Chi Y.A. Tsao

Mechanical Properties of the MG-Based Amorphous/Nano Zirconia Composite Alloy

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 .

Microstructure Characteristics of Spray-Formed and Melt-Spun Al85Nd5Ni10 and Al89La6Ni5 Bulk Hybrid Composites

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.

Hot Workability of the Mg65Cu20Y10Ag5 Amorphous/ NanoZrO2 Composite Alloy within Supercooled Temperature Region

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.

Spray-Formed 50Si-50Al Alloy Deformed in the Semi-Solid State

50Si-50Al alloys were synthesized by the Spray Forming process, and were further deformed in the semi-solid state. The spray-formed microstructure consists of small equiaxed primary Si in 3-5 μm scale. For semi-solid deformation at 650°C at solid fraction (fs) of 0.4, the stress increases initially to reach the peak stress, then decreases to a plateau level, followed by last stage of stress climbing, and the effects of the deformation speeds on the stress level are significant, but not large. For deformation at 750°Cat fs of 0.31, the stress increases initially, then increases at a slower rate, followed by last stage of fast climbing, and the lower the deformation speed is, the higher the deformation stress. The average deformation stress needed at 650 °C was much higher that that needed at 750 °C. The deformation was inhomogeneous at 650°C to become pieces of connected semi-solid clusters, while the deformation was much more homogeneous at 750°C.

AZ80 Mg Alloy Synthesized by Spray Forming and its Extrudability

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.