engleski

Microstructure of Al-Ag-Zn Alloys

The precipitation sequence in ternary aluminium rich Al?Ag?Zn alloys, after rapid quenching to RT from a temperatu- re, 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 age- ing 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 mec- hanisms 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 slow- ly 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 tempera- ture. As the temperature increased, an initial increase of diffraction line intensities of both α and ε ´ phases was obser- ved, 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.

X-ray powder diffraction; microstructure; phase transitions; aluminium-based ternary alloys; unit-cell para- meters; crystallite size; electron microscopy

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