Microstructure of Al–Ag–Zn Alloys*

Stanko Popović,^a Željko Skoko^a and Goran Štefanić^b

^a Physics Department, Faculty of Science, University of Zagreb, P.O.B. 331, 10002 Zagreb, Croatia
^b Ruđer Bošković Institute, P.O.B. 180, 10002 Zagreb, Croatia
* Corresponding author: E-mail: spopovic@phy.hr, zskoko@phy.hr, stefanic@irb.hr

Abstract
The precipitation sequence in ternary aluminium rich Al–Ag–Zn alloys, after rapid quenching to RT from a temperature, T_t (820 K), higher than the solid-solution temperature, T_ss, was found to be: GP zones (fcc) → ε´ (hcp) → ε (hcp). The as-quenched alloys contained GP zones having 3 to 4 nm in diameter as observed by XRD and TEM. During ageing of the quenched alloys at 420 K GP zones increased in size, remaining fcc and coherent with the α-phase (fcc). In parallel, metastable precipitates, ε´, were formed; their unit-cell parameters depended on the solute content. Two mechanisms of ε´ nucleation were suggested on the basis of composite diffraction-line profiles, discontinuous nucleation and a direct GP zones to ε´ transition. The unit-cell parameters of the equilibrium phase, ε, observed in the alloys slowly cooled from T_t to RT, depended on the solute content. The alloys, that had been quenched from T_t to RT, aged at 420 K for 50 days and then prolongedly aged at RT, being two-phase system (α + ε´), were studied in situ at high temperature. As the temperature increased, an initial increase of diffraction line intensities of both α and ε´ phases was observed, due to lattice strain annealing. A shift of diffraction lines due to thermal expansion took place. A small anisotropy of thermal expansion of ε´ was noticed. Above ≈ 500 K a gradual dissolution of ε´ in the matrix (M, α-phase) started, as manifested in an enhanced decrease of diffraction-line intensities. Finally, solid solution was formed; T_ss depended on the alloy composition. On cooling, the alloys underwent reversal changes, exhibiting a temperature hysteresis (10 to 20 K). The dependence of unit-cell parameters of ε´ on temperature during cooling was little different from that on heating. At RT, after a complete heating and cooling cycle, unit-cell parameters of the precipitates were close to those of the equilibrium ε-phase, while diffraction-line profiles were not composite any more.

Keywords: X-ray powder diffraction, microstructure, phase transitions, aluminium-based ternary alloys, unit-cell parameters, crystallite size, electron microscopy