatial correlation.

Abstract: To understand the structure of SiC–oxide interface more in detail, we propose a profiling theory of Si and C emission into SiC layer during oxidation. Simulations of the depth profiles of Si and C interstitials results in the structures analogous with those observed from a spectroscopic ellipsometry. To determine the diffusivities of Si and C interstitials, we performed capacitance–voltage measurements for examining the re-distribution profiles of nitrogen after oxidation and compared between observed and calculated profile. The calculated nitrogen profiles showed good fits to the observed ones in the case of self-diffusivity of C interstitials magnified by several 10 times for literature value. Finally, we discuss the validity of the proposed theory.

Abstract: In fast diffusion, the impurity diffusion coefficient is much greater than the self-diffusion coefficient. The pair mechanism is here considered to explain fast diffusion. Formulations for the formation of the pair are based upon pseudopotential theory.

Abstract: Using the full-potential-full-electron-linearized-augmented-plane-wave (FLAPW) method, we carried out ab initio calculations of the electronic structure for austenitic steels of different compositions. The total set allows to find a system of equations for obtaining the value and sign of the interatomic bond energy between Fe-Fe, Fe-Mn, Fe-Cr, Fe-Ni; Ni-Ni; Cr-Cr, Mn-Mn and Mn-Cr. It is obtained that both Cr-Cr and Cr-Fe bonds are attractive, however the Cr-Cr bonds are stronger. The interaction between interstitial carbon (nitrogen) and metallic atoms is calculated. The interstitial atoms change the metal-metal interaction between the nearest neighbors.

Abstract: The results of highly sensitive FTIR investigation, ab initio calculations and rate equation modeling of the early stages of oxide precipitation are compared. The attachment of interstitial oxygen to VOn is energetically more favorable than the attachment to On for n  6. For higher n the energy gain is comparable. The point defect species which were detected by highly sensitive FTIR in high oxygen Czochralski silicon wafers are O1, O2, O3, and VO4. Rate equation modeling for I, V, On and VOn with n = (1..4) also yields O1, O2, O3 to appear with decreasing concentration and VO4 as that one of the VOn species which would appear in the highest concentration after RTA.

Abstract: Successful theoretical models of vacancies, self-interstitials and oxygen dynamics during an ingot growth lead us to apply these models results also to silicon wafers. The modern silicon technology uses successfully the RTA (rapid thermal annealing) in order to form MDZ (magic denude zone) in individual CZ Si wafers. The effect of RTA is based on the utilization of vacancies for control of oxygen precipitation. The question about the theory of kinetics of vacancies and interstitials, which describes its behavior within RTA, is still opened up to now. This work deals mainly with the nucleation of vacancies during RTA concerning various cooling rates and initial states.

Abstract: Titanium alloys are excellent implant materials for orthopedic applications due to their desirable properties, such as good corrosion resistance, low elasticity modulus, and excellent biocompatibility. The presence of interstitial elements (such as oxygen and nitrogen) causes strong changes in the material’s mechanical properties, mainly in its elastic properties. Study of the interaction among interstitial elements present in metals began with Snoek’s postulate, that a stress-induced ordering of interstitials gives rise to a peak in the mechanical relaxation (internal friction) spectra. In the mechanical relaxation spectra, each species of interstitial solute atom gives rise to a distinct Snoek’s peak, whose temperature and position depend on the measurement frequency. This effect is very interesting because its peculiar parameters are directly related to the diffusion coefficient (D) for the interstitial solute. This paper presents a study of diffusion of heavy interstitial elements in Ti-35Nb-7Zr-5Ta alloys using mechanical spectroscopy. Pre-exponential factors and activation energies are calculated for oxygen and nitrogen in theses alloys.