Decomposition in Al-Zn alloys: Part II. Decomposition during continuous cooling
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T. LEWANDOWSKI
AND
KARL
B. RUNDMAN
A small angle X - r a y scattering study (SAS) has been made of decomposition during continuous cooling in four binary AI-Zn alloys with compositions spanning the miscibility gap and in two t e r n a r y alloys, each containing 22 at. pct Zn plus small amounts of Sn and Mg. Plots of log ~m (wavelength receiving maximum amplification during the quench) v s log Q (quench rate) yield slopes of approximately - 1/3 for all alloys, indicating that coarsening plays an important role during the quench. In addition, measurements of integrated area under the SAS spectra indicate that decomposition is essentially complete in the quenched condition for all of the alloys studied. SPINODAL decomposition has received considerable theoretical attention within the last 15 years, particularly with r e g a r d to isothermal decomposition. However, only scant reference has been made to the decomposition process occurring during a continuous cooling cycle. Huston, Cahn, and Hilliard 1 (HCH) have attempted to analytically describe spinodal decomposition during continuous cooling and deFontaine 2 has, by an iterative technique, simulated the decomposition event in an A1-22 at. pct Zn alloy at several different cooling rates. These are, to these authors' knowledge, the only attempts that have been made to predict s t r u c ture as a function of cooling rate in spinodally decomposing systems. It is our intent, in this paper, to present data obtained during continuous cooling experiments conducted with several A1-Zn alloys, and to compare our data with the theoretical predictions of Huston, Cahn, and Hilliard.
rates shown in Table II are the maximum slopes on the t e m p e r a t u r e - t i m e trace for each media recorded on a Tektronix model 543B oscilloscope. Fine chromel-alumel thermocouple wires were spot-welded on either side of the specimens (chromel on one side and alumel on the other) to obtain an average temperature reading a c r o s s the specimen during cooling. Note that this experimental setup yielded quench rates of approximately 3.1 x 104 ~ for an ice water quench, a value quite close to that determined by Rundman 3 by m e a s u r ing the r e s i s t a n c e drop through a specimen during the quench. Within 10 seconds of quenching the specimens were t r a n s f e r r e d to a liquid nitrogen bath to prevent further decomposition at room temperature. The small angle scattering (SAS) measurements were performed as described in Part I, except the specimens were kept at approximately - 2 5 ~ to prevent decomposition during the measurement.
EXPERIMENTAL PROCEDURE
EXPERIMENTAL RESULTS
The six alloys used in this study, having nominal compositions given in Table I, were prepared from 99.99 material. The cast t e r n a r y alloy specimens designated A-Sn and A-Mg were extruded from the cast ingot into an 0.63 cm thick plate. The a s - c a s t binary alloys designated A-22, A-37, A-45 and A-52 were hot rolled to a thickness of 0.38 cm. The specimens of all alloys were cold rolled to the thi
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