Papers by Author: Jason S.C. Jang

Abstract: The (Zr53Cu30Ni9Al8)99.5Si0.5 bulk metallic glass (BMG) rods, 3 mm in diameter, with different crystallization fraction for compression test were prepared by isothermal annealing the as-cast BMG rods at the temperature in the middle of supercooled temperature region for different time period in vacuum, respectively. The result of compression test revealed that the fracture strength of these samples increases with the crystallization ratio at the beginning, then decreases rapidly at 40 vol.% crystallization fraction. In addition, the best mechanical performance with 1970MPa yield strength and 3 % plastic strain occurs at the sample with 30 vol.% nanocrystalline phase (with an average size about 50 nm). This was suggested these homogeneous distributed nanocrystals which embedded in the matrix may act as obstacles to branch the primary shear band into multiple shear band and result in improving the ductility.

Abstract: In this study,the (FePt)94-xCu6Nbx (x=0, 2.87, 4.52, 5.67) alloy films were prepared by co-sputtering. The effects of Nb addition content and heat treatment on the microstructure and magnetic properties of the polycrystalline FePtCu films are reported. Our previous experiments showed that the ordering temperature of the (FePt)94Cu6 films reduced to 320 °C, which is much lower than that of the FePt alloy. However, the grain growth after heat treatment limited the practical application in recording media. By adding the Nb content in the (FePt)94Cu6 film, the grain sizes of the films can be adjusted from 50 to 18nm, even for the films annealed at temperature as high as 600°C. DSC traces of as-deposited disorder films at different heating rates, to evaluate the crystallization of the order phase, revealed that the addition of Nb enhanced the activation energy of ordering from 87 kJ/mol to 288 kJ/mol for the (FePt)94-xCu6Nbx (x=0 and 2.87, respectively) films. The reduction of the grain size and the corresponding increase in the activation energy of the Fe-Pt-Cu-Nb films might result from the precipitation of the Nb atoms around the ordering FePt phase. The (FePt)94-xCu6Nbx (x=2.87) film showed a coercive force of 13.4 kOe and the magnetization of 687 emu/cc.

Abstract: In this study, (Cu36Zr48Al8Ag8)100-xSix (x = 0–1) amorphous alloy rod with (2~4) mm diameter were prepared by arc melting. The thermal properties and microstructure development during the annealing of amorphous alloys have been investigated by the combination of differential scanning calorimetry (DSC),, X-ray diffractometry (XRD) and Vickers indentation. The XRD result reveals that all these as-quenched, (Cu36Zr48Al8Ag8)100-xSix alloys exhibit the broad diffraction patterns of amorphous phase. A clear Tg (glass transition temperature) and supercooled region (about 102 K) were revealed for all of those amorphous alloy rods. The crystallization temperature (Tx), (ΔTx) , and micro-hardness of (Cu36Zr48Al8Ag8)100-xSix amorphous alloys is increased as the Si content.

Abstract: This study is to investigate tribological behavior of brake lining materials by hot pressing commercial friction powders with ceramic powders prepared by TEOS / boehmite sol-gel. The stoichiometric ratios of TEOS / boehmite sol-gel were kept constant but calcinated at different temperature to fabricate different homemade ceramic powders. The various phases of ceramic powders such as γ-Al2O3, δ-Al2O3, θ-Al2O3, α-Al2O3, cristobalite and mullite were formed during the preparation process starting from TEOS / boehmite sol-gel solution. The XRD observations reveal the final compositions of these homemade powders were strongly related to the calcining temperature. The brake lining specimens made from TEOS / boehmite sol-gel calcinated at higher temperature show better tribological performance. The brake lining specimen with α-Al2O3 and mullite which were transformed from TEOS / boehmite sol-gel calcinated at 1300°C shows the most stable friction coefficient and the lowest mass loss during wear tests.

