Papers by Keyword: Surface Segregation

Abstract: This work investigated the surface tension and kinematic viscosity of the multicomponent Fe_73.5Cu₁Nb₃Si_13.5B₉ melt. A relationship was found between surface tension and kinematic viscosity, which manifests itself in a synchronous change in these quantities at temperatures of 1600 and 1780 K. In the temperature range 1600–1780 K, there is a sharp increase in surface tension upon heating and the same decrease upon cooling. The increase in surface tension during heating was explained by the appearance of a large number of free Nb atoms as a result of the dissolution of the residual crystalline phase in the mushy zone, and their diffusion to the melt surface. The drop in the surface tension on cooling below 1780 K is associated with the liquid–liquid structure transition (LLST), which stimulates the outflow of Nb atoms from the surface in order to form new stable clusters. The LLST manifests itself in a change in the activation energy of a viscous flow, which is higher in the high-temperature region and corresponds to the motion of larger clusters with a length scale of about 1 nm.

Abstract: Preparation of aluminum substrates for surface segregation enhancement of insoluble lead deposition was achieved. Sever plastic deformation 'SPD' of Al sheets was done using surface mechanical attrition treatment 'SMAT' in air. Scanning electron microscope SEM of etched Al substrates cuts showed micro-cavities both on the surface and in-depth. Orientation effects and surface inclusions of Pb inside Al surface found at 40 and 50 Hz - SMAT Al by X-Ray diffraction and energy dispersive of X-Rays EDX. Concluding that SMAT frequency limits used enhanced surface inclusions without annealing that could improve adhesion of industrial protective Pb coatings.

Abstract: The Cu–3%Ag alloy was prepared by continuous unidirectional solidification technique. The composition distribution of continuous unidirectional solidification Cu–3%Ag alloy was analyzed. The effect of different mold temperatures on the composition distribution of alloy was studied. The results show that when the mold temperature is low, continuous unidirectional solidification Cu–3%Ag alloy has surface segregation. The morphology of segregation phase mainly presents stripe and water drop shape. When the mold temperature is high, the surface segregation begins to decrease and eventually disappear.

Abstract: The Cu/Ni, Ni/Cu and Cu/Ni/Si film/substrate and film/film/substrate systems were prepared by magnetron sputtering method to investigate the surface segregation. The chemical composition of film was analyzed by Auger Electron Spectroscopy (AES). The microtopographies of the Cu/Ni surface and the cross section of the film were observed by Transmission Electron Microscope (TEM), where the blocky distribution of Ni-rich area on surface of Cu film and columnar grains was observed in the specimen. It is found that the thickness of sputtered film has stronger effect on the composition of segregation layer near the surface than that of the annealing time. The surface segregation could be ascribed to the fast vertical diffusion of the substrate atoms through the columnar grain boundaries and the subsequent lateral surface diffusion.

Abstract: The objective of this present research was to study the surface liquid segregation (SLS) of 7075 aluminum alloy formed by semi-solid squeeze casting together with the gas induce semi-solid technique (GISS). The rheocasting times of 3, 5, 10, and 20 seconds, were used in GISS processes. The cross sectional areas of specimens near the mold were used to measure the surface segregation. The results revealed that longer rheocasting time gave higher amount of solid fraction in the semi-solid slurry. In addition, it was found that when solid fraction increased, the surface segregation increased.

Abstract: The energetic study of B effects on the oxidation of γ-TiAl alloys are performed by using the first-principles method based on Density Functional Theory (DFT) in this paper. The surface and interface segregatation of B as well as of the surface adsorption of O are discussed. B is found to preferentially segregat to TiAl subsurface with respect to γ-TiAl bulk. The B segregation at surface decreases oxygen coverage in the initial oxidation process of γ –TiAl alloys, thereby it is beneficial to the decrease of the growth of γ–TiAl alloys oxide film. In the initial oxidation process, oxygen prefers to stay in the vicinity of surface Ti atoms, and B addition is beneficial for the decrease of the growth of A1₂O₃ and TiO₂. After the formation of Al₂O₃ oxide film, B is energetically favoured stayed at interstitial site of α-Al₂O₃ (0001)/γ-TiAl (111) interface, and enhances the adhesion of this interface.

Abstract: First-principles calculations have been carried out to investigate the incipient oxidation mechanism of FeCrAl alloy. It is found that Al,Y,Cr atoms energetically prefer surface sites, and the driving force of Y segregation to surface is strongest. The surface segregation of Y, Al and Cr will suppress the outward diffusion of Fe, form the tight coherent films of Al₂O₃,Cr₂O₃ and RE oxides which can restrain oxygen inward diffusion, as a result, leading to the decrease of the growth of oxide films. The O adsorption process at Fe surface are found to be spontaneous, and our calculations predict Al, Cr, Y segregation at Fe surface is beneficial for decreasing the oxidation rate of FeCrAl alloy. The interaction between O and Fe, Cr, Al, Y atoms exists both ionic and covalent binding characteristics. Also Al, Y alloying increases ionic and covalent binding between Al, Y and O, which speed selective oxidation of Al and Y, and hence improves the oxidation performance of FeCrAl containing Y alloy.

Abstract: Particle surface component and valence state of nickel-based superalloy FGH96 powders fabricated by argon gas atomization have been measured by X-ray phoelectron spectrometer (XPS) to recognize the effect of powder size, surface segregation and heat treating on the oxidation behavior of powders. Type and phase structure of prior particle boundaries (PPBs) precipitated in HIPed superalloy have been studied also. The results showed that prior particle surface segregation and oxidation happened during the powder collection and storage with the C, O, Ti elements enrichment and Ti, Cr, Al, Zr elements oxidation in the surface respectively. During the heat treating of 1150/2h, normal segregation element Ti and Zr enriched more apparently in the particle surface, on the contrary, negative segregation elements Ni, Al and Co diffused into the particle inner. Furthermore, the higher oxygen level on particle surface enhanced to form much more oxides such as ZrO₂, TiO₂ and Cr₂O₃. During HIP process of 1150/2h/150MPa, the oxide ZrO₂ as the nuclei accelerated the preferential precipitation of MC-type carbides which forms the continuous harmful PPBs. Definitely, PPB precipitation depended on the prior particle surface segregation and oxidation, surface elements Ti, Cr and Nb enrichment and oxidation reaction enhancement in high temperature such as 1150. Small powders always had little segregation in the surface and thin oxidation layer, but had more oxygen content per weight for its higher specific surface. So using some powders with optimum particle size and lower oxygen content can help to reduce the PPB precipitation.

Abstract: The discontinuous transition of surface concentration due to the change of bulk concentration in isosteric/isothermal surface segregation has been discussed quantitatively according to the modified Darken model. The criteria for observing the discontinuous transition in surface segregation has been derived on the basis of the Bragg-William expression.

Abstract: Depth profile of subsurface chemical composition and free volume in segmented polyurethane-urea/clay nanocomposites was studied by angle resolved X-ray photoelectron spectroscopy (ARXPS) and Doppler broadening energy spectroscopy (DBES) using slow positron beam. The ARXPS studies revealed increasing N/C atomic ratio (hard segment to soft segment ratio) at any given depth for the clay loaded samples compared to the neat polymer. DBES study revealed significant microstructure modification with clay loading. Self segregation of hard and soft segments in neat polymer and their interspersing with clay loading was observed from DBES measurements.