Microstructure of Al–Ag–Zn Alloys*

Stanko Popović,a Željko Skokoa and Goran Štefanićb

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

The precipitation sequence in ternary aluminium rich Al–Ag–Zn alloys, after rapid quenching to RT from a temperature, Tt (820 K), higher than the solid-solution temperature, Tss, 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 Tt to RT, depended on the solute content. The alloys, that had been quenched from Tt 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; Tss 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