engleski

XRD Study of Al-Ag-Zn Alloys at High Temperatures

The precipitation sequence in Al-Ag-Zn alloys (having electron to atom ratio 2.9 and the Ag/Zn ratio from 4.0 to 0.2, the total atomic fraction of solute being up to 0.086), after rapid quenching from the solid solution temperature, TSS, to room temperature, RT, was found to be: GP zones (fcc) →  ’ (hexagonal) →  (hexagonal) [1-3]. The size of GP zones in “ as-quenched” alloys was of the order of 3 o 4 nm, as observed by TEM and from XRD broadening. The growth rate of GP zones, during ageing at 420 K after quenching, increased with the Zn content. GP zones remained fcc during ageing and coherent with the  -phase (matrix, fcc) but had a smaller unit-cell parameter than the  -phase. In parallel with growth and dissolution of GP zones, metastable precipitates  ’ were formed ; their unit-cell parameters depended on the alloy composition and probably on the mechanism of nucleation as concluded from composite XRD line profiles. A heterogeneous nucleation of  ’ was observed, but a direct GP zones →  ’ transition also took place in alloys with higher Zn content. The unit-cell parameters of the equilibrium phase,  , observed in alloys slowly cooled from TSS to RT, depended on the alloy composition. By extrapolating the unit-cell parameters of  to zero fraction of Zn values in good agreement with the corresponding data for the  phase in Al-Ag alloys were obtained. The Al-Ag-Zn alloys, quenched from TSS to RT and aged at 420 K for 50 days and then aged at RT for 38 years, and being two-phase systems,  + ’ , were studied at high temperature. As the temperature increased, a slight increase of diffraction line intensities of both  and  ’ phases was observed, due to lattice distortion annealing. A gradual shit of diffraction lines due to thermal expansion took place. A small anisotropy of thermal expansion of the  ’ phase was noticed. At higher temperatures (above  500 K) a gradual dissolution of the  ’ phase in the  -phase took place. This process was manifested in an enhanced decrease of its diffraction line intensities. Finally, solid solution was formed ; TSS depended on the alloy composition amounting  720,  700 and  680 K for the alloys with the Zn to Ag atomic ratio of 1.00, 0.50 and 0.20, respectively. On cooling, the alloys underwent reversal changes, exhibiting a temperature hysteresis: the onset of precipitation during cooling was observed at a temperature of about 10 to 20 K lower than the temperature at which precipitates dissolved on heating. The dependence of unit-cell parameters of the  ’ phase on temperature during cooling was little different from that on heating. At RT, after a completion of the heating and cooling cycle, unit-cell parameters of the precipitates were much closer to those of the equilibrium  -phase, while XRD line profiles were not composite any more. [1] S. Popović and D. E. Passoja, J. Appl. Cryst. 4 (1971) 427-434. [2] D. E. Passoja, S. Popović and P. Barrand, Met. Trans. 5 (1974) 715-721. [3] S. Popović, Cryst. Res. Technol. 19 (1984) 1351-1358.

X-ray powder diffraction; Al-Ag-Zn; alloys; precipitation

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