Modulated structures and position of the coherent spinodal curve in Al–Zn(22 at%)–Cu(x) alloys

Abstract

The character of the coarsening of modulations in dependence on quenching temperature Ta is studied in Al–Zn(22)–Cu(0 to 3) alloys using X‐ray small angle scattering and TEM measurements (TEM micrographs and diffraction patterns) in the temperature (Ta) range from 70 to 240 °C (XSAS) and 70 to 180 °C (TEM), respectively. The temperature of the coherent spinodal curve is estimated in the range between (230 ± 10) °C. This value agrees fairly well with the results of other authors. The TEM observations of structure revealed in the examined Ta range a periodic structure relevant to spinodal decomposition. The periodicity of modulations is confirmed by electron diffraction patterns where a clear effect of sidebands near the matrix spots is observed. The addition of copper has no remarkable influence on the spinodal temperature.

Information about the Position of the Coherent Spinodal Curve in the Al−Zn System by Means of the Course of the SAXS Intensity at Elevated Temperatures

“…1 the course of TlIfl versus T , obtained for the A1-21.5 at% Zn alloy is shown. From the extrapolation of the straight line of the TIIf1-T plot occurring above Trhn the position of the coherent spinodal curve Tcohsp is evaluated t o be situated a t about (220 f 10) "C. This value is not far from those determined by other authors for this alloy, namely Tcohsp = (230 f 10) "C, by means of the analysis of the wavelength of the spinodal structure in dependence on T , when approaching Fcohsp from lower T , performed by SAXS [16] and TEM investigations [16, 181 or evaluated from the SAXS results obtained below 100 "C [17].…”

“…From these results it can be concluded that the information about the position of Teohsp gained by means of our method fits fairly well the data obtained by other authors. Furthermore, the results presented in [12] and [17] are additionally drawn (see the dashed and the dash-dotted line, respectively) as well as experimentally determined points taken from other authors (full circle [16], square [17], and empty circles [18 to 201). The meaning of the full triangle is explained in the inset of Fig.…”

On the mode of decomposition of AlZn alloys

“…Such quantitative experiments were not applied to investigate Al-Zn alloys in the vicinity of the coherent spinodal till 1977. The main reason could be that a t elevated temperatures the decomposition processes are remarkably accelerated and that in [27], and Dutkiewicz et al [28] tried to find information on the position of the coherent spinodal by means of SAXS 125 to 281 and TEM investigations 127, 281. Their procedure is based on the idea that the time evolution of the spinodal structure comprising GP zones in their later stage of coarsening may provide reliable information on the mechanism initiating the decomposition of the alloy.…”