Defect and Diffusion Forum Vols. 326-328

Abstract: The mechanical properties of metals with a body-centered cubic (bcc) structure, such as Nb, Ta, V, and their alloys, are modified with the introduction of interstitial impurities, such as O, N, C, or H. These metals can dissolve great amounts of O and N, for example, to form solid solutions. The interstitial solute atoms (ISA) in metals with a bcc structure occupy octahedral sites and cause local distortion with tetragonal symmetry. So ISA in these metals forms an elastic dipole that can align along one of the three cubic axis of the crystal. In the present paper, the torsion pendulum technique was employed for the investigation of various interactions among the metallic matrix and different interstitial solutes in the Nb-46wt%Ti alloy. From the relaxation spectra, we obtained the diffusion coefficients, pre-exponential factors, and activation energies for nitrogen in the Nb-46wt%Ti alloy.

Abstract: sotope exchange of oxygen ¹⁸О₂ with oxides Mn₂O₃ and Mn₃O₄ was investigated in the temperature range of 300700°C. It was established that the content of the isotope O¹⁸ in mechanically activated powders was noticeably higher than in the initial ones. Already at relatively low annealing temperatures isotope exchange leads to nearly the maximum possible under the given conditions replacement of atoms ¹⁶O to ¹⁸O. The isotope exchange parameters connected with the processes at the boundaries of the particles of mechanoactivated powder and within their volume were determined.

Abstract: The anelastic behavior of the ferroelectric ceramics (Pb)(Zr/Ti)O₃ (PZT) and (Pb/La)(Zr/Ti)O₃ (PLZT), with Zr/Ti = 65/35, La = 5 at.% and 8 at.%, was investigated in the region of the ferroelectric phase transitions. Anelastic spectroscopy experiments were performed in an acoustic elastometer system, operating in a kilohertz bandwidth, at temperatures rising from 300 K to 770 K, at a heating rate of 1 K/min, under pressure of 10^-5mbar. Anelastic measurements on PZT showed only one anomaly, associated with the occurrence of a ferroelectric-paraelectric phase transition, while the PLZT data showed two anomalies, which were associated with the following transitions: the ferroelectric-paraelectric phase transition and a ferro-ferroelectric phase transition between distinct rhombohedral ferroelectric phases. The behavior of the relative variation of the elastic moduli with temperature, near the phase transitions, which describes the change in the type of coupling between strain and the order parameter in ferroelectric-paraelectric phase transition, with the increase of lanthanum amount and, linear coupling in the strain and order parameter type to PZT ceramic and linear coupling in the strain but quadratic in order parameter type for PLZT ceramics.

Abstract: This paper is centred on a compact finite differences method for the calculation of two-dimensional viscous flows through complex geometries. The immersed boundaries are set through body forces that allow for the imposition of boundary conditions that coincide with the computational grid. Two different flow configurations are simulated. First, the flow past a cylinder with square cross-section inside a plane channel is calculated. The computed average drag coefficient and Strouhal number are compared to data available in the literature. The agreement between the results is good. The second flow configuration analyzed is the flow through a porous matrix composed of equal size staggered square cylinders. Flow visualization results are shown. The work presented in this paper illustrates the potential of the immersed boundary method in general and of this implementation in particular to simulate the flow through porous matrices.

Abstract: Hydrogen mobility has been studied at high temperature by absorption experiments in the Ni₅₂Ti₄₈ alloy, which does not transform martensitically but rather behaves like a so-called strain glass. The results obtained have been compared with those deduced from an anelastic relaxation occurring in this alloy below the strain-glass transition temperatures. An accurate analysis of the anelastic data has confirmed the conclusion that the relaxation is related to H rather than to the glass transition. Its relaxation time obeyed a Voogel-Fulcher type of temperature dependence. Combining absorption and anelastic results, the H diffusion coefficient in the B2 lattice structure of this alloy could be studied from 1200 K down to 170 K. The agreement between the absorption and mechanical spectroscopy data was satisfactory. The activation energy (0.33 eV) deduced from a Vogel-Fulcher representation of the H diffusion coefficient D was sensibly lower than earlier determinations (0.44-0.50 eV) from Arrhenius plots. The high temperature data of Ni₅₂Ti₄₈ alloy, compared with the ones available in the literature for other NiTi SMA in their B2 structure, show a substantial independence of D on the alloy composition.