Papers by Keyword: Niobium

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Authors: M.S. Blanter, V.V. Dmitriev, Andrei V. Ruban
Abstract: t is common knowledge that interstitial-interstitial interaction influence on the Snoek relaxation. We used a computer simulation of this effect in the Nb-O alloy to test the adequacy of various models of the O-O interaction and clarify the mechanism of this effect The energy calculations in the first twelve coordination shells have been performed by the projector augmented-wave (PAW) method as implemented in the Vienna ab initio simulation package (VASP). The energies have been calculated in different ways which vary in the method of determination the energy of non-interacting O-O pairs. The energies calculated on the various variants are similar but in one case there is O-O repulsion in all first twelve coordination shells, whereas in another one can see attraction in four of twelve shells. Internal friction Q-1 was calculated as a sum of the contributions from individual interstitial atoms in different environments, each of which being assumed to be the Debye function. It is assumed that long-range interaction of oxygen atoms affects the distribution of these atoms and the energy of each interstitial atom in the octahedral interstices before a jump and after a jump. The Monte Carlo method is used for simulating short-range order of interstitial atoms and for calculating values of energy changes. Comparison of the calculated temperature and concentration dependence of the Snoek peak with the published data showed that the PAW supercell calculation of the O-O interactions in Nb describes the behavior of the interstitial solid solution adequately. It proves also that the impact of interstitial atom concentration on the Snoek relaxation is connected to the mutual attraction of these atoms.
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Authors: Hong Li Wu, Xin Qing Zhao, Jian Xu, Chun Gen Zhou, Hui Bin Xu
Abstract: A first principle study was performed to discuss the high temperature oxidation mechanism of NiTi alloys with the special emphasis on the effect of Nb on the oxidation behavior. The calculation results suggest that the Nb atom prefers the Ti site in Ni(Ti,Nb). The addition of Nb will not only reduces the electron density of Ti-d and Ni-d states near the Fermi energy level but the their electron contributions to the p-orbital of Ti. In addition, the Nb atom increases the formation energy of the Ti defect, which will decrease the diffusion of Ti atoms. All these Nb-induced effects account for the improvement of high temperature oxidation resistance, which agrees well with the experimental results.
1471
Authors: Marcio W.D. Mendes, António Carlos P. Santos, Francisca de Fatima P. Medeiros, Clodomiro Alves Jr., A.G.P. Silva, Uilame Umbelino Gomes
Abstract: The aluminothermic reduction is a highly exothermal reaction between a metal oxide and aluminium. Conventionally this reaction is ignited by an electric resistance and the reaction products after cooling are in the form of a rigid block of mixed metal and aluminium oxide. In this work a new process of aluminothermic reduction is presented, in which the reaction is ignited by a hydrogen plasma. The niobium oxide and aluminium powders are high energy milled for six hours to form particles constituted of oxide and aluminum. Stoichiometric, substoichiometric and superstoichiometric mixtures were prepared. The mixture was placed in a stainless steel tube (the hollow cathode) inside the reactor chamber. The chamber was firstly evacuated. Then hydrogen at low pressure was introduced. In the following an electric discharge between the cathode and the anode localized just above the cathode ignites the plasma. The plasma heats the particles on the surface of the powder layer and starts the reaction that proceeds in each particle since the reactants are intimately mixed. The heat generated by the reaction propagates deeper in the layer until the whole mixture reacts. Substoichiometric mixtures can be used because hydrogen takes part of the reduction. The Nb2O5 – Al starting powder mixture and the products of the reaction are characterized by laser grain size measurement and X-Ray diffraction (XRD). The products are in form of powder or agglomerates of particles. Phases of reaction products was determined by XRD analysis and the particle size trough SEM.
599
Authors: Hugo Ricardo Zschommler Sandim, Dierk Raabe
Abstract: The recrystallization behavior of coarse-grained niobium depends on the nature of its deformation microstructure. In this regard, a longitudinal section of a high-purity coarse-grained niobium ingot was cold rolled to a thickness reduction of 96% followed by annealing in vacuum at 800°C for 1 h. Metallographic inspection in cold-rolled and annealed specimens was carried out in a field emission gun scanning electron microscope (FEG-SEM). Microtexture was determined by electron-backscattered diffraction (EBSD) coupled to the FEG-SEM. The use this technique has evidenced details of the boundary character and subgrain structure found in partially recrystallized regions. The early stages of primary recrystallization are associated to the presence of high-angle lamellar boundaries found in the cold-worked state. Abnormal subgrain growth has been evidenced as a viable mechanism for nucleation of recrystallization.
519
Authors: L. Sanz, B. Pereda, B. López
Abstract: Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (≈0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and ≈0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (ε = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.
1170
Authors: Xiao Jian Wang, Jian Yu Xiong, Yun Cang Li, Peter D. Hodgson, Cui E Wen
Abstract: Current orthopaedic biomaterials research mainly focuses on developing implants that could induce controlled, guided and rapid healing. In the present study, the surface morphologies of titanium (Ti) and niobium (Nb) metals were tailored to form nanoporous, nanoplate and nanofibre-like structures through adjustment of the temperature in the alkali treatment. The in vitro bioactivity of these structures was then evaluated by soaking in simulated body fluid (SBF). It was found that the morphology of the modified surface significantly influenced the apatite inducing ability. The Ti surface with a nanofiber-like structure showed better apatite inducing ability, than the nanoporous or nanoplate surface structures. A thick dense apatite layer formed on the Ti surface with nanofiber-like structure after 1 week soaking in SBF. It is expected that the nanofibre-like surface could achieve good apatite formation in vivo and subsequently enhance osteoblast cell adhesion and bone formation in vivo.
85
Authors: Klaus Hulka, A. Kern, U. Schriever
Abstract: This article offers an overview on the ways and means of the application of Nb in quenched and tempered high-strength steel plates. Thereby, the outstanding role of Nb to control the austenite microstructure during rolling or heat treatment and to contribute to effective precipitation hardening is discussed. It is shown, that Nb is very effective to retard the transformation processes during quenching. For high-strength constructional steels with up to 1100 MPa yield strength and for wear resistant steels, the improvement in strength and toughness properties in Nb microalloyed steels as compared to Nb-free steels is pointed out. A remarkable effect is the improvement in toughness and brittle fracture resistance due to a very fine microstructure and finely dispersed Nb carbonitrides in a martensitic microstructure. Fields of application for the new Nb microalloyed high-strength structural steels are presented too.
519
Authors: Marco Antônio Schiavon, I.V.P. Yoshida, A.C. Silva, Wilson Acchar
Abstract: Ceramic matrix composites (CMC) were prepared by the active-filler-controlled polymer pyrolysis process (AFCOP) using a polysilsesquioxane resin filled with metallic niobium and alumina powders. Samples containing 60 wt% of polysilsesquioxane and 40 wt% of metallic niobium and alumina powders mixtures were homogenized, uniaxially pressed and pyrolysed in an alumina tube furnace up to 1400 °C, under argon flow. The ceramic products were characterized by X-ray diffraction (XRD), thermogravimetry (TGA), differential thermal analysis (DTA), Fourier transform infrared (FTIR) and energy-dispersive (EDS) spectroscopies. XRD analysis of the products showed the presence of crystalline phases such as NbC, Nb3Si, Nb5Si3, SiC, crystoballite and mullite. Thermogravimetry data of the composites presented low weight losses at 1000 °C. DTA curves showed an endothermic peak at 1350 °C, which was associated to the beginning of carbothermic reduction and/or the formation of silicon oxide and carbide. In addition, an exothermic peak at 1400 °C was associated to the formation of the mullite phase.
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