Papers by Author: Chi Y.A. Tsao

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Authors: Y.P. Hung, K.J. Wu, Chi Y.A. Tsao, J.C. Huang, P.L. Hsieh, Jason S.C. Jang
Abstract: The current study applies the inclusion of thermally stable ceramic nano powders into the light weight AZ61 Mg base alloys via spray forming plus extrusion. The combination of spray forming and severe extrusion provides a new processing route for fabricating nano composites, with uniform distribution of the nano fillers. Parallel tries using the PM route followed by the same severe extrusion can also yield sound composites but the dispersion of the nano SiO2 appears to be less uniform. The processed composites are characterized in terms of microstructure examination, thermal analysis, phase identification, and mechanical testing.
Authors: Shih Han Wang, Chi Y.A. Tsao
Abstract: Magnesium materials are gaining an increasing interest especially in transportation industries. The goals are the reduction of fuel and the reduction of the green house gas CO2. To achieve these goals, the weight of vehicles must be reduced. Compared to Al based materials, Mg based materials offer the possibility of saving up to 30% in weight. Unfortunately the latter suffer from poor workability. The workability of AZ91B has been proven to be improved significantly by employing the rapid solidification process, Spray Forming. In this study, the AZ80 Mg alloy was synthesized via Spray Forming process. The microstructures were characterized and compared with conventionally cast materials. The workability of the spray-formed AZ80 Mg alloys was studies via a simple extrusion process. The effects of the extrusion speed and temperature on the workability were investigated.
Authors: C.H. Chiang, Chi Y.A. Tsao
Abstract: The coarsening behavior of spray-formed A-35wt%Si and A-45wt%Si alloys in the semisolid state were studied. The coarsening kinetic and Si size distributions after coarsening at various temperatures and times in the semi-solid state were investigated. The coarsening exponents, n, the rate constants, K, in the LSW theory, and the modified rate constants were determined from previous works. The coarsening activation energies of the two spray-formed alloys were calculated from the slopes of ln(KT) vs. 1/T plots. The Si size distribution functions after holding at various temperatures and times in the semisolid state were determined for the two spray-formed alloys. The effects of the stokes-motion in addition to pure diffusion mechanism on the variations of coarsened Si size distribution were also investigated.
Authors: Jason S.C. Jang, S.C. Lu, L.J. Chang, T.H. Hung, J.C. Huang, Chi Y.A. Tsao
Authors: L.J. Chang, G.R. Fang, Jason S.C. Jang, I.S. Lee, J.C. Huang, Chi Y.A. Tsao
Abstract: Mg65Cu20Y10Ag5 Amorphous/ nano ZrO2 composites alloy powder were fabricated through the combination method of melt spinning and mechanical alloying (MA). The melt spun amorphous matrix ribbons are ground into powders and mixed with 3 vol.% ZrO2 nano particles in the planetary mill. After then formed by hot pressing in Ar atmosphere under the pressure of 700 MPa at the temperature of soft point which measured by TMA (Thermal mechanical Analysis). The hot-pressed bulk composite specimens are compression tested at different temperature within the supercooled temperature region. The flow stress was found decrease with increasing temperature dramatically when the temperature exceeds the middle temperature of supercooled region. The specimens after compression test were examined by X-ray diffractometry and SEM to investigate its crystallinity and fracture mechanism.
Authors: Y.T. Chen, Chi Y.A. Tsao, C.H. Chiang
Abstract: The cooling slope technique has been developed in recent years, which controls the nucleation and growth of the primary grains during solidification to achieve fine and non-dendritic microstructures. In this study, A356 Al alloys were processed through a modified cooling slope technique to obtain fine, non-dendritic microstructures, in which the cooling rate of the cast crucible was controlled. Three process parameters, namely pouring temperature, inclined slope angle, and the cooling rate of the cast crucible, were varied during the processing. The cooling slope was water-cooled with a constant water flow rate. The solid fraction and the size distributions of the primary grains along the vertical and horizontal positions of the cast ingots were measured individually. The macro-segregation was examined in terms of the distribution of the solid fraction. The yields of the ingots were calculated for studying the efficiency of the cooling slope technique. The effects of the three process parameters on the microstructures, macro-segregation, and yields were studied by the Taguchi method.
Authors: L.J. Chang, J.H. Young, P.T. Chiang, Jason S.C. Jang, J.C. Huang, Chi Y.A. Tsao
Abstract: Mg-based composites are fabricated through mechanical alloying (MA) the Mg65Cu20Y10Ag5 amorphous alloy spun and mixed with 1-5 vol.% spherical nano-sized ZrO2 particles in the planetary mill, after then formed by hot pressing in Ar atmosphere under different pressures at the temperature 5 K above the glass transition temperature (Tg). The microstructure characterizations of the resulting specimens are conducted by means of XRD, FEG-SEM, and TEM techniques. It is found that the nano-sized ZrO2 dispersed Mg-based composite alloy powders can reach to a homogeneous size distribution (about 80 nm) after 50-hour mechanical alloying. After hot pressing of these composite alloy powders under the pressure of 1100 MPa at 409K, a 96% dense bulk specimen can be formed. Throughout the MA and hot pressing, the amorphous nature of the Mg65Cu25Y10Ag5 matrix is maintained. The hardness of the formed bulk Mg-based composites (with 3 vol.% nano-sized ZrO2 particles) can reach to 370 in Hv scale. In addition, the toughness of the formed bulk Mg-based composites presents an increasing trend with the content of nano-sized ZrO2 particles and can reach to 8.9 MPa m .
Authors: K.F. Chang, F.H. Chen, S.K. Fan, Chi Y.A. Tsao
Abstract: Mg bulk metallic glass rod, ribbon and composite plate were synthesized via injection casting, melt spinning, and spray forming, respectively. The BMG composite plate has various microstructure combinations, from fully amorphous phase to mixture of nanocrystals, crystals and amorphous phase. XRD analyses were done on the rod and ribbon and throughout the vertical locations of the plates at the maximum thickness. Glass transition temperature (Tg) and onset crystallization temperature (Tx) were measured with DSC, and GFA (Glass Forming Ability) was estimated with various GFA models. The microhardness of the spray-formed Mg-Cu-Gd layered composite plate at various positions and Mg-Cu-Gd rod and ribbon were measured.
Authors: M.L.T. Guo, Chi Y.A. Tsao, J.C. Huang, Jason S.C. Jang
Abstract: The microstructure characteristics of the spray-formed and melt-spun Al85Nd5Ni10 and Al89La6Ni5 alloys were studied. The spray forming process was demonstrated to produce a bulk scale hybrid composite consisting of amorphous and nanostructured phases directly without the need of an amorphous precursor. The spray-formed Al89La6Ni5 deposit (~1 mm in thickness) were partially amorphous, and the amorphous phase came from the undercooled liquid droplets upon deposition. The as-spray-formed Al85Nd5Ni10 deposit (~20 mm in thickness) was completely crystallized due to the devitrification of the retained amorphous phase to nano-scale secondary crystals upon deposition. Primary crystals (~1 μm) are dispersed uniformly in the bulk spray-formed amorphous/or partial amorphous composites and many distinctive deformation twins also are observed in the crystals, however, not twins found in the corresponding completely devitrified ribbon. This is mainly because of the stirring and impacting force offered by high velocity droplets during spray forming and the mismatch of thermal expansion coefficient between primary crystals and adjacent amorphous matrix.
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