Woo-Sang Jung

Ab initio calculation of interfacial energies between transition metal carbides and fcc iron

An ab initio study was carried out on coherent and semicoherent interfacial energies for fcc Fe/MCs (NaCl structure, M = Ti, Zr, Hf, V, Nb, Ta) systems. The group V transition metal carbides have lower coherent and semicoherent interfacial energies than group IV transition metal carbides. Also, the coherent interfacial energies for fcc Fe/MCs systems are lower than those for bcc Fe/MCs systems. The difference between semicoherent and coherent interfacial energy for fcc Fe/MC systems becomes larger as the misfit increases. The semicoherent interfacial energies at relaxed interfaces Fe/TiC, Fe/ZrC, Fe/HfC, Fe/VC, Fe/NbC and Fe/TaC were 0.600?J?m?2, 0.661?J?m?2, 0.946?J?m?2, ?0.050?J?m?2, 0.320?J?m?2 and 0.380?J?m?2, respectively. In order to maximize precipitation strengthening effect in austenitic steel, VC is the most favorable precipitate under consideration.

Atomistic study of temperature dependence of interaction between screw dislocation and nanosized bcc Cu precipitate in bcc Fe

Molecular dynamics simulations of the interaction between a screw dislocation and a 3.5 nm diameter bcc Cu precipitate in bcc Fe have been performed in the twinning and antitwinning direction between 10 and 400 K. The results indicate a significant temperature dependence in the antitwinning direction, whereby the screw dislocation bypasses the Cu precipitate by Orowan looping below 200 K, but shears the precipitate above 300 K. The transition in interaction mechanism is caused by a screw dislocation assisted martensitic transformation of the Cu precipitate, which significantly diminishes above 300 K. The screw dislocation shears the precipitate at all temperatures between 10 and 400 K in the twinning direction. Thus, transformation of the precipitate induces additional precipitate strengthening below 200 K in the antitwinning direction, which drastically decreases with increasing temperature above 300 K.

Modified embedded-atom method interatomic potentials for the Nb-C, Nb-N, Fe-Nb-C, and Fe-Nb-N systems

Modified embedded-atom method (MEAM) interatomic potentials for Nb-C, Nb-N, Fe-Nb-C, and Fe-Nb-N systems have been developed based on the previously developed MEAM potentials for lower order systems. The potentials reproduce various fundamental physical properties (structural properties, elastic properties, thermal properties, and surface properties) of NbC and NbN, and interfacial energy between bcc Fe and NbC or NbN, in generally good agreement with higher-level calculations or experimental information. The applicability of the present potentials to atomic-level investigations to the precipitation behavior of complex-carbonitrides (Nb,Ti)(C,N) as well as NbC and NbN, and their effects on the mechanical properties of steels are also discussed.

An ab initio study of the energetics for interfaces between group V transition metal Nitrides and bcc iron

An ab initio study was carried out on interface energies, misfit strain energies and electron structures at coherent interfaces between bcc Fe and Nitrides (XNs) (NaCl structure, X = V, Nb, Ta). The interface energies at relaxed interfaces Fe/VN, Fe/NbN and Fe/TaN were −0.051 J m−2, −0.226 J m−2 and −0.643 J m−2, respectively. The influence of bond energy was estimated using the discrete lattice plane/nearest neighbour broken bond model. It was found that the dependence of interface energy on the type of nitride was closely related to changes of the bond energies between Fe, X and N atoms before and after formation of the interfaces Fe/XN. The misfit strain energies in Fe/VN, Fe/NbN and Fe/TaN systems were −0.052, 0.178 and 0.005 eV per 16 atoms (Fe 8 atoms and XN 8 atoms). The misfit strain energy became larger when the difference in lattice parameters between the bulk Fe and the bulk XNs increased.

Modified embedded-atom method interatomic potentials for the Ni–Co binary and the Ni–Al–Co ternary systems

Interatomic potentials for the Ni–Co binary and Ni–Al–Co ternary systems have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The potentials describe structural, thermodynamic, deformation and defect properties of solid solution phases or compound phases in reasonable agreements with experiments or first-principles calculations. The results demonstrate the transferability of the potentials and their applicability to large-scale atomistic simulations to investigate the effect of an alloying element, cobalt, on various microstructural factors related to mechanical properties of Ni-based superalloys on an atomic scale.

Transmission-EBSD Using High Current Electron Beams

Transmission-EBSD (t-EBSD) also known as Transmitted Kikuchi Diffraction (TKD) is becoming increasingly popular among researchers in the field of nano/microstructure [1-6]. It offers a higher resolution than conventional EBSD while utilizing commercially available hardware and software. This enables large-scale quantitative measurements of nanoand ultra-fine-grained materials. However, recent studies have shown that severe drifting and carbon contamination may occur during the scanning of large areas with fine step sizes [6-7]. Additionally, it has been known that Kikuchi band intensity decreases significantly with increasing projection angle and detector distance from SEM’s optical axis [7-9].

First-principles study on the thermal expansion of Ni-X binary alloys based on the quasi-harmonic Debye model

In this study, we use the quasi-harmonic Debye model to predict the coefficient of thermal expansion of Ni- X binary alloys. The method bridges between the macroscopic elastic behavior and thermodynamic properties of materials without an expensive calculation of the volume dependence of the phonon density of states. Furthermore, the Grüneisen parameter is derived from the volume dependence of the Debye temperature, which is calculated from the first-principles elastic stiffness constants. The experimental coefficient of thermal expansion (CTE) of pure nickel is well reproduced, especially in the low temperature region. Among the few alloying elements tested, Al is predicted to slightly decrease the CTE whereas Mo and W are more effective in reducing the CTE. For the cases of Ni-X binary alloy systems, where the variation in the CTE is relatively small, the method used here appears to perform better than certain other formulations that rely entirely on the energy vs. volume relationship.

Surface modification of cast inconel 740 superalloy by heat-assisted friction stir processing

Cast In740 Ni-based alloy with large grains generally show remarkable high temperature strength. However, this alloy is still have insufficient surface microstructure-dependent properties. In this study, for improvement of surface properties, surface modification of a cast In740 Ni-based superalloy was successfully performed by friction stir processing using a conventional two-horsepower milling machine and by additional heating to facilitate plastic flow of the hard alloy. Without using a high-power heavy stirring machine, a notable reduction in grain size of 2.9 μm was achieved and a corresponding 30% increase in Vickers hardness was observed. The microstructure in the stir zone was analyzed in terms of the grain size and precipitate distribution. The result of the potential dynamic polarization test and in-situ acoustic emission monitoring show that electrochemical corrosion resistance was improved by this surface modification process.