Abstract: Based on the thermodynamic calculation, two phase separated Zr-based BMGs (Zr63.8Ni16.2Cu15Al5 and Zr66Cu15.3Ni8.7Al10) which developed by the authors previous study were selected for investigating their crystallization behavior and thermal stabilities by means of differential scanning calorimetry (DSC), and X-ray diffractometry. The results show that the Zr66Cu15.3Ni8.7Al10 amorphous alloy exhibits higher GFA than the Zr63.8Ni16.2Cu15Al5 amorphous alloy. But the Zr63.8Ni16.2Cu15Al5 amorphous alloy presents higher activation energy of crystallization (227 kJ/mole and 188 kJ/mole for Zr63.8Ni16.2Cu15Al5 and Zr66Cu15.3Ni8.7Al10 BMGs, respectively). However, Zr66Cu15.3Ni8.7Al10 amorphous alloy contains less atomic percentage of Cu and Ni elements (with positive heat of mixing) may result in forming less volume phase separation as well less interface area between these separated amorphous phase. Overall, the Zr66Cu15.3Ni8.7Al10 amorphous alloy exhibits longer incubation time at higher annealing temperature in comparison with the Zr63.8Ni16.2Cu15Al5 amorphous alloy, suggesting that the amorphous alloy which contains fewer amounts of Cu and Ni elements would have better thermal stability.

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.

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.

Abstract: The (Cu42Zr42Al8Ag8)100-xSix amorphous alloy rods, x =0 to 1, with 3 mm in diameter were prepared by Cu-mold drop casting method. The glass forming ability, thermal properties and microstructure evolution was studied by differential scanning calorimetry (DSC), and X-ray diffractometry (XRD). The XRD result reveals that these as-quenched (Cu42Zr42Al8Ag8)100-xSix alloy rods exhibit a broaden diffraction pattern of amorphous phase. The crystallization temperature and GFA (glass forming ability) of (Cu42Zr42Al8Ag8)100-xSix alloys increase with the silicon additions. The highest Trg (0.59) and γ value (0.405) occurred at the (Cu42Zr42Al8Ag8)99.75Si0.25 and (Cu42Zr42Al8Ag8)99.5Si0.5 alloy. In addition, both of the activation energy of crystallization and the incubation time of isothermal annealing for these (Cu42Zr42Al8Ag8)100-xSix alloys indicates that the (Cu42Zr42Al8Ag8)99.25Si0.75 alloy posses the best thermal stability among the (Cu42Zr42Al8Ag8)100-xSix alloy system.

Abstract: The results of atmosphere-controlled tensile test in various conditions (with different strain rate at different temperature under vacuum, air, or water vapor atmosphere) revealed that the addition of boron and chromium would improve the elongation as well as ultimate tensile strength (UTS) of the Ni-19Si-3Nb based alloys over a wide range of temperature under air and water vapor atmosphere. The UTS and elongation can reach to 1270 MPa and 14%, respectively at 873K in each kind of atmosphere. On contrary, the alloy without boron addition only presents ductile mechanical behavior in vacuum. This is evident that boron and Cr elements present positive effect on suppressing the environmental embrittlement in air and water vapor atmosphere from room temperature to 1073 K for the Ni-19Si-3Nb base alloy. In addition, both of UTS and elongation present quite insensitive on the strain rate when test at the temperature below 973 K. However, the UTS exhibits very dependent on the strain rate when test temperature above 973 K, decreasing the ultimate tensile strength with decreasing strain rate.

Abstract: In the current study, the amorphization behavior of mechanically alloyed Ni57Zr20Ti22Pb1 powder was examined in details. The conventional X-ray diffraction results confirm that the fully amorphous powders formed after 5 hours of milling. The thermal stability of the Ni57Zr20Ti22Pb1 amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the glass transition temperature (Tg) and the crystallization temperature (Tx) are 760 K and 850 K, respectively. The supercooled liquid region

 is 90 K. The appearance of wide supercooled liquid region may be mainly due to the Pb additions which cause the increasing differences in atomic size of mechanically alloyed Ni57Zr20Ti22Pb1 powders.