Papers by Author: L.J. Chang

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 (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: The Mg65Cu25Gd10-xNdx (x=0 ~ 10) amorphous alloy rods with 3~6 mm in diameter were prepared by Cu-mold injection method. The thermal properties and mechanical properties of these amorphous alloys have been investigated by DSC, SEM with EDS capability, X-ray diffractometry (XRD) and Vickers hardness test. The XRD revealed that these entire as-quenched Mg65Cu25Gd10-xNdx alloy rods exhibit a broaden diffraction pattern of amorphous phase. A clear Tg (glass transition temperature) and supercooled region (about 30~60 K) were revealed for all of those Mg65Cu25Gd10-xNdx alloys. In addition, the single stage crystallization of the Mg65Cu25Gd10 alloy was found to change into two stages crystallization when the Nd element was added into this alloy. In parallel, the crystallization temperature (Tx) and supercooled region (
Tx) presents a decreasing trend with increasing Nd content. The lowest liquidus temperature (Tl, about 721 K) occurs at the Mg65Cu25Gd8Nd2 alloy. In addition, The Mg65Cu25Gd8Nd2 alloy exhibits the high γ value (0.416, defined as γ= Tx/Tg+Tl), a relatively high Trg (0.59, defined as Trg = Tg/Tl) and the highest hardness in these alloys.

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 .

Abstract: A series of Mg-based alloys with composition of Mg65Y10Cu25-XAgX, x = 0, 5, 10, were selected for investigating the microstructure evolution of the Mg-based nano/amorphous-composite alloy powder synthesized by the combination method of melt-spinning and mechanical alloying (MA). The microstructure characterization of the alloy powders was conducted by means of DSC, XRD, FEG-SEM, and TEM techniques. The result of XRD reveals that the entire as-quenched alloy ribbons exhibit a broaden diffraction pattern of amorphous phase. After 50 hours milling the mixture of amorphous alloy ribbons with 5 vol.% of nano-sized ZrO2 by planetary mill, the ZrO2 dispersed magnesium composite alloy powder can reach to a homogeneous size distribution. In parallel, the MA composite Mg-based alloy powders still remain an amorphous state by the characterization of X-ray diffraction and the DSC analysis. A clear Tg (glass transition temperature) and most wide supercooled region (about 44 K) were revealed for both the Mg65Y10Cu20Ag5 alloy ribbon and the MA magnesium composite powder. In addition, the result of TEM observation also revealed that the ZrO2 with average particle size of 80 nm distributed homogeneously in the amorphous matrix of the Mg65Y10Cu20Ag5 /5 vol.% ZrO2 composite alloy powder. The interface between the ZrO2 dispersoid and the amorphous matrix of the composite alloy powder exhibits a very good bonding condition.