Browsing by Author "Li, BJ"

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Aging kinetics of heat-treated Zn-4Al-3Cu alloy
(1999-06-18)
Li, BJ
;
Chao, CG
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Mechanical properties and 95 degrees C aging characteristics of zircon-reinforced Zn-4Al-3Cu alloy
(1996-03-01)
Li, BJ
;
Chao, CG
;
交大名義發表
;
材料科學與工程學系
;
National Chiao Tung University
A process for preparing zinc alloy castings containing dispersions of zircon particles is described. Composites were prepared by stirring zircon particles in Zn-4Al-3Cu (ZAS) alloy melts and subsequently casting these melts in permanent molds. It was found that additions of zircon resulted in an increase in the sliding wear resistance and in the proportional limit in compression. The aging characteristics of the ZAS alloy have also been investigated by hardness tests, dilatometry technique, and transmission electron microscopy observations. There are two kinds of precipitates that occur during the aging process. The alpha-phase precipitates from the eta phase in the early stage of aging and the copper-rich epsilon-phase precipitates from the eta phase in the later stage of aging. Therefore, there are two peaks in the hardening curve caused by both alpha-phase and epsilon-phase precipitation. The alpha-phase precipitation induces the dimensional shrinkage, and the copper-rich epsilon phase precipitation results in dimensional expansion. Zircon particles existing in ZAS alloy reduce the maximum shrinkage from 353 X 10(-6) for the monolith to 167 x 10(-6) for the composite. Two groups of parallel alpha-phase plates had formed within the eta dendrite during aging at 95 degrees C. The orientation relationship between the alpha phase and matrix was determined as [(1) over bar 101](//[l (1) over bar 0](alphaalpha), (11 (2) over bar 0)(eta)//(111)(alpha).
Phase transformation of Zn-4Al-3Cu alloy during heat treatment
(1999-04-01)
Li, BJ
;
Chao, CG
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The phase transformation in Zn-4 Al-3 Cu alloy employing various solution-treatment temperatures (230 degrees C to 325 degrees C) was studied by means of microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The starting microstructure of the as-cast Zn-4Al-3Cu alloy consists of an alpha phase (aluminum-rich, fee structure) in the eta matrix (zinc-rich, h.c.p. structure) prior to solution-treatment. A platelike epsilon phase with 3-mu m length and 0.5-mu m thickness was found in the eta phase matrix after solution-treating the as-cast material at 240 degrees C for 1 hour. The epsilon phase was then dissolved gradually back into the eta matrix above that temperature. A four-phase transformation, alpha + epsilon --> T' + eta, was observed from the temperature 250 degrees C to 310 degrees C, wherein the T' phase formed at the interface of epsilon platelet and eta phase matrix. This T' phase was further identified as a rhombohedral structure. As the solution-treatment temperature was increased to above 310 degrees C, the epsilon phase was completely dissolved back into the eta matrix and numerous beta phase particles were distributed uniformly in the eta matrix. The beta phase subsequently decomposed at room temperature to a fine alpha phase embedded in the eta matrix. For the materials solution-treated above 250 degrees C, the microhardness of the eta matrix increased in 40 minutes during natural aging, which was associated with the formation of fine epsilon phase of 0.15 mu m diameter. The orientation relationship between this fine epsilon phase and eta phase was determined as [(1) over bar 011](eta) // [(1) over bar 011](epsilon), (01 (1) over bar 2)(eta) // (01 (1) over bar 2)(epsilon).