Arturo Bronson

The effect of oxygen partial pressure on the stability of Magneli phases in high temperature corrosive wear

Abstract Titanium carbide with the presence of the Magneli phases has been considered for corrosive wear at temperatures greater then 1000 K. To investigate the effects of the Magneli phases (or non-stoichiometric titanium oxides, TinO2n − 1) on the wear behavior of TlC or titanium oxides, experiments must be performed under known oxygen partial pressures under which the non-stoichiometric compounds are stable in a vacuum or flowing system. The TiOC stability diagram, which clearly shows the oxygen partial pressure dependence of the Magneli phases in high temperature investigations, has been developed for the temperature of 1673 K from the thermodynamic data available for the titanium oxides and TiC. The gaseous atmospheres needed for stabilizing the Magneli phases during material processing and in experimental wear research are briefly discussed.

Compatibility of Refractory Metal Boride/Oxide Composites at Ultrahigh Temperatures

Abstract : The compatibility of the HfO2/HfSi2/HfB2 phases has been investigated in an argon atmosphere at 1800 deg, 1700 deg and 1600 deg C. At the three temperatures investigated, HfB 2 was determined to be relatively inert to either the oxide or silicide phases when examined with the scanning electron microscope and optical microscope. At 1800 deg C, a reactive product was found between the hafnia and hafnium dislicide phases with a wavy interface moving, parabolically with respect to time. The microstructural analysis of the interface suggests that liquid contributes to the interfacial reactions. In addition, two-solid phases (Hf 2 Si and HfSi) were formed at temperature along with silicide and silicate liquids located between HfSi2 and HfO2. At 1600 deg C for annealing times from 4 to 50 hours, the HfSi 2 /HfO 2 interface does not react as inferred from the acquired constant thickness of the silicide layer. Ceramic Composites, High Temperatures Materials Boride-Oxide Composites, Compatibility, Silicide-Oxide Composites.

Analysis of the scribing technique for determining the corrosive wear of an Fe-21wt.%Cr-19wt.%Ni alloy

Abstract To investigate the mechanical-chemical interactions, a triboelectrochemical system has been developed to measure experimental parameters during corrosive/abrasive wear. The experimental technique measured the transient current response of a rotating cylindrical electrode after a Vickers diamond stylus impacted the surface with a force ranging from 0–40 N. The surface of the electrode was slightly activated with increasing scribing loads. The impact of the stylus on the rotating electrode produced a scribing action, which caused an indenting process followed by plowing on the surface. The surface topography of the scars showed a plastically deformed region surrounding the diamond shaped scar created by the Vickers diamond. The scribing on the rotating cylindrical electrode was explained by considering the behavior of microindentation and by calculating the impulse of deformation. The impulse calculated for each scratch followed a linear relationship with respect to force with an adequate correlation. In contrast, the scratch current density ( i s ) which quantifies the metal dissolution during scribing correlated poorly with the impact force, even though a corrected area was used to determine i s . For light impact forces ( F N ) the plastic zone was calculated for each scratch according to Hills equation developed for explaining microindentation. The plastic zone contributes an area of activation, which can significantly decrease the ohmic potential drop and the scratch current density during the electrochemical measurements.

Influence of near-surface microstructures on the transient current response in Fe-Cr-ni alloys during scratch tests

Metals and alloys are protected from the corrosive environments in which they are used by the presence of a passive film on the surface. However, the passivity of a metal surface cannot be preserved if mechanical forces rupture this protective film from the surface. Thus, during corrosive wear, the process of wear essentially destroys the passive film, which then leaves a bare metal surface for the corrosion reaction to start taking place once again. In an industrial application of a metal, the process of passivation, depassivation, and repassivation kinetics must be completely understood if the life of a metal part is to be prolonged. The purpose of this article is to study the kinetics involved in the corrosive wear of Fe-Cr-Ni alloys by the scratch technique and to determine the relationship between the electrochemical response of these alloys and the near-surface microstructures.

The effect of indenter shape on the transient current response and surface microstructures in Fe-16wt.% Cr-16wt.%Ni alloy

Abstract The effect of indenter shape on the transient current response of Fe-16 wt.%Cr-16 wt.%Ni alloy has been investigated in an 0.01 M H2SO4-0.01 M KCl electrolyte. Various Rockwell hardness indenters with C-Brale, Vickers, 1 16 in and 1 8 in balls at their tips have been used in this study to produce scratches of different shapes on the alloy when the specimen was being rotated at 1500 rev min−1 in the given electrolyte. The transient current increases linearly with increasing impact force of the stylus. The transient current response in itself indicates various stages corresponding to passivation, depassivation and repassivation kinetics of the samples. The details of the kinetics have been discussed in this paper. The scanning electron micrographs indicate that the area of the scratches increased with increasing applied load, and the morphologies of the scratches appear to be quite different in air and electrolyte. The scratches are usually more elongated in the electrolyte.

