X-RAY DIFFUSE SCATTERING AND STRUCTURE OF SUPERSATURATED SOLID SOLUTION NaCl-AgCl

A.V. Dobromyslov

Institute of Metal Physics, Ural Division of Russian Academy of Sciences, 18 S.Kovalevskaya str., GSP-170, Ekaterinburg, 620219, Russia

Keywords: diffuse scattering, decomposition, ionic compound, NaCl-AgCl.

At present time there is a comparatively small number of investigation devoted to the study of the mechanism of decomposition of a supersaturated solid solution of ionic compounds. However, in order to establish common regularities of the mechanism of decomposition of a supersaturated solid solution of metallic and non- metallic system one must take into account the importance of studying the special features of the diffuse scattering and the structure of compounds with ionic type of binding. For this reason , the purpose of this investigation was the study of the diffuse scattering and change in the structure fine parameters in the course of aging of a supersaturated solid solution NaCl-AgCl.

Specimens of NaCl-AgCl. alloy containing 10, 17, and 30 mol.% of AgCl were investigated. Large-grain specimens were used to study the initial stage of the decomposition; for the second stage of decomposition single crystals were employed. The alloys composition was determined by X-ray technique using linear concentration dependence of the crystal lattice parameter of solid solution. The specimens were homogenized at 500^oC for 1 h and quenched in a stream of air or ejection on a metallic block. Cu Ka radiation was used for the X-ray analysis.

The distribution of the diffuse scattering intensity at the initial stages of decomposition was studied in detail in the reciprocal lattice space of alloys: NaCl-17 mol.%AgCl and NaCl-30mol.%AgCl. In general the regularity of variation of the diffuse scattering of these alloys can be described as follows. In the vicinity of (000) the diffuse effects show up in the form of halo. In the neighborhood of non-zero reciprocal lattice points the shape of diffuse effects and intensity depend on the type of Miller indices hkl and the content of AgCl in alloy. In the vicinity of reflections with even hkl indices the diffuse effects are crescent-shaped. If the condition S h²_i >20 is met, the diffuse effects are transformed into two diffuse maxims (type II satellites). In the neighborhood of reflections with odd hkl the diffuse effects for alloy NaCl-17 mol.%AgCl exhibit diffuse rings (halos), while for NaCl-30mol.%AgCl alloy they are crescent-shaped. The intensities of diffuse effects near reflections with odd hkl are considerably weaker than those near reflections with even hkl.

The appearance of halo-shaped, crescent-shaped or type II satellite diffuse effects can be explained only on the assumption that in a solid solution there exist spherical particles preferentially enriched in silver ions (GP zones). The average size of GP zones, determined from the slope of the diffuse scattering intensity curve is 3,45 nm, while the spacing between GP zones, calculated according to the intensity maximum position, is 16.9 nm. Electron microscopy investigation of the crystal structure at this stage of decomposition has revealed the presence of white spherical particles measuring about 10 nm and arranged in rows of <100>.

After several months aging at room temperature a transition to the second decomposition stage occurs. The analysis of diffuse scattering observed in X-ray patterns of alloy NaCl-10mol.%AgCl reveals the same peculiarities of diffuse scattering which are typical for alloy Al-20wt.%Zn: rods due to diffuse scattering along the <111> directions and an identical extinction law for diffuse effects in the vicinity of different reflections. The number of rods near each reciprocal lattice point varies from 2 to 4: for type (h00) it is four; for type (hk0) it is equal to two; for type (hkl) it is again four. The intensity of rods located near odd hkl reflections is considerably weaker than that of rods near even hkl reflections.

The best explanation of experimental data was obtained in the case when the emergence of diffuse scattering was associated with the existence of elastic distortion fields near coherent particles lying in the matrix {111} planes. Due to the crystalline and geometric binding between precipitates and the matrix there emerge rhombohedral distortions. The features of diffuse scattering established experimentally were explained on the base of the proposed structure model. In particular, the disappearance of diffuse effects in the vicinity of (211) reflections has been explained. The diffuse scattering observed in this alloy at a latter stage of aging can be considered as a result of modulation of the {111} plane spacing with account taken of correlation in a direction perpendicular to these planes. The existence of an intermediate metastable phase is also analyzed.

The existence of an intermediate metastable phase AgCl in the course of decomposition of the solid solution NaCl-AgCl suggests the following transformation sequence for this system:

GP zones->intermediate metastable phase AgCl_R-:cubic phase AgCl.