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Viola L. Acoff

Bio: Viola L. Acoff is an academic researcher from University of Alabama. The author has contributed to research in topics: Intermetallic & Wire bonding. The author has an hindex of 17, co-authored 38 publications receiving 1199 citations.

Papers
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TL;DR: In this article, the authors investigated the nanoscale interfacial evolution in Cu-Al wire bonds during isothermal annealing from 175°C to 250°C, where the kinetics obey a parabolic growth law until complete consumption of the Al pad.

188 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of initial thickness reduction by cold roll bonding and cyclic cold-roll bonding/annealing on the formation and development of the titanium aluminide phase has been investigated in detail.
Abstract: In this study, multi-layered Ti/Al composites were prepared using a cyclic cold roll bonding/annealing process. The effect of initial thickness reduction by cold roll bonding and cyclic cold roll bonding/annealing on the formation and development of the titanium aluminide phase has been investigated in detail. The phases formed were characterized using light microscopy equipped with digital image analysis system and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The thermodynamics and kinetics of the diffusion reaction were investigated using differential scanning calorimetry (DSC) and enthalpy (H), entropy (S) and capacity (C) thermodynamic calculations. The diffusion reaction mechanism at this stage was discussed.

182 citations

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TL;DR: In this article, a high Nb containing TiAl based alloy was produced using accumulative roll bonding (ARB) followed by two-stage reaction annealing from Ti, Al, and Nb elemental foils.
Abstract: The high Nb containing TiAl based alloy, Ti–46Al–9Nb (at.%), was produced using accumulative roll bonding (ARB) followed by two-stage reaction annealing from Ti, Al, and Nb elemental foils. Well-bonded sheet materials were successfully obtained with an initial rolling reduction of 50%. Two-stage annealing of 600 and 1400 °C was employed to promote the formation of intermetallic compounds. The cooling rate was controlled to produce the lamellar structure. The scanning electron microscopy and X-ray diffraction indicated that there were no room temperature solid-state reactions between the Ti/Al/Nb foils, even at 10 cycles. Besides the annealing time and temperature, rolling strain also has a significant effect on the production of the intermetallic compound. Differential thermal analysis was used to characterize the effect of rolling strain on the solid-state reaction and phase formation in the composites during annealing. It is clear that the first annealing stage produced a complete reaction of the Al layers forming the intermetallic compounds TiAl 3 and NbAl 3 . The desired composition and structure of the intermetallic compounds were achieved after the second stage annealing.

110 citations

Journal ArticleDOI
TL;DR: In this paper, cold roll bonding of multi-layered bimetals has been modeled using the slab method, and the effect of anisotropy has been included. And the numerical results from the model were compared with the experimental results.

90 citations

Journal ArticleDOI
TL;DR: In this paper, the authors measured the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn 3.8Bi, Sn3.8Ag-0.7Cu, and Sn 0.7 Cu, using the sessile-drop method.
Abstract: In this study, the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn-3.5Ag-4.8Bi, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu (wt.%), were measured on copper substrates at different temperatures. Measurements were performed using the sessile-drop method. Contact angles ranging from 30° to 40° after wetting under vacuum with no fluxes and between 10° and 30° with rosin mildly activated (RMA) and rosin activated (RA) fluxes were obtained. The Sn-3.5Ag-4.8Bi exhibited the lowest contact angles, indicating improved wettability with the addition of bismuth. For all soldering alloys, lower contact angles were observed using RMA flux. Intermetallics formed at the solder/Cu interface were identified as Cu6Sn5 adjacent to the solder and Cu3Sn adjacent to the copper substrate. The Cu3Sn intermetallic phase was generally not observed when RMA flux was used. The effect of temperature on contact angle was dependent on the type of flux used.

81 citations


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TL;DR: The microstructural and mechanical properties of Inconel 718 were determined on the specimens manufactured by selective laser melting (SLM) of prealloyed powder as mentioned in this paper, showing that columnar grains of supersaturated solid solution with internal microsegregation of Nb and Mo, demonstrated by fractions of Laves eutectic or its divorced form in interdendritic regions.
Abstract: The microstructural and mechanical properties of Inconel 718 were determined on the specimens manufactured by selective laser melting (SLM) of prealloyed powder. High- density (99.8%) cylindrical specimens were built with four orientations (0°, 45°, 45°×45° and 90°) in relation to the building and scanning directions. Because of directional, dendritic-cellular grain growth, microstructure of the as-built specimens was characterized by columnar grains of supersaturated solid solution with internal microsegregation of Nb and Mo, demonstrated by fractions of Laves eutectic or its divorced form in interdendritic regions. Such a heterogeneous microstructure is unsuitable for direct post-process aging and makes the alloy sensitive to subsolidus liquation during rapid heating to the homogenizing temperature. In homogenized and aged condition, the alloy received a very good set of mechanical properties in comparison with the wrought material. In heat-treated condition, like in as-built condition, weak anisotropy of properties was found, manifested by lower Young's modulus, yield strength and tensile strength of the specimens extended along the build direction in comparison to the values for the other variants of the specimens. This is attributed to the fact that the grains maintained their geometric and crystallographic texture obtained during solidification.

512 citations

Journal ArticleDOI
TL;DR: Gamma titanium aluminides display attractive properties for high temperature applications as discussed by the authors, including microstructure, deformation mechanisms, and alloy development, which has led to the production of gamma-titanium aluminide sheets.

415 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated solid-state reactive diffusion between Ti and Al in the temperature range of 520-650 degrees C by employing multi-laminated Ti/Al diffusion couples.
Abstract: Solid-state reactive diffusion between Ti and Al was investigated in the temperature range of 520-650 degrees C by employing multi-laminated Ti/Al diffusion couples. In samples annealed up to 150 h intermetallic TiAl3 is the only phase observed in the diffusion zone and the preferential formation of this compound in Ti/Al diffusion couples was predicted using an effective heat of formation model. The present work indicated that both Ti and Al diffused into each other and the growth of the TiAl3 layers occurred mainly towards the Al side. The TiAl3 growth changes from parabolic to linear kinetics between 575 and 600 degrees C, characterized by activation energy of 33.2 and 295.8 kJ mol(-1), respectively. It is suggested that the low-temperature kinetics is dominated by the diffusion of Ti atoms along the grain boundaries of the TiAl3 layers, while the reaction at the TiAl3/Al interfaces in the high-temperature regime is limited by the diffusion of Ti atoms in the Al foils as a result of increased solubility of Ti in Al with increasing temperature. (c) 2006 Elsevier B.V. All rights reserved.

254 citations

Journal ArticleDOI
TL;DR: In this article, a polyethersulfone-silver composite membrane was fabricated via a simple phase inversion method by using silver nitrate (AgNO3) as an antibacterial agent and polyvinylpyrollidone (PVP) of molecular weight 10,000, 40,000 and 360,000 Da as dispersant in the dope formulation.

237 citations

Journal ArticleDOI
TL;DR: The effects of process parameters on bonding, mainly including process and surface preparation conditions, have been analyzed and it has been showed that it is easy for bcc structure metals to bond compared with fcc and hcp structure metals.

235 citations