Microstructure of Al-Ag-Zn Alloys

Popović, Stanko and Skoko, Željko and Štefanić, Goran (2008) Microstructure of Al-Ag-Zn Alloys. Acta Chimica Slovenica, 55 (4). pp. 793-800. ISSN 1580-3155

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Abstract

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.

Item Type: Article
Keywords: X-ray powder diffraction; microstructure; phase transitions; aluminium-based ternary alloys; unit-cell para- meters; crystallite size; electron microscopy
Date: 2008
Subjects: NATURAL SCIENCES > Chemistry
NATURAL SCIENCES > Physics
Divisions: Faculty of Science > Department of Physics
Project code: 119-0982886-1009, 098-0982904-2952
Publisher: Slovenian Chemical Society
Depositing User: Gordana Stubičan Ladešić
Date Deposited: 30 Oct 2017 15:13
Last Modified: 30 Oct 2017 15:13
URI: http://digre.pmf.unizg.hr/id/eprint/5646

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