Variations of zinc content in solid solution during reversion treatment in an Al-4.4 At. Pct Zn Alloy
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In a previous work ~it has been shown that the thermoelectric power (TEP), measured at 20 ~ is insensitive to G P zones existing in A1-Zn alloys. This result has been verified for large precipitates sizes (0 = 40 A). The T E P seems to depend only on the zinc content remaining in solid solution in the matrix. Therefore this T E P property allows to follow, especially, the reversion of G P zones. This has been already verified in a qualitative w a y ? Thus the purpose of the present work is to follow quantitatively the evolution of the zinc content in solid solution by T E P measurements and to compare these results to those obtained by the measurement of the integral intensity of X - R a y small angle scattering (XSAS), Moreover it will be shown that the temperature at which T E P measurements are carried out, has no influence on zinc content determination. The studied composition of the A1-Zn alloy is 4.4 at. pct Zn; after an annealing treatment (24 h at 450 ~ the specimens are heated at 350 ~ during 1 h and then directly quenched to the aging temperature T A = 45 ~ the aging duration is 48 h in order to develop G P zones with an average diameter of about 50 ~,, The latter reversion of G P zones is carried out at 130 ~ TEP is measured either at 20 ~ using a previously described apparatus? or as a function of temperature T ( - 150 ~ ~ T ~ 2 0 0 ~ using an apparatus based on the same conceptioh. AS is the T E P difference between the alloy and pure aluminium; the temperature difference between the two ends of the sample is about 5 ~ Measurements are carried out during heating at 80 ~ samples are 0.15 mn thick strips. The same samples are used for XSAS experiments. The integral intensity measurements are performed at room temperatures. A high power rotating anode generator with a linear detector is used; a multichannel analysis system connected to a computer allows fast measurements. The X - R a y b e a m is focused in the detector plane by a curved quartz monochromator. The sample and X - R a y beam are under vacuum, in order to limit air scattering. We first investigate the influence of temperature Ton T E P measurements. At 45 ~ the miscibility gap relative to G P zones in A1-Zn alloys is located at 2.2 at. pct. 4 Then in order to compare the T E P evolution as a J. M. PELLETIER, G. VIGIER, F. LIVET, and C. MAI are with the Groupe d'Etudes de M6taiiurgie Physique et de Physique des Mat6riaux, Insa de Lyon, Villeurbanne Cedex, France. Manuscript submitted January 10, 1980. METALLURGICAL TRANSACTIONS A
function of temperature of an A1-4.4 at. pct Zn alloy aged at 45 ~ and therefore containing G P zones (State 1) to that of a solid solution having the same residual solute concentration (state 2), we have plotted on Fig. I the curves AS = f ( T ) for these two states; the curve relative to a solid solution with 4.4 at. pct Zn has been added (state 3). Three temperature ranges may be observed: - - f o r temperatures lower than T~ (T I -'~ 60 ~ curves for state (1) and (2) are identical; thus in this whole temperature
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