Phase Transformations in Quench-Aged Zn-Al-Si Alloys
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PHASE TRANSFORMATIONS
IN QUENCH-AGED
Zn-Al-Si
ALLOYS
Y.H.ZHU,* T. SAVASKAN,** AND S. MURPHY* * University of Aston, Gosta Green, Birmingham B4 7ET, U.K.; ** Technical University of Nova Scotia, P.O. Box 1000, Halifax, N.S. Canada. ABSTRACT Two zinc-aluminium based alloys with 30% Zn and 60% Zn and small additions of silicon have been quench-aged and the phase transformations studied in relation to the hardness. It was found that the peak hardness increase is due to the formation of a" and a" phases and evidence has been m obtained for the transformation of these to a' and a'. A cellular reaction U,' - U + r was observed in the first stage oT ageing in the alloy with 68% Zn.
INTRODUCTION This work is part of an investigation of solid state phase transformations in Zn-Al based alloys with Cu and/or Si [1] . No previous work has been published on Zn-Al-Si alloys but binary Zn-Al alloys have been investigated in detail. Numerous reports have shown that over the range of compositions studied, the first products of ageing are G.P.zones, followed by two zinc-rich transitional phases, a" and a' [2-7]. The final products of ageing are zinc (n) and an Al-rich solid soTution (af) Recent work on binary alloys with 60% or 70% Zn have [8,9] shown that decomposition of the initial supersaturated solid solution involved two stages of ageing after the formation of G.P. zones. The first products were f.c.c. metastable phases formed by a cellular reaction. The cellular reaction was interpreted as an extension of the monotectoid reaction, with the compositions of the products given by the monotectoid phase boundary extrapolated below 3400 C to the ageing temperature [8,9]. In the second stage the zinc-rich f.c.c. equilibrium phases n and a.
phase decomposed into the
Ternary alloys containing silicon have potential commercial use, and in this work the quench-ageing behaviour of two low Si-alloys of different zinc contents have been studied.
EXPERIMENTAL Alloys of composition Al-28.9%Zn-3.7%Si (AlZn3OSi5) and Al-61.5%Zn4.8%Si (AlZn60Si5) were induction melted and cast into mild steel ingot moulds. The ingots were then hot-rolled from 400°C to 2 mm thick strip. 0 Solution treatment was carried out in sealed argon-filled capsules at 400 C 0 for 2 days (AlZn3OSi5) or 35 °C for 1 day (AlZn6OSi5). Following quenghing in iced-water, ageing was carried out at temperatures from 21°C to 170 C. A computer controlled X-ray diffractometer was used to investigate the structural changes shown by the hardness measurements. CTEM work was carried out using a JEOL 1OOB microscope, and STEM and analysis with a Philips EM400.
Mat. Res. Soc.Syrp. Proc. Vol. 21 (1984) Published by Elsevier Science Publishing Co., Inc.
836 RESULTS Ageing behaviour Age hardening occurred in both AlZn3OSi5 and AlZn6OSi5 when aged in the range from room temperature to 170 C, and multistage ageing curves are shown in Figures 1 and 2. oFor AlZn3OSi5, three hardness peaks were observed. on ageing at 100 C, a small peak at 9 minutes, the principle peak
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