Recalculation of several metastable phase lines of the AlZn system

Abstract

Based upon new results of the position of the upper limit of the α–α′ miscibility gap, the positions of the (i) metastable αm–α′m miscibility gap and (ii) chemical as well as coherent spinodal curves are recalculated. Furthermore, the position of the so‐called ‘spinodal of GP zones’ is determined by means of latest experimental data. It turned out that by use of the applied methods (i) all the calculated curves are in a “reasonable” order and (ii) the positions of the coherent spinodal curve and the “spinodal of GP zones” are not far from each other.

Evaluation of the critical size for the nucleation of precipitates in AlZn alloys

“…Note that in this alloy the upper limit Tfor the homogeneous nucleation of precipitates, i.e. GP zones, is placed at (203 2) "C and the metastable line of the a;-phase is situated at (243 f 2) "C [14]. Thus, the mode of the heterogeneous formation of precipitates at lattice defects can proceed in a well pronounced way.…”

Evaluation of Several Free Transformation Enthalpies Occurring in AlZn Alloys

“…where R = 8.31435 J/(mol K). Taking the course of the a-a' miscibility gap in the T range from 550 to 624.5 K (277 to 351.5 "C) as given in Table 1 As already successfully performed in [3], (2) is extrapolated to T down to 273 K.…”

“…Using the chemical potentials of the A1 and Zn atoms, gmix writes gmix(x, '1 = x Apzn(X, 7') + (1 -X) APAI(x, 7') 9 (3) where Api = pipoi stands for the chemical potentials of the element i in the mixture and in the pure substance, respectively. Using (1) and the well-known Gibbs-Duhem relation In Fig.…”

“…As a result of this review, the positions of the upper limit of the a-a' miscibility gap as well as the eutectoid temperature and the respective zinc concentrations were corrected. For details see a recent paper of LoMer et al [3], where the effects of these changes of the respective phase lines on gmix and the positions of the metastable a,-ak phase lines and the chemical as well as coherent spinodal curves were considered.…”

“…In [3], neither the course of the chemical potentials pi and of gmix as a function of the zinc content x and temperature T nor transformation enthalpies from the supersaturated solid solution a, to metastable or stable phases were taken into account. For the latter problem, also in [2] only the general formulae but not numerical calculations had been presented.…”