Phase relations of a silicide/silica reaction couple at 2273 K

The phase relations of a zirconium silicide/silica reaction couple have been investigated at 2273 K in air. After annealing times from 4 to 49 hours, the reaction couples, created by encapsulating zirconium disilicide in a quartz ampule, developed an interdiffusion zone and an inner core consisting of solidified zirconium silicide. The interdiffusion zone consisted of a silica layer, zirconia precipitates, and prior liquid silicide globules dispersed in prior liquid silica. Zirconia precipitates formed from the oxidation of the silicide melt between the protective silica layer and the two liquid regions of silicide and silica, as a result of oxygen diffusion. At 2273 K, the inner core consisted of liquid zirconium silicide (ZrSi)L, although the microstructure analysis indicated formation of proeutectic ZrSi2 and a eutectic microstructure of ZrSi2 and Si upon solidification. A constant oxygen potential developed within the interdiffusion zone and protected the silicide from oxidation even after 49 hours at temperature.

Numerical analysis of certain solutions of laplace's equation to calculate the ohmic potential drop after scribing

Abstract The accuracy of certain solutions to Laplaces equation for various electrode geometries is examined and compared to the accuracy of new solutions based on a Taylors series expansion. Whereas previous solutions are accurate only for cases where the counter-electrode is remotely placed from the working electrode, the new solutions are highly accurate for all values or relevant geometric parameters. In particular, for an electrode with a non-uniform current or uniform current distribution, the ohmic drop computed with an equation assuming an infinite spacing can deviate substantially from the correct potential drop for a counter-electrode near a working electrode. For the specific case of a rectangular electrode, the solution computed here provides a better fit to experimental data than available solutions in the literature. A direct benefit to the use of these results is the freedom from having to make the decision as to what constitutes a large enough separation (or spacing) between the working- and counter-electrodes. In addition, error bounds on the use of the new solutions are provided for the benefit of the user.

Solution of Laplace's Equation to Calculate the Ohmic Potential Drop for Hexagonally‐Shaped Scratches

Laplaces equation is solved to calculate the ohmic potential drop resulting from hexagonally-shaped scars. The solution also generalizes the results previously presented for rectangular and diamond-shaped scratches. For scars having identical lengths and widths, the ohmic potential drop decreases, as expected, in the following order Φ (rectangular)>Φ (hexagonal)>Φ (diamond). For equal areas, however, it is possible for the diamond- and hexagonal-shaped scratches to produce either a larger or smaller ohmic potential drop than the rectangular scratch

High-Temperature Liquid Metal Infusion Considering Surface Tension-Viscosity Dissipation

In considering the significant effect of the surface tension-viscosity dissipation driving the fluid flow within a capillary, high-temperature liquid metal infusion was analyzed for titanium, yttrium, hafnium, and zirconium penetrating into a packed bed. A model of the dissipation considers the momentum balance within the capillary to determine the rate of infusion, which is compared with the Semlak-Rhines model developed for liquid metal penetration into a packed bed assumed as a bundle of tubes mimicking the porosity of a packed bed. For liquid Ti, the penetration rate was calculated from 0.2 µs to 1 ms and rose to a maximum of 7 m/s at approximately 1 µs; after which, the rate decreased to 0.7 m/s at 1 ms. Beyond 10 µs, the decreasing trend of the rate of penetration determined by the model of dissipation compared favorably with the Semlak-Rhines equation.

Feasibility Studies of Encapsulated Particles With Heat Absorbing Medium at 800–1300°C for Concentrating Solar Power Technology

The feasibility of using liquid Al or B2O3 encapsulated in SiC particles was studied by using thermodynamic analysis and fluid-solid analysis at temperatures ranging from 800 to 1300°C. Alloy melts of the Al-Si and Fe-Al-Si systems were considered for absorbing and desorbing energy for a high temperature energy storage (TES) unit incorporated in a concentrating solar power scheme. Boria was also evaluated instead of metallic melts and compared with the traditional NaNO3-KNO3 molten salt as a TES medium. In addition to determining the enthalpies for sensible heat and phase transformations, the phase equilibrium was determined for possible reactions at the liquid Al/SiC and B2O3/SiC interfaces by calculating their thermodynamic stability. The transport of encapsulated SiC particles within a fluid and their effect on the thermal conductivity is discussed toward the efficacy of the thermal energy storage.Copyright