Phase transformation of Zn-4Al-3Cu alloy during heat treatment
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DUCTION
THE Zn-4 Al-3 Cu alloy is a widely used commercial material for fabricating various construction parts through die casting. It is also a useful mold material in the trial mold applications for stamping and injection molding. The addition of copper results in an increase of strength of the alloy; however, the material will suffer dimensional instability over a period of time at ambient temperature. The dimensional expansion is related to the copper-rich ε phase precipitated in the h matrix during aging at slightly elevated temperature after casting.[1] The phase transformations of heat-treated Zn-Al-Cu alloys have been well investigated in the isothermal process by many researchers, such as Ko¨ster,[2] Gebhardt,[3] and Murphy.[4] As a result, most of the ternary phase diagrams were established. The aging of zinc alloys containing 19 to 38 pct Al and 0 to 4 pct Cu take place in the following reactions suggested by Savaskan and Murphy[5] and Lo¨bery:[6] b 1 ε → a 1 h at ;276 7C (eutectoid reaction) a 1 ε → T' 1 h at 268 7C (four-phase reaction) One can see in the preceding reactions that the b phase is unstable at room temperature after quenching. It decomposes into a and h phases in 10 hours.[5] Some investigations[4,7] indicated that the ε phase was a stable phase at the
B.J. LI, formerly Graduate Student, Institute of Materials Science and Engineering, National Chiao Tung University, is Scientist, Materials Research Laboratories, ITRI, Hsinchu 31015, Taiwan, Republic of China. C.G. CHAO, Professor, is with the Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan, Republic of China. Manuscript submitted October 27, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature above 268 7C and would be replaced by a stable phase T ' during long-time aging below 268 7C. In contrast, Kovacheva et al.[8] found that the ε phase dissolved in h phase above 260 7C through the measurement of differential thermal electromotive force in Zn-Al-Cu alloy. The crystal structure of T' phase (58 pct Al, 30 pct Cu, 12 pct Zn) present in the high-aluminum and high-copper zinc alloys was investigated by Arndt and Moeller[9] and Murphy.[10] They had established that T' has a rhombohedral ˚ , a 5 27.4 deg) with CsCl-type cells. structure (a 5 8.68 A Most of the investigations were concentrated in the highaluminum zinc alloys. However, little work has been done on phase transformation of Zn-Al-Cu alloy with lower aluminum content. From the equilibrium ternary phase diagram of Zn-AlCu alloy,[4] it can be inferred that in Zn-4Al-3Cu alloy, the h phase (zinc-rich, hcp structure), ε phase (CuZn4, hcp structure), and a phase (aluminum-rich, fcc structure) are the stable phases below eutectoid temperature. On the other hand, the stable phases are b, h, and ε phases above that temperature. In the present work, the phase transformations of the primary h phase in an as-cast Zn-4 Al-3 Cu alloy during solution-treatment are investigated. II.
EXPERIMENTAL PROCEDURE
The alloy used in this stu